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If you are not located in the United States, you'll have -to check the laws of the country where you are located before using this ebook. - -Title: The Body at Work - A Treatise on the Principles of Physiology - -Author: Alexander Hill - -Release Date: August 31, 2020 [EBook #63089] - -Language: English - -Character set encoding: UTF-8 - -*** START OF THIS PROJECT GUTENBERG EBOOK THE BODY AT WORK *** - - - - -Produced by Paul Marshall, Turgut Dincer and the Online -Distributed Proofreading Team at https://www.pgdp.net (This -file was produced from images generously made available -by The Internet Archive) - - - - - - - - - -Transcriber’s Notes: - - Underscores “_” before and after a word or phrase indicate _italics_ - in the original text. - Equal signs “=” before and after a word or phrase indicate =bold= - in the original text. - Small capitals have been converted to SOLID capitals. - Illustrations have been moved so they do not break up paragraphs. - Old or antiquated spellings have been preserved. - Typographical errors have been silently corrected. - - - - -THE BODY AT WORK - - -[Illustration] - -[Illustration] - -[Illustration] - -[Illustration: FIG. 1.—PHOTOMICROGRAPHS OF CELLS OF THE CORTEX OF THE -CEREBELLUM AND CEREBRUM. - -_For description see p._ x. - -_Frontispiece._] - - - - - THE BODY AT WORK - - A TREATISE ON THE PRINCIPLES - OF PHYSIOLOGY - - BY - ALEX HILL, M.A., M.D., F.R.C.S. - SOMETIME MASTER OF DOWNING COLLEGE, CAMBRIDGE - - WITH 46 ILLUSTRATIONS - - LONDON - EDWARD ARNOLD - 1908 - - [_All rights reserved_] - - - - -PREFACE - - -Few subjects are as well provided with text-books as physiology; yet -it may be doubted whether the interests of the amateur of science have -been adequately cared for. From his point of view there are certain -obvious drawbacks to even the most admirable of text-books. Writing for -medical students, their authors assume that their readers have passed -through two years of preliminary training in physics, chemistry, and -biology; they take for granted that they will have the privilege of -supplementing their study of the theory of physiology with practical -work in a laboratory; they treat all parts of the subject with equal -thoroughness. In this book I have endeavoured to describe the phenomena -of life, and the principal conclusions which have been drawn as to -their interdependence and as to their causes, in language which will -be understood by persons unacquainted with the sciences upon which -physiology is based. I have omitted all reference to experimental -methods and to the technique of the science, save when a knowledge of -the means by which information has been obtained is essential to a -comprehension of its bearing. I have passed over such sections of the -subject as are generally considered unsuitable for ordinary discussion. -And since this book neither aims at being an introduction to the -systematic study of physiology, nor poses as an aid in the preparation -for professional examinations, I have treated with some thoroughness -the more recondite and the more suggestive results of recent research, -and have tried to indicate the trend of modern thought regarding -problems as yet unsolved. I have endeavoured to reflect the intrinsic -interest of the science apart altogether from its medical applications. - -An author who attempts the popular exposition of a science must -stand sufficiently far away from his subject to lose sight of its -details, whilst keeping its outlines clearly in view. The difficulty -of finding such a position is probably greater in the case of -physiology than in that of any other science. Few of its conclusions -are indisputable—even those which seem to be most in accord with -the balance of evidence. If my treatment of any vexed questions is -unjustifiably dogmatic, this will, I trust, be attributed to the -desire to present a definite picture, and not to forgetfulness of -considerations which seem to call for qualified statements. All -physiologists will agree that a book which recorded every piece of -evidence which is difficult to reconcile with the views generally -adopted would not only extend to an inordinate length, but would leave -a very indefinite impression on the mind of the reader. - -In many cases the value of a conclusion depends upon the reputation for -insight and accuracy of the physiologist who recorded the observations -upon which it is based. It is no want of appreciation of the genius of -the workers who have contributed most largely to the advance of the -science which has led me to omit, save in a few classical instances, -the names of all authorities. It is solely due to a desire to lighten -this book of all details not essential to the comprehension of the -propositions which it sets forth. - -The illustrations are reproductions of blackboard drawings. A few of -them have already appeared in my _Physiologist’s Notebook_ and _Primer -of Physiology_; but the large majority are now printed for the first -time. - - ALEX HILL. - _November, 1908._ - - - - -CONTENTS - - - CHAPTER PAGE - I. PROLEGOMENA 1 - II. THE BASIS OF LIFE 6 - III. THE UNIT OF STRUCTURE 26 - IV. THE FLUIDS OF THE BODY 37 - V. INTERNAL SECRETIONS 84 - VI. DIGESTION 96 - VII. RESPIRATION 164 - VIII. EXCRETION 194 - IX. THE CIRCULATION 217 - X. MUSCLE 248 - XI. THE NERVOUS SYSTEM 293 - XII. SMELL AND TASTE 364 - XIII. VISION 372 - XIV. HEARING 404 - XV. SKIN-SENSATIONS 423 - XVI. VOICE AND SPEECH 431 - - INDEX 441 - - - - -LIST OF ILLUSTRATIONS - - - FIG. PAGE - 1. PHOTOMICROGRAPHS OF BRAIN-TISSUE _Frontispiece_ - 2. THE ORGANS OF THE CHEST AND ABDOMEN xi - 3. MUCOUS GLAND, CAPILLARIES, AND CONNECTIVE-TISSUE SPACES 38 - 4. BLOOD-CORPUSCLES 60 - 5. SPLEEN-PULP, WITH PHAGOCYTES 81 - 6. DUODENUM AND NEIGHBOURING ORGANS 99 - 7. A LOBULE OF THE LIVER AND LIVER-CELLS 160 - 8. THE DIAPHRAGM DURING INSPIRATION AND EXPIRATION 172 - 9. THE KIDNEY 197 - 10. THE HEART IN LONGITUDINAL SECTION 218 - 11. HORIZONTAL SECTION OF THE HEART, SHOWING ITS VALVES 225 - 12. SECTION OF THE WALL OF A SMALL ARTERY 233 - 13. KYMOGRAPH 238 - 14. SPHYGMOGRAPH 244 - 15. BLOOD-PRESSURE TRACINGS 245 - 16. MINUTE STRUCTURE OF MUSCLE-FIBRES 262 - 17. THE BICEPS MUSCLE IN ACTION 286 - 18. ELECTRIC ORGAN OF SKATE 289 - 19. MINUTE STRUCTURE OF A NERVE-FIBRE 296 - 20. GANGLION-CELLS WITH NEURO-FIBRILLÆ OF THE LEECH 298 - 21. THE DEVELOPMENT OF THE GRANULES OF THE CEREBELLUM 304 - 22. TIGROIDS AND NEURO-FIBRILLÆ 321 - 23. MINUTE STRUCTURE OF THE CORTEX OF THE CEREBELLUM 339 - 24. MINUTE STRUCTURE OF THE CORTEX OF THE CEREBRUM 347 - 25. FUNCTIONAL AREAS OF THE CORTEX OF THE CEREBRUM 352 - 26. TASTE-BULBS 368 - 27. HORIZONTAL SECTION OF THE EYE 373 - 28. DEVELOPMENT OF THE CRYSTALLINE LENS 374 - 29. PURKINJE’S SHADOWS OF THE VESSELS OF THE RETINA 375 - 30. RETINA ADAPTED FOR OBSCURITY AND FOR BRIGHT LIGHT 377 - 31. SIMULTANEOUS CONTRAST 383 - 32. FORMATION OF AN IMAGE ON THE RETINA 391 - 33. THE FORM OF THE EYEBALL IN SHORT SIGHT, NORMAL SIGHT, - AND LONG SIGHT 392 - 34. THE BLIND SPOT 394 - 35. AN OPTICAL ILLUSION 398 - 36. AN OPTICAL ILLUSION 401 - 37. AN OPTICAL ILLUSION 402 - 38. EXTERNAL, MIDDLE, AND INNER EAR 411 - 39. THE COCHLEA 414 - 40. THE ORGAN OF CORTI 415 - 41. NERVES OF THE CORNEAL EPITHELIUM 424 - 42. TOUCH-CORPUSCLES 427 - 43. PACINIAN CORPUSCLE 428 - 44. THE GLOTTIS 432 - 45. THE LARYNX IN LONGITUDINAL SECTION 433 - 46. THE LARYNX FROM THE RIGHT SIDE 435 - - - - -NOTE ON THE FRONTISPIECE - - -Four photomicrographs of cells or parts of cells of brain-tissue, -coloured by the chrome-silver method (_cf._ p. 293). - -=A.= Cell of Purkinje from the cerebellum of a man aged 45. At the -bottom of the photograph is seen the rounded cell-body, with the -commencement of its axon. The summit of the cell-body bears an -elaborately branched system of dendrites, spread out in the plane of -the section. - -=B.= A single basket-cell of the cortex of the cerebellum (very highly -magnified). The oval cell-body gives origin to four dendritic processes -which branch. Thorns are to be seen on the larger process which -ascends on the right. From the same process, near its origin, springs -a delicate axon which thickens as it proceeds to form a basket at the -right hand lower corner of the photograph. Two other branches of the -same axon, which form baskets around other Purkinje-cells, are faintly -visible, although out of focus. - -=C.= Seven or eight pyramids from the cortex of the cerebrum of a -hedgehog. A little below the centre of the photograph is seen a large -pyramid with a single thorny apical process which bifurcates, several -basal dendrites and an axon. In the upper part of the photograph are -seen the apical processes of a number of pyramids of which the bodies -were not included in the section. - -=D.= The margin of the cortex (subiculum cornu Ammonis) from the same -specimen. A single row of pyramids extends across the photograph. They -are remarkable for the richness of branching of their basal processes, -which has earned for the cells which comprise this sheet the name of -“double pyramids.” - -All four sections were cut vertically to the surface. - -[Illustration: FIG. 2.—DIAGRAM SHOWING THE RELATIVE POSITIONS OF THE -ORGANS OF THE CHEST AND ABDOMEN. - - The ribs from the first to the tenth have been cut - across in the lateral line. The eleventh and - twelfth ribs do not reach sufficiently far forwards - to be cut. With the exception of a short segment - near its junction with the ascending colon, the - small intestine has been removed. The trachea is - seen to divide into bronchi beneath the arch of - the aorta. The right lung has three, the left - two lobes. The kidneys are situate behind all - the other viscera. On their upper ends rest the - two suprarenal capsules. The lower edge of the - right lobe of the liver follows closely the line - of the ribs and costal cartilages. Below the - left lobe of the liver the stomach comes to the - anterior abdominal wall. The transverse colon - (large intestine) comes to the anterior wall below - the stomach. Below the latter the wall is in - contact chiefly with coils of small intestine. The - vermiform appendix rests on the posterior wall. - Spleen and pancreas are not shown in the diagram.] - - - - -THE BODY AT WORK - - - - -CHAPTER I - -PROLEGOMENA - - -Physiology is the science of the body at work. It is the study of life. -Anatomy records how plants and animals are constructed. It maps and -measures. Physiology ascertains what they do, endeavours to explain how -they do it, and conjectures why. - -A knowledge of structure is essential to the right understanding of -function; but the physiologist does not contemplate structure with -a view to divining possibilities of action. He has no interest in -structure as such. To him it is a matter of perfect indifference -whether the tendon of a muscle is at its origin or its insertion. He -would rather not know which end of the muscle terminates in a tendon. -It is waste of his time to notice such a fact, save for the negative, -the protective value of the information. If he did not know how the -muscle and tendon are related, he might possibly imagine the muscle as -doing something of which it is incapable. Observers of living things -are often credited with studying structure with a view to determining -function. The reverse is the true order of thought and observation. -Living things perform certain acts. Having no inherent knowledge of -our own microcosm which enables us to say how it works, we cannot, -by reflecting upon our own internal operations, explain its various -activities. Nor can we make use of the results of introspection when -endeavouring to account for the acts of other beings. Our knowledge -of how things are done is altogether extrapersonal, objective. It is -the result of trial, failure, success in the use of apparatus, our own -essays, or those of others. The body is a combination of organs—a -term used somewhat loosely to designate any piece of the animal -mechanism which has a distinct function to perform. The physiologist -studies the results of the activity of an organ. He watches it in -action, and endeavours to explain the process by which it produces its -effects. Then follows the anatomist, who, taking it to pieces, examines -it with the utmost thoroughness which scalpel and forceps or microscope -allows, with a view to ascertaining whether its structure will support -the physiologist’s hypothesis as to its mode of action. This in the -vast majority of cases has been the history of scientific progress. -The physiologist has preceded the anatomist in drawing inferences as -to the manner in which things are done. The anatomist, after a further -examination of structure, has either admitted the plausibility of -his explanation, or has interposed the objection that the part was -incapable of working in the way supposed. - -This comparison of anatomy and physiology must not be pushed too far. -Enough has been said to emphasize the distinction between them. The -one treats of form, the other of function. The one looks at structure, -the other at action. Anatomy in its limited and logical sense has -nothing to do with the uses of a part; its business is to measure it. -Physiology has nothing to do with the measurements of parts; its duty -is to watch for movement. Every living thing may be contemplated either -in its statical or in its dynamical aspect. Physiology looks at it from -the latter point of view. - -Surveying his province, the physiologist asks himself: “Who are my -subjects? What am I to find out about them? What methods, in addition -to direct observation, may I use to obtain this information?” His -oversight embraces all living things. It is no longer reasonable to -make a distinction between human and animal physiology, or between -the physiology of animals and the physiology of plants. No human -being can take all science for his field. If he contents himself with -scratching its surface, he will assuredly raise but a meagre crop, and -that mostly weeds. But he is far behind the spirit of his age if he -declines to sow in his own little patch seeds of thought which have -blossomed in other localities, however remote. The man whose purpose -in studying physiology is to obtain a knowledge of the working of -the healthy human body, in order that he may know how to set right -the accidents, perversions, and premature decay to which human flesh -is prone, would remain an empiric of the most rigid type did he not -apply to the elucidation of his problems all conclusions reached from -the study of other organisms which are likely to prove pertinent. -There would be no science of human physiology had observation and -experiment been limited to Man. There would be no science of medicine, -it may be added, had not the mode of working of the human body, and -the influence of drugs upon it, been inferred from the results of -experiments upon animals—experiments which could never have been made -upon men. Blisters, blood-letting, mercury-poisoning, would still be -the physician’s remedies for all human ills. “Give the watch a good -shaking. It sometimes does good. If that fails, I cannot advise you -what to do, as I know nothing about the working of a watch.” Even -though we open the living human body, as must be done for the purpose -of making good such defects as are amenable to surgical treatment, -and for a little while observe its wheels go round, we are unable, -from fear of damaging the wheels, to introduce the mechanical tests -which would tell us how and why they revolve. The man must be allowed -to recover with uninjured organs. But, thanks to anæsthetics, there -is no test which may not be applied to a live animal with as much -propriety as to a dead one. Anæsthetics abolish the distinction, in its -ethical applications, between life and death, because we are under no -obligation, as in the case of the human being, to allow an animal to -recover. Many experiments upon animals will be recorded in this book, -and since the book is intended for the general public, who have been -singularly misled regarding the nature and methods of vivisection, an -opportunity is taken thus early of insisting that anæsthetics have -made all things, not only possible, but legitimate. It is unnecessary -to commence the description of each experiment with the statement that -the animal was first placed in a condition of complete anæsthesia, -or to end it with the statement that it was destroyed before it had -recovered from the effects of the anæsthetic. The reader may take these -facts for granted. In discussing the propriety of operating upon a -living but unconscious animal, we are playing a word game as old as -Plato’s day. What is life? What is the relation of the personality to -the animal machine which it occupies and operates? For a few minutes -a heart removed from the body continues to beat. In a physiological -sense it is alive, although the body from which it was removed is dead. -Yet the personality does not reside in the heart, as many generations -of philosophers believed. It is merely an accident that the body dies -when the co-ordinating mechanism, the heart, ceases to pump blood -through its vessels. Nor is the personality limited to the brain. -Without the sense-organs which place the brain in relation with the -body, and owing to the movements of the body—by which the sources of -sensations of smell, sight, hearing are ascertained—with the world -of which it forms a part, there would be no personality, no Ego. Is -it, then, coextensive with the body which exhibits it? A soldier -returning crippled from the wars does not finish out his days with his -personality curtailed. We are no nearer than was Plato to a definition -of life. Such a discussion soon takes us out of the realm of science. -Science is limited to the sphere in which the whole is greater than the -part. Take away consciousness, and personality ceases. Guarantee that -consciousness shall never return. The animal is dead. When considering -the propriety of vivisection we must regard life and consciousness -as inseparable. There can be no question of right or wrong in regard -to experiments on a dead animal, even though a sensitive mind, from -association, shrinks from contemplating them. A person who dislikes the -idea of dissecting a dead animal is influenced by purely subjective -and personal considerations; nor is he prompted by sympathy with an -unconscious animal when he recoils from the spectacle of its still -moving organs. The term “vivisection” conveys too large a meaning. A -negative term is needed, some word which will hold the emotion of pity -in check. Pity is misplaced when devoted to the unconscious subjects -of physiological experiment; and, happily for animals, as for Man, -anæsthetics suspend conscious life. Only a person who has undergone a -surgical operation can understand how resolutely the intellect declines -to adopt as part of itself things which have not come within its own -experience. The nurse’s testimony, that a long interval separated the -placing of the mask upon the face and the commencement of that dull -half-consciousness which gradually reawakened into interest in one’s -surroundings cannot be set aside. The nurse says that during that -interval knife, saw, and cautery were busy at their work. Her story -is accepted, but it is not believed. All physiological operations are -conducted under anæsthetics. In by far the larger number the experiment -is continued until life terminates, under anæsthetics. The only ground -upon which an objection to vivisection can be based is the ground that -it involves the infliction of pain, and it is with regard to this -that the greatest misapprehension exists in the public mind. Only in -experiments which have for their object the study of the effects of the -removal of a certain part, the diversion of a duct, the elimination -of the control of a particular nerve, is there any possibility, under -existing conditions, that an animal will suffer. In such experiments -as these, observations cannot commence until after the animal has -recovered. The operation is conducted under anæsthetics, and with the -utmost precautions, to prevent any disturbance of the animal’s general -health. The injury is in almost all cases of a comparatively limited -nature, and it is certain that it involves very little pain to the -animal when it has recovered from its anæsthesia, since, thanks this -time to aseptic surgery, there is no inflammation or other secondary -trouble. - -The field of physiology embraces the phenomena exhibited by all living -things, whether plants or animals. The vegetable physiologist works -in one part, the comparative physiologist in another. The work of the -human physiologist is more limited in scope. Yet there are few problems -relating to Man’s mechanism concerning which the physiologist can have -direct knowledge. His theories are based upon the results obtained by -experimenting upon animals. - - - - -CHAPTER II - -THE BASIS OF LIFE - - -Protoplasm was defined by Huxley as “the physical basis of life.” It -is the material substance which lives. There is no life in anything -which does not consist of, or is not supported upon, or permeated by -a system of filaments of protoplasm. Huxley’s definition indissolubly -links in thought protoplasm and life. But it is doubtful whether the -definition is in any sense axiomatic. The adjective “physical” has too -narrow a range. If the biologist could say to the chemist, “Here is a -substance which was alive. If I could restore to it the energy which -it has lost, if I could impart to it the movement which I recognize -as life, it would again be alive,” he would offer the chemist a -substance susceptible to the methods of his science, something which -he could analyse. If, approaching the physicist with a group of -chemical products, he could say, “Into these protoplasm broke up on -dying. I cannot assure you that while it was alive they were combined -into molecules within your meaning of the term. There may be no such -‘substance’ as protoplasm in the sense in which you understand the -word, but so long as this mass lived these various familiar compounds -were bound together in a supermolecular form. Death was their falling -apart. If I could cause them to recombine, they would be alive,” he -would give the physicist a problem within the range of his methods. -The physicist could devise a method for measuring these units. The -science which can weigh an electron, the thousandth part of an atom, -need not fear failure in its attempt to gauge the size of units of -structure composed of groups of heavy molecules, albumins, globulins, -and other proteins,[1] with the inclusion, perhaps, of fats, sugars, -inorganic salts. But herein lies the biologist’s dilemma. He cannot -assert that there exists a homogeneous substance, protoplasm. He cannot -assert that there exists a definite tectonic grouping of heterogeneous -substances which, so long as it is maintained, constitutes a physical -basis capable, and alone capable, of exhibiting the phenomena of life. -Protoplasm is still a hypothetical substance—a name. Truly, in the -absence of nitrogen-containing compounds of very complicated chemical -constitution there is no life. All living things yield on chemical -analysis approximately the same nitrogenous substances. No one can say -whether the capacity for living is dependent upon the molecular—that -is to say, the chemical—constitution of the basis, or whether it -is dependent upon the arrangement of its molecules, its form. It is -even open to question whether instability, the capacity for incessant -change, both in chemical composition and in form, be not the condition -which differentiates living matter from dead. “Physical basis” is too -hard a term for this elusive concept of the matter which exhibits life. - -If it were possible by a process of elimination to ascertain the -substances which must be present in protoplasm, the physiologist might -formulate a reasonable hypothesis as to the nature of this “basis.” -But there is no part of any living thing, or, at any rate, no part -which is not microscopic in its dimensions, which can be pointed -out as protoplasm and nothing besides. It is impossible to isolate -anything which can be described as pure protoplasm. Nor is it possible, -by comparing various tissues which are acknowledged to be rich in -protoplasm, to ascertain what chemical substances are common to them -all. - -If it were feasible, by analysing a number of specimens of protoplasm, -to make sure that, although _x_ is absent from one, _y_ from another, -and _z_ from a third, some one thing, _P_, is always present, then _P_ -might be regarded as the physical basis, even though it were evident -that _P_ alone was not protoplasm. Protoplasm would be _P_ combined -with either _x_, _y_, or _z_. Globulins and albumins and other proteins -are always present, but in varying proportions; but it is impossible to -make certain that either of these chemical substances is more important -than the rest. Nor is it possible to assert of either that it is -essential. - -Chemically, protoplasm is a mixture of substances, chiefly proteid, -in a condition in which it is capable of manifesting the phenomena -of life. But whether it be more complex and of heavier molecule than -either globulin, nucleo-protein, albumin, fibrin, or any other of the -nitrogenous compounds which take its place when it is dead; or whether -it be as simple as either of these, but differ from them all in its -instability, in the constant flux of its atoms, which causes it at one -time to incline towards one of them, at another time to another, are -questions which cannot at present be answered. - -The uncertainty as to the chemical nature of protoplasm is responsible -for an unfortunate irregularity in the use of the term. It is _ex -hypothesi_ the most active, the most living part of an animal cell. -If the cell has a nucleus and an envelope, the protoplasm must lie in -the space between the two. This part of the cell is therefore often -termed, without qualification, the “cell-protoplasm.” Frequently the -abuse of the word is carried still further. Young cells, leucocytes, -nerve-cells, etc., which have no envelope, consist of a nucleus -embedded in soft cell-substance. The latter is termed its protoplasm. -The cell is described as consisting of nucleus and protoplasm, the term -assuming an anatomical signification. Not only is such a use of the -term bad, because it indicates a confusion of thought, but it brings -with it a train of ambiguities. What are the limits of the protoplasm? -If the cell-body be firmer towards its exterior than it is within, is -the denser substance protoplasm, or is it not? It has not the qualities -which are attributed to protoplasm in so marked a degree as has the -substance which it surrounds. Hence a distinction is made. The one -is “ectoplasm,” the other “endoplasm.” Within the cell-body are many -collections, often in the form of granules, of substances which have -not the protoplasmic attributes. They constitute the “deuteroplasm” -of certain cytologists. But these enclosed substances may be as far -removed from protoplasm as starch grains. It is absurd to use the -termination “plasm” for such well-defined products of cell activity -as these. The subject is, unfortunately, obscured by conflicting -terms. Nomenclatures which were invented with the object of giving -definiteness to our ideas have served but to perplex them. The term -“protoplasm” should be reserved as a synonym for the substance which -is most alive, the substance in which chemical change is most active, -the substance which has in the highest degree a potentiality of growth. -Anatomical distinctions are better expressed in anatomical terms. We -shall treat of such distinctions when considering the organization of -the cell. - -In the meantime it may be well to consider the attributes which appear -to belong to this most living substance. Its chemical composition can -be inferred only from the compounds found on analysis to be present -in a mass of organized substance which there is reason for thinking -was rich in protoplasm while it was alive. The compounds found vary -within certain limits. The quantity of water associated with these -compounds is still more variable. Water is essential to the existence -of protoplasm. Its power of combining with water in variable quantities -is one of its characteristics. Tissue rich in protoplasm yields on an -average about 75 per cent. of water. Part of the protoplasm within a -cell holds more water associated with it, part less. - -Closely associated with its power of holding water is its tendency to -assume an architectural form. In large vegetable cells, such as those -of the hairs within the flowers of Tradescantia, the protoplasm may be -seen, under the microscope, arranged in threads containing granules -which are incessantly streaming up and down them. The spaces between -the threads are filled with water. Such mobile protoplasm cannot be -said to have a structural form. But in the greater number of cells, and -especially in animal cells, the protoplasm is disposed in a network, -with usually a tendency for the strands of the network to set in lines. -In attempting to define these very variable networks, the microscopist -is obliged to speak with caution. He finds it very difficult to -distinguish between appearances which he is justified in regarding as -inherent in the cell-substance, whether alive or dead, and appearances -which he may have induced by the action of reagents whilst preparing -the tissue for examination. Rarely can he assert that he sees a network -in a living cell. When examining a dead cell, he is bound to recognize -that the preservatives and hardening reagents which he used may have -caused the proteins to coagulate in a particular pattern. If he obtains -the same pattern with several different methods, he infers that the -appearance which he sees is that of a structure existing in the living -cell; but he is never quite sure that it is not an arrangement produced -by reagents after death. - -The tendency of protoplasm to dispose itself in the form of a network -or sponge-work is of the greatest interest in its bearing upon the -theory of its activity in effecting chemical change. The body itself, -as we shall find later, is a network of tissues enclosing lymph. -The lymph in the tissue-spaces contains foods and waste products in -solution. The tissues are constantly taking from it the former, and -discharging into it the latter. Every cell is, microscopically, a -tissue. The strands of its protoplasm are perpetually sorting foods -from its cell-juice, adding to its cell-juice waste products. By -diffusion, foods, including oxygen, pass from lymph to cell; waste -products, including carbonic acid, pass from cell to lymph. If water -be added to gum, the gum swells. The mixture is homogeneous. Diffusion -takes place slowly through the mucilage. When water is taken up by -protoplasm, the protoplasm swells; but the mixture is not homogeneous. -The protoplasm expands as a wet sponge expands, although the relation -of the enclosing reticulum to the water which it encloses is far more -complicated. It is, as it were, a sponge made of gum. Some water is -combined with the protoplasm; the remainder fills its spaces. There is -an active surface relation between the free water and the protoplasmic -threads. As water rises in a capillary tube, as it passes from the -inside to the outside of a flannel shirt, so it circulates within the -cell. - -=Irritability= is a property commonly attributed to protoplasm, -but it is a little doubtful whether there be not again some danger -of an illogical use of terms. An amœba, one of the unicellular -organisms found in ponds, has the power of moving. If a piece of a -water-plant—the stalk of duck-weed is a suitable object—be examined -with the microscope, these little animals are usually to be found -upon its surface. They feed upon algæ more minute than themselves. -When they come in contact with something suitable for food, their -body-substance flows around it. The food is coagulated. So much of it -as is digestible is digested; the remainder is extruded. Constantly -parts of the body-substance are protruded, other parts retracted, in -the search for food. Such movement is a response to stimulus. Stimuli -received at one part of the body-substance are transmitted to another. -The body-substance is irritable. It acknowledges stimuli; it conducts -them. But if the amœbæ are watched until, owing to lack of oxygen or -other cause, they die, their irritability comes to an end. It is a -phenomenon of life. Again the physiologist is in a dilemma. Either -protoplasm is not protoplasm when death has supervened, or protoplasm -is not irritable as such. It is somewhat paradoxical to ascribe to the -physical basis of life a property which depends upon its being alive. - -Yet the influence on protoplasm of anæsthetics makes it difficult to -understand how it can be either physically or chemically a substance -which loses its form or changes its constitution whenever it ceases -to display the usual evidences of its existence. Chloroform and -similar agents suspend irritability. Yet irritability returns as their -influence passes off. They appear to hold it in check without—at any -rate visibly—changing the nature of the irritable substance. - -All parts of the minute body-substance of an amœba are equally -irritable. In higher animals irritability is concentrated in the -nervous system. The form of irritability to which consciousness is -adjunct is restricted to the cortex of the great brain. - -Chloroform and similar agents are termed “anæsthetics” because they -abolish the irritability of the cortex of the great brain, before -their effects upon other parts of the nervous system are sufficiently -pronounced to endanger the working of the animal machine. Pain ceases -to be felt before the dose of anæsthetic is sufficient to suspend -the irritability of the centres of reflex action. All protoplasm, -whether animal or vegetable, is susceptible to the influence of these -agents. They cause it to enter into a state which resembles death -in all respects save the impossibility of revival. There is a great -demand in the Paris flower-market for white lilac in the winter. The -plant cannot be forced until after a period of rest. By withholding -water and placing the bushes in a cool, shady place, horticulturists -endeavour to send them prematurely into their winter sleep. Recently -it has been found that from three to four weeks can be gained by -placing the bushes for a couple of days in an atmosphere charged -with the vapour of ether. Some change of state is evidently produced -in protoplasm by anæsthetics. It ceases to be capable of receiving -or transmitting stimuli. But we cannot picture the change as being -sufficiently pronounced to justify the hypothesis that so long as it is -irritable protoplasm is a complex substance which is resolved, as it -loses its irritability, into simpler compounds familiar to the chemist. -Perhaps it would be more correct to say, we cannot picture these -chemical substances as reuniting into protoplasm when the effect of the -anæsthetic passes off. Rather are we driven to think of living matter -as a mixture of many substances in a state of molecular interchange, -and to suppose that the activity of this interchange is diminished by -anæsthetics. - -=Chemical activity= is a property of protoplasm. In its network -combinations and decompositions are effected more extensive in range -than any which a chemist can cause to occur in his laboratory. From -ammonia, carbonic acid, and water, a plant makes albumin. A chemist -cannot make albumin, no matter how complex may be the nitrogenous -substances which he endeavours to cause to combine. Albumin is resolved -by animals into water, carbonic acid, and urea. Cells of the gastric -glands set a problem which puzzles the chemist by making hydrochloric -acid from sodic chloride without the intervention of a “stronger” acid. -Many other illustrations of the same kind might be cited. Although the -tissues of animals act chiefly as destroying agents, their protoplasm -is not without constructive power. There is apparently no limit to the -capacity for synthesis of plants. The chemistry of living things may -be divided into two provinces, absolutely antagonistic in the series -of reactions which they comprise. The one series is constructive, -synthetic; the other destructive, analytical. Construction involves -the locking up of energy. It is endothermal. Destruction results in -the setting free of energy. It is exothermal. To accomplish synthesis -energy must be added. Plants obtain it from the sun’s rays. Animals -disperse energy, set free by the analysis of substances formed in -plants, in maintaining their bodies’ warmth and movement. - -The chemistry of the laboratory and the chemistry of protoplasm present -certain contrasting features. A chemist reaches the compound which he -wishes to form by effecting a series of interchanges. For example, he -wishes to form uric acid by uniting a nucleus contained in lactic acid -with urea. First he introduces chlorine and ammonia into the molecule -of lactic acid. He makes trichlorlactamide. Then he heats (supplies -energy to) a mixture of trichlorlactamide and urea. Two of the chlorine -atoms carry off hydrogen atoms from the urea. A third leaves the -trichlorlactamide with its ammonia. Water also breaks away. Uric acid -remains. - - Trichlorlactamide Urea Uric Acid - - CCl₃CH.OH.CO.NH₂ + 2(NH₂)₂CO = C₅H₄N₄O₃ + NH₄Cl + 2HCl + H₂O. - -In this example the trichlorlactamide may be said to exchange its -chlorine and ammonia for urea. When he planned the reaction, the -chemist foresaw what would happen. He knew that if he weakened the grip -of the lact radicle upon them, chlorine and hydrogen, chlorine and -ammonia, oxygen and hydrogen, would take the opportunity of getting -away together. The lact radicle and urea would be left with dangling -arms, which must “satisfy their affinities” by linking up. It would be -rash to assert that any reaction is impossible to Nature’s chemistry; -but it may safely be said that the reactions which protoplasm effects -are, so far as we know them, of a different type from this laboratory -example. Uric acid is the chief excrement of birds. It is made in -the liver. If the liver is shut off from the circulation, lactate of -ammonia is excreted in the place of uric acid. It is therefore, in all -probability, lactate of ammonia which the liver transforms into uric -acid. We cannot pretend to say how this is done, although an empirical -formula for the change might be drafted easily enough. - -Lactate of ammonia has the formula NH₄C₃H₅O₃. Uric acid, C₅H₄N₄O₃, -contains a much higher percentage of nitrogen. It could be produced -from lactate of ammonia by the condensation of the nitrogen-containing -nucleus and the addition of a sufficient amount of oxygen to complete -the oxidation of the superfluous carbon and hydrogen into carbonic -acid and water. It is of little interest to count the number of atoms -concerned in this process. If a bird be fed upon urea, or even upon -various salts of ammonia, its liver will change them into uric acid. -Lactate of ammonia is the nitrogen-containing compound with which the -liver has normally to deal. It can handle almost any other combination -of nitrogen with equal ease. In the protoplasm of the liver the atoms -in the molecule of lactate of ammonia are rearranged. The molecules are -condensed; water is set free; oxidation occurs. It seems almost as if -molecules, when in contact with protoplasm, lose their individuality. -Their atoms fall into new groups. Chains which the chemist finds -so difficult to break—chains from which he can remove a link only -by insinuating another and a stronger—are, when in contact with -protoplasm, groups of isolated links. The links rearrange themselves. -They join into new circlets, larger, smaller, more open, closer. As -grains of sand on a metal plate group themselves in harmony with the -vibrations caused in the plate by drawing a violin bow across it, so -the atoms answer to the forces which set protoplasm vibrating. There -is no waste of force. The chemist may need to enclose sawdust and lime -in a crucible heated in an electric furnace if he wishes to compel -them to combine as carbide. He supplies energy enormously in excess of -the amount which the new compound will lock up. Under the influence of -protoplasm the reactions which occur are exactly proportional to the -amount of energy supplied. Or, if it be a reaction by means of which -energy is set free, it occurs spontaneously. No energy is absorbed in -setting it going. All the energy liberated is effective. The chemist -very frequently needs to heat a substance in order to cause it to -decompose, even though it be falling from a less stable to a more -stable state. - -Vital chemistry and mineral chemistry are so widely different in their -methods that one is tempted to think of them as different in kind. We -find it very difficult to look at both from the same point of view. -Men’s minds are preoccupied with the things that they have to do for -themselves. The chemistry of the laboratory is seen as a science -circumscribed by the laboratory walls. If it were possible to stand -outside, it would be evident that it is only a part of the science of -molecular change. Matter changes its state under the influence of -force. Many rearrangements are effected by the chemist which do not -occur in nature. He has an almost infinite range of action. Yet many -of the rearrangements of matter and force which are occurring in the -dandelion on his window-sill (if the fumes of sulphuretted hydrogen -have not killed it) he is unable to reproduce. It is largely a question -of waste. Nature works with greater precision than the chemist; but the -chemist could do all that Nature does if he had but the same control of -force. - -We have spoken of the reactions which occur in protoplasm as divisible -into two great series—the one ascending, constructive, endothermal; -the other descending, destructive, exothermal. In the one series -energy is locked up; in the other series it is set free. Synthesis and -analysis are names applied to the two series respectively. Synthesis -is characteristic of plants, although analysis is also perpetually -occurring. Plants fix carbon from the air and liberate oxygen. They -also respire, setting free carbonic acid. Analysis is characteristic of -animals, although synthesis is not excluded. - -Of the chemical processes which occur in plants very little is known. -Few halting-places between raw materials and finished products can be -marked. The final products are sugars and starches, oils, proteins, -and a vast number of other substances—alkaloids, glucosides, etc. -Condensation, dehydration, and deoxidation are the methods by which the -synthesis of these compounds is accomplished. These methods are adopted -simultaneously in varying degree. The large group of bodies known as -sugars and starches are, with few exceptions, built on the C₆H₆ model; -in fruit-sugar, C₆H₁₂O₆, six atoms of carbon are linked to one another -and to six molecules of water. The formula of starch is (C₆H₁₀O₅)ₙ. Not -only has water been removed from the molecule, but an unknown number of -molecules have been linked together. This condensation and dehydration -is effected whenever sugar carried in cell-sap is deposited as starch -in seeds or tubers. These compounds are hexatomic. The chemist -pictures them as made by the union in the first place of six atoms. As -small drops unite to form larger ones, so small molecules, under the -direction of the protoplasm of plants, close together. - -The reactions which characterize animal protoplasm are of a different -kind. They belong to the descending series. Close molecules are -unfolded. Water is incorporated with them. Hydrogen and carbon are -oxidized into water and carbonic acid. The conversion into sugar of -glycogen or of starch may be taken as an illustration of expansion. -Starch, (C₆H₁₀O₅)ₙ, becomes maltose, C₁₂H₂₂O₁₁, and then dextrose, -C₆H₁₂O₆. The grouped molecule of starch opens out. The breaking of the -double molecule of maltose into two molecules of dextrose is a further -illustration of progress towards simplicity. Hydration, union with H₂O, -accompanies this expansion. Hydrolysis is the secret of almost all -digestive acts. Starch is hydrolysed into sugar, fat hydrolysed into -glycerin and fatty acid, proteins hydrolysed into peptones. - -All the chemical transformations which protoplasm is able to accomplish -are of the nature of fermentations. The term =fermentation= was -first applied to the effervescence which occurs in grape-juice when -its sugar is being converted into alcohol, carbonic acid gas, and -certain substances which appear in relatively small quantities. -It was discovered later that the yeast which effects this change -is a unicellular plant. The term “fermentation” was extended to -the production of vinegar from alcohol, and eventually to all such -reactions as are carried out by living organisms, or by the secretions -or products of living organisms, without the destruction of the agent -which is effective in the process. A ferment is an organic body which -brings about changes in other bodies without itself undergoing change. -At the end of the process, however prolonged, there is as much ferment -as there was at the beginning, and its chemical nature is the same. -Rennin has been made to curdle nearly a million times its weight of -milk, pepsin to digest half a million times its weight of fibrin. As -the ferment is not consumed, there is no relation, except one of speed, -between the ferment and the quantity of fermentable substance which -it is able to transform. We said that a ferment is an organic body. -It is necessary to introduce the qualification “organic,” because -certain reactions termed “catalyses” which occur in mineral chemistry -resemble fermentations in respect of the non-destruction of the agent -which serves as intermediary. If a solution of cane-sugar containing -a very small quantity of sulphuric acid is boiled, the cane-sugar is -“inverted.” It is changed into a mixture of fruit-sugar and levulose. -The ferment invertin of the gastric juice and of intestinal juice -produces a similar effect; and just as invertin remains unchanged, so -also the sulphuric acid is found in the mixture unchanged in nature and -in amount after an unlimited inversion of cane-sugar. Great stress was -formerly laid upon the similarity between fermentation and catalysis. -It has now been shown that catalytic actions are not necessarily of the -same nature as fermentation, although the results and, as far as is -visible, the means are similar. For example, finely divided platinum -(or, better, palladium) causes an indefinite quantity of oxygen and -hydrogen to unite. The reaction comes within the category of catalyses. -But it is widely different from a fermentation. The metal causes -hydrogen to condense, and actually absorbs it into its surface layer. -In the liquid form hydrogen cannot resist combination with oxygen. This -may be termed a “physical phenomenon,” adopting the common distinction -between chemistry and physics. There is no reason for thinking that -fermentations can be explained in so simple a way. They may, however, -be grouped under the designation “catalyses.” As the initial conditions -and final results are similar, it is inevitable that fermentations and -catalyses should obey the same “laws” as to mass action, speed, effect -of accumulation of products of action, and the like; but it does not -follow that invertin and sulphuric acid produce their effects in the -same way. Fermentations are instances of catalysis, but all catalytic -actions are not fermentations. - -So far from dwelling upon the resemblance between fermentation and -the catalysis of mineral chemistry, chemists nowadays incline to -regard fermentation as essentially a reaction of life. It is very -difficult, when attempting to present ideas which are new to thought, -to adapt, without ambiguity, existing words. It would be absurd to -talk of a substance removed from yeast or bacteria or blood-corpuscles -by a process which involves cooling with liquid air, grinding with -powdered glass, solution in water, precipitation with absolute alcohol, -and resolution in water, as alive. Yet, unlike any known mineral -product, it is easily killed. Ferments are not destroyed by cold, -but their activity is arrested. They are most active at about the -body temperature. Their activity is annihilated by heating them, in -solution, to the temperature at which albumin coagulates—a little -over 50° C. Although they are not alive, their behaviour very closely -resembles that of living matter. They can be obtained only from living -things. They produce their effects even though they are present in -almost infinitely small quantity. It is impracticable to make a -chemical analysis of a ferment, owing, in the first place, to the very -small amount available for analysis, and, in the second place, because -of the impossibility, with existing methods, of obtaining a ferment -pure. The amount of ferment present in even a great mass of yeast, -or in many pounds of salivary gland or pancreas, is extremely small. -However prepared, it is always accompanied with proteid substances. -It is impossible to say whether ferments, like proteins, have heavy -nitrogen-containing molecules. The fact that they are not diffusible -suggests that they have. - -It would be straining language to term fermentation a phenomenon of -life; worse, to define life as a sequence of fermentations. Yet it -is safe to say that all the chemical changes carried out by living -organisms are fermentations. Fermentation and the chemistry of life are -almost synonymous terms. - -A very large number of ferments are already known. Each has its own -specific work to do: “To every fermentable substance is fitted a -ferment, as a key to a lock.” It will be understood, from what has -been already said regarding our inability to determine the composition -of any ferment, that we cannot say whether or not these various -ferments differ one from another in chemical constitution. They are -classified according to their action, and not according to their -nature. Those which build up are termed “synaptases” (συνάπτω, I -unite); those which decompose, or hydrolyse, “diastases” (διάστασις, -separation). The termination “ase” is added to the name of the -substance upon which the ferment acts, except in cases in which other -terms have already become so general as not to be displaceable: -amylase, hydrolysing starch; sucrase, inverting cane-sugar; protease, -hydrolysing proteins. Unfortunately, there is little uniformity in -this nomenclature; amylopsin, invertin, pepsin, are terms used as -often as those terminating in “ase.” As a distinguishing termination, -“in” or “sin” is less desirable than “ase,” owing to the fact that -it has been appropriated already as the termination of the names of -albuminoids—_e.g._, gelatin, chondrin, mucin. - -The various ferments are substances which protoplasm sets aside for -specific purposes. Primitively, contact with the substance to be -fermented determined the nature of the ferment assigned to the task. -There are reasons for thinking that protoplasm still retains its power -of making a suitable response; cases may be cited in which the lock -presented to protoplasm shapes the wards of the key. In such cases the -fermentable substance provokes the formation of the ferment. But, for -the most part, in situations where particular ferments are regularly -needed, protoplasm has acquired the habit of making such ferments and -no others. The cells of salivary glands accumulate ptyalin, the cells -of gastric glands accumulate pepsin, during the intervals between meals. - -The capacity of protoplasm for producing a new ferment when it is -needed is shown by such examples as the following: Blood-plasm contains -a variety of proteid substances. If a solution of white of egg be added -to it, the mixture is clear and uniform. Yet egg-albumin is treated by -the blood as a foreign body, a poison. When injected into the veins of -a living animal, some of it is excreted by the kidneys, some destroyed -in the blood-stream. If several successive doses of egg-albumin are -injected into an animal (it is most convenient to inject it into the -peritoneal cavity), the power of the blood to destroy the intruder is -greatly increased. If now a specimen of blood be taken, and the plasma -or serum mixed with egg-albumin, the mixture is no longer clear. The -egg-albumin is precipitated. The blood of the animal thus “prepared” -has developed a ferment, termed a “precipitin,” which throws down -egg-albumin. If instead of egg-albumin, which, although a foreign body, -is comparatively innocent, a substance which is distinctly poisonous, -toxic, be injected into an animal, the first dose, if a large one, will -prove fatal. If, however, the first dose be small, and succeeding doses -progressively larger, the animal acquires the power of tolerating a -quantity of the poison much larger than would have proved fatal in the -first instance. A classical example of this, because it afforded an -opportunity of directly observing under the microscope the difference -between “unprepared” blood and blood from an immune animal, is the -acquisition by a mammal of the power of tolerating the injection of the -blood of an eel. Eel’s blood contains a toxin which destroys the red -blood-corpuscles of a mammal. The dissolution of the blood-corpuscles -may be watched with the microscope. If successively increasing doses of -serum of eel’s blood be injected into the body of a rabbit, the rabbit -acquires the power of resisting the toxin. Further than this, the serum -of the immune rabbit injected into a rabbit which has not been prepared -confers immunity upon the latter. If the blood of the prepared animal -be mixed with the blood of an unprepared rabbit and with eel’s serum, -and the mixture examined under the microscope, it will be seen that -red blood-corpuscles are no longer dissolved. The immune serum is able -to save the blood-corpuscles of the unprepared blood from destruction. -During its course of preparation the rabbit developed an antitoxin. - -If germs of diphtheria are injected into the blood of a horse, the -first injections give rise to marked febrile symptoms. After a number -of injections the horse becomes completely tolerant of the virus. Not -only does its blood develop sufficient antitoxin to protect it against -the toxin of diphtheria, however large may be the quantity injected -into its system, but the serum of the prepared horse, when injected -beneath the skin of a child suffering from diphtheria, carries with it -sufficient antitoxin to destroy the toxin which has gained admission to -the child’s blood. - -Many more instances might be cited of this capacity of developing -“antibodies” of protoplasm. The leucocytes of the blood are incessantly -adapting their chemistry to the needs of the economy. All the tissues, -it may be supposed, possess the power of developing resistant ferments; -but the leucocytes (Fig. 4) are the undifferentiated cells, the -maids-of-all-work. They have not specialized as makers of ptyalin or -makers of pepsin. They are not completely given up to lifting weights, -like muscles, or carrying messages, like nerves. - -Bacteria are the world’s scavengers. To them ultimately belongs the -task of reducing organic matter to the salts which plants reorganize. -The cycle of life would be broken if bacteria were suppressed. No -sooner has an animal fallen than these little agents commence their -beneficent task of resolving its carcass into air and soil. Birds -and insects may interrupt their work. They may steal portions of the -derelict, use them for fuel, or patch them between their own ribs. But -they, too, will soon lie breathless on the ground; and the bacteria -are always ready to finish their interrupted task. Why should they -wait until the slight change occurs, important to us, but of little -consequence to them, which marks the transition of living protoplasm -into dead proteins? There is nothing in the constitution of protoplasm -which makes it harder to break up than protein. There is no quality -inherent in living matter which makes it resistant of decay. We resent -the officiousness which prompts bacteria to obtain entrance into the -ship while it is still under full sail, with a view to commencing the -work of demolition. Deep in our minds lies the conviction that it is -contrary to the rules of Nature. We are especially annoyed at the many -ruses bacteria adopt to disguise their personalities. The bacteria of -the soil we can keep at a proper distance. But bacteria of the stream, -bacteria of milk, bacteria of the breath that would betray us with -a kiss! It is hard to recognize that they are fairly and squarely -playing their part. Birds and insects we can beat off with our hands. -Our invisible enemies are everywhere. They are constantly insinuating -themselves through scratches in the skin, through abrasions in the -mouth, through surfaces of the intestine left unprotected owing to -the desquamation of its epithelium. But if we are constantly open to -attack, we are policed by myriads of zealous leucocytes, ever ready to -reduce the invaders to impotence. The germs which have found entrance -fire off a toxin. The leucocytes reply with an antitoxin. There is -absolutely no limit to the power of protoplasm to protect itself, if -only it be not taken by surprise. It can resist any organic poison if -it is allowed a sufficient time to produce the antipoison. The ferment -of pancreatic juice, trypsin, is a poison which is unlikely to find its -way into the blood. When injected it produces disastrous results owing -to its immense activity in digesting proteins. An animal “prepared” by -the injection of successive doses of trypsin develops an antitrypsin. -Injection of pancreatic juice no longer does it any harm. Tapeworms -which live in the intestines are bathed in pancreatic juice; they are -constantly exposed to its digestive action. They are not digested, -because they secrete an antibody which prevents the development of -the activity of trypsin. It is not in this case, strictly speaking, -antitrypsin. It is antikinase, a substance which, if extracted from the -bodies of tapeworms and added to pancreatic juice, renders it incapable -of digesting albumin. The antikinase does not destroy trypsin, but -destroys kinase, the co-operation of which is essential to its activity. - -Not only has protoplasm the power of meeting with an antiferment any -ferment which might prove prejudicial to its own integrity; but after -it has been once attacked it continues to defend the vulnerable spot. -Its tactics are, it must be confessed, somewhat like those of the dusky -warrior who, during his first lessons in the art of boxing, made a -point of covering with his fist the place where he had just been hit; -but even its power of remembering its last injury is of supreme value -to the human race. Before the age of sanitary science, and even, in -certain backward communities, in these days of its beneficent rule, -conditions producing disease were not necessarily set right as soon -as the epidemic was over. The close-packed inhabitants of a ghetto -were continuously exposed to germs of typhoid fever, small-pox, -whooping-cough. But after their protoplasm had once responded to -the need for the production of an antigerm, it either continued for -many years to keep a stock in hand, or it kept the recipe within -easy reach. The memory of protoplasm is amazing. It is commonly said -that vaccination is an absolute protection for seven years. There is -no doubt but that the immunity from small-pox which it induces, if -gradually lessening, lasts for life. The disease, if it attacks a -person who has been vaccinated in infancy, is relatively harmless. - -Inoculation, vaccination, is the boxing-master’s method of utilizing -the self-protective instinct of the dusky warrior. Knowing that his -pupil will for a long while continue to cover an injured spot, he asks -himself: “Where is he most likely, when it comes to a serious contest, -to be hit?” Then he gives him a gentle tap in that particular place. -Does he need to know how to defend himself against small-pox? Give him -cow-pox. Is he likely to receive a knock-down blow from typhoid fever? -Just show him what it feels like to have a gentle shake. Educate his -protoplasm to make antityphoid ferment, by giving him the typhoid germ -in such an attenuated form that it cannot do him any harm. - -The chemistry of protoplasm is a science which is growing rapidly, -or, to speak less arrogantly and more correctly, our knowledge of the -ways of protoplasm, the Chemist, has greatly increased during the last -few years. We can but watch protoplasm at work. Our experiments, so -called, are but windows which we open in the walls of his laboratory. -We cannot take the work out of his hands. The methods of mineral -chemistry are useless in this search for knowledge. And, naturally, -the longer we watch, the more details do we discover in what seemed at -first a generalized procedure. We recognize that several manipulations -are required in the carrying out of a reaction which hitherto we -believed to take place in a single stage. This is not the place in -which to give an account of a subject regarded as belonging, owing to -its applications, to the province of pathology. But Nature is one, -however many be the companies into which we divide the explorers of -her secrets. We have attempted the merest outline of the observations -made up to the present, and have submitted the results for the sake of -the light which they throw upon the way in which ferments are prepared -as they are wanted to meet the needs of normal every-day digestion and -metabolism, rather than for the purpose of showing the methods by which -protoplasm combats disease. - -Amongst the chemical phenomena of life is respiration. =Respiration= in -this very general sense means oxidation. The force which is exhibited -in living is obtained from the union of organic materials with oxygen -under the direction of protoplasm. This is true of plants as well as -of animals. It is true even of the subdivision of bacteria, termed -anaerobic, because they cannot live in air. They secrete ferments which -enable them to decompose compounds which contain oxygen, in order that -they may use the oxygen for respiration. It might have been supposed -that green plants which are receiving radiant energy from the sun -would convert this energy into the forces which enable protoplasm to -display the phenomena of life. But this is not so. The energy which -green plants obtain from the sun is used in constructive metabolism, -and not in maintaining life. Life-force, if we may use the expression, -is derived from the oxidation of the substances which the sun’s rays -enable the plant to make. A plant, equally with an animal, respires. -The distinction between the constructive metabolism of a plant and its -respiration may be brought out in a striking way by administering to it -sufficient anæsthetic to stop the former without stopping the latter. -It may be paralyzed without being killed. If a water-weed—potamogeton -is the most convenient—enclosed in a bell-glass filled with water and -inverted over a dish of water, is placed in sunshine, bubbles of gas -rise from the plant. They accumulate at the top of the bell-glass. -If the gas be removed and analysed, it is found to be oxygen with a -small admixture of carbonic acid. If a second bell-glass containing -water-weed be exposed under the same conditions in all respects, save -that a small quantity of chloroform is added to the water, the gas -that collects at the top of the bell-jar will be much less in amount. -It will be found to be carbonic acid without admixture of oxygen. -The power which chlorophyll possesses of decomposing carbonic acid -with fixation of carbon and liberation of oxygen is suspended by the -anæsthetic; whereas respiration is not interfered with. - -Lastly, we must attribute to protoplasm a =capacity of growing=. The -activity of protoplasm depends upon constant molecular interchange. It -incorporates molecules of food. It excorporates molecules of waste. If -food is abundant and “vitality” exuberant, it takes in more than it -gives out. It grows. - -If we attempt to formulate a definition of protoplasm, we find that our -ideas are far from clear, owing to want of knowledge. The questions, -What is protoplasm? What is life? are equally unanswerable. Their -definition is reciprocal. Protoplasm is the substance, the material, -which exhibits life. Life is the complex of phenomena exhibited by -protoplasm. All parts of the body are alive, in their degree. The -nucleus of a cell lives, as well as its cell-body. Its capsule may be -less alive—that is to say, less vibrant—than the soft cell-substance -which it encloses; but it lives. So-called intercellular substance, or -matrix, is alive. In growing cartilage the matrix does not behave as a -dead substance. It does not crack and gape under the pressure of the -dividing and multiplying cell-bodies which it contains. If the windows -of a house were endowed with the power of spontaneously enlarging, the -walls would be crushed. They would bulge, break, tumble. The matrix -of cartilage offers as little resistance to the enlargement of the -cells which it encloses as the plasma of blood to the multiplication -of blood-corpuscles. It grows with the cell-bodies, and must be -considered as divisible into areas, each of which is the periphery of -a cell. Muscle is alive. So, too, are bone, teeth, hair, nails. But as -we proceed outwards we find the quality of aliveness growing less and -less apparent, until at last we acknowledge that it is unrecognizable. -Vibrations diminish in amplitude and in rapidity, until the material of -which the body is made appears to be at rest. - -Biologists apply the term “protoplasm” to the _most living_ substance -of which plants and animals are composed. It may be that there is an -entity, protoplasm. It may be that in certain situations this exists in -an unmixed state. It may be that the degree of aliveness of a tissue -or constituent part of a tissue varies as the quantity of protoplasm -which it contains. The tendency of protoplasm to dispose itself in a -reticulum in the meshes of which other substances accumulate favours -such a view. The cells of the deeper layers of the skin are rich in it. -The superficial layers are composed chiefly of keratin. It is possible -that the network opens out, and its strands grow thinner and thinner, -as keratin accumulates. But it cannot be demonstrated that this is the -case. There is no completely satisfactory reason for concluding that -the life of a cell of the skin resides in its protoplasmic network, -while its keratin is inert. - -Many attempts have been made to prove that living cells contain -something which dead cells do not contain; but no evidence which will -bear sifting has, as yet, been adduced in support of this thesis. - -FOOTNOTE: - -[1] Proteïn, _subs._, proteïd, _adj._, general terms for complex -nitrogenous substances, such as albumin (white of egg), the less -soluble globulins, fibrin of blood, casein of milk, etc. - - - - -CHAPTER III - -THE UNIT OF STRUCTURE - - -Immediately after its discovery in the seventeenth century, the -compound microscope was applied to the study of minute plants and -animals, their organs and tissues. In this connection and for this -purpose the microscope has steadily improved, until perfection has -almost been attained. Calculations based upon the physical properties -of refracting media show that the limits of the assistance which it -can give to the eye have been very nearly reached. One of the first -results of the application of the microscope to the study of parts -of plants was the discovery of their cellular structure. Robert -Brown, looking at slices of cork, saw that its tissue is divided into -compartments. It is difficult to ascertain who it was that first used -the word “cell.” The resemblance of a slice of vegetable tissue or the -surface view of a petal of a flower to honeycomb is so striking that -the same comparison probably occurred to the mind of everyone who saw -it. Further study with better instruments showed that the cells are not -empty. Each cell contains cell-juice, or cell-substance, and in the -centre of the cell-substance a miniature cell, the nucleus. Naturalists -therefore extended the connotation of the term. A cell was no longer a -space with enclosing walls; it had contents. A nucleus was invariably -a constituent of the cell. The cell was regarded as an anatomical -unit, consisting of a wall, cell-contents, and nucleus. In 1839 -Theodor Schwann, using his microscope in the study of animal tissues, -recognized the similarity between animals and plants. Animals also, he -discovered, are aggregations of cells. He enunciated the =Cell Theory=. -Philosophers are always ready to generalize. It is their business. -Seeing that vast numbers of organisms are single cells, that they -feed, breathe, divide, and reproduce their kind—in fact, carry out all -the functions of life—as isolated cells, they conceived the idea that -a visible plant or animal is a community of cells, each an organism -in itself. As bees are units of a swarm, as men and women are units -of a state, cells are units which for the sake of mutual protection -remain associated in a multicellular body. The physiological or -sociological aspects of this conception we shall consider shortly; but -the anatomical basis of the cell theory was laid without a sufficient -testing of the facts upon which it rests; or, rather, one ought to -say that, although the axiom, enunciated by Virchow when he applied -the cell theory to tumours and other morbid growths, _Omnis cellula a -cellulâ_, holds good, the applications of the theory which certain of -its later exponents have made are not necessary sequents. - -Every plant, every animal, commences its existence as a single cell. -An organism which is permanently unicellular divides. Each of the -separate cells into which it divides is a new individual. Higher -plants set aside certain cells as ovules, which in due course, after -conjugation with pollen grains, grow into plants. In the same way the -ova of animals, by repeated cell division, reproduce the species. The -individual commences as a single cell. Its complicated body, composed -of various organs and various tissues, is formed by the multiplication -of cells. Each of the innumerable cells of which it is composed has -the structure, and may therefore be presumed capable of performing -all the various functions, of a unicellular organism. But it does not -follow that the cells retain their individuality. Even unicellular -plants (_e.g._, the extraordinary vinegar and tan fungi, myxomycetes) -may for a time merge their individuality in a common mass formed by the -aggregation of many cells. - -The cells of higher plants are not always, or even generally, -anatomically distinct. Their protoplasm, the essential part of every -cell, is united with the protoplasm of neighbouring cells by threads -which traverse the cell-walls. The cells of the connective tissues -of animals are united into a web, or syncytium. This is especially -noticeable during early stages of growth. Nerve-cells are connected -together by conducting filaments (neuro-fibrillæ). It is possible -that nerve-cells and the muscle-fibres which they innervate are from -the beginning united by nerve-filaments—that the nerve-cell and -muscle-cell grow apart without severing this thread-like connection. -Certain anatomists regard the nerve strand which connects a cell in -the central nervous system with a number of muscle-fibres, placed, -it may be, at a great distance from the nerve-cell, as the bridge -which has never been broken in the process of cell division and -displacement, which made one primitive cell into a nerve-cell and a -group of muscle-cells. Muscle-fibres are not separate cells, but cell -complexes. Each muscle-fibre contains scores, in some cases hundreds, -of nuclei (Fig. 16). It is a cylinder, perhaps 2 inches long, in which -cell division is incomplete. Tendons are bundles of exceedingly slender -fibres which lie side by side, like silk threads in a skein. The row of -cells which gives rise to a tendon undergoes incomplete cell division. -Their nuclei divide, and a small quantity of soft body-substance is -set apart for each nucleus. The rest of the mass consists of fused -cells. It constitutes a continuous rod, which becomes fibrillated as -it grows. Vegetable cells are separated by cell-walls. Animal cells -tend to develop intermediate partitions; but the partitions are so -thick that they can no longer be described as walls. In cartilage the -cell-bodies are embedded in a great mass of intercellular substance, -or matrix. In this intercellular substance elaborate developments may -take place. Elastic fibres may make their appearance in it to form -elastic cartilage, as in the case of the epiglottis. In these various -instances, although it is perfectly true that tissues are formed by -cell division, the cells are not, strictly speaking, separate units. -They are not completely divided one from another. It is impossible to -recognize their anatomical boundaries. - -But there is a much more serious difficulty in applying the cell -theory—the difficulty of deciding what are the essential parts of -a cell. Long ago it was recognized that many animal cells—white -blood-corpuscles, for example—have no cell-wall. It was therefore -decided that cell-body and nucleus are the only essential parts. But -what is to be said of the red blood-corpuscles of mammals? (Fig. 4). -Are they cells? They have neither cell-walls nor nucleus; nor does -their substance present the structure which is usually associated with -the “body-substance” of cells. They are not produced, if the view held -by many histologists be sound, by cell division, in the ordinary sense -of the term, but appear as spots, gradually growing into discs inside -the body of a blood-forming cell. The discs are extruded when they -reach their full dimensions. Yet the tissue, blood, is composed of -these blood-discs and the intermediate substance blood-plasm. Mammalian -blood might be dismissed as a non-cellular fluid secretion containing -formed elements, if it were not for its history. In all animals below -mammals the red corpuscles are cells with nuclei and cell-bodies. The -absence of nuclei in mammals is due to the recognition by Nature of the -fact that, as the blood-cells will never be called upon to divide, it -is a waste of material to provide each of them with a nucleus. Not only -would the nucleus be useless, but it would take up space, diminishing -the capacity of the corpuscle for carrying hæmoglobin. The process of -cell division is in consequence curtailed. There are, it is true, other -ways of looking at this problem. The cells which line the bloodvessels -stand in some sort of nutritive relation with the blood. When the -lining cells of the bloodvessels are injured or inflamed, the blood -clots. But here again it is somewhat straining a point to say that -these lining cells are the cells of the blood, and the blood a kind of -intercellular substance; especially as a distinction would have to be -made between mammals with non-nucleated blood-corpuscles and birds with -complete blood-cells. - -The physiologist, if he is to feel sure of his ground, needs to know -the minute anatomy as well as the naked eye anatomy of the body. But -what is there that he does not need to know? He must be chemist, -physicist, biologist, pathologist, and expert in various other branches -of science. Microscopic anatomy, or histology, as it is commonly -termed, will be called upon in this book only when it has evidence to -give which bears directly on physiological problems. We have dwelt -at some length upon the cell theory because the physiologist needs -starting-points. He needs to have in his mind a conception of the -fundamental structure of the body. Protoplasm is the material which -lives. We begin with protoplasm albeit our conception of protoplasm is -so difficult to formulate that we are obliged to admit that in using -the term we are almost guilty of playing with words. Protoplasm is -the most living substance. The substance which is most alive always -presents itself to us as an imperfectly transparent, viscous material, -which proves on analysis to contain a large quantity of certain -proteins mixed with various organic and inorganic compounds. Protoplasm -is organized into, or distributed amongst, cells, which in any given -tissue present a fairly uniform size. What determines the size of -cells? Speaking generally, cells are small—say about 0·01 millimetre -in diameter. In early stages of growth, cell division occurs as soon -as the cell attains to something like this size. It would seem that -when nutriment is abundant cells add to their protoplasm more than they -lose. Having attained certain dimensions at which the conditions most -satisfactory for cell life reach their limit, cell division occurs. The -big drop falls into two smaller drops, each of which grows more rapidly -than the big one was growing at the time when it began to divide. -But if there be an optimum size for nutritive purposes, this limit -is suspended in many cases, and for various reasons. Take the ovum -itself as an example. It is vastly bigger than the cells into which -it divides. The yolk of a hen’s egg is, when first formed, a single -cell. By the time the egg is laid cell division has already set in. In -the embryo there are cells which surpass the average dimensions—the -unexplained “giant cells” which appear in the liver as soon as it can -be recognized as such (_cf._ p. 65). These disappear from the liver, -but are for a time evident in the spleen. The large cells found in the -marrow of bone, some with a great single nucleus, others containing a -bunch of separate nuclei, also show that there is no fixed limit of -size. It is generally considered that the giant cells of marrow—or, -at any rate, those which are multinucleated—are leucocytes which are -engaged in scooping out the bone; consuming the hard tissue on the -inner surface of the hollow cylinder in order that, by deposition of -new material on the outside of the cylinder, the size of the whole -bone may be increased—leucocytes battening on bone which, owing to -interference with its blood-supply, is breaking down. They have not -time to divide. Nourishment is superabundant. Although much too large -for a vigorous standard of cell life, they continue to grow, putting -off the duty of cell division until the supply of nutritious food -begins to run short. - -The most remarkable variations in size are to be found amongst the -cells of the nervous system. It may be given as one of the most -distinctive characters of nervous tissue that its cells have no fixed -or standard dimensions. A nerve-cell enters into connection with -other nerve-cells and with muscle-fibres by means of branches, or -cell-processes, as they are termed. The cells may be globular, as in -the sympathetic system, or star-shaped. Each cell gives off a certain -number of processes, which divide like the branches of a tree, and one -process which may run for a very long distance without dividing. This -latter thread-like process places it in communication either with a -distant part of the central nervous system or with the muscle-fibres -which it controls. By means of such a thread a cell in the spinal cord -may be connected with muscle-fibres of the hand or of the foot. The -thread is really a bundle of filaments (neuro-fibrillæ) which separate -to supply a number of muscle-fibres. It is, in its whole length, a -part of the cell in which it originates. The size of the cell varies -as the number of filaments in this bundle (termed the “axon”), and -possibly also as their length. Hence it comes about that nerve-cells -may be amongst the smallest, or they may be the very largest, in the -body. The so-called “granules” of the cortex of the cerebellum and -of the cerebrum are almost as small as red blood-corpuscles (Fig. -23). Each of them has five or six minute branched processes and an -exceedingly delicate axon. The large cells of the cerebral cortex, -which send their axons far down the spinal cord, and the large cells of -the spinal cord which supply the muscles of the body, have a diameter -ten or twelve times as great as that of a granule. But larger still -are the nerve-cells which supply the electric organs of the torpedo -and other electric fishes (p. 295); and largest of all are the cells -which innervate the curious “fishing-rods” of the strange angler fish -(_Lophius piscatorius_). It is difficult, owing to their irregular -shape, to say how large these cells are; but they are visible to the -naked eye. - -The anatomical unit of structure is the cell. Cells are the bricks of -which the body is built. Some are large, others small, as befits the -part which they take in the construction of the body. If the tissue be -merely a supporting tissue, connective tissue, cartilage, bone, its -cells are uniform in size and small. If it have functions to perform -which in some cases are carried out best by small cells, in other cases -by large ones, the cells are adapted in size to the work that they have -to do. Of the various kinds of wandering cells, some—the bone-forming -cells (osteoblasts), for example—are small; others—the bone-eating -cells (osteoclasts)—relatively large. Nerve-cells, like telephone -exchanges, are large or small according to the size of the area which -each supplies. - -All animals of complex organization, from starfishes and sea-urchins -to Man, are inhabited by =motile cells=. In addition to the bricks -which enter into the construction of its fabric, each fixed in its -place and definitely united to its neighbours, the animal contains -leucocytes which wander through its tissue-spaces or float down -the streams of lymph or blood. We are disposed to speak of these -wanderers as inhabitants of the body, to distinguish them from the -elements which enter into the construction of their habitation. It -is difficult to avoid the temptation of describing the body as a -habitation. Allegorical as Aristotle’s distinction between body and -soul—between the habitation and that which inhabits—may seem, when -contrasted with the exact language of modern science, it would save -many a periphrasis if we might still use the monosyllable “soul.” The -fixed tissues constitute a unity, bound together by nerves. The work -done by glands and muscles is done in response to directions conveyed -by nerves. It is impossible to say where the control of the nerves -ceases—to point out any fixed tissue which is not co-ordinated with -other tissues, nor susceptible to the influence of the environment -as impressed upon the central nervous system, through the medium of -sense-organs. The fixed tissues constitute a habitation for the “soul.” -They share in a common life. The wandering cells are as independent -of control as the parasites which occasionally find entrance into the -body. Each must have a soul of its own in Aristotle’s sense. Like -parasites, they carry on all the business of nutrition, respiration, -cell division, without reference to the needs of the fixed tissues. -They take what they require from the lymph as it leaves the intestines -loaded with the products of digestion; they take it from the lymph -in the tissue-spaces; they take it from the blood. When nutriment -or oxygen runs short, they do not share the privations of the fixed -tissues. Only indirectly is their well-being affected by that of -the body as a whole; only accidentally is the death of the body the -occasion of their death. The same might be said of such parasites as -the “blood-worms” of Egypt, or the trypanosomes (the cause of “sleeping -sickness”) of Equatorial Africa. Occasionally, in the rare disease -lymphocythæmia leucocytes multiply exceedingly, not, apparently, -in response to a call for their presence in large numbers, but in -defiance of the needs of the economy, and with baneful results. To the -indispensable services which wandering cells render, frequent reference -will be made. In the present connection, and while we are searching -for the principles of construction of the animal body, it would be -desirable, if we could do so, to define the status of wandering cells. -If they entered the body from without, they would be parasites of -commensal type, intruders who share in the food and shelter of the body -in return for service. But they do not enter from without. They are -cells of the growing body which, detaching themselves from the cells -which are forming tissues, assume a wandering life. They are not to be -recognized in the embryo until development is considerably advanced. -Their origin is far from clear, but histologists believe that, although -they are not recognizable as wandering cells in the earliest stages -of growth, they, or rather their parent cells, are set apart at a -very early date. Probably they are not formed in the embryo proper, -but in the “extra-embryonic area,” from which they emigrate into the -embryo. In this sense they come in from outside. But, after all, the -extra-embryonic area equally with the embryo is a product of the ovum. -Looking at the body as a whole, we recognize a common life, a soul -in Aristotle’s sense, which inhabits the framework of fixed tissues; -and at the same time we see a multitude of independent cells, each an -organism in itself, produced, like amœbæ, from similar independent -cells by cell division, absorbing the body-fluids, consuming invading -germs and fragments of decaying tissues, dying, disintegrating, in -their turn absorbed. Wandering cells are autonomous in the largest -sense. - -All multicellular plants and animals are formed by division of a -primitively single cell, the segments remaining in contact. As the -scale of life is ascended, the cells which are massed together in -the body, whether of a plant or of an animal—we are still unable to -find any word other than body for the thing as a whole—tend more and -more to differ in appearance. Some are large, others small. Some have -cell-walls; others have none. Some remain “protoplasmic”; others are -largely composed of “metaplasm.” Better terms are wanted to connote -“most living substance” and “less living substance” respectively. It -would be easy to coin suitable words, but, alas! the nomenclature -of physiology is already hopelessly encumbered, and there is little -prospect that a bad word will die when a good one is available in its -stead. Differences in structure indicate differences in function. A -division of labour has set in. The cell starts with capacities for -every function. Its particular situation renders it desirable that -it should cultivate one capacity at the expense of the rest. It -specializes in a particular direction. If it happens to be placed -in the centre of the body on the course of the bloodvessels which -bring to the embryo food and oxygen from its mother, it develops a -great capacity for taking up food. It accumulates in its substance -a vast quantity of nutriment which it cannot consume, holds it, and -passes it on into the blood-stream as it is required. Thus the liver -is formed. In the embryo it attains to a great size, equal to about -one-half the whole body-weight; but whether storing food be its chief -function at this stage, or whether the other special functions for -which it is responsible are equally important, remains a question -for further research. In subsequent life its main work is to store -food. After birth, when the child prepares its own food by processes -of digestion in its stomach and intestines, the blood-supply of the -liver is so modified that the blood from the digestive organs is -passed through it. Now and for the rest of life the liver is the -storehouse of food, the larder of the body. It is a reservoir from -which supplies are distributed as required. A liver-cell retains many -primitive characters. It is soft and destitute of envelope. But under -the microscope it appears, unless it be taken from a starving animal, -unlike any other cell (Fig. 7). It is always loaded with masses of -glycogen. Sometimes it contains fat globules also. This is perhaps the -simplest of all instances of specialization of function. An amœba can -take up food. Presumably it always absorbs as much as it can get, the -simple law of growth with cell division making it impossible for it -ever to get too much. The cells which in the liver are so fortunate as -to be placed on the route along which food is carried into the body -retain the appetite of an amœba, but lose its capacity for growth and -cell division. They return to the blood-stream, when it is deficient in -food, the stores which they took up when food was in excess. - -The specialization of a gland-cell is opposite in kind to that of a -liver-cell. It takes up no more food than it requires, but it has -developed a great capacity of producing from the food a substance which -would no doubt be needed for its own purposes were it an isolated cell, -but which the gland-cell places at the service of the body as a whole. -An amœba can digest proteid substances. A cell of the pancreas produces -the ferment necessary for the digestion of proteins, and secretes it -into the alimentary canal. - -To take another instance of specialization. An amœba responds to -stimulation by changing its shape. It contracts in one direction, -expands in another. A muscle-fibre has developed the capacity of -contraction at the expense of all other functions. During the course -of its growth it changes from a round cell into one that is elongated. -The elongation is in the direction in which it acts with greatest -efficiency. Its cell-substance is very highly specialized in order that -it may have the maximum capacity of contraction in this direction. - -Sensory cells develop to a maximum the capacity of responding to -external force; nerve-cells, the capacity of conducting the impulses -generated in sensory cells. The body is a republic in which every -citizen develops to the highest degree the capacity of doing the thing -which his situation makes it desirable for him to do. - -The possibility of isolated cell life, and the necessity within -certain limits of cell division, have led biologists to dwell too -much upon the independence of the separate cells of which the body is -composed. Protoplasm organizes itself into cells, but cells are not -necessarily anatomically distinct. They may be the partially separate -elements of a syncytium, or there may be but the faintest traces of -cell separation. The objection to looking upon cells as isolated, -self-complete units does not hold good to the same extent when they are -viewed from a physiological standpoint. A cell is an administrative -area. For purposes of nutrition, respiration, and cell division it is -autonomous. It is responsible for its own local affairs. If a part is -cut off from it, this part loses its vitality; this, at least, is the -conclusion drawn from the atrophy of the axons of nerves when they are -cut off from the cells of which they are outgrowths. Apparently we must -understand by “the cell,” when speaking of the cutting off of a part, -the portion of the cell which retains the nucleus; although we must -be careful not to lay too much stress upon the nucleus as the centre -of cell life. Red blood-corpuscles, as already pointed out, have no -nuclei, and yet they live. Cell growth, estimated by mere increase in -size, does not depend upon the nucleus. Many cells of the skin and its -appendages increase considerably after the nucleus shows changes which -clearly indicate that it is far advanced towards decay. But increase -in protoplasm, cell growth in a legitimate sense, and especially cell -division, are dependent upon the presence of an active nucleus. While, -therefore, histologists no longer formulate the cell theory in the -restricted terms in which it was enunciated some years ago, they still -regard the cell as the unit of structure and unit of function. The body -is built of cells, and whatever is done by the body as a whole is done -by its individual cells. - - - - -CHAPTER IV - -THE FLUIDS OF THE BODY - - -From one-fourth to one-third of the whole body is fluid. If the skin -be regarded as a water-tight bag, three-fourths or rather less of its -contents are solid, one-fourth liquid; and even its apparently solid -contents, the tissues, contain much water. Water is an essential -constituent of protoplasm. It is also present in cell-juice. The -estimate given above does not include the fluid within the cells, but -only the fluid with which the cells are bathed. In a general sense -this extracellular fluid, excluding blood, is termed =lymph=. It -occupies the spaces of a gauzy “connective tissue,” which connects, -or separates—the terms are equally appropriate—muscles, nerves, -glands, and other tissues of specialized function. Nowhere, except, in -a fashion, in the spleen, does blood come in contact with a cell. The -lymph which more or less surrounds them is the bath from which cells -receive their food and oxygen, into which they excrete carbonic acid -and tissue-waste. The network of lymph-spaces is traversed by capillary -bloodvessels with walls composed of flattened connective-tissue cells. -Such cells are usually spoken of as elements of an “endothelium.” As -the epithelium covers the surface of the body, so endothelium lines -its cavities. Endothelial cells are thin scales or tiles with sinuous -borders dovetailed one into another. That the tiles which form the -walls of capillary vessels are not cemented together in any proper -sense is shown by the facility with which white blood-corpuscles, -leucocytes, by their amœboid movements, push them asunder when making -their way from the blood-stream into the tissue-spaces, or _vice -versa_. They offer no more resistance to a leucocyte than a pair of -curtains hanging in front of a door offers to a child. Yet so long as -the endothelial cells are alive they keep their edges in such close -apposition as to constitute a continuous membrane which shuts off -blood from lymph. They are always close enough together to prevent red -blood-corpuscles from escaping from the capillary vessels; but their -resistance to the passage of the different constituents of plasma -varies greatly. The membrane which they compose is more complete and -less pervious, or less complete and more pervious, in accordance with -the nature of the tissues which surround it, and their varying needs. -The blood-passages of the liver may be described as filters. The escape -of red blood-corpuscles into lymphatic vessels is prevented, but they -offer practically no resistance to the plasma. Plasma—“lymph,” as -it is termed as soon as it is outside bloodvessels—passes through -the walls of the capillaries of the liver unchanged in constitution. -Where they traverse glands (other than the liver), muscles, skin, -and various other structures, the walls of capillary vessels, while -offering practically no resistance to water and diffusible salts which -can pass through membranes, prevent proteid substances from passing -from blood to lymph, except in extremely small quantities. In this way -an exquisite balance is automatically maintained. Water and salts pass -out as they are needed. But they never pass out in excess, because the -protein-containing blood-stream tends to keep them in, in virtue of the -same attractive force which enables it to suck in the oxidized products -thrown into the lymph by the tissues. Whatever a tissue needs it takes -from the lymph. Suppose that bone is being formed. Large quantities of -lime and phosphates are needed for the calcification of the cartilage -in which it is modelled. The cartilage absorbs lime and phosphates -from the lymph which bathes it. Lime salts and phosphates immediately -begin to diffuse from blood into lymph. The hurrying blood-stream -brings up further supplies from the walls of the intestine, products -of digested milk and other foods. Lymph contains (although not in the -same proportions) everything which blood contains. Many an analogy may -be found in the world of economics, although no illustration would be -sufficiently complete. From the lymph tissues take the fuel that they -need, the oxygen with which to burn it, the foods for their own repair, -the raw materials for their arts. Into it they throw their smoke, their -drainage, the slag and refuse of their factories. The blood replaces -the supplies as they disappear. It absorbs all waste. Lymph occupies -streets, market-place, passages, corridors. The blood-stream is a -closed system, rolling down the streets and through the market-place, -on its never-ceasing circuit from port and mine to open air and open -sea. From the alimentary canal it picks up food and fuel; the lungs -give it oxygen, and disperse its carbonic acid; the kidneys purge it of -non-gaseous waste. - -[Illustration: FIG. 3.—A DUCTULE AND TWO ACINI OF A MUCOUS GLAND -OF THE MOUTH, WITH A MUSCLE-FIBRE CUT LONGITUDINALLY; CAPILLARY -BLOODVESSELS AND CONNECTIVE TISSUE. - - Stellate connective-tissue cells form a labyrinth of - intercommunicating lymph-spaces which separate the - gland-cells and the muscle-fibre from the walls - of the capillary bloodvessels. The capillaries - contain circular red blood-corpuscles and nucleated - leucocytes. Some of the leucocytes are squeezing - their way either out of a capillary into a lymph-space - or _vice versa_. A granular leucocyte is to be seen - in a lymph-space at the bottom of the picture.] - -The facility with which the constituents of blood pass out to the -lymph, and the constituents of lymph pass into the blood, depends -upon the condition of the walls of the capillary vessels. Water -and substances dissolved in water might pass through the wall of a -capillary vessel in either of three ways—by filtration, by osmosis, -or by secretion. A filter is a porous barrier, which allows water and -all substances dissolved in water to traverse it. The solution passes -through unchanged in composition. Only solid particles are kept back. -The rapidity with which fluid passes through a filter varies as the -difference between the pressure on the one side and the pressure on -the other. A membrane does not allow of filtration. Water and things -dissolved in water pass through it by osmosis. Some things it will -not allow to pass; such, for example, as gum, mucin, white of egg. To -others it offers resistance in varying degrees. Most of the things that -can diffuse through a membrane are capable of crystallization; but the -membrane exercises some control over the passage of even crystallizable -substances when in solution. If a membranous tube containing water in -which proteins, sugar, and various salts are dissolved is hung in a -basin of pure water, the proteins remain in the tube; the sugar and -the salts pass through its wall into the surrounding water. But they -pass at different rates. Those of small molecular weight pass more -quickly than those whose molecule is heavy. After a time a condition of -equilibrium is established. No more salts pass out of the tube. If now -the contents of the tube and the contents of the basin are analysed, -it will be found that the tube contains all the proteins, some of the -sugar, and some of each of the salts, although not in the proportions -in which they were present at the commencement of the experiment. The -water in the basin contains some sugar and some of each of the salts, -but not in the same proportions in which they are found in the tube. -As a matter of fact, the same number of molecules would be present, -per unit volume, on each side of the membrane—in the tube and in the -basin. In this respect the percentage composition of the two solutions -would be the same. But some of the molecules being heavy, others light, -the weight of salts which unit volume of the solution in the tube -would contain would not be the same as the weight of salts in unit -volume of the solution in the basin. A membrane exerts a discriminating -action on the substances which pass through it. Secretion is osmosis -in disguise. It may be even filtration in disguise. A gland-cell (like -an amœba) takes things up and passes them out without regard to their -osmotic equivalent. It seems to exercise a choice. It seems to act -in disregard of the laws both of filtration and of osmosis. So, at -least, it appears to us when we are looking at the result in ignorance -of what has happened inside the living cell. The passage from blood to -lymph and _vice versa_ through the wall of a capillary vessel is in -certain situations or at certain times a mere process of filtration; -at others a process of restricted filtration. If the wall is behaving -as a perfect membrane, it is a process of diffusion, or osmosis. It -seems unnecessary to regard it, in any case, as a process of secretion. -The more widely the capillaries are dilated, the less resistance do -they offer to exudation. The narrower their calibre, the greater is -the restraint which they place on the escape or entrance of fluid. -When the skin of the palm of the hand is not sufficiently thick to -protect the soft tissues beneath it from the injurious effects of the -prolonged pressure of an oar or an axe, the capillary vessels of the -under-skin dilate; more lymph transudes; the skin is raised up as a -blister. The same thing happens when the capillaries are dilated and -paralyzed by scalding water. The fluid of a blister has much the same -constitution as blood-plasm, except that it contains less proteid -substance. These results might be regarded as purely mechanical—the -direct effects of pressure or heat upon the membranous capillary wall. -But the “vital” element is more important. The capacity of endothelium -to act as a barrier depends upon its nutritive condition—its vital -integrity, as it might be termed; which no doubt in the last resort -means its chemical relation to the fluids which bathe it. Now and again -blebs, like blisters, are formed on the skin—the herpes which appears -about the mouth; urticaria, which is more generally distributed; and -various other cutaneous disorders. Frequently a connection can be -traced between these eruptions and the consumption of a particular -food. An attack of urticaria results not uncommonly from eating -lobster, mussels, rook-pie, or some few other articles of diet. Various -things—bad fish, for example—may produce the same effect; but -shell-fish have an especially evil reputation. If extract of lobster -or of mussels be injected into the blood of an animal, the amount of -lymph which leaves the blood is markedly increased. The extract acts -as a poison upon the endothelium of the capillary walls. It increases -its permeability in all conditions in which lymph escapes in undue -quantity from the blood-stream, or escapes more rapidly than it is -absorbed; the nutritive condition of the endothelium is disturbed. Its -unusual permeability is due in part, no doubt, to the dilatation of the -capillary tube, the stretching of its membranous wall; but it is due -also to the diminished vigour of the endothelial cells. They have lost -to a certain extent their capacity for holding their edges in perfect -apposition. - -When the circulation is sluggish, owing to the inefficiency of the -heart, the tissues become œdematous. In other words, lymph accumulates -in the tissue-spaces. When the skin of a healthy person is pressed, it -returns to its natural position as soon as the pressure is removed. -If there is a tendency to dropsy—for ages the term “hydropsia” has -been thus familiarly clipped—the finger leaves a pit behind it when -pressed upon the skin. It is some little time before the lymph in the -connective-tissue sponge readjusts the surface. Excessive escape of -lymph from the blood, or its insufficient return into the blood, may -also be the result of obstruction to the flow in the great veins. When -the veins of the leg are varicose, the weight of the column of blood -in the distended vessels impedes its circulation. After standing, the -tissues about the ankle become œdematous. The œdema disappears on lying -down. A hardening (cirrhosis) of the liver impedes the circulation of -the blood which comes to it through the portal vein from the walls of -the alimentary canal. The capillaries of the stomach and intestine -are distended. Lymph accumulates in the abdominal cavity, producing -ascites, another form of dropsy. - -It is almost hopeless to attempt to disentangle the various factors -which disturb the balance between blood and lymph—excessive outflow -from blood, deficient inflow from lymph, stretching of the endothelium -of the capillary tubes, imperfect nutrition and consequent imperfect -apposition of the endothelial scales, increased permeability of the -scales. The exudation which accompanies inflammation would seem to be -due to the diminished vitality of the endothelium rather than to a -mechanical factor, such as increased blood-pressure in the capillaries, -and their consequent distention. Ascites is, apparently, a purely -mechanical result of the resistance offered to the passage of blood -through the liver; but pleurisy, the accumulation of lymph in the -space between the lungs and the chest-wall, cannot be explained in -the same way. There is no undue pressure on the vessels in which the -blood circulates through the inflamed pleura (the investing membrane -of the lungs and lining membrane of the chest), yet the walls of the -capillaries fail to maintain a proper balance between blood and lymph. - -Hitherto we have spoken of the lymphatic system as a labyrinth of -communicating spaces containing stagnant fluid, which is kept in a -fitting state by egress and ingress out of and into blood. Such a -mental picture is substantially correct. But the system is complicated -by the presence of lymphatic vessels. Cells of the connective-tissue -sponge-work arrange themselves side by side. They flatten into -endothelial scales. The borders of the scales close up. They form -lymphatic channels, wider than blood-capillaries, but strictly -comparable in every other respect. The lymph capillaries unite into -larger vessels. The larger vessels are connected by cross-branches; -they form plexuses. Their walls are strengthened with fibrous tissue. -Like the veins, they are abundantly provided with valves, which -check any tendency to a backward flow on the part of the fluid which -they contain. Lymphatic plexuses surround and accompany the larger -bloodvessels. They are disposed on the surface of muscles and glandular -tissues. They are abundant beneath the skin. Nearly three centuries -ago the lymphatic vessels of the mesentery, which collect products of -digestion, especially fat, from the walls of the alimentary canal, -were recognized owing to the milkiness of their contents after a -meal. They were, on this account, termed “lacteals.” Other lymphatic -vessels, owing to their transparent walls and colourless contents, are -not easily seen; but they are readily injected with mercury or other -fluids which render them conspicuous. In the upper part of the thigh, -in the armpit, or in the neck, they are about large enough to admit a -crow-quill. Those from the lower limbs, from the viscera, and from the -walls of the abdomen converge to a receptacle which lies in front of -the spinal column. The receptaculum chyli is continued upwards as the -thoracic duct, which pours the lymph into the great veins of the left -side of the neck and of the left arm just where they join together. - -The thoracic duct provides for the overflow of lymph from the spaces -of the body. There is no circulation of lymph. Lymph from the liver -and from the intestines is constantly draining into the thoracic -duct, and thus returning to the blood-stream by a short direct route, -entering it without the necessity for reabsorption through the walls of -capillary vessels. By no means all of this fluid has exuded from the -blood-stream. Much of it is water which was poured into the stomach -as gastric juice, and into the intestines as the secretions of the -pancreas and other glands, or imbibed through the mouth and absorbed -by the lymphatics of the alimentary canal. The remainder of the water -taken up from the alimentary canal enters its bloodvessels. The diluted -blood flows to the liver, loaded with digested products which the -liver will store. As the blood parts with them the additional water -which has served for their transport exudes from the capillaries of -the liver into lymphatics, which empty it into the thoracic duct. -Large quantities of water are used in washing out digested products. -Secreted into the alimentary canal by the digestive glands, it passes -out through its wall as the vehicle of digested products. Collected -by lymphatic vessels, it is either carried directly into the thoracic -duct, or passed from lymph into blood, carried by blood to the liver, -again transferred from blood to lymph, and borne by the lymphatic -vessels of the liver to the thoracic duct. - -Water exuded from blood into lymph may be reabsorbed into the blood -near the place where it was poured out, or it may reach the blood via -the thoracic duct. It would seem that the former is the natural, the -latter the emergency route; the former the course taken when an organ -is tranquil, the latter a necessity when the organ is active. If the -large lymphatic vessels of a limb are cut, no lymph escapes from them -so long as the limb is at rest. When the muscles contract lymph begins -to flow. If the limb is flexed and extended by hand, lymph flows. If -the muscles are squeezed or massaged, lymph flows. As the flow is set -up both by active contraction of the muscles and by passive movements -in which the muscles do not take part, it clearly must be due to -external pressure on the lymphatic vessels. As they are provided with -valves, squeezing them converts them into pumps. The fluid which they -contain is bound to go forwards. Additional fluid is squeezed into -them from the tissue-spaces. To a large extent, therefore, the outflow -of lymph from contracting muscles is to be explained as the result -of the pressure which the swelling muscles exert upon the lymphatic -vessels within their sheaths. But there is another factor which must -not be overlooked, although it cannot readily be estimated. When a -muscle is actively contracting its bloodvessels dilate. There is a -greater exudation of lymph; and reabsorption by blood is not equal to -the exudation. The surplus leaves the limb by the lymphatic vessels. A -gland is never at rest. In the intervals between the ejection of its -secretion its cells are preparing materials for the next outflow. Lymph -is always flowing from a gland; its amount increases as the activity -of the gland increases. More lymph leaves the blood when the gland -is exceptionally active than when it is relatively quiet. Some of it -is not reabsorbed into the blood. A certain proportion of the waste -products of the active gland are hurried away by the overflow system in -the direction of the thoracic duct. - -Lymph is the reservoir of nutriment upon which every cell in the body -draws. It is improbable that in health and under normal conditions -the activity of any organ is ever restricted for want of sufficient -food. As food is removed from lymph, it is instantly replaced by fresh -food from the blood. There is some evidence—not very clear—that the -removal of waste products offers greater difficulty than the renewal -of supplies of food. When the activity of muscles has been excessively -prolonged they ache. It has been supposed that their unwillingness to -do more work is due, not to the exhaustion of the food which they use -up when contracting, but to the inadequacy of the lymph and blood to -carry off all refuse. This, at least, is the explanation of fatigue -which is usually offered, although it is difficult to understand why -the arrangements for removing waste products which have worked to -perfection for eight hours should during the ninth hour become rapidly -ineffective. - -If a frog’s muscle, cut out of the body, has been made to contract -until it refuses to work any longer, it again responds to stimulation -after a solution of salt has been passed through its bloodvessels. -The salt-solution brings no food; the only thing it can do is to -wash away waste products. But this experiment upon a tired, isolated -muscle does not necessarily throw light upon the nature of fatigue in -muscles under normal conditions. The isolated muscle is using up, in -contracting, food which it has stored. Cut off from the circulation, it -has no means of getting rid of the lactic acid and other products into -which food is changed. They may well have accumulated to a poisonous -extent long before all the food has been used up. Hardly more cogent -is the argument based upon the benefit which a tired man experiences -from hot baths, massage, and the like. They take away the feeling -of tiredness, but it does not follow that this result is due to the -removal of waste products. Quickening the circulation of blood brings -about renewal of the lymph. Renewal of lymph means fresh supplies of -food as well as removal of waste products. Even human muscles are not -perfect as machines. They will not work for an unlimited spell. There -comes a time when they must have rest. Something goes wrong in the -admirable adjustment which has hitherto provided exactly the right -amount of food and exactly the necessary freedom from the products of -action. A feeling of fatigue is the signal that the apparatus is not -in a condition to work longer; but whether this feeling is due to a -dislocation of the balance of supply and loss, or to some deterioration -of the apparatus which calls for rest and renovation, it is at present -impossible to say. It is not due to the exhaustion of muscle food. A -more powerful stimulus, the urgency of fright or some other strong -emotion, or an electric current applied directly to the muscle or -its nerve, will still induce vigorous contraction. The muscles of a -hare that has been coursed until it can run no farther still contain -glycogen, muscle food. - -Glycogen is stored in the liver. Fat, if it is assimilated in excess of -the needs of the body, accumulates in the connective tissues. Proteins, -if in excess, are either destroyed by oxidation, or partly destroyed -and partly converted into fat. Increasing the amount and richness of -the food does not, if nutrition is already at its best, improve the -quality of the blood. The surplus of food is either stored or burnt. -The composition of lymph is unaffected. Its quality is not improved by -taking more food than enough. A perfect balance is maintained. Every -cell is able, when conditions are normal, to obtain as much nutriment -as it needs. It cannot get more. It cannot lay by food and shirk work. -If it did it would grow. Reaching its optimum size, it would divide. -Additional tissue would be formed. But when it does more work it needs -more food; and it is a matter of common experience that the system is -so adjusted that food is supplied to the tissues, not reluctantly, -but with a slight tendency towards generosity. Working harder than -usual, they find the lymph by which they are bathed somewhat richer in -the materials that they need than the necessities of the case demand. -They are able not merely to obtain all they want, but a little more. -Activity favours growth. - -Many attempts have been made to show that if a part of the body has -more than its share of food it grows to an excessive size. John Hunter -grafted a cock’s spur into its comb. It grew to monstrous dimensions. -Such a result favours the view, but it is not quite conclusive. -Undoubtedly the comb was richly supplied with blood, but it does not -follow that the cells of the spur were able in their new situation -to take advantage of this supply. Besides, the spur when projecting -from the head was not subject to the accidents to which it was -exposed whilst on the leg. Its size was not kept down by friction. -Nor was it as hard and compact as it would have been in its normal -situation. It is scarcely possible to devise any experiment that would -be satisfactory now that the relations between blood and lymph and -lymph and tissues are understood. In certain pathological conditions, -however, hypertrophy is the result of the hyperæmia of chronic -inflammation; and there is little doubt that, if we could arrange for a -certain group of cells to receive lymph richer in food and freer from -waste products than the perfect adjustment of supply to needs normally -allows, the cells would grow. - -Under perfectly healthy normal conditions growth can be induced only -by use. Nature supplies the fuel which is used during activity, -and a balance of food available for the construction of additional -machinery. The muscle which is called upon to do work develops a -greater capacity for work. - -When nutrition is not at its best, the growth of muscle may be favoured -by external pressure which squeezes lymph out of its tissue-spaces, -and therefore leads to increased exudation from the blood. It is not -improbable that in badly nourished tissues the circulation of blood is -somewhat torpid and the lymph stagnant. A feeble circulation usually -results in some œdema. The muscles, or rather the connective tissue -which envelops and penetrates them, feels doughy, instead of being, -as it should be, firm and elastic. Under these conditions massage is -undoubtedly of service. Squeezing the muscles displaces lymph, and, -if the pressure is properly directed, drives it along the lymphatic -vessels. Fresh lymph exudes from the capillary bloodvessels, and the -muscle-fibres, surrounded with a more abundant supply of nutriment, -benefit, as, in a vigorous person, they benefit from use. - -Lymph is an exudate from blood. Its composition therefore depends -upon that of blood-plasma, but it tends to differ from it owing to -the influence of two causes. In the first place, the walls of the -capillary bloodvessels restrict exudation. Red blood-corpuscles cannot -pass through them. Proteins which are non-diffusible are, according -to the circumstances of the tissues, held back to a greater or to a -less extent. The pseudo-capillaries of the liver let them pass, as has -already been said. The capillaries of the limbs restrict their passage -to such proportions as, it may be supposed, are absolutely necessary -for the nutrition of the tissues. In the second place, tissues remove -food from lymph and add to it waste products. Hence the lymph issuing -from a limb, after full contact with the tissues, contains less of the -former and more of the latter—less sugar, for example, and rather -more oxidized nitrogenous substances, lecithin and other things termed -collectively “extractives,” because they can be extracted from dried -blood or lymph by ether. The reaction of lymph is alkaline. After a -time it coagulates, but coagulation is slower, and the clot less firm -than in the case of blood. - -As the composition of lymph depends upon the source from which, and the -conditions under which, it has been obtained, it is unnecessary to -state the results of a chemical analysis. It suffices to say that lymph -contains all the substances which are present in the plasma of blood, -but not necessarily in the same total amount or in the same relative -proportions. Speaking generally, leucocytes are present in about the -same numbers as in blood—6,000 to 8,000 to the cubic centimetre; -but leucocytes are everywhere present: in blood, in the lymph, in -lymph-vessels, in the tissue-spaces. As they are not passively floating -bodies like red blood-corpuscles, but active migratory organisms, -they tend to accumulate in one situation and withdraw from another, -in accordance with the opportunities which the different localities -afford. They desert effused lymph, blisters, ascitic fluid, and the -like. They are not found in the lymph in the pericardium. There are -fewer in the lymph coming from the intestines after a meal than in the -same lymph during the intervals between meals. Their departure from -effused lymph might easily be explained. It is not so easy to account -for their comparative absence from the lymph in the lacteals when it -is heavily charged with fat and other products of digestion. Such -leucocytes as are present at this time are loaded with fat granules -which they have stolen from the chyle, as the lymph in the lacteals is -usually termed. One would need to be very intimate with a leucocyte -before one ventured to give reasons for all its movements. Lymph -contains the same proteid substances as blood, and in the same relative -proportions, but usually in smaller quantity. - -Incidental reference has been made to the great lymph-spaces—peritoneal, -pleural, and pericardial. The brain and spinal cord are separated from -their outer membranes by a lymph-space. There are also spaces within -the brain—the ventricles—and a central canal in the spinal cord. The -aqueous and vitreous humours of the eye are also lymph-spaces, although -the latter contains some remnants of tissue. The joint cavities are -lymph-spaces. So also are the bursæ which surround tendons or separate -them from bones. It is not, however, justifiable to include all these -cavities in a single category, either from the point of view of their -purpose, their mode of formation, or the nature of their contents. The -peritoneal, pleural, and pericardial spaces are parts of the great -primitive body-cavity, or cœlom. The two first are potential rather -than actual. Normally they contain just sufficient fluid to moisten -the apposed surfaces of the endothelium which lines their walls and -covers the organs which they contain. There is no fluid in them which -can be collected and labelled “peritoneal” or “pleural” fluid. The -purpose of the spaces is to allow of movement without friction—in the -one case of the intestines, in the other of the lungs. It is possible -to take a spoonful or so of fluid out of the space which surrounds the -heart. It has the usual composition of lymph. It contains proteins, -but is not spontaneously coagulable. Leucocytes are absent, a fact -which probably accounts for its not clotting. The fluid inside the -cerebro-spinal system is extremely dilute. Its principal salt—its -principal constituent, indeed—is sodic chloride. It contains hardly -a trace of proteins, and these in a modified condition—proteoses. It -also contains pyro-catechin, a benzoic alcohol. This substance has -long been recognized as a constituent of cerebro-spinal fluid, owing -to the fact that, like sugar, it reduces copper salts when heated with -them in an alkaline solution. It appears to be one of the products of -proteid decomposition. Although exuded as lymph from the bloodvessels -of the chorioid plexuses, the composition of cerebro-spinal fluid has -been profoundly changed by the activity—it might almost be called the -digestive activity—of the epithelium which lines the cerebro-spinal -canal. There is a theory that the ancestors of all vertebrate animals -were organized on a very different plan from that of their distant -descendants. Our cerebro-spinal canal was their stomach and intestine. -It would appear that the lining epithelium of these organs, although -disused for millions of years, cannot resist the temptation to digest -the lymph which they contain! The fluid in joints contains mucin (the -essential constituent of mucus), or a substance resembling mucin. In -this case the joint-membrane has added something to lymph without -removing or destroying any of its other constituents. - -Other illustrations might be given showing how the plasma of blood is -altered in composition while it is passing out of, or after it has -passed out of, capillary bloodvessels. Perhaps it would be more logical -to start on the outer side of the walls of the capillaries; since blood -may, very properly, be regarded as a tissue, dependent, like all other -tissues, upon diffusion from lymph for the nutrient materials that -it needs. In the wall of the alimentary canal it receives supplies -_via_ the lymph. It drops them in the liver, its _garde-manger_, to -pick them up again as they are wanted. The torrent of lymph which the -thoracic duct discharges into the veins of the neck conveys the fat -which could not traverse the walls of the capillary bloodvessels, and -much of the reserve of food which the blood had deposited in the liver. -Only about one-quarter of the fluid of the body (one-thirteenth of the -body-weight) is included within the blood-system; but this enclosed -fluid, owing to the fact that it is kept in circulation by the heart, -replenishes and purifies the much larger quantity which does not -circulate. The unenclosed lymph has in particular situations a chemical -composition which varies widely from that of the blood. Imagine a -marsh through which a river flows—the vast plains of water-plants on -the Nile above Fashoda, for example. There is a constant interchange -between the flowing water of the river and the stagnant water of -the marsh. In any given part of the marsh the quality of the water -will depend upon what it has been able to take from, and what it has -given back to, the river; upon what the water-plants have taken from -it, and what they have added to it. Boats which cannot penetrate the -walls of reed keep to the open channel of the Nile. Fish swim, now in -the river, now in the narrow passages and open pools of the marsh. -So it is, in a way, with the fluid in the spaces and cavities of the -lymphatic system and in the bloodvessels which traverse them, and with -its migratory inhabitants. In our extravagant analogy read leucocytes -for fish. Fish have two reasons for wandering from river to marsh. -Amongst the water-weeds they hunt for food; they seek quiet places in -which to breed. In this matter the analogy holds good. A leucocyte may -be overtaken with cell division anywhere—in the blood-stream or in a -lymph-vessel. But cell division very rarely occurs except in certain -favoured spots. The breeding-places chosen by leucocytes are sheltered -situations in connective tissue where the blood-supply is abundant, -and the eligibility of such a spot is much increased by its being near -to a field where their services are likely to be called for. The nests -of connective tissue made by the leucocytes are of three kinds, termed -respectively diffuse adenoid tissue, lymph-follicles, and lymphatic -glands. The connective tissue beneath the mucous membrane of the -whole of the respiratory tract—trachea, bronchi, and bronchioles—is -diffuse adenoid tissue. It presents no special structure, but its -spaces are packed with leucocytes in various stages of cell division, -and young leucocytes, or lymphocytes, as they are usually named. Some -of the lymphocytes make their way into the blood or into the lymph. -Others, acquiring their full dimensions, scour the epithelium which -lines the respiratory tract for germs and other foreign bodies which -are drawn into the tract with inspired air. They may be seen pushing -aside the cells of the lower strata of the epithelium, on their way to -the surface, or returning to the subepithelial connective tissue with -germs, or particles of soot, or débris of epithelial cells which they -have taken into their substance (Fig. 4, B). - -The tonsils are examples of follicular lymphoid structures. They lie -one on either side of the entrance to the gullet, between the two -folds (the anterior and posterior pillars of the fauces) by which -the soft palate is continued to the side of the tongue. Normally the -tonsil is not visible, but when inflamed it may project sufficiently -to be seen; and its surface may then be covered with mucus and pus. -It is liable to become enlarged in childhood, owing to chronic -inflammation. A section of the tonsil shows it to consist of clusters -of lymph-follicles lying beneath the mucous membrane. The term -“follicle” is unfortunate. It conveys no idea of the form or structure -of one of these masses of lymph-cells; and it is, besides, applied to -things of an entirely different character—for example, the pits of -mucous membrane which sink down between the masses of lymphoid tissue -in the tonsil. The expression “follicular tonsillitis” does not refer -to the lymph-follicles, but to the epithelial pits. It is a condition -in which a drop of pus is to be seen in the mouth of each of the pits. -A lymph-follicle is a small rounded clump of connective tissue, denser -on its periphery than in its centre. Its bloodvessels are disposed -chiefly on the periphery. Lymphatic streamlets arise in the centre. -Its outer portion is closely packed with dividing lymph-cells and -young leucocytes, which as fast as they are formed migrate towards -the centre, and eventually escape from the follicle by the lymphatic -vessels. The connective tissue which invests and separates the -follicles is full of leucocytes. Removal of the tonsils is followed by -no ill effects. They are not essential to our well-being. Nevertheless, -they have important functions to perform. They are barracks crowded -with leucocytes, which guard the pass into the alimentary canal. Their -leucocytes incessantly patrol the mucous membrane, capturing germs, -removing fragments of injured epithelium, striving to make good the -mischief to which this part of the alimentary canal is peculiarly -liable. The enlargement of the tonsil which results from frequent -sore throat is a response to the demand for an increase in the supply -of these little scavengers, in order that they may cope, not only -with objectionable things outside the walls, but with the still -more pernicious germs which during an attack of sore throat succeed -in breaking through the epithelium. It is the invaders which elude -the vigilance of the leucocytes that cause fever and other general -symptoms. Other notable groups of lymph-follicles are found in the -middle portion of the small intestine, where they form oval patches, -about three-quarters of an inch long by half an inch broad—Peyer’s -patches. The leucocytes which are developed in them search the walls of -the intestine for germs. During an attack of enteric fever the patches -become inflamed, and one of the greatest risks which the patient -runs is the risk of ulceration of a patch and the perforation of the -intestinal wall. - -The abundant provision for the multiplication of leucocytes shows that -the destruction of these cells must occur on an equally large scale. -Every day large numbers die. Where this occurs, and how their dead -bodies are removed, is not certainly known. Doubtless they are eaten -by their fellows, their substance oxidized, and the products—carbonic -acid, water, and nitrogenous waste—thrown into the lymph. There is -some reason for thinking that a part of the nitrogenous waste is -excreted in the form of uric acid (_cf._ p. 216). The daily production, -and consequent destruction, of leucocytes shows that their metabolism -is a factor which cannot be overlooked when we are making up the body’s -accounts. - -The fixed tissues receive their nutriment in a digested condition. -Leucocytes digest it for themselves. In many cases, although not in -all, the cells of fixed tissues last throughout life, so far as their -outer form is concerned, although their molecules are oxidized and -replaced by new material. It is not improbable, therefore, that there -is a difference between the metabolism of the fixed tissues and the -metabolism of leucocytes. The whole of a wandering cell, its nucleus -included, breaks down and has to be removed. We do not know that -this occurs in the case of a fixed cell. On the strength of evidence -which points, apparently, to a chemical relationship between nuclear -substances and uric acid, it has been inferred that the two chief -nitrogenous products which are excreted by the kidney are divisible -into the one which in the main represents the oxidation of fixed cells, -urea, and the other, uric acid, largely derived from the oxidation of -wandering cells. - -The valiant leucocytes do their best to cope with all the rubbish, -whether living or dead, that needs removal. They flock to any situation -in which germs are numerous or tissue has been destroyed. If all goes -well they take the foreign matter into their substance—dead tissue is -matter foreign to the body—and either digest it in the course of their -ordinary progress, or retreat with it, if they cannot digest it, to the -nearest lymphatic gland. But in their efforts to reach objectionable -matter they are apt to wander too far from the healthy lymph from which -they obtain oxygen for their own respiration. Unable to breathe, they -die. They lose the power of extruding pseudopodia. Their extensible, -prehensile processes are drawn in. Assuming a globular form, they float -helplessly in what once was lymph. Their body-proteins are largely -changed to fat. As “pus cells,” they are thrown off in the discharge -from an ulcer, or accumulate in the cavity of an abscess. A pus cell is -a dead and fattily degenerated leucocyte. - -The third kind of breeding-place of leucocytes, a lymphatic gland, -has a more elaborate structure than the tissues with which we have -already dealt. Lymphatic glands are about the size of beans, and of -the same shape. They are found in the course of lymphatic vessels in -situations where they are not exposed to pressure, such as the back of -the knee, the groin, the front of the elbow, the armpit, in the neck -above the collar-bone, and on either side of the sterno-mastoid muscle, -behind the angle of the jaw. There are a number in the abdomen and -in the thorax. Each lymphatic gland is invested by a strong fibrous -capsule. Its artery enters, and its vein and efferent lymphatics leave, -the concave side (the hilus) of the gland. The lymphatic vessels -which bring lymph to it pierce the capsule on its convex side. It is -divisible into two parts: (1) The adenoid tissue which surrounds the -artery and its branches; (2) the open network of “lymph-ways” which -invest this adenoid tissue. Leucocytes divide in the adenoid tissue. -The young lymphocytes drop out into the lymph-ways. As a stream of -lymph, brought by the afferent vessels, is always flowing into the -lymph-ways, and out by the efferent vessel or vessels, the lymphocytes -are carried with it towards the thoracic duct. A lymphatic gland is -therefore an organ for adding leucocytes to lymph in the course of the -lymph-stream. It has, however, another and equally important function. -Leucocytes which have picked up germs or other foreign matter pass on -with the lymph to a lymphatic gland. After entering its lymph-ways -they leave the lymph-stream, squeeze into the adenoid tissue of the -gland, and there come to rest with their burden. They remain in the -gland until the foreign matter is digested, or, if it be indigestible, -until they undergo dissolution, when the particles of soot or pigment -are deposited from their débris in a harmless state. When the skin is -tattooed, much of the Indian ink and other pigment remains where it was -inserted with the needle, but some of it is picked up by leucocytes and -carried to the nearest lymphatic gland. - -Lymphatic glands are barriers which stop the spread of infection. -They are the stations to which our police carry captured germs. The -skin of the heel is abraded. Germs from the soil, or elsewhere, which -have accumulated in a dirty stocking—owing to the warm moisture -enclosed by an impervious boot, the woollen covering of the foot is a -peculiarly healthy place for germs—enter the opened lymph-spaces of -the subcutaneous tissues. Leucocytes hasten to the spot. They seize the -invaders with their pseudopodia, engulf them in their body-substance, -enter lymphatic vessels, and are rolled away by the lymph-stream. The -instinct which brings them in ever-increasing numbers to the breach -in the protecting skin can be explained only in terms of force. From -our own conscious action to the causes which determine the movements -of a leucocyte, or of an amœba, is so deep a drop that we prefer -to recognize in the latter a merely chemical attractive force. -“Chemiotaxis” we term the influence which draws leucocytes to the place -where food is abundant; although it is also the place, one must admit, -where in the interests of the body as a whole they run great risk of -asphyxiation. It is appetite which draws a schoolboy to a bun-shop; a -sense of duty prompts a fireman to risk his life in a chamber filled -with smoke. We have no desire to humanize a leucocyte; but it is -difficult to emphasize too strongly its independence. It would be -absurd to use terms which imply that a leucocyte has a self-directive -power; yet it is equally misleading to describe its migration to -the seat of injury, its retreat with ingested germs to a lymphatic -gland, its wriggling from the lymph-ways of the gland into the shelter -of its adenoid tissue, in terms which imply that the forces which -direct it are known, and their mode of action understood. The success -which attends the inroads of germs is due to their amazing capacity -for multiplication when they reach lymph or blood. It is useless to -attempt to form an idea of the rapidity with which they divide, since -we have no data upon which to base calculations. If the leucocytes -fail to deal with the first few that enter, germs soon swarm within -the lymph-vessels. This leads to an inflammation of the walls of the -vessels, which may then be seen as red lines beneath the skin. These -red lines lead upwards towards the nearest lymphatic gland. The glands -in the space behind the knee are not usually affected when the focus of -infection is in the foot. The red lines can be traced up the inner side -of the knee and the front and inner side of the thigh to the groin. The -glands in this situation swell until they can be easily felt. If the -mischief is in the hand, the gland at the elbow may be affected, but -most of the lymphatics pass by it on their course to the glands in the -armpit. If a sore throat is the source of infection, the glands beneath -the angle of the jaw enlarge. Thus various glands block the further -progress of infection. In doing this their resources may be strained to -the uttermost; they may enlarge, become tender, grow soft, fill with -pus, break down and discharge the pus without the aid of a surgeon’s -knife, although as soon as pus is recognizable within them it is wise -to let it out. If germs pass through these first stations into the -lymph-vessels beyond them, abscesses are formed in other situations. A -condition of “blood-poisoning,” so called, is set up. - -The readiness with which leucocytes sacrifice themselves in their -efforts to remove germs and decaying tissue is a matter of almost -every-day experience. The fatty matter produced in the sebaceous -glands of the skin normally overflows on to the surface. It serves to -render the skin supple and impervious to water. Germs get into one of -the sebaceous glands of the face or of the eyelid. The contents of -the gland begin to decompose. Leucocytes enter it for the purpose of -removing the putrescent substance. They lose their vitality and turn -into pus corpuscles. The pimple or the stye bursts, and pus and fatty -matter are discharged together. - -That the conversion of leucocytes into pus cells is due to want of -oxygen has been shown by the following experiment: A minute piece of -phosphorus is placed beneath the skin. Leucocytes gather round the spot -with a view to removing the tissue which the phosphorus has destroyed. -But phosphorus has so strong an affinity for oxygen that it exhausts -the supply in the area of tissue which surrounds it. The leucocytes -die before reaching the tissue immediately adjacent to the piece of -phosphorus. Their dead bodies form round it a raised ring of pus cells. -We can explain this readiness of leucocytes to sacrifice themselves -in their efforts to reach foreign matter which needs to be removed, -only by saying that the attraction of the food is greater than the -repulsion of lymph destitute of oxygen. An amœba placed in comparable -circumstances gives up the quest of food, however strongly chemiotaxic, -and retreats towards water which contains oxygen sufficient to provide -for its respiratory needs. - -=Blood.=—A portion of the body fluid is enclosed within vessels and -kept in circulation by the heart. The heart pumps blood into the aorta. -This trunk gives off large arteries, which in turn divide until the -finest capillary vessels are reached. The capillary tubes reunite to -form veins, which, with the exception of those which collect food -from the digestive organs, convey the blood right back to the heart. -The veins which drain the stomach and intestines (the organs in which -food is prepared for absorption) and the spleen (the organ in which -worn-out red blood-corpuscles are in a sort digested) break up in the -liver into a second set of small vessels. The pseudo-capillary vessels -of the liver reunite to form the hepatic veins, which add the blood -that has passed through that organ to the rest of the blood which is -passing up the inferior vena cava to the heart. A second capillary -circulation is found in the kidney also. - -The heart is four-chambered (Fig. 10). Its left ventricle drives the -blood round the systemic or greater circulation, the blood returning -to the right auricle. The right ventricle drives the blood through the -lesser or pulmonary circulation, from which it returns to the left -auricle. The walls of all bloodvessels, except capillary tubes, are -sufficiently thick to prevent the escape of any of the constituents of -blood. To support the pressure of the blood which they contain, the -arteries and the larger veins need walls of considerable thickness. The -walls of the capillaries allow an interchange between blood and lymph -in the manner already described (_cf._ p. 39). - -Blood fresh from the lungs, whether still in the pulmonary veins or in -the systemic arteries, is scarlet in colour. Venous blood is darker and -purple-red, the depth of its tint varying with the extent to which it -has parted with its oxygen. It looks less opaque than arterial blood. -With this exception, the physical properties and chemical composition -of blood are remarkably constant in all parts of the body. Arterial -blood contains more oxygen, venous blood more carbonic acid. Other -chemical differences can be recognized, but they are relatively very -small. The constancy in the constitution of blood is its most notable -character. Bleeding, unless excessive, does not greatly affect it. -The number of corpuscles is of course diminished, but even these are -replaced with great rapidity. The plasma, after bleeding, soon recovers -its proteins and salts. A similar readjustment occurs if normal saline -solution (water containing 0·9 per cent. sodic chloride), or even a -strong solution of salt, is injected into the blood. Within certain -limits it is very difficult to disturb the balance of its constituents. -It gets rid of substances added in excess, or replaces substances -removed, with remarkable facility. If sugar (glucose) be injected into -a vein, it escapes through the capillary walls into the lymph. After -a short interval the lymph contains more sugar than the blood. If an -excess of protein, whether of a kind foreign to the blood or its own -serum-albumin, be injected, it is removed by the kidneys. The blood has -various sources from which it can draw out reserves of anything that is -lacking, and various ways of getting rid of anything that is in excess. -It draws upon the lymph in the tissue-spaces for water. It discharges -salts into the lymph. It also takes salts from the lymph. It draws upon -the liver for sugar, and probably for proteins also. In a starving -animal the blood still contains sugar long after fresh supplies have -ceased to reach it from the intestines. The lungs remove its carbonic -acid. The kidneys free it from everything which cannot be otherwise -removed. It is essential to the well-being of the organism as a whole -that a uniform standard of composition should be maintained by the -blood. - -[Illustration: - - FIG. 4.—RED BLOOD-CORPUSCLES - PRESENTING, SOME THE SURFACES, OTHERS THE EDGES, OF - THEIR DISCS, TOGETHER WITH SINGLE REPRESENTATIVES - OF FOUR TYPES OF LEUCOCYTE. - - A, the most common type, highly amœboid and - phagocytic. Its protoplasm is finely granular, - its nucleus multipartite. B, a leucocyte closely - similar to the last, but larger, and containing an - undivided nucleus. It is shown with a cluster of - particles of soot in its body-substance. C, a young - leucocyte, or “lymphocyte.” D, a coarsely granular - leucocyte. Its granules stain brightly with acid - dyes—_e.g._, eosin or acid fuchsin.] - -_Composition._—The structural composition of the blood, and the -relation of its several constituents to each other, is best studied -under the microscope. A thin transparent membrane in which blood is -circulating through small vessels—the web between the toes of a -frog’s foot, the mesentery, the membrane of a bat’s ear—affords an -opportunity of observing blood in circulation. In any of the smaller -vessels, whether artery or vein, a column of red corpuscles is seen -moving in the axis of the stream. This column is surrounded by a layer -of clear plasma. Amongst the red corpuscles a few leucocytes may be -detected floating placidly down the current. Others are seen in the -peripheral layer of plasma, tending to creep along the wall of the -vessel rather than submit to be moved forward, as passive objects, by -the current. If an irritant be applied to the membrane, the vessels -dilate; yet, notwithstanding their wider calibre, the current becomes -slower. The red corpuscles mass together. Apparently their constitution -is slightly altered by this commencing inflammation, in such a manner -that they cease to be clean, independent discs which slide past each -other like small boats on a river; they exhibit a tendency to stick one -to another. In the capillary vessels leucocytes may now be observed, -not merely creeping along the inner surface of the endothelium, but -squeezing themselves between its scales; making their way out of the -vessel into the tissue-spaces through which the vessel passes. Such an -observation gives the clue to the functions of the several constituents -of the blood. The red corpuscles carry oxygen in chemical combination -with their colouring matter. From them it passes into solution in the -plasma; from the plasma through the walls of the capillary vessels -into lymph; the tissues take it from the lymph as they require it. As -fast as it is removed from lymph it is renewed from plasma. Carbonic -acid excreted by tissue cells is dissolved in lymph. From lymph it is -transferred to plasma. The reception of carbonic acid by these fluids -is not quite so simple as the transference of oxygen from blood to -lymph. It is aided by the presence of alkaline carbonates which are -always ready to form “acid” salts: not acid to litmus-paper—the blood -is always alkaline—but containing more than one unit of acid to one -of base. Sodic carbonate has the formula Na₂CO₃. With an additional -molecule of carbonic acid it becomes Na₂CO₃CO₂(HO)—bicarbonate. When -in solution it can hold still more carbonic acid. If carbonic acid -were merely dissolved in lymph and plasma, it would be impossible for -the blood to carry it away with sufficient rapidity; just as it would -be impossible for blood to bring sufficient oxygen were it not for the -colouring matter (hæmoglobin) which forms a temporary, easily divorced -union with it. But from a physical point of view it comes to the same -thing. As the tension of oxygen in plasma falls, it dissolves more from -the hæmoglobin. When the tension of oxygen in lymph is less than its -tension in plasma, the former borrows from the latter. If the tension -of carbonic acid in lymph is higher than in blood, it passes to the -blood. The rapidly circulating blood at frequent intervals traverses -the lungs. The whole blood of the body is exposed to air in the lungs -once every minute. Oxygen tension being higher in pulmonary air than in -venous blood, this gas is taken up. Carbonic acid tension being higher -in venous blood than in pulmonary air, this gas escapes. The plasma in -the capillary vessels which traverse the tissues exchanges gases with -the lymph with very great rapidity. - -The specific gravity of blood varies from 1·056 to 1·059. The -corpuscles are heavier than the plasma. Its reaction to test-paper is -alkaline, owing to the presence of bicarbonate of soda and disodic -phosphate. The alkalinity is greatest when the body is at rest; it is -diminished by severe muscular exercise. Blood contains about 5,000,000 -red corpuscles, and 7,000 or 8,000 leucocytes, to a cubic millimetre. -Red blood-corpuscles are biconcave discs destitute of nucleus, and, so -far as can be seen, devoid of any investing membrane. Seen in profile -they appear biscuit-shaped, because the centre is hollowed out. Their -largest diameter is 7·5 micromillimetres (¹/₃₂₀₀ inch)—a measurement -of great importance to anyone who works with a microscope, because it -serves as a standard by which to estimate the size of other objects. -They are soft, but fairly tough and highly elastic. In circulating -blood a corpuscle may occasionally be seen to catch on the point where -two capillary vessels unite. It bends almost double under the pressure -of the column of corpuscles behind it, and then springs forward. - -A red corpuscle is a vehicle for hæmoglobin. If blood is diluted with -water, or if it is alternately frozen and thawed, the hæmoglobin -separates from the corpuscles, which can then be seen as colourless -discs. Hæmoglobin constitutes 40 per cent. of the weight of a -moist corpuscle, or 95 per cent. of its weight after it has been -dried. This is an enormous charge for a corpuscle to carry, and the -question of how it carries it has been much discussed. It is not in -a crystalline state. A corpuscle examined by polarized light is not -doubly refractive. Microscopists know that if there were any crystals -in the corpuscle it would appear bright on a dark ground when the -Nicholl prisms are crossed. It cannot be in solution, since the water -which the corpuscle contains would not suffice to dissolve it. It must -be combined with some constituent of the corpuscle. But whether it -is uniformly distributed throughout the disc, or in a semifluid form -enclosed in spaces in a sponge-work; or whether the corpuscle is a -hollow vesicle enclosing fluid hæmoglobin—a view which was long ago -maintained, and has recently been revived—are questions which still -await further evidence. - -Red blood-corpuscles, properly so called, are found only in vertebrate -animals, although invertebrate animals, from worms upwards, possess -genuine blood, and in some of them it contains hæmoglobin, or a -similar pigment in the form of globules. These might be likened to -the non-nucleated corpuscles of mammals, but it must be remembered -that the non-nucleated cells of mammals have been evolved from the -nucleated blood-corpuscles of birds, reptiles, amphibians, and fishes. -Below fishes red blood-cells are not found. Hæmoglobin is usually -dissolved in the blood of invertebrate animals. It is impossible to -trace any relationship between the coloured globules of invertebrates -and the blood-cells of fishes. The coloured globules must be regarded -as deposits or accretions of hæmoglobin held together by a proteid -substance. - -The nucleated red corpuscles of submammalian vertebrates multiply by -cell division while circulating in the blood-stream. A good subject -in which to look for dividing corpuscles is the blood of a newt in -spring-time, when rapidly increasing activity calls for an additional -supply. There is nothing to distinguish the method of division of a -nucleated blood-corpuscle from that of any other cell. - -The life-story of the red blood-corpuscles of mammals is one of the -most fascinating that the histologist has to tell. He wishes that -he could tell it with assurance; but, unfortunately, there are many -uncertainties, due to conflicting testimony, in its earlier chapters. -It is unlikely that a blood-corpuscle lives for long. A month or six -weeks is probably the term of its existence. The rapidity with which -the stock is replenished after bleeding shows that there must be ample -provision in the body for making blood-corpuscles. The rate at which -they disappear after they have been added in excess shows that there -is an equally effective mechanism for destroying them. If half as many -again as the animal already possesses be injected into its veins, the -number is reduced to its normal limit in about ten days. It is clear -that they can be made and can be destroyed with great facility, and -it seems a legitimate inference that production and destruction are -constantly taking place. Regarding the way in which they are destroyed -there is no uncertainty. We shall refer to this subject when describing -the functions of the spleen. But how are they made? We can sketch their -history in outline, but the evidence is conflicting with regard to all -matters of detail. - -In early stages of embryonic life all red blood-corpuscles are -nucleated, as they are permanently in birds and the other classes -of vertebrates below mammals. In embryonic mammals they multiply by -division whilst circulating in the blood, just as in the newt. But -it is generally believed that this is not the most important source -of new ones. During the earliest stages of growth they are being -formed in enormous numbers. Such instances of division as can be seen -in circulating blood appear to be all too infrequent to account for -their rapid multiplication, and there can be no doubt but that a more -complicated method of production is more important. Their formation -is described as taking place “endogenously.” Certain cells termed -“vaso-formative,” or “vaso-sanguiformative,” reach a considerable size, -and become stellate in form, or branched. Their nuclei divide without -the cell dividing. Each nucleus accumulates a little hæmoglobin round -it. A space filled with fluid appears inside the cell. The nuclei -project into this space. Then they drop off with their envelopes of -hæmoglobin. The outer shell of the big vaso-formative cell becomes -the wall of a capillary bloodvessel. By its branches it links up with -other vaso-formative cells, making a network of vessels. The fluid -inside it is the plasma of the blood. The nuclei and their envelopes -are blood-corpuscles. This, if it be a true story, is a comprehensive -way of making bloodvessels and blood at the same time. Doubts have been -thrown upon its accuracy, but many leading histologists strenuously -maintain that this description is correct. - -At a certain period all nucleated red corpuscles disappear from -mammalian blood. Non-nucleated corpuscles take their place. How are -the latter formed? For a short stage of embryonic life nucleated cells -containing blood-pigment are seen, or are supposed to be seen, in the -liver—there is, unfortunately, great difficulty in distinguishing -them with certainty from young liver-cells; later they are seen in -the spleen; throughout the whole of life they are to be seen in the -marrow of bone. The nucleated cells give origin to the non-nucleated -corpuscles. It is hardly legitimate to call these cells persistent -embryonic corpuscles. Yet the chain which connects the cells which -in the embryo are capable of dividing into pairs of nucleated red -blood-corpuscles, and the cells which, assuming the rôle of parent -cells, do not accumulate hæmoglobin for their own purposes, but for the -benefit of the red corpuscles which split off from them, is probably -unbroken. In this sense they are persistent embryonic corpuscles which -have deserted the blood-stream, and have taken shelter in certain -tissues which are particularly favourable for cell division. The -situations in which they hide themselves are singularly suggestive. In -the liver there is an abundant supply of nutriment, more abundant than -in any other part of the body of the embryo. Later, in the spleen, red -blood-corpuscles are being destroyed. Materials available for making -new ones must therefore be set free. The inside of a hollow bone is -a peculiarly sheltered situation. The fat cells of marrow accumulate -there after a time; but within some bones the marrow develops very -little fat; hence it shows the red colour, which is due to its abundant -bloodvessels. This “red marrow” is the most important seat of the -manufacture of red blood-corpuscles in adult life. Unfortunately, when -we try to answer the question, How are they formed? we are obliged to -speak with caution. Some histologists assert that the nucleated cells -divide, and that one of the two daughter cells accumulates hæmoglobin, -and loses—that is to say, extrudes—its nucleus. Others maintain -that the nucleated cells become irregular in form; that hæmoglobin -accumulates in the projecting portion of the cell; that this projecting -portion breaks off as a non-nucleated corpuscle. It would be indiscreet -at the present time to pronounce in favour of either of these reports, -although the decision is of theoretical importance. If the former -account be true, red blood-corpuscles are nucleated blood-cells which -have lost their nuclei. If the latter account be in accordance with -fact, it is hardly justifiable to regard them as cells. They are parts -of cells which finish their existence independently of the cell body -and nucleus to which they belong. As circumstantial evidence, favouring -the theory that cell division is normal and the nucleus subsequently -lost, may be pleaded the existence in marrow, and also in the embryonic -liver and spleen, of certain very peculiar cells. These cells have long -been known as giant cells, and all attempts at accounting for them -have broken down. They are relatively of immense size: their diameter -may be twenty times as great as that of a red blood-corpuscle. Each -contains a huge irregular, bulging nucleus. Hence the cells are termed -“megacaryocytes” (big-nucleus cells). They must not be confounded -with the polycaryocytes (cells with several nuclei), which eat up -degrading bone, although it must be confessed that megacaryocytes and -polycaryocytes appear to be genetically connected. It is supposed -that megacaryocytes consume the nuclei which red corpuscles extrude -during the process of their conversion from nucleated cells. Traces -of nuclei, or things which often look like nuclei, are found in their -body-substance. Their own overgrown misformed nuclei appear to be -the result of an excess of nuclear food. It is certainly remarkable -that megacaryocytes are not found below mammals. They do not occur -in any animal in which red blood-corpuscles retain their nuclei. -Polycaryocytes are found in numbers in the bones of growing birds. -They are evidently scooping out bone from situations in which it has -to be displaced in order that the shape of the bone as a whole may be -changed. But there are no megacaryocytes in birds. On the other hand, -megacaryocytes are present in the liver, and later in the spleen, -of mammals at the periods when blood-formation is occurring most -actively in these organs. From the liver they disappear early. In -most mammals they disappear from the spleen about the time of birth; -but in some—the hedgehog, for example—they are found in the spleen -throughout the whole of life. - -Hæmoglobin is a substance which has the property of uniting with oxygen -to form oxyhæmoglobin—a compound from which the oxygen is, again, -very readily withdrawn. It is extremely soluble, but may be made to -crystallize by adding alcohol to blood, after setting the hæmoglobin -free from the corpuscles by freezing and thawing. From the blood of Man -and most other animals it crystallizes in the form of rhombic prisms, -whether in the oxidized (oxyhæmoglobin) or non-oxidized condition. -The addition of oxygen does not affect its crystalline form; although -crystalline, it is absolutely non-diffusible. This is due to the great -size of its molecule, which is probably larger than that of any other -substance which is capable of crystallizing. - -The percentage composition of hæmoglobin conforms closely with that -of albumin and other proteins, with this most important difference: -it contains a definite proportion of iron—0·336 per cent. That the -percentage of carbon, hydrogen, nitrogen, sulphur, and oxygen should -agree with that commonly found in proteins is inevitable, since it -may be split into a part which contains all the iron, hæmatin, and a -proteid part resembling albumin; and the latter constitutes 96 per -cent. of its weight. - -There is no doubt but that its value as a vehicle of oxygen depends -upon the presence of iron. In the matter of taking up and dropping -oxygen, hæmatin behaves somewhat in the same manner as hæmoglobin; -whereas if iron be removed from hæmatin the “iron-free hæmatin” -loses its respiratory value. It is almost certain that a molecule of -hæmoglobin contains a single atom of iron. On this supposition its -molecular formula may be calculated. It is not quite the same for all -animals, although the variations are slight. For the blood of the horse -it is as follows: - - C₇₁₂H₁₁₃₀N₂₁₄S₂FeO₂₄₅. - -This means a molecular weight of 16708. We give the figures, because -the properties of hæmoglobin will be better understood if its -prodigious molecular weight is borne in mind. In a sense, the reason -for the great size of its molecule is not far to seek. The atomic -weight of iron (Fe = 56) is much greater than that of either of the -other elements contained in hæmoglobin. The molecule needs to be -very great to float an atom of iron. As it is, the corpuscles are -heavier than the plasma which surrounds them, in the proportion of -about 13 to 12. Although hæmoglobin is a crystallizable substance, -its immense molecule is absolutely non-diffusible. It cannot pass -through a membrane. This is of no consequence as regards the relation -of hæmoglobin to the walls of the capillary bloodvessels, since it -is contained in corpuscles; but it is of great importance as regards -its relation to the discs which carry it. A very small quantity of -enveloping substance suffices to prevent it from diffusing into the -plasma of the blood. The great molecules are held together and isolated -from the fluid in which they float by a minimal amount of insoluble -globin. - -The iron needed for the making of hæmoglobin is obtained both from meat -and vegetables. The constituents of an ordinary diet provide from 2 to -3 centigrammes of iron a day. The whole of the blood contains about -4·5 grammes. When corpuscles are being destroyed in the spleen, the -iron which their pigment contains is largely reabsorbed and rendered -available for further use. The iron in a mixed diet is more than -sufficient to counterbalance any loss. Milk contains extremely little -iron. Before birth the liver and spleen accumulate a store of iron -which lasts until the end of the nursing period, unless this be unduly -prolonged. If it be prolonged, the child is apt to become anæmic. -Iron has been administered in the treatment of anæmia ever since its -presence in the red clot of blood was recognized a hundred and fifty -years ago. Physicians are agreed that in the anæmia of young people it -is of value; but observations made with a view to obtaining definite -data as to the increase in number of blood-corpuscles which results -from the administration of iron, without any other alteration in the -diet or the habits of the patient, have not given accordant results. -Some observers have obtained an increase with organic compounds of -iron, others with inorganic compounds; some are in favour of small -doses, others of very large ones. As in the treatment by drugs of -other abnormal conditions, it is difficult to isolate the effect of -the drug from the effects of improvements in the general regimen. Yet -physicians agree that iron accentuates the beneficial effects of fresh -air and improved diet. - -When the surface of the body is struck, the effect of the blow is -marked at first by redness. There is nothing to show that small -bloodvessels have been ruptured and blood effused beneath the skin. -Next day the injured area is reddish-purple. The bruise turns blue, -green, yellow, and eventually disappears. In the process of absorption, -oxyhæmoglobin undergoes decomposition. First its proteid constituent is -removed, leaving a coloured pigment containing iron, termed “hæmatin”; -soon reduced by loss of oxygen to hæmochromogen. When Sir George Stokes -first described the spectrum of blood (_cf._ p. 185), he showed that as -hæmoglobin may exist in an oxidized and in a non-oxidized condition, -distinguished by their spectra, so also may the coloured residue -which is left after the proteid constituent has been removed from -hæmoglobin. This coloured residue he termed, when oxidized, “hæmatin”; -when not oxidized, “reduced hæmatin.” Stokes’s reduced hæmatin is -now termed “hæmochromogen.” Hæmochromogen stands for the coloured -nucleus of hæmoglobin. Although it is not present in hæmoglobin as -hæmochromogen—hence we must not speak of hæmoglobin as made of a -protein, _x_, plus hæmochromogen, _y_—it is to its coloured residue -that hæmoglobin owes its value as a carrier of oxygen. Later, iron -is removed from hæmochromogen, leaving hæmatoidin, a substance often -found at the seat of old hæmorrhages, where it may remain unchanged -for a very long time. Hæmatoidin is apparently identical with the -yellow pigment of bile, bilirubin. The green colour which shows itself -in the bruise seems to indicate that the more oxidized bile-pigment, -biliverdin, is formed in the first instance. Red corpuscles, when -destroyed in the spleen, pass through transformations similar to those -which blood undergoes when effused beneath the skin. Their protein is -used by the phagocytes which eat them. Their iron is reserved for the -use of the blood-forming cells of the red marrow of bone. The pigment -which remains as the residue of hæmoglobin is carried by the splenic -vein to the liver, which secretes it as bile-pigment. So much of the -bile-pigment as is reabsorbed by the wall of the alimentary canal is -eventually excreted as the pigment of urine. - -Such is the history of the changes which blood-pigment undergoes within -the living body. To a certain extent its chemistry can be followed in -the laboratory; but it must be remembered, when we are treating of the -chemistry of a substance as complex as hæmoglobin, that the products -which can be obtained from it in the laboratory are not necessarily -those into which it is transformed in the body. In the laboratory -oxyhæmoglobin is easily changed into methæmoglobin, a substance of the -same percentage composition, but with its oxygen more firmly fixed. -Methæmoglobin can be decomposed into a proteid substance and hæmatin. -Hæmatin, when acted on by reducing agents, becomes hæmochromogen. -Hæmochromogen, when subjected to such a reducing agent as a mixture -of tin and hydrochloric acid, gives rise to coloured bodies closely -resembling bile-pigments—not as they are secreted by the bile, but as -they appear in the urine. It is impossible to prove that the changing -colours of a bruise indicate a sequence of chemical transformations -from hæmoglobin to bile-pigment, but it is not improbable that such -a description is correct. The test commonly used to ascertain the -presence of bile-pigment, _i.e._, bilirubin, is the play of colours -which it exhibits when oxidized by fuming nitric acid. From yellow it -turns to green, to blue, and then to purple, more or less reversing -the colours of the bruise. It is fairly certain that effused blood -undergoes changes along lines which, if not identical with those -through which blood passes on its road to bile-pigment, are at any rate -very similar. - -=Coagulation of Lymph and Blood.=—Two or three minutes after blood has -been shed it begins to clot. In ten minutes the vessel into which it -has been received may be inverted without spilling the blood. After a -time the jelly, holding all the corpuscles, shrinks from the sides of -the jar. It squeezes out a transparent, straw-coloured fluid—serum. -The clot continues to contract until, in a few hours, about one-half of -the weight of the blood is clot, the other half serum. Lymph coagulates -like blood, but most specimens clot more slowly, and the product is -less firm. - -When the process is watched through the microscope—a few drops of the -almost colourless, transparent blood of a lobster afford an excellent -opportunity of studying the formation of the clot—innumerable -filaments of the most delicate description are seen to shoot out from -many centres. They multiply until they constitute a felt-work. In -the case of blood obtained from a vertebrate animal, this felt-work -holds the corpuscles in its meshes. Its filaments exhibit a remarkable -tendency to contract. They shorten as much as the enclosed corpuscles -allow. - -The filaments may be prevented from entangling the corpuscles by -whipping the blood, from the instant that it is shed, with a bundle of -twigs or wires. The fibrin collects on the wires, while the corpuscles -remain in the serum. If this fibrin is washed in running water until -all adherent serum and corpuscles are removed, it appears as a soft -white stringy substance which, when dried, resembles isinglass. - -Clotting is a protection against hæmorrhage. As it oozes from a scratch -or tiny wound, blood clots, forming a natural plaster which prevents -continued bleeding. It has little if any influence in resisting a -strongly flowing stream of blood. But a clean cut through a large -vessel is an accident which rarely happens as the result of natural -causes. It is not the kind of injury to which animals are liable. When -an artery is severed by a blunt instrument, the muscle-fibres of its -wall contract. They occlude the vessel. The blood clots at the place -where the vessel is injured, and plugs it. This happens also when a -surgeon ties an artery. He is careful to pull the ligature sufficiently -tight to crush its wall. His sensitive fingers feel it give. He stops -before the thread has cut it through. As will be explained later, the -clotting of blood is promoted by contact with injured tissue. If in -tying an artery its wall be not crushed, the blood in it may remain -liquid. When it is skilfully tied, the blood clots, forming a firm plug -which is practically a part of the artery, by the time that the silk -thread used in tying it is thrown out, owing to the death of the ring -of tissue which it compressed. After a tooth has been extracted, the -cavity is closed and further bleeding stopped by clotted blood. - -When large vessels have been severed, the copious hæmorrhage which -follows induces fainting. For a short time the heart stops, or beats -very feebly. The blood-pressure falls. The bloodvessels contract. A -clot has time to form. An emotional tendency to faint at the sight of -blood is a provision for giving the various causes which stop bleeding -an opportunity of coming into play. It is a useful reflex action, -always supposing that the person who is liable to it faints at the -sight of his own blood. Amongst other reasons for the greater fortitude -of women—they are far less subject to this emotional reflex than -men—might be alleged the circumstances of life of primitive people. It -was the part of their women-folk to dress wounds, not to receive them. - -The phenomenon of coagulation has attracted attention from the earliest -times. It was a phenomenon that needed explanation, and culinary -experience suggested analogies close at hand. Hippocrates attributed -the clotting of blood to its coming to rest and growing cold. The blood -which gushed from a warrior’s wound formed a still pool by his side. -It set into a jelly as it cooled. Until the second quarter of the -nineteenth century this theory was deemed sufficient. It then occurred -to two men of inquiring mind to institute control experiments. John -Davy placed a dish of blood upon the hob. William Hunter kept one -shaking. In both experiments the blood clotted more quickly than it did -in vessels of the same size, containing the same amount of the same -blood, left upon the table. - -Even before this date an observation had been made regarding the -circumstances in which clotting occurs, which has thrown much light -upon the causes of the phenomenon. In 1772 Hewson gently tied a vein -in two places. At the end of a couple of hours he opened the vein. The -blood was still liquid, but clotted in a normal manner after it was -shed. Scudamore showed that blood clots more slowly in a closed than in -an open flask. A new theory, as little trustworthy as Hippocrates’, was -based upon these observations. Blood clotted because it was exposed to -air. A record of all observations of the circumstances of coagulation, -and of all the theories to which they have given rise, would make an -exceptionally interesting chapter in the history of human thought. -It would bring into singular prominence stages in the development of -what is now known as the “scientific method.” Not that Science has a -method of her own. Philosophers of all classes would follow the same -method if their data allowed of its application. The peculiarity of the -data with which Science deals is that they can be brought to a test -of which the data of historical, or political, or economic theory are -not susceptible. They can be confronted with control experiments. The -control experiment is the alphabet and the syntax of the scientific -method. No hypothesis is admissible into the pyramid of theory until -it has passed this test. A natural phenomenon is observed. Every -measurement which is applicable is taken and recorded—time, weight, -temperature, colour. Scientific observation implies the tabulation of -all particulars which are capable of statistical expression. Reflecting -upon the relation of the phenomenon to other phenomena of a like -nature, the philosopher—it is the philosophy of physiologists which -interests us—formulates an hypothesis as to its cause. At this point -the real difficulty of applying the scientific method begins. It is -easy to formulate hypotheses. It is very difficult to devise control -experiments. An experiment must be arranged which will provide that, -while all other conditions in which the phenomenon has been observed -to occur are reproduced, the condition which was _ex hypothesi_ its -cause shall be omitted. This digression into the philosophy of science -may seem to be somewhat remote from our line of march, but it may -perhaps hasten our progress in the comprehension of the story of -physiology. There is no other science in which the control experiment -plays an equally important part. Unless this is realized, the whole -trend of experimental work will be misunderstood. Scudamore explained -coagulation as due to contact with air. Based on the observations we -have cited, no hypothesis could have seemed more reasonable. With -a view to checking this hypothesis, blood was received into a tube -of mercury. It coagulated in the Torricellian vacuum. Scudamore’s -hypothesis, like many earlier and later, when confronted with a control -experiment, was turned away, ashamed. - -Clotting is a property of plasma. Red corpuscles play no part in -the process. Coagulation does not occur in a living healthy vessel. -It occurs when the vessel, and especially when its inner coat, is -injured. It is hastened by contact with wounded tissues, especially -with wounded skin. Contact with a foreign body also starts coagulation. -If a silk thread is drawn through a bloodvessel, from side to side, -fibrin filaments shoot out from the thread, as well as from the wound -inflicted on the vessel by the needle which was used to draw it through. - -Plasma contains a substance which sets into fibrin. It has been -termed “fibrinogen.” It is present in lymph, and in almost all forms -of exuded lymph. If sodium chloride (common salt) is added to plasma -until it is half saturated—until it has dissolved half as much as -the maximum quantity which it can dissolve—fibrinogen is thrown down -as a flocculent precipitate. It can be redissolved and reprecipitated -until it is pure. When fibrinogen was separated from plasma a step was -taken towards the explanation of coagulation. Under certain conditions -fibrinogen sets into fibrin. The question which then presented itself -for solution was as follows: What is the substance which, by acting -upon or combining with fibrinogen, converts it into fibrin? The clue -to the solution of this question was obtained from the consideration -of certain observations made by Andrew Buchanan in 1830, but long -neglected, because their significance was not understood. Buchanan had -observed that some specimens of lymph exuded into a lymph-space—the -peritoneal cavity, for example—will clot; others will not. He noticed -that they clot when, owing to puncture of a small bloodvessel during -the process of drawing them off, they are tinged with blood. Determined -to ascertain which of the constituents of blood is effective in -rendering non-coagulable effusions capable of clotting, he added to -them in turn red blood-corpuscles, serum, and the washings of blood -clot. Either of the two latter was found to contain the clot-provoking -substance. Thirty years later a German physiologist prepared fibrinogen -from effused lymph by precipitating it with salt. He also treated -serum in a similar way, precipitating a protein which he termed -fibrinoplastin. When these two substances were dissolved and the -solutions mixed, he obtained a clot, which he regarded as a compound of -fibrinogen and fibrinoplastin. Subsequently he found that the mixture -did not always clot, but he discovered that if he coagulated blood -with alcohol, and washed this residue, the washings added to the mixed -solution just referred to invariably produced a clot. Thinking that the -substance which he obtained from his alcohol-coagulated blood could -not be proteid, he termed it “fibrin-ferment.” He neglected the control -experiment. He failed to ascertain whether or not all three substances -were needed. Had he tried adding fibrin-ferment to fibrinogen, he -would have discovered that the further addition of fibrinoplastin -was unnecessary. He did not ascertain, as he might have done, that -the weight of fibrin formed is somewhat less, not greater, than the -weight of fibrinogen used. (Fibrinogen gives off a certain quantity of -globulin when it changes into fibrin.) He was also wrong in supposing -that the water which he added to alcohol-coagulated blood dissolved -no protein. His “fibrin-ferment” is always associated with a protein. -Since it may also be obtained from lymphatic glands, thymus gland, and -other tissues which contain lymphocytes, it has been inferred that -it is itself a protein, of the class known as nucleo-proteins. The -fact that it is destroyed at so low a temperature as 55° C. has been -supposed to confirm the theory that it is a protein. But with regard to -the chemical nature of fibrin-ferment, as of all other ferments, we are -at present in the dark. Under ordinary circumstances, when blood clots, -the fibrin-ferment, or plasmase, or thrombin—it has received various -names—is set free by leucocytes. Fluids which contain fibrinogen clot -on the addition of a “ferment” which is either secreted by leucocytes -or set free from leucocytes when they break up—as they are very apt -to do, as soon as the conditions upon which their health depends are -interfered with. - -Freshly shed blood contains minute particles, termed “platelets,” in -diameter measuring about a quarter that of a red blood-corpuscle. When -the inner coat of a vessel is injured, platelets accumulate at the -injured spot. They form a little white heap, from which coagulation -starts. Evidently they supply the ferment, or a precursor of the -ferment. As yet their origin has not been traced. They are too large to -be the unchanged granules of granular leucocytes, but that they are in -some way derived from leucocytes seems probable. - -The further study of coagulation has shown that the conditions under -which it occurs are more complicated than the simple explanation -just given would seem to imply. This explanation holds good, so far -as it goes, but facts connected with the details of the process have -recently been brought to light which warn the physiologist that as yet -his theory of coagulation is incomplete. - -The presence of salts of lime has an important relation to coagulation. -If blood is received into a vessel in which has been placed some -powdered oxalate of potash, or soap, or any other chemical which -fixes lime, the blood does not coagulate. All other conditions are as -usual, but lime is withdrawn from the plasma. The non-coagulation of -oxalated plasma was interpreted as indicating that lime, under the -influence of fibrin-ferment, combines with fibrinogen to form fibrin; -that fibrinogen altered by fibrin-ferment combines with lime. This -hypothesis was based upon the analogy of the curdling of milk. Milk -cannot curdle if lime be absent. If rennin (milk-ferment), prepared -from milk from which lime has been removed, be added to a solution -of caseinogen (the coagulable protein of milk), also prepared from -lime-free milk, no curd is produced. The addition of a few drops of a -solution of chloride of lime results in the immediate curdling of the -mixture. Evidently rennin so alters caseinogen as to bring it into a -condition to combine with lime. But the analogy does not hold good for -blood. In the case of plasma, lime acts, not upon fibrinogen, but upon -the fibrin-ferment—or rather upon a precursor of fibrin-ferment—in -such a way as to render it effective. Leucocytes produce a prothrombin, -which in contact with lime salts is converted into thrombin, which -coagulates fibrinogen. - -Fibrinogen is the substance which fibrin-ferment combined with salts -of lime changes into fibrin. Yet even now the story is not complete, -if the theory of coagulation is to be brought up to date. A perfectly -clean cannula is passed into an artery of a bird. If it be thrust -well beyond the place where the vessel has been cut, if the vessel -be tied so gently as to avoid injury to its inner coat, and if the -blood which first passes through the cannula be allowed to escape, the -blood subsequently collected will not clot. It contains fibrinogen, -lime salts, and fibrin-ferment, ordinarily so called; but the ferment -is ineffective. The addition to the blood of a fragment of injured -tissue, or of a watery extract of almost any tissue, immediately sets -up coagulation. This observation brings fibrin-ferment into line with -other ferments. Digestive ferments are secreted as zymogens, which -require to be influenced by a kinase before they acquire fermentative -activity. So, too, must thrombogen be changed into thrombin, under the -influence of thrombokinase, before it can act upon fibrinogen. Almost -all tissues yield the kinase which actuates fibrin-ferment. The utility -of this provision is manifest. A bird’s blood contains everything -necessary to form a clot with the exception of thrombokinase. The -injury which brings the blood into contact with a broken surface -supplies this ferment of the ferment. Fibrin-ferment, rendered active, -at once changes fibrinogen into fibrin. The same interaction is -necessary before the blood of a mammal is susceptible of clotting. But -a mammal’s blood is even readier to clot than is the blood of a bird; -for not only will a broken surface provide it with thrombokinase, -but the leucocytes contained within the blood, when injured, also -yield it. And the leucocytes are exceedingly sensitive of any change -of circumstance; on the slightest indication that conditions are not -normal they set free, perhaps owing to their own disintegration, the -kinase which turns thrombogen into thrombin. - -There is a constitutional condition, fortunately rare, in which blood -does not coagulate. A person subject to this abnormality is said to -suffer from hæmophilia. It is alleged that this condition is due to -deficiency of lime in the blood; and the deficiency of lime is said to -be due to excess of phosphates. The subject suffers from phosphaturia. -His kidneys get rid of the superabundance of phosphates by excreting -them in combination with lime. If this explanation be correct, there -is a chronic insufficiency of lime in the blood, because it is being -constantly withdrawn in the process of removing phosphates. - -The difficulty in the way of establishing a complete theory of the -coagulation of blood increases when the phenomena of incoagulability -are considered. Blood may be rendered incapable of clotting in a -variety of ways. Leeches and other animals which suck blood have the -capacity of rendering it incoagulable. If the heads are removed from -a score of leeches, thrown into absolute alcohol, dried, ground in a -pepper mill, extracted with normal saline solution, a dark turbid -liquor is obtained. This liquor, after filtration and sterilization at -a temperature of 120° C., injected into the veins of an animal, renders -its blood incoagulable. - -The preparation sold by druggists under the name “peptone,” when -injected into the veins of a dog, renders its blood incoagulable. -Commercial “peptone” is a mixture of many substances. Its -anticoagulation-effect is not due to the peptone which it contains. -It has been supposed to be due to imperfectly digested albumin and -gelatin (proteoses), but products of bacteric fermentation (toxins -and ptomaines) are more probably the active bodies. Not only is -the peptonized blood of a dog incoagulable, but if this blood be -injected into the veins of a rabbit (an animal upon which the direct -injection of peptone has no effect), it diminishes the coagulability -of the rabbit’s blood. If peptonized blood be mixed in a beaker with -non-peptonized blood, it prevents the coagulation of the latter. There -is little doubt but that the poison, whatever it may be, acts upon the -leucocytes; and there are some reasons for thinking that the poison is -not contained in the “peptone,” but is secreted by the liver of the -animal into which the “peptone” has been injected. - -A still more remarkable property in relation to coagulation must be -assigned to leucocytes. The blood of a dog which has been rendered -incoagulable by injection of peptone recovers its coagulability after -a time. If a further injection of “peptone” be made, the animal is -found to be immune. Injection of “peptone” no longer renders its -blood incoagulable. In a similar manner the blood develops a power of -resisting the action of agents which induce its coagulation whilst -circulating in the vascular system. Nucleo-proteins contained in -extracts of lymphatic glands and other organs when injected into the -veins of living animals cause their blood to clot, provided they are -injected in sufficient quantity. If they are injected in quantity less -than sufficient to induce coagulation, they render the animal immune -to their influence. A larger quantity given to an animal thus prepared -fails to take effect. This brings the phenomena of coagulation and -resistance to coagulation to the verge of chemistry. They extend into -the domain in which pathology reigns. Tempting though it be to record -other facts with regard to these phenomena which recent investigation -has brought to light, it is probably judicious to leave the problem at -the frontier. Across the frontier lies a fascinating land, rich with -unimaginable possibilities for the human race. Settlement is rapidly -proceeding in this country, which is charted, like other border-lands, -with barbarous names: “antibodies,” “haptors,” “amboceptors,” “toxins,” -“antitoxins,” and the like—finger-posts to hypotheses which show every -sign of hasty and provisional construction. But certain facts stand -out, in whatever way theory may, in the future, link them up. The virus -of hydrophobia, modified by passing through a rabbit, develops in human -beings, even when injected after they have been infected, the power of -resisting hydrophobia. The serum of a horse which has acquired immunity -to diphtheria aids the blood of a child, which has not had time to -become immune, in destroying the germs of this disease. It is a contest -between the blood and offensive bodies of all kinds which find entrance -to it, whether living germs or poisons in solution; with victory -always, in the long-run, on the side of the blood, provided its owner -does not die in the meantime. And not only is the blood victorious -in the struggle with any given invader, but having repulsed him, it -retains for a long while a property which neutralizes all further -attempts at aggression on his part. In the past, physicians have fought -disease with such clumsy weapons as mercury, arsenic, and quinine. Now -they anticipate disease. In mimic warfare with an attenuated virus the -blood is trained to combat. Smallpox which has been passed through -the body of a cow is suppressed by the blood’s native strength. The -exercise develops skill to deal with the most virulent germs of the -same kind. In cases in which physicians cannot anticipate disease in -human beings, they train the blood of animals to meet it; and, keeping -their serum in stock, they can, when the critical moment arrives, -reinforce the fighting strength of the patient with this mercenary aid. - -=The Spleen.=—The spleen is placed on the left side of the body, and -rather towards the back. It rests between the stomach and the inner -surface of the eighth, ninth, tenth, and eleventh ribs. It is quickly -distinguished from other organs by its brown-purple colour, a sombre -hue to which it owed its evil reputation with the humoralists. The -liver’s yellow bile tinged man’s mental outlook, preventing him from -seeing objects in their natural brightness; but the spleen made black -bile, which, mounting to the brain, displayed its malign influence upon -the action of that organ, as, or in, the worst of humours. - -The spleen is invested with a capsule of no great toughness. Inside the -capsule is “spleen-pulp.” When the fresh organ is cut across, it is -seen that, although most of the pulp is of the colour of dark venous -blood, it is mottled with light patches. In some animals—the cat, -for example—these whitish patches are small round spots, regularly -arranged at a certain distance from the capsule. The distinction -into “red pulp” and “white pulp” marks a division into two kinds of -tissue with entirely different functions. The white pulp is lymphoid -tissue, lymph-follicles developed in the outer or connective-tissue -coat of the branches of the splenic artery. Its function is to make -lymphocytes, of which, for reasons which will shortly appear, the -spleen needs an abundant supply. The constitution of the red pulp is -entirely different, and peculiar to the spleen. The branches of the -splenic artery divide in the usual way into smaller and still smaller -twigs until the finest arterioles are reached; but these arterioles -do not give rise to capillary vessels. At the point at which in any -other organ their branches would attain the calibre of capillaries, -the connective-tissue cells which make their walls scatter into a -reticulum. They are no longer tiles with closely fitting, sinuous, -dovetailed borders, but stellate cells with long delicate processes -uniting to constitute a network. The blood which the arterioles bring -to the pulp is not conducted by closed capillary vessels across the -pulp to the commencing splenic veins. It falls into the general -sponge-work. The venules commence exactly in the same way as the -arterioles end. Stellate connective-tissue cells become flat tiles -placed edge to edge. The endothelium of an arteriole might be likened -to a column of men marching shoulder to shoulder, three or four -abreast; the connective tissue of the pulp, to a crowd in an open -place. The column breaks up into a crowd. On the other side the crowd -falls into rank as the endothelium of veins. The capsule and the red -pulp are largely composed of muscle-fibres. These relax and contract -about once a minute. By their contraction the blood is squeezed out of -the sponge. - -If the spleen be enclosed in an air-tight box (an oncometer), from -which a tube leads to a pressure-gauge—a drum covered with thin -membrane on which the end of a lever rests, or a bent column of -mercury on which it floats—the pressure-gauge shows the changes in -volume of the spleen. The long end of the lever, which records the -variations of pressure in the gauge, may be made to scratch a line -on a soot-blackened surface of travelling paper. A record of the -variations in volume of the organ, which can be studied at leisure, -is thus obtained. It shows that the spleen is sensitive to every -change of pressure in the splenic artery. Small notches on the tracing -correspond to the beats of the heart. Larger curves record the changes -of blood-pressure due to respiration. A long slow rise and fall marks -the rhythmic dilation and contraction of the spleen itself. - -One of the three large arteries into which the cœliac axis divides -delivers blood to the spleen direct from the aorta. The splenic vein -joins the portal vein shortly before it enters the liver. Thus the -spleen is placed on a big vascular loop which directs blood, not long -after it has left the heart, from the aorta, through the spleen, to the -liver. - -The peculiar construction of the splenic pulp which brings the blood -more or less to rest within its sponge-work, and the transmission to -the liver of the blood which leaves the spleen, indicate that it is an -organ in which blood itself receives some kind of treatment. It is not -passed through it, as it is through all other parts of the body, in -closed pipes. The spleen is a reservoir, or a filter-bed, into which -blood is received. - -[Illustration: FIG. 5.—A MINUTE PORTION OF THE PULP OF THE SPLEEN, -VERY HIGHLY MAGNIFIED. - - Stellate connective-tissue cells form spaces - containing red blood-corpuscles and leucocytes. - In the centre of the diagram is shown the mode of - origin of a venule. It contains two phagocytes—the - upper with a nucleus, two blood-corpuscles just - ingested, and one partially digested in its - body-substance; the lower with two blood-corpuscles.] - -The red blood-corpuscles of mammals are cells without nuclei, and with -little, if any, body-protoplasm. They are merely vehicles for carrying -hæmoglobin. We should deny to them the status of cell, if it were -possible to prescribe the limit at which a structural unit ceases to be -entitled to rank as a cell. They are helpless creatures, incapable of -renewing their substance or of making good any of the damage to which -the vicissitudes of their ceaseless circulation render them peculiarly -liable. It is impossible to say with any approach to accuracy how long -they last, but probably their average duration is comparatively short. -The spleen is a labyrinth of tissue-spaces through which at frequent -intervals all red corpuscles float. If they are clean, firm, resilient, -they pass through without interference. If obsolete they are broken up. -In the recesses of the spleen-pulp, leucocytes overtake the laggards of -the blood-fleet, attach their pseudopodia to them, draw them into their -body-substance, digest them. The albuminous constituent of hæmoglobin -they use, presumably, for their own nutrition. The iron-containing -colouring matter they decompose, and excrete in two parts; the iron -(perhaps combined with protein); the colouring matter, without iron, -as the pigment, or an antecedent of the pigment, which the liver will -excrete in bile. Hæmoglobin is undoubtedly the source of bilirubin, and -general considerations lead to the conclusion that it is split into -protein, iron, and iron-free pigment in the spleen; but the details -of this process have never been checked by chemical analysis. Neither -bile-pigment nor an iron compound can be detected in the blood of the -splenic vein. The only evidence of the setting free of iron in the -spleen is to be found in the fact that the spleen yields on analysis -an exceptionally large quantity of this metal (the liver also yields -iron), and that the quantity is greatest when red corpuscles are being -rapidly destroyed. - -As a rule, it is very difficult to detect leucocytes in the act of -eating red corpuscles; but under various circumstances their activity -in this respect may be stimulated to such a degree as to show them, -in a microscopic preparation, busily engaged in this operation. The -writer had the good fortune to prepare a spleen which proved to be -peculiarly suitable for this observation (Fig. 5). His method was an -example of the way in which a physiological experiment ought not to -be conducted. Having placed a cannula in the aorta of a rabbit, just -killed with chloroform, he was proceeding to wash the blood out of its -bloodvessels with a stream of warm normal saline solution, when the -bottle from which the salt-solution was flowing overturned. Fearing -lest an air-bubble should enter the cannula, he hastily poured warm -water into the pressure-bottle, and threw in some salt, in the hope -that it would make a solution of about 0·9 per cent. The salt-solution -was allowed to run through the bloodvessel for rather more than an -hour. When sections of the spleen were cut, after suitable hardening, -every section was found to be packed with leucocytes gorged with red -corpuscles. Some of the corpuscles had just been ingested; from others -the hæmoglobin had already been removed. It may be that, for some -unknown reason, the destruction of red corpuscles was occurring in this -particular rabbit with unusual rapidity at the time when it was killed; -but it seems more probable that the animal’s leucocytes were provoked -to excessive activity by changes in the red corpuscles brought about by -salt-solution which was either more or less than “toxic.” As a score -of attempts to reproduce the experiment, with solutions of different -strengths, have failed, it is impossible to be sure that this is a -valid explanation. - -There must be something in the condition of worn-out red corpuscles -which either makes them peculiarly attractive to predatory leucocytes -or renders them an exceptionally easy prey. It does not require much -imagination to picture the drama which is enacted in the spleen. -Slow-moving leucocytes are feeling for their food. The majority of red -corpuscles pass by them; a few are held back. The leucocytes, like -children in a cake-shop, cannot consume all the buns. A selection -must be made, and preference is given to the sticky, sugary ones. Red -corpuscles when out of order show a tendency to stick together. When -blood is stagnating in a vein, or lying on a glass slide in a layer -thin enough for microscopic examination, its red discs are seen after -a time to adhere together in rouleaux. The parable of a child in a -cake-shop is not so fanciful as it may appear. - -The differentiation of function of organs is not as sharp as was -formerly supposed. Evidence of their interdependence is rapidly -accumulating. The activity of various organs is known to result in -the formation of by-products termed “internal secretions,” which -influence the activity of other organs, or even of the body as a -whole. The spleen enlarges after meals. This may be merely connected -with the engorgement of the abdominal viscera which occurs during -active digestion, or it may indicate, as some physiologists hold, that -an internal secretion of the spleen aids the pancreas in preparing -its ferments. The spleen enlarges greatly in ague and in some other -diseases of microbial origin. This has been regarded as evidence -that it takes some part in protecting the body against microbes. But -whatever may be the accessory functions which it exercises, they are -not of material importance to the organism as a whole, seeing that -removal of the spleen causes no permanent inconvenience either to men -or animals. Its blood-destroying functions are taken on by accessory -spleens, if there be any, and by lymphatic glands. The marrow of bone -also becomes redder and more active. Under certain circumstances, red -corpuscles, or fragments of red corpuscles, are to be seen within -liver-cells; but it is uncertain whether blood-destruction is a -standing function of the liver. - - - - -CHAPTER V - -INTERNAL SECRETIONS - - -=Thyroid Gland.=—On either side of the windpipe, rather below the -thyroid cartilage (Adam’s apple), lies a somewhat conical mass of -tissue. The two masses are connected by an isthmus; lobes and isthmus -make up the thyroid gland. The whole weighs about an ounce. In health -it is so soft that only the finger of an anatomist could detect it -through the skin and the thin flat muscles which connect the hyoid -bone and the thyroid cartilage with the breast-bone. It makes no -visible prominence on the front of the neck. The thyroid gland is, -however, liable to enlargement, especially amongst the people who live -in certain districts. In the Valais, “goitre,” as it is termed, is -so frequent that anyone walking up the Rhone Valley is sure to meet -a number of persons—for the most part women—whose swollen necks -overhang their collar-bones, like half-filled sacks. Goitre is even -more common in the Valle d’Aosta, on the Italian side of the Alps. In -England this condition, comparatively rare, is known as “Derbyshire” or -“Huntingdonshire” neck. - -In the majority of cases the tumour in the neck develops slowly, and -does not reach its full dimensions until after middle life. Goitre in -this form, although inconvenient, causes no serious discomfort. But -when it appears in early life, it is associated with an extraordinary -complex of malformations and ill-performed functions. The condition -into which a goitrous child sinks is known as cretinism. With the -exception of the skull-case, its skeleton does not attain to its proper -proportions; and, since the soft parts do not equally submit to arrest -of growth, the dwarf is heavy and ungainly, with large jowl and -protuberant abdomen. The appearance of distortion is extraordinarily -heightened by hypertrophy of the skin and the subcutaneous connective -tissue. Ears, eyelids, nose, lips, fingers, are thick and heavy. The -hair and nails are coarse. The skin is folded, wrinkled, rough. - -The bodily ungainliness of a cretin has its counterpart in the -deformity of his mind. He is an idiot whose deficiency is chiefly -marked by apathy. - -Cretinism exhibits itself in varying degrees. The description that we -have just given would not be accurate for all. For the sake of brevity, -we have chosen a case which might be that of a goitrous cretin of a -certain type, or that of a cretin whose thyroid gland, in lieu of -showing what looks like overgrowth, has failed to properly develop. -Nothing is more remarkable with regard to this organ than the fact -that the condition associated with its overgrowth and the effects of -its atrophy, or inadequate growth, are the same. A consideration of -the function of the gland will suggest an explanation of this seeming -paradox. - -The inconvenience caused by goitre induced surgeons, about twenty-five -years ago, to remove the tumour in simple uncomplicated cases. Owing -to the accessibility of the gland, the operation is both safe and -easy; but its removal was found to be followed by symptoms of a very -serious nature, especially overgrowth and œdema of subcutaneous -tissue, muscular twitchings and convulsions, mental dulness. About -the same date, physicians recognized that the disease myxœdema—so -called because the œdema is not watery, as in dropsy, but firm and -jelly-like—is due to deficiency of the thyroid gland. - -No other organ of the body has so weird an influence upon the -well-being of the whole. No other organ has an equally mysterious -ancestral history. Assuredly the thyroid gland was not always such -as we see it now. In prevertebrate animals it must have been quite -different, both in structure and in function. From fishes upwards, -however, its structure is always the same. It is composed of spherical -vesicles or globes. Every globe is lined by a single layer of cubical -epithelial cells. Its cavity is filled with a homogeneous semi-solid -substance known as “colloid.” The globes are associated into groups -or lobules. They are in contact with large wide lymphatic vessels. -The organ has a lavish supply of blood. It is also well supplied with -nerves. Colloid is the secretion of the epithelial cells which line -the globes. As these globes have no openings, the secretion must be -passed by osmosis into the lymphatic vessels. There is abundant reason -for believing that by this route the products of the gland reach the -blood, and are distributed by the blood to all the tissues of the body. -And here it is important to notice that associated with the thyroid -gland are certain very small masses of tissue termed “parathyroids.” -There may be four of these—two on the course of the large arteries -which supply the thyroid gland from above, two related with the almost -equally large arteries which supply it from below; but the number -varies. The parathyroids do not contain vesicles. They are solid -masses of epithelial cells, traversed by bloodvessels and lymphatics. -Yet, like the epithelial cells of the vesicles, they secrete colloid. -Granules of this substance are to be seen within their cells. We cannot -pass over the parathyroids without this reference, since, small though -they are, they seem to be quite as important as the thyroid gland -itself, judging from the effects which follow their removal. - -In all vertebrate animals the thyroid gland has the characters which -we have described. What was it like in the ancestors of the vertebrate -races? Its microscopic appearance in vertebrates, the only animals in -which we know it, is so anomalous as to convince an histologist that -it is a makeshift; it looks like an organ which, at a period no longer -visible through the mists of time, had a quite different function to -perform. This function it has lost—some other organ has taken it -on—yet it must do something which is useful to the organism. Otherwise -it would not have been preserved. It has been retained for the sake -of its by-function, for the sake of the internal secretion which it -produces. This is now the only work it has to do. - -What was its prime function? It is an axiom of biology that an -animal in its individual development recapitulates, albeit with many -omissions and abbreviations, the ancestral history of its race. The -thyroid gland appears in the embryo as a diverticulum of the anterior -wall of the pharynx. It is remarkable in being a single, median, -unpaired diverticulum, whereas almost all other organs are bilaterally -symmetrical. The parathyroids are formed on the two sides in connection -with certain of the branchial pouches. In its earliest development the -thyroid gland resembles any other gland—a salivary gland, for example. -Until a late stage it retains its connection with the back of the -mouth. Occasionally indications of this primitive connection persist -throughout life. In most cases the place where the duct of the thyroid -gland used to open is clearly marked. At the back of the tongue—too -far back to be seen without the aid of a dentist’s mirror—there -is a =V=-shaped row of large papillæ (papillæ circumvallatæ). Just -behind the meeting-point of the two limbs of the =V= a pit is to be -seen—foramen cæcum. This pit is the vestige of the mouth of the duct -of the thyroid gland which opened into the pharynx in the ancestors of -fishes. It is an inconceivably long time since fishes diverged from -other races of animals. We do not know which of the various orders of -invertebrate animals now existent most nearly resembles our prepiscine -ancestor. The organ which has developed into the thyroid body of -mammals may possibly have disappeared from all the other descendants of -the common stock from which vertebrates and their nearest relatives in -the invertebrate sub-kingdom were evolved; but it is much more likely -that it has been preserved, and is still performing its prime function -in the higher invertebrate animals. Probably it is a functional organ -in a cuttle-fish or a scorpion or a worm, but so unlike the thyroid -gland of vertebrates that we fail to recognize its homology. There are -other instances in the body of the persistence of an organ long after -it has fallen into such ruin that not even archæologically-disposed -biologists can guess what it was like, or what purpose it served in -the days when it was at the height of its efficiency; but perhaps -there is none other which so pregnantly illustrates the physiological -doctrine of functional interdependence. Nature shows herself amazingly -conservative in retaining primal organs—the pituitary body, the -thymus gland, the thyroid gland, the suprarenal capsules—organs which -millions of years ago forgot the very rudiments of their craft; but -her conservatism is not mere force of habit. Although she no longer -has any use for the wares which she created these pieces of apparatus -to make, she cannot do without their refuse. Even the vermiform -appendix may have its use. Dr. Gaskell’s theory of the thyroid gland -involves a transformation so fantastic that it would provoke a smile -of incredulity were we to set it forth without a prologue far more -lengthy than our space permits. Yet Dr. Gaskell may be right. We can -but guess as to the nature of the prime functions of the thyroid and -parathyroids. For many geological epochs they have not been exercised. -But whatever else they did when they were indispensable constituents -of the organism, their activity was accompanied by the secretion of -colloid. Colloid is not made by other organs; therefore the otherwise -obsolete thyroids are retained. It is of course not impossible that, in -a certain degree, Nature, like a thrifty housewife, finds a new use for -superseded apparatus; but we are probably justified in believing that -the use is never really new. Not wanting the organ for its original -specific purpose, Nature relegates to it alone work which hitherto it -had shared with other of her tools. - -A comparatively short while ago the attention of physiologists was -wholly concentrated upon the obvious or prime functions of organs. -Muscles contracted. The stomach digested. The pancreas secreted -pancreatic juice. The brain made thought. Now they understand, to put -it somewhat metaphorically, that gastric juice is made in the calves -of the legs; the ferment of pancreatic juice in the small intestine; -thought of a certain emotional quality in the large intestine. The -chemistry of the laboratory is far behind the body’s chemistry. -We cannot detect in the blood coming from contracting muscles the -stimulant—possibly a precursor of pepsin—to which the stomach reacts, -although the magical benefit of exercise seems to prove that there -is a chemical connection between the activity of the muscles and the -activity of the glands of the alimentary canal. It has been proved by -experiment that a substance produced in the epithelium of the small -intestine is the messenger upon whose call depends the potency of -pancreatic juice. The clearing of the brain effected by a judicious -pill shows that poisons of some kind are absorbed into the blood from -an overloaded large intestine. None of the organs lives altogether for -itself. The chemical products which it throws off, absorbed by the -blood, regulate the activity of other organs. Formerly the several -parts of the body were looked upon as independent. Their activity was -regarded as a direct response to the commands of the nervous system. -If it varied in kind, the variations were supposed to depend upon the -quality of the nervous impulses which reached the organ. Evidence is -rapidly accumulating that many exhibitions of function are evoked by -the calls of “hormones,” or chemical messengers, not by command of the -nerves. - -Internal secretions, using the expression in its general sense, are -necessary for the co-ordination of the work of the various parts of -the animal mechanism. Colloid is the internal secretion of the thyroid -gland and of the parathyroids. Unlike most other internal secretions, -it is a substance easily analysed, and startlingly definite in its -chemical characteristics. It is composed mainly of a protein which -contains iodine. From this protein a substance termed “iodothyrin” -may be obtained, of which no less than 10 per cent. is iodine; but -it is uncertain whether iodothyrin is preformed in the gland. The -exact nature of the active substance of the internal secretion of the -thyroid gland matters little. Whether it be iodothyrin or a protein, -its activity depends upon the fact that it contains iodine in large -quantity. Iodine amounts to from 0·3 per cent. to 0·9 per cent. of the -weight of the whole thyroid gland in Man. - -Iodine is very widely distributed in Nature. It is present in the air, -in rain-water, in herbage. It is also present in all parts of the -body, although in quantities which are infinitely minute. It is found -in sea-water, and is relatively abundant in marine plants. There is -no reason for supposing that it is deficient in districts in which -_goitre_ is common. It would appear more likely that the soil has -properties which result in the fixing of iodine in a form in which it -is not available for plant-food, and that in consequence animals are -unable to obtain a sufficient supply. Careful analyses have shown that -the thyroid glands of sheep bred in mountainous districts where goitre -is common contain but one-thirtieth part as much iodine as the thyroid -glands of sheep bred in places where goitre is rare. In ancient times -burnt sponge and seaweed were esteemed useful in the treatment of -goitre. Later, iodide of potassium given internally, and tincture of -iodine as an outward application, were the approved remedies. It is -now known that myxœdema and certain forms of goitre may be checked, -and even cured, by administering uncooked thyroid gland or even -tabloids of dried extract. Fortunately, it is not necessary to inject -it subcutaneously; the iodine-containing compound is so stable as to -resist the action of gastric juice. - -Iodine stored in the thyroid and parathyroid glands is distributed -to all the tissues. The remarkable symptoms which indicate that the -tissues are not receiving an adequate supply may occur under either of -two conditions. Iodine may be deficient in the food, or the thyroid -gland may be incompetent; the former is the commoner cause. And here -we see the explanation of the formation of a goitre. By increasing the -size of the organ which selects iodine, Nature attempts to obtain and -store an adequate supply for distribution to the tissues. - -Cretinism has been observed in animals. If attention were directed to -this inquiry, it might be found that it is not so exceedingly rare as -would be judged from the few observations that have been recorded. A -cretin, if a wild animal, falls an easy prey. If a domesticated animal, -little trouble is taken to insure its survival. A myxœdematous pig -is a dwarf with coarse, sparse hair, thick, warty hoofs, large jowl, -heavy ears. It is apathetic. A piglet presenting these characteristics -is not altogether uncommon in a litter. Among chickens and pigeons, -also, individuals appear which might, judging from their uncouth -appearance and mental dulness, be suffering from cretinism. The only -way of proving that this is the case is to feed them on thyroid glands; -it does not matter from what animal the gland is obtained. Operative -cretins, produced by removing the thyroid gland soon after birth, -recover their natural characters on a diet containing a daily allowance -of thyroid gland. The coarse hairs, or wiry towsled feathers, fall off, -and are replaced by a smooth, supple growth. The thickened skin becomes -soft and pliant. Mental apathy gives place to alertness. They make up -for lost time by growing more rapidly than other animals of the same -age, which have not been operated upon, although they never surpass the -normal stature. - -=Suprarenal Capsules.=—Each of the kidneys is capped by a pyramidal -body weighing about ⅛ ounce. Small though it be, this organ is -essential to life. As Dr. Addison was the first to discover, in 1855, -its disease results in a cycle of symptoms which invariably has a fatal -termination. A college friend of the writer suffered from “slackness.” -Before he had finished a set of tennis, he abandoned the game, and -spent the rest of the afternoon lying on the grass, wrapped in a rug. -After hall, although he earnestly desired to conquer the subtleties of -the Greek grammar, he fell asleep over his books. As his countenance -was not ruddy merely, but bronzed like that of a man who has just -returned from a yachting cruise, he was the butt of many a joke. -Although already a qualified medical man, who had been in practice—he -had come to the University with a view to adding the degree of M.D. to -his M.R.C.S.—he had no suspicion that he was ill. Thought he wanted -“freshening up.” Took a trip across the Atlantic. Stumbled over a rope -on landing; broke his thigh. Spent two months in a New York Hospital, -but the bone did not mend. At last, the surgeons, growing anxious, sent -him back to London. He was seen by a leading physician, who told him -that he was suffering from Addison’s disease. Two months later he died -of failure of the heart. Disease of the suprarenal capsules is usually -of tuberculous origin. Its symptoms: muscular weakness and excessive -liability to fatigue; abnormal pigmentation of the skin; lowered -blood-pressure, and consequent sensitiveness to cold; cardiac weakness. -As the pigmentation of the skin and mucous membrane is not invariable, -and since it may occur without disease of the capsules, it is not -improbable that it is due to disease of the abdominal sympathetic -ganglia, which are usually affected at the same time as the capsules. - -The suprarenal capsules are composed of columns of epithelial cells, -which radiate from a large vein in their centre. They are abundantly -supplied with blood and with nerves. The cells near the vein are much -larger than those in the peripheral portions of the columns. Amongst -them are nerve-cells resembling those of the sympathetic system. - -The history of the suprarenal capsules is almost as obscure as that of -the thyroid gland. In the embryo they are relatively very large—larger -at one period than the kidney. At this period bloodvessels are formed -in them with great rapidity by a curious process of boring through and -channelling out of their cells. There are other facts connected with -their development in the individual and their varying form in different -classes of vertebrate animals which point to a “previous existence,” -but there is nothing to indicate that they were ever open glands. In -all vertebrates they are closed masses of cells, the only function of -which, so far as we know, is to produce an internal secretion; but the -importance of this chemical messenger in bringing about the proper -working of other organs is almost startlingly evidenced by the collapse -which follows disease, or removal of the organ which produces it. - -The suprarenal capsules yield a substance which has been termed -“adrenalin.” It contains nitrogen, is crystallizable and dialysable; -but its chemical relationships have not been made out as yet. It is not -destroyed by boiling, nor by digestion with gastric juice. Injected -into a vein, it causes, amongst other effects, an immense rise in -blood-pressure, even though the amount injected be extraordinarily -small. Applied locally as a wash or spray, a solution of 1 part in -10,000 produces marked blanching of the surface; and it is useful, -in consequence, as a means of checking bleeding in small operations, -especially those on the eye or the nose. It is a most energetic poison. -Even ¼ milligramme is sufficient to kill a rabbit. In short, adrenalin -acts like the most powerful drugs known to physicians; and this drug, -manufactured by the suprarenal capsules, is constantly added to the -blood. Disastrous consequences follow a failure in the regular supply. - -The tone of the vascular system is maintained by adrenalin. The nature -of its influence upon muscles is not known, but probably the complete -loss of muscular strength, which is one of the most noticeable symptoms -of disease of the suprarenal capsules, is an indirect result of the -lowering of blood-pressure. The muscles, it must be remembered, make up -about one-third of the weight of the body of a muscular man. For the -exchange of their waste products for food, they are dependent upon an -efficient circulation. They are unable to display their normal vigour -when the vascular system is not up to its work. - -=The Pituitary Body= is another ductless gland of dubious history. It -is a round body, the size of a small marble, which occupies a deep -recess in the floor of the skull, beneath the centre of the brain. -It is composed of epithelial cells collected into irregular groups. -No homologue of the pituitary body can be found in the invertebrate -sub-kingdom. Its strange mode of development in vertebrate animals—it -is present in them all, from fishes to mammals—and the mystery in -which its prevertebral existence is hidden, provoke to speculation. -We must be content to state that it is undoubtedly masquerading under -an assumed name. “Pituitary body” is reminiscent of a long-abandoned -theory that it secretes fluid into the upper chamber of the nose. - -Disease of the pituitary body is associated with a perversion of -growth even stranger than that due to disease of the thyroid gland. -The condition has been termed “acromegaly,” to indicate that all -extremities—toes, fingers, nose, lips, tongue—undergo enlargement. - -With these three organs—the thyroid gland, the suprarenal capsules, -and the pituitary body—we must leave the subject of internal -secretions. Each of these organs is a ductless gland. Each has a -history which the zoologist is unable to transcribe. The document is -a palimpsest, the earlier script so faint as to be illegible beneath -the dark letters which a new era has written over it. Even the modern -script is smudged and blotted. The laws which it sets forth seem, as -a rule, to be destitute of sense, but a sinister meaning is evident -at times. We are tempted to regard these codes as obsolete, until the -mischief which follows their suppression calls our startled attention -to the fact that they are, in the most lively sense, extant. Myxœdema, -Addison’s disease, acromegaly, are ominous warnings that the three -ductless glands are no mere monuments of a past epoch, which owe -their survival to Nature’s indolence. They teach us that we must not -attribute the persistence of such organs to a conservatism which -resists innovation, or suppose that they would long ago have been -wiped off the statute-book if her inertia could have been overcome. -Undoubtedly Nature gives us many excuses for adopting this attitude of -mind. The “chestnuts” on a horse’s legs, the “dew-claws” of a dog’s -foot, are vestiges which would have disappeared if every part of the -body had to establish its claim to be regarded as useful before it -became entitled to share in the common supply of food; so, at least, we -are disposed to think. But, tempting though it be to attribute to sheer -conservatism the retention of an organ which has been superseded in its -original functions, and for which we cannot recognize any new use, it -is a temptation which must be severely checked. It is safer to suppose -that the fact that it has been retained is _prima-facie_ evidence that -the body has need of it. - -There can be no doubt as to the importance of the internal secretions -of the three chief ductless glands. What about other organs—the glands -which make external secretions, for example? Does each of them make -also an internal secretion which influences the activity of other -organs? It is very difficult to prove the production of internal -secretions by such organs as the salivary glands, the pancreas, the -kidneys, because all the effects which result from their removal may -be due to the suppression of their external secretions. It is almost -impossible to distinguish the consequences which might be due to -the abolition of an internal secretion from those which ought to be -attributed to the loss to the body of the chief functions of the organ. -Certain physiologists are inclined to think that all organs—not only -the glands, but the liver, spleen, muscles, etc.—produce chemical -messengers which are discharged into the blood; and recent discoveries -tend to justify this view. As the time approaches when milk will be -wanted for the nourishment of offspring, it begins to appear in the -breast. Hitherto this has been attributed to nervous control. It is now -known that the secretion is provoked by a chemical messenger. If this -messenger, extracted from the organ in which it is formed, be injected -into the veins of an animal which has no call to secrete milk, it sets -up a condition of activity in its mammary glands. Such an illustration -of the possibilities of chemical, as distinguished from nervous, -control inclines us to attribute the harmonious working of the body in -large measure to the mutual influence of its several parts, instead of -invoking in every case, as used to be the custom, the directing power -of a somewhat bureaucratic nervous system. - -It is curious to note that an internal secretion is essentially a drug. -Faith in drugs has suffered eclipse in latter days, and with good -reason. The medicines of fifty years ago so little resembled Nature’s -pharmacy that there is cause enough for astonishment at the credulity -of a generation that believed them to be charms by the exhibition of -which they could direct the working of the body. To be quite just, -our forebears did not exactly adopt this view. They still believed in -remedies. Docks grew in the same hedgerow as nettles. Therefore the -juice of the dock was an antidote to nettle-stings. Washerwomen found -wasps vexatious, but, fortunately, “blue-ball” cured the pain of their -stings, and prevented the swelling which otherwise would have occurred. - -A new pharmacology is rapidly developing. The physiological action of -every substance likely to be of service as a drug is put to the proof. -Having ascertained what is wrong, and knowing exactly what effects his -drugs are capable of producing, the physician devises the adjustment -which he may attempt without risk of making matters worse. He then -seeks, if possible, a chemical messenger near akin to the messenger -whom Nature herself would send; at least, this is the ambition of the -modern pharmacologist. - - - - -CHAPTER VI - -DIGESTION - - -=The Canal.=—The prospect presented by a widely open mouth is too -familiar to need description, but a few details may be pointed out. -The teeth are, or should be, thirty-two in number. Starting from -the middle line of either jaw, the two first are incisors, with -chisel-shaped cutting edges. If they meet, as they ought to do, their -edges are ground flat. The third tooth is the canine, with a more or -less pyramidal crown. Then two premolars, or “milk-molars,” as they -are often termed, because they are the only grinding teeth of the -first dentition. Twenty is the full complement of teeth in a child. -Lastly, three strong grinders—the molar teeth. The third molar, or -wisdom-tooth, is evidently disappearing in the human race. In civilized -people, whose brains are large and jaws small, it does not appear -until about the twentieth year. Sometimes it tries to squeeze through -the gum of a jaw not large enough to carry it, and causes trouble by -becoming “impacted” beneath the ascending ramus. Not infrequently it -fails to appear. It may be truly said that the increasing wisdom of -the human race is responsible for the postponement of its development, -although this is hardly the circumstance to which it owes its name. A -fold of mucous membrane—the frenulum linguæ—connects the under side -of the tongue with the floor of the mouth. On either side of this may -be seen the opening of a duct common to the submaxillary and sublingual -salivary glands. The opening of the duct of the parotid gland is not -so easy to find. It pierces the mucous membrane of the cheek opposite -to the base of the second molar tooth of the upper jaw. The parotid -gland lies just below the ear, behind the jaw. The saliva which it -secretes is a watery fluid containing little beside salt and a weak -ferment. It serves to moisten the food as it is being crushed by the -molar teeth. The submaxillary and sublingual secretions contain, in -addition to the ferment, ptyalin, mucus which the tongue mixes with the -masticated food as it forms it into a bolus suitable for swallowing. -The dorsal surface of the tongue is covered by papillæ, which rasp -the food against the palate. Of these the greater number are pointed, -or filiform. The remainder are flat-topped, or fungiform. The two -varieties may be distinguished with a lens, especially on the sides of -the tongue. Usually the fungiform papillæ are the redder. In fever, -when the tongue is densely furred, they stand out as bright red spots. -The back of the tongue is crossed by a =V=-shaped row of papillæ of -larger size, each surrounded by a slight fossa and a vallum, and hence -termed “circumvallate.” Very minute organs of sense—taste-bulbs—stud -the mucous membrane which lines the fosse. - -The hard palate ends in a muscular curtain—the soft palate—the -central portion of which—the uvula—depends lower than the rest. -On either side the soft palate splits into two folds; the anterior, -continued to the side of the tongue; the posterior, to the pharynx. -These folds, since they bound the gateway into the pharynx, which is -known as the “fauces,” are termed the “pillars of the fauces.” The -tonsil lies between the anterior and posterior pillars of the fauces, -but does not appear as a prominence unless inflamed or enlarged. - -The pharynx hangs as a bag from the base of the skull. It, like all -the rest of the alimentary tract, is lined by mucous membrane. “Mucous -membrane” is not a happy term. It does not denote that the epithelium -secretes mucus. It may or may not possess this property. Nor does it -imply that it has a different origin from the skin—that it arises -from hypoblast, the inner layer of the rudiment from which the embryo -grows. The term is applied to all internal, and therefore moist, -surfaces, whether they arise from hypoblast, as in the case of the -lining of the greater part of the alimentary tract, or whether they -are involutions of epiblast as in the case of the mouth and also of -the extreme lower end of the alimentary tract. Almost the whole of the -alimentary canal is, in the first instance, a tubular cavity in the -interior of the embryo, lined by hypoblast. This cavity communicates -with the yolk-sac, but has no openings on the exterior until it joins -up with two epiblastic pits—one the stomodæum, or mouth-cavity, at -the anterior end; and the other the proctodæum, at the posterior end -of the body. The distinction between the middle closed portion of the -alimentary canal and its two secondary openings suggests morphological -speculations, into which we have not space to enter, as to the ancestry -of the vertebrates. The majority of anatomists believe that the -primitive canal is represented in the middle portion, and that, in -prevertebrate animals, it opened to the exterior in a different way. -The pharynx is 4½ inches long. It is enclosed by three thin muscles, -which overlap from below upwards—the constrictors of the pharynx. -The anterior attachment of the superior constrictor is to the jaw; of -the middle constrictor to the hyoid bone; of the inferior constrictor -to the thyroid cartilage. Above the soft palate the nasal chambers -communicate with the pharynx by the posterior nares. Below the hyoid -bone, which is easily felt in the neck as a bony arch just above the -thyroid cartilage (Adam’s apple), the windpipe, or trachea, joins the -pharynx by a single pear-shaped orifice—the rima glottidis. When we -consider the mechanism of swallowing, we shall study the arrangements -which prevent food, passed through the fauces, from entering either the -nasal chambers above or the windpipe below and in front. At the level -of the lower border of the thyroid cartilage the pharynx becomes the -relatively narrow œsophagus. This tube, which lies behind the trachea, -and slightly to its left side, passes with a straight course to the -abdomen. It traverses the chest, lying behind the heart, pierces the -diaphragm, and just beneath it joins the stomach. Its length is about 9 -inches. The stomach is a sickle-shaped bag. It has two apertures—the -cardiac orifice, or junction with the œsophagus; and the pyloric -orifice, or junction with the small intestine. It is so folded on -itself that these two apertures are not more than 4 inches apart. Its -outline may be drawn on the body-wall with a piece of charcoal from a -point an inch below and an inch to the left side of the lower end of -the breast-bone, the position of the cardiac orifice, to a point about -4 inches below the end of the breast-bone, and an inch or two to the -right side of the mid-line of the body, the position of the pyloric -orifice, with a slight curvature to represent the upper border; to -represent the lower border the same two points are joined by a bold -curve, bulging upwards to the nipple, outwards to the side of the body, -and downwards some distance on the abdomen (_cf._ Fig. 2). - -[Illustration: FIG. 6. - - The stomach has been cut across a short distance - from the pyloric valve, and removed, to show the - viscera which lie behind it. The descending aorta - and the vena cava rest upon the vertebral column. - They are crossed by the pancreas and the transverse - portion of the duodenum. The head of the pancreas - is enclosed by the curvatures of the duodenum. The - ducts of the liver and pancreas are seen entering - the descending duodenum side by side.] - -Such an outline represents the form and position of the stomach when -distended; but it is to be understood that its dimensions depend upon -the amount of its contents. It is capable of holding about 7 pints. -The junction of œsophagus and stomach is closed by a muscular ring, or -sphincter muscle—the cardiac sphincter; the junction of stomach and -intestine is guarded by a much stronger pyloric sphincter. The average -diameter of the small intestine is about 1½ inches. It is wide enough, -therefore, to admit two fingers. The length of the tube is about 22 -feet. Its first part is termed the “duodenum,” because its length -equals the breadth of twelve fingers—_i.e._, about 9 inches. The -remainder is divided arbitrarily into jejunum and ileum. The duodenum -makes three sharp curves. First it inclines upwards and to the right, -then vertically downwards, then horizontally to the left, and finally -forwards. The ducts of the liver and pancreas open by a common orifice -into the descending portion. Its horizontal portion is bound firmly to -the vertebral column. After this the whole of the small intestine is -supported by the mesentery, a double fold of peritoneum which allows it -to hang freely in the abdominal cavity. The mesentery is attached to -the back of the body-wall. Commencing on the left side of the second -lumbar vertebra, its line of attachment inclines obliquely downwards -and to the right, across the vertebral column, for about 6 inches. -Measured from its attached edge to the edge which bears the intestine, -it has a width of about 8 inches. Its free border has, as already -said, a length of 22 feet. Its measurements being as just stated, it -is clear that it must be folded backwards and forwards upon itself, -like a goffered frill. In the right groin the small intestine joins the -large intestine, or colon. It does not, as might have been expected, -simply dilate into the large intestine, but enters it on its mesial -side, its orifice being guarded by the ileo-colic valve. In other -words, the large intestine projects downwards beyond this orifice, -as the cæcum coli. In many animals the cæcum is of great length and -capacity. In the human embryo it begins to assume a similar form; but -a very small portion only (the so-called “cæcum” of human anatomy) -dilates to the calibre of the colon. The real cæcum retains throughout -life its embryonic calibre. It has a length of about 3½ inches, and a -diameter of not more than ¼ inch. This is the “vermiform appendix,” of -ill fame, which must be looked upon as one of Nature’s misfits. Its -great liability to become inflamed is commonly explained as due to the -tendency of such articles of food as pips, the fibre of ginger, flakes -from the inside of enamelled saucepans, etc., to become lodged in its -cavity. But whether this explanation be correct or no—and there are -reasons for thinking it somewhat fanciful—it is much to be wished -that the process of evolution would hasten the disappearance of this -functionless vestige of a cæcum. As there is no tendency towards the -inheritance of characters due to mutilation, and since the surgeon’s -knife now prevents this death-trap from claiming its toll of possible -parents, we must look upon the rudimentary cæcum, with its liability -to inflammation, as a permanent burden on the human race. In justice -to the appendix, however, it must be pointed out that it has acquired -its criminal reputation during the past twenty years. The frequency of -appendicitis has increased so enormously during this period that it -ought to be possible to correlate its prevalence with the introduction -of the cause upon which it chiefly depends. - -The colon has a length of about 5 feet. Its greatest width, about -3 inches, is at its commencement, but it is everywhere much wider -than the small intestine. Whereas the wall of the small intestine is -smooth externally, the wall of the colon is sacculated. Three muscular -bands constrict it longitudinally; circular bands at intervals of -about 1 inch or 1½ inch throw it into pouches. It ascends on the -right side, lying far back against the body-wall, to which it is -bound by peritoneum, which in this part of its course covers only its -anterior surface. Having touched the under side of the liver, it loops -forwards and to the left side, crossing the middle line just above the -umbilicus. On the extreme left side it touches the spleen, getting -very near to the back of the abdominal cavity. It then descends on the -left side, again bound to the body-wall by peritoneum, although not -so closely as on the right side, until it reaches the inner lip of -the crest of the hip-bone. From here onwards the fold of peritoneum -which attaches it allows it a free movement. This portion of the large -intestine, the sigmoid flexure, may even fall over into the right -groin. Lastly it curls backwards into the pelvis, as the rectum. - -Movement of the contents of the alimentary canal may be favoured by -judicious pressure, or massage. From the description of the situation -of its several parts given above, it will be understood that if the -right hand be placed on the abdomen immediately beneath the ribs, with -the fingers well round to the left side, the stomach will be covered. -Pressure from left to right will tend to drive its contents towards the -pyloric valve. The small intestine is so irregular in its course as to -preclude the possibility of following it with the hand. Pressure first -on one side and then on the other, with a general tendency to work -from above downwards, tends to press forward its contents; but, owing -to its circular form and strong muscular walls, it is not in much -need of help. Very different is the position of the large intestine -in this respect. Its calibre is much greater, its wall is sacculated, -its contents comparatively firm. If the palm of the hand be placed -above the right groin and pressure directed upwards, the cæcum coli -and ascending colon are emptied. If pressure be directed from the -extreme right side just below the ribs, across the middle line to the -left side, the transverse colon is emptied. The descending colon needs -pressure from above downwards on the left side; the sigmoid flexure, -pressure above the left groin, downwards, and towards the middle line. - -The inner wall of the œsophagus is smooth, save for the wrinkles into -which it is thrown when not distended; but from the cardiac orifice -of the stomach onwards the mucous membrane of the alimentary canal -exhibits folds and other projections which serve many purposes. -They serve to delay the food, keeping it longer in contact with the -secreting surface. They increase the area pitted with tubular glands; -they increase also the area through which absorption of the products -of digestion occurs. On the inner surface of the stomach the folds -produce a reticulated pattern. In the upper portion of the small -intestine, especially the duodenum, there are prominent transverse -shelves (valvulæ conniventes). No definite folds occur below the -upper three-fourths of the small intestine, with the exception of the -constrictions of the transverse colon already referred to, which affect -the whole thickness of its wall. Throughout the whole of the small -intestine the mucous membrane projects in finger-like processes, or -villi, which give it a characteristic velvety appearance. The villi are -longest in the duodenum. - -Lymph-follicles occur at intervals in the intestine. In the ileum they -are collected into patches (Peyer’s patches), on the side opposite to -the line of attachment of the mesentery. They serve both for the supply -of phagocytes, which hunt any germs that have penetrated the mucous -membrane, and also as stations to which germ-laden phagocytes retreat. - -The wall of the intestine is composed of mucous membrane, submucous -tissue, and muscle. The mucous membrane is everywhere pitted with -tubular glands, termed in the stomach “gastric glands,” and in the -intestines, both small and large, “crypts of Lieberkühn.” Their -relation to the wall might be exemplified by taking a block of dough -about 6 inches thick and pushing a pencil vertically into it almost -down to the table on which it rests. The holes should be made as close -together as possible, since, especially in the stomach, extremely -little tissue intervenes between the tubes of gland-cells. If the piece -of dough were placed upon a folded cloth, the cloth would represent the -muscularis mucosæ, a layer properly regarded as a constituent of the -mucous membrane. The fibres of this coat are disposed in two or three -sheets, the fibres of one sheet crossing those of the next. By their -contractions they squeeze the ends of the crypts, and probably wobble -them about, expelling their secretion. Beneath the muscularis mucosæ is -a layer of connective tissue, the submucosa, which contains abundant -lymphatic channels, bloodvessels, and nerves. At the pyloric end of the -stomach, the tubes of gland-cells tend to pierce the muscularis mucosæ. -In the first part of the duodenum, certain tubes, having pierced this -layer, branch in the submucosa. A layer of racemose glands is thus -formed—the glands of Brunner. Outside the submucosa is the muscular -coat proper, composed of plain muscle-fibres, except in the upper part -of the œsophagus, where the fibres are striated. It consists of an -inner and an outer sheet, the fibres being disposed circularly in the -inner, longitudinally in the outer sheet, with a slight departure from -this regular arrangement in the wall of the stomach. On its outside the -canal is invested by peritoneum, a layer of flattened epithelial cells -supported by connective tissue. The abdominal wall also is lined with -peritoneum. The smooth moist surface of the peritoneum covering the -intestines glides on the peritoneum lining the abdominal wall. Between -the two is a “potential” space. In dropsy, fluid accumulates within -this space. In a healthy condition the apposed surfaces are merely -moist. - -The movements of the intestines are of two kinds. At all times they -exhibit swaying movements, in the production of which the longitudinal -fibres play the chief part, although the circular fibres also contract. -The object of this undulation is to thoroughly mix the contents of -the gut with its secretions. If pills of subnitrate of bismuth are -administered, and their progress observed by the aid of Röntgen rays, -they are seen to oscillate backwards and forwards on their way down -the canal. The slower vermicular movement which squeezes the contents -forwards is called “peristalsis.” It resembles the progressive -contraction of an elastic tube which may be effected by drawing it -through a ring, but is rather more complicated. At the point at which -it is occurring the circular coat is sharply contracted. Above this -it is also somewhat contracted; below it is relaxed. The longitudinal -fibres, using the constricted portion as a _point d’appui_, pull up the -segment of the intestine which lies immediately below it, drawing it -off the contents of the tube as a glove from a finger. - -When food is swallowed, it falls down the œsophagus, aided by slight -peristalsis. As soon as sufficient has accumulated on the upper surface -of the cardiac valve of the stomach, the valve relaxes; at the same -time a stronger peristalsis of the lower portion of the œsophagus -squeezes its contents into the stomach. Food remains in the stomach -until it has reached a certain stage of digestion, the chief object -of which is its subdivision into small particles. Until this stage -is reached, the pyloric valve is firmly closed. The contractions of -the wall of the stomach drive its contents round and round—down the -greater and up the lesser curvature—mixing them thoroughly with the -gastric juice (_cf._ p. 124). As the acidity of the mixture increases, -the peristaltic contractions of the stomach become more vigorous, -until, the pyloric valve relaxing, the food is little by little driven -into the duodenum. - -The alimentary canal has an abundant supply of nerves from the vagus -and the sympathetic systems. It contains also within its own wall an -enormous quantity of nerve-fibres and nerve-cells. They are disposed -as two plexuses, one in the submucosa, the other between the circular -and longitudinal muscular coats. In a specimen successfully stained -with methylene-blue, they are so abundant as to give the impression -that every plain muscle-cell may have its own separate nerve-twig. -Nevertheless, the contraction of the muscle-cells may take place -independently of all nerve-influence—independently, even, of the local -mechanism, the plexus referred to above. Nicotin applied to the wall -of the intestine paralyses the local nerves; yet rhythmic contractions -still occur. They are, however, no longer progressive. They do not -drive the contents of the intestine forwards. Co-ordinated contraction -is observed so long as the local mechanism is intact, even though all -external nerves have been cut. The intestines have their own nerve -cells and fibres, which, acting as a linked system of reflex centres, -provide for the harmonious contraction of their walls. External nerves, -sympathetic and splanchnic, convey impulses which either intensify the -movements or inhibit them, as need may be. - -In the matter of its nerve-supply, the alimentary canal stands apart -from the other organs of the body. It may be supposed that it presents -a more primitive condition. Its muscular fibres have the power of -contracting spontaneously. The pressure of the contents of the tube -acts as a stimulus. When the fibres are stretched, they contract. -When the tube is dilated, its muscles endeavour to restore it to its -normal calibre. Such direct action would not, however, provide for the -forward passage of its contents. To bring about peristalsis, a nervous -mechanism is needed, as abundant and complicated as that which ensures -the progress of a slug or a worm. To deal satisfactorily with the -various contents of the tube—liquid, solid, gaseous—the mechanism -must be capable of complicated adjustments. The dilated portions of -the tube—stomach, cæcum coli, rectum—require special arrangements of -muscle and nerve. Nor is the canal altogether independent of the rest -of the body. To a large extent its work is carried on without regard -to the activities of other organs, yet it is not wholly free from the -control of the central nervous system. It is regulated by means of both -afferent and efferent nerves of the vagus and sympathetic. Even the -brain has something to say with regard to the way in which it shall -contract. It is a matter of common experience that emotional influences -may affect the movements of the stomach and intestines—“His bowels -yearned.” - -Normally, vomiting is due to irritation of the endings of the vagus -nerve in the stomach, although the afferent impulses may have other -sources. Touching the upper surface of the epiglottis with the finger -will provoke the reflex. So also will stimulation of the olfactory -nerves by a foul smell. In this latter case the emotion of disgust to -which the odour gives rise brings about the reflex action. A flow of -saliva precedes the act of vomiting. A deep inspiration is then taken, -in order that for a time the lungs may be independent of a fresh supply -of air. The glottis is closed, the diaphragm fixed. Contraction of the -abdominal wall presses the stomach against the diaphragm; its cardiac -sphincter relaxes, and its contents are squirted into the œsophagus, -which undergoes a forcible retrogressive peristalsis. - -It is interesting to note the difference between carnivora and -herbivora in regard to vomiting. Carnivora swallow fur and other -indigestible materials, as well as many unwholesome things which they -need to be able to return. A dog can, apparently, vomit at will. -Never, while in a state of nature, do herbivora need to return the -contents of the stomach. No provision is made for vomiting. A heifer -which has strayed into a dewy clover-field is not unlikely to die -from the effects of distension of its paunch, if relief be not given -by opening it with a knife. In a horse the cardiac sphincter is -strong, the pyloric weak. Pressure on the stomach tends to drive its -contents through the pyloric valve into the duodenum, not backwards -into the œsophagus. The stomach is not so placed as to allow of its -being compressed between the wall of the abdomen and the diaphragm. -Horses cannot vomit. It is a mistake to suppose that they suffer from -sea-sickness. In rough weather they sweat, their limbs tremble, they -go off their feed; but these symptoms are probably due to the fatigue -which results from excessive anxiety to maintain their balance, and -to fear. We can never know their feelings, but there is no reason for -supposing that they experience the sensation of nausea. - -Vomiting is a frequent symptom of cerebral disturbance. The -fluctuations of pressure which the brain experiences as it rocks about -on its “water-bed” within the skull is the cause of sea-sickness. Yet -the motion of a ship may produce violent headache without nausea, the -brain only, not the stomach, appearing to be troubled by the motion. -Not that headache is a pain “inside the head.” Nor is it properly -described as a pain in the scalp, although the messages which are felt -in consciousness as headache originate in the endings of the nerves -of the skin which covers the skull. The excessive sensitiveness of -these nerves is due to vaso-motor conditions, usually the dilation, -occasionally the constriction, of the bloodvessels of the scalp. But -the vaso-motor condition is sympathetic with the disturbance of the -brain; and the special urgency or efficiency of the messages from the -skin results from their being delivered into excited brain-tissue. -Nausea and headache are equally symptoms of the irritability of the -brain caused by the motion of the ship. In one case messages from the -stomach, in the other case messages from the scalp, acquire undue -importance, owing to the agitated condition of the brain-tissue -through which they pass. Not uncommonly the voyager, who wakes in -the morning reconciled to the changes of pressure which he has -experienced while recumbent, finds, when he stands upright, that the -base of his brain is as sensitive as ever. Visual sensations also -contribute to the brain-disturbance. So, too, do the movements of -endolymph in the semicircular canals (_cf._ p. 335). It is, indeed, -possible that this last factor is more important than the variations -in pressure on the surface of the brain. Probably it accounts for the -after-image of rolling which almost everyone experiences for at least -a day after leaving the ship. Its cause being cerebral, the tendency -to sea-sickness can be controlled by drugs which, like the bromides, -chloral, alcohol, etc., deaden the brain. - -=Salivary Glands.=—The secretion which accumulates in the mouth is -the combined product of the sublingual, submaxillary, and parotid -glands. It is a very thin, watery solution containing not more than 0·5 -per cent. of solid substance. If red litmus-paper is moistened with -saliva, it becomes blue, showing that the secretion is alkaline. It -contains a ferment, ptyalin, which digests starch. The action of this -ferment can be demonstrated by holding in the mouth for half a minute -some warm starch mucilage—boiled arrowroot, for example. It quickly -loses its viscidity owing to the conversion of starch into sugar. -Chemically this change may be demonstrated by adding iodine-water to -a specimen of the starch before and after action. Before the starch -is taken into the mouth the iodine turns it blue (a characteristic -reaction for starch). After it has been exposed to the digestive action -of the saliva, iodine fails to colour the mixture, which now contains -no starch. All the starch has been converted into dextrin and sugar. -If unboiled arrowroot is placed in the mouth, some sugar is produced, -but the process of conversion is very slow. It is almost impossible to -digest raw starch in the mouth sufficiently to render it insusceptible -to the colouring action of iodine. The sugar produced by the action of -ptyalin is of the same nature as that which appears during the malting -of barley. It is therefore termed “maltose.” It closely resembles -grape-sugar, but is not identical with it. - -=The Secretion of Saliva.=—The accessibility of the salivary glands, -and especially of the submaxillary, has led to their being used for a -very large number of experiments. They have been studied with the aim -of coming to an understanding of the mechanism of secretion in general. -The glands consist of tubes of gland-cells, each tube suspended in -a basket of connective tissue, in a bath of lymph (_cf._ Fig. 3). -Innumerable capillary bloodvessels traverse the lymph-bath. The -arteries which carry blood to the gland are supplied with nerves, which -regulate their calibre, and therefore determine the amount of blood -which passes through the capillaries into which they break up. The -glands also are supplied with nerves which influence their functional -activity. Nutrient substances and oxygen pass out of the blood into -the lymph. Carbonic acid passes into the blood from the lymph. Waste -products are either carried away in the lymph-stream, or make their -way through the walls of the capillaries into the blood. Many problems -present themselves for solution. How does the amount of work done by -the gland affect its supply of blood? Does the quantity of saliva -secreted vary directly with the pressure of lymph in the spaces by -which the gland is surrounded? Is this pressure wholly dependent upon -the pressure of the blood? Are the substances secreted by the gland -supplied as such by the blood, or does the gland make the ptyalin and -mucus which it secretes? If it makes its secernable products, what -materials does it abstract from the blood for the purpose of their -manufacture? Does it use the whole of these materials, whatever they -may be, or does it use part only and return the residue to the lymph? -Does it make its products only when it is actively secreting, or is it -always making them, and storing them in its cells in order that it may -have a supply to discharge when called upon by the stimulation which -results from the presence of food in the mouth? Is their discharge -merely a washing out due to the rush of fluid which occurs when the -bloodvessels are dilated, or can the gland-cells expel their products -in response to nervous action? In what way do the nerves of the -gland influence secretion? Do they call for increased production, or -increased output, or both? These are some of the problems which the -exposed situation of the submaxillary gland allows physiologists to -tackle. - -By means of a very simple operation, the ducts of one or both parotid -or submaxillary glands can be brought to the skin, and made to pour -their secretions on to the surface instead of into the mouth. The flow -under various circumstances can be watched. The saliva can be collected -and measured. - -The nerves of the submaxillary gland are easily isolated. A nerve -leaves the seventh (or facial), crosses the drum of the ear, comes -out through a minute crevice in the skull, and runs for some little -distance as a separate nerve before it applies itself to the lingual -branch of the fifth, which runs along the side of the tongue. Owing to -its passage across the tympanic cavity (drum of the ear), it is termed -“chorda tympani.” As its fibres are very small, they can be recognized -wherever they form a part of the lingual nerve. They leave the lingual -to go to a ganglion, the submaxillary ganglion, from which the gland -is supplied. The gland also receives branches from the sympathetic -nerve which ascends the neck. The last-named branches accompany the -facial artery. Stimulation of either of these nerves causes the gland -to secrete. The flow of saliva which follows stimulation of the chorda -tympani is much more copious than that which follows stimulation of -the sympathetic, and as a rule it contains far less organic matter, -although about the same amount of mineral salts. Under normal -conditions the activity of the chorda tympani is brought into play in -a reflex manner by impulses which travel up the nerves of taste (the -lingual and glosso-pharyngeal) to the cerebro-spinal axis; but almost -any other nerve will serve as an afferent path. The gland may also, as -we shall presently explain, be called into activity by the cortex of -the brain. - -It is certain that in the case of the submaxillary gland secretion -is not the direct result of increased blood-pressure. It is not a -case of filtration from the blood through certain membranes and -cells into the salivary duct. Atropin (belladonna) dilates the -bloodvessels, increasing blood-pressure, but it stops secretion. -After belladonna-poisoning, the mouth, like the skin, is hot and dry. -Other drugs there are which provoke a certain amount of secretion, -even after the bloodvessels going to the gland have been tied. It is -possible, by stimulating the chorda tympani, to obtain a pressure in -the fluid in the duct very much greater than that in the bloodvessels -which supply the gland. Here we have clear proof that secretion is not -filtration. Filtration is the passage of fluid through a filter-bed -from a higher to a lower pressure. In filtration, moreover, soluble -diffusible salts accompany the water. The saliva contains only half as -much of these diffusible salts as the blood. Therefore the gland tissue -stops half the salts. Secretion is an active process carried out by -the gland-cells, under the influence of nerves, in opposition to the -laws of filtration. The gland-cells determine how much water shall pass -through them and what percentage of salts shall accompany the water. - -How does a gland-cell make the substance which it secretes? There is no -reason for supposing that the ptyalin or the mucus which the salivary -glands secrete is present in the blood, either ready formed, or, as it -were, half formed, in combinations which can be easily broken up. All -the evidence obtainable points to the conclusion that the gland-cells -take out of the lymph proteid materials from which they manufacture -the peculiar substances which they secrete. During rest, granules -accumulate in the cells. During activity they disappear. It has been -shown in the case of the gastric glands that these granules consist -of the special ferment which the gland secretes, in an inactive form. -It may be that it is combined with a substance which prevents it from -exerting its digestive action on the cells within which it is made; -damped, as gunpowder is damped during transit. Or it may be that it is -not a finished ferment; it may need a further addition to its molecule. -During activity, while the granules disappear, proteins accumulate at -the bases of the cells, giving to a tube of gland-cells the appearance -of a peripheral non-granular zone. This proteid substance must have -come from the lymph, and the inference seems inevitable that the cells -have taken into their protoplasm a supply of material which will -serve for the manufacture of additional granules. Each gland-cell -is therefore an independent unit. By its own activity it takes up -materials from the lymph, out of which it manufactures its own special -products. It stores its products until they are wanted. Then by its -own activity it extrudes them into the lumen of the gland-tube. It -has, indeed, been shown that, when the nerve going to a salivary -gland is stimulated, the gland shrinks, notwithstanding the great -dilation of its bloodvessels. Under the influence of the stimulation -the granules in the gland-cells imbibe water, swell up, and escape -from the cells. The cells discharge their accumulated stores, in the -first instance, more rapidly than they take up materials (even fluid) -from the blood. For its knowledge (if the term may pass) of what is -wanted the gland-cell is dependent upon messages which reach it through -the nervous system. These messages take origin in the endings of the -sensory nerves of the mouth, pass up to the brain, and are reflected -down the nerves to the gland. So accurate is the information conveyed -to the glands, that when a horse transfers the work of mastication from -one side of its mouth to the other, as it is in the habit of doing -about every quarter of an hour, the flow of saliva from the parotid -gland on the masticating side is increased; on the other side it is -diminished. Two or three times as much saliva is poured out on the one -side as on the other. - -Not only is the amount of saliva poured out in response to stimulation -proportional to the needs of mastication, but the kind of saliva is -adapted to the nature of the food. In a dog—and this is an observation -which can be made only on an animal which lives on a mixed diet—it is -possible to determine the amount of the two kinds of saliva secreted -and the relation of flow to food. When meat is given to the animal, the -submaxillary gland yields its secretion; when it is fed on biscuit, -abundance of the watery parotid saliva is poured forth. A mouthful of -sand also causes the parotid saliva to flow, in order that the sand may -be washed out of the mouth. - -More remarkable than the response to direct stimulation is the effect -produced by the sight and smell of food. When meat is shown to a dog, -submaxillary saliva begins to flow; when it is offered bread, parotid -saliva is secreted. And the activity of the glands is not merely a -nervous reflex independent of the animal’s mind. The moment the dog -realizes that it is being played with—that there is no intention of -giving it the coveted food—the flow of saliva ceases. An emotion -may check secretion when every physiological condition is demanding -it. This is the explanation of the Rice Ordeal. Dry rice provokes a -flow of saliva in the mouth of all save the guilty man. Response to -mental impressions is a matter of the greatest consequence in the -physiology of digestion. It holds good in the case of the secretion -of gastric juice equally with that of saliva. The sight and smell of -food sets the juice flowing into the stomach, and the more desirable -the food, the more attractive its appearance, the more stimulating -its smell, the more rapidly does the secretion flow. Here we touch -upon a theme which hardly needs exhaustive treatment. It is not -the stoutest people who eat the most, although an impartial survey -of one’s well nourished friends will show them to be persons who -“take kindly to their victuals.” A small quantity of food perfectly -digested is more nourishing than much food which the digestive organs -do not efficiently prepare for assimilation. Good digestion waits on -appetite; and appetite, in civilized man, is something more than a mere -physical need of food. The hunger which leads to the bolting of food -without pleasurable anticipation, without mastication, without any -consideration of the quality of the viands, is a harmful craving which -ends in imperfect assimilation. It is more profitable to toy with a -_hors d’œuvre_ than to engulf, unthinking, a plateful of beef. But we -have said enough to suggest reflections to those who take no thought -as to what they shall eat or what they shall drink; and few who take -thought need to be convinced. - -=The Stomach.=—The sight and smell of food, its presence in the -mouth, and the performance of mastication, which induces a secretion -of saliva, gives rise at the same time to a flow of gastric juice. -It is psychic stimulation and the act of eating which cause gastric -juice to ooze from the gland-tubes of the stomach at the commencement -of digestion, not the stimulation of nerve-endings by food which has -passed down the œsophagus. As a consequence of gunshot wounds, or -as the result of operations performed for the purpose of relieving -patients whose œsophagus has become blocked, numerous cases have been -recorded in which a fistulous opening into the stomach has made it -possible to study the interior of this organ. Such cases present an -opportunity of watching the digestion of various foods introduced -through the opening, and of collecting gastric juice for purposes -of analysis. A similar condition has been established in animals by -operative means. The œsophagus having been cut, and the cut end sutured -to the margins of an aperture in the skin, food taken by the mouth -escaped by this opening instead of passing into the stomach. A similar -opening was made into the stomach for the insertion of food, and for -the purpose of studying the effects of reflex stimulation of the -gastric glands. As soon as food was introduced into the mouth, gastric -juice began to flow. The advantage of this experimental method lies in -the fact that the juice secreted was a pure juice—not mixed with food, -as in all the earlier experiments in which, the stomach being opened -without diversion of the œsophagus, the presence of food within it was -the stimulus which led to secretion. No juice flowed in the absence of -stimulation; nor was the secretion normal in composition when provoked -by a mechanical stimulus, such as the tickling of the gastric mucous -membrane by a feather. - -My lord the stomach! He is not the only, nor is he the chief, agent -in digestion; but with him rests the decision as to whether the food -offered to the alimentary tract is suitable in quality and quantity. He -is offended if it be not offered with all the circumstance and ceremony -which becomes his rank. As an intimation that he is about to receive -food, he accepts the news from the mouth that its nerve-endings are -subject to mechanical stimulation. But the chewing of indiarubber would -produce a like effect. The stomach, therefore, confers with the organs -of taste and smell. If their report is favourable, he argues that the -substance which the teeth are crushing will justify an outflow of -gastric juice. He responds most generously when prolonged mastication -assures him that he may trust to receiving the food in a sufficiently -subdivided state. At our peril we neglect to propitiate my lord. Not -always debonair when treated with consideration, he is morose or -petulant when slighted. Never content with lip-service, he exacts the -labour of teeth and tongue and palate. The tribute we offer may be of -the best—savoury, wholesome, well cooked, well chewed—but if it be -not tendered with some degree of love, if thoughts are concentrated -on other things, if no attention is devoted to the meal, if no sense -of liking accompanies our offering, my lord the stomach on his part -affords the viands an indifferent reception. In consulting our own -tastes we are to a large extent consulting the needs of the stomach. -Ravenous and excessive feeding is not an exhibition of taste; it is a -return to the instinct of the savage, who was never sure that he would -get his full share, and was afraid to trust that another meal would be -obtainable when nature declared it due. Some degree of epicureanism -is favourable to digestion. The flow of gastric juice in the stomach -occurs reflexly in response to the emotion of appetite, to stimulation -of the nerves of taste and smell, to the obscure sensations which -accompany the activity of the muscles of mastication. - -The gastric juice secreted in a day amounts probably to about 8 or 9 -pints. To this we must add, when considering the quantity of fluid -which passes through the stomach, the saliva, which certainly reaches -as much as 2 pints, and the beverages taken with food. - -Gastric juice collected in the manner described above is a clear, -colourless, inodorous fluid. It is very acid, and so powerfully peptic -as to digest its own weight of coagulated white of egg. Its solid -constituents amount to 0·5 per cent. They consist of the two ferments -pepsin and rennin, with traces of proteins and mucin, and various -inorganic salts. Its acidity is due to free hydrochloric acid to the -amount of 0·2 per cent. This acid is more or less in combination with -the pepsin. In pure gastric juice hydrochloric acid is the only acid -present; but when mixed with food the juice contains other acids also, -especially lactic. - -When food first reaches the stomach, the alkaline saliva which -accompanies it neutralizes the acidity of the gastric juice. For some -time, probably about half an hour, the conversion of starch into sugar -is still carried on by the ptyalin of the saliva, owing chiefly to the -difficulty which the gastric juice encounters in permeating the masses -of masticated food. The _Bacillus acidi lactici_ is always present in -the stomach. It converts some of the sugar into lactic acid; of this a -small quantity is further changed into butyric and acetic acids, with -the formation of carbonic acid and hydrogen gas. After a while the -lactic acid is absorbed, and hydrochloric acid alone remains. - -The secretion by the gastric glands of so powerful a mineral acid -as hydrochloric has always aroused interest. How is it possible for -the gland-cells to produce it without injury to them selves, or for -the stomach to contain it without self-digestion? Many chemical and -physical theories have been advanced in the belief that they rendered -the process of its production less difficult to understand. All such -theories are, however, inadequate to explain the secretion as a -discontinuous process, which occurs only as a response to demand. That -the source of the acid is the sodic chloride which the gland-cells -take from the blood does not need assertion, but we cannot picture -the process by which this exceedingly stable compound is decomposed -otherwise than on the assumption that weaker acids, or, rather, acid -salts, are also absorbed by the cells, and that, in accordance with the -laws which govern the composition of salts in solution, an exchange -of acids occurs. If sodic chloride and any acid salt—acid phosphate -of sodium, for example—are in solution in water, the salts do not -retain their form as we know them when isolated by crystallization. -The mixture contains “free” hydrochloric as well as “free” phosphoric -acid. It may be assumed that within secreting cells a similar exchange -of acids takes place. By a process which we term “vital,” the acids are -kept apart, and the hydrochloric acid is extruded by the cells. In the -present state of knowledge this vital action is mysterious; but it is -no more mysterious than the isolation of pepsin, or any other metabolic -event which occurs within a cell. - -The proteolytic ferment pepsin is active only in an acid medium. Yet -apart from its digestive function as an ally of pepsin, hydrochloric -acid by itself also exerts a valuable disintegrating action on certain -constituents of the food. Possibly the most important results of the -presence of free hydrochloric acid in the great chamber into which food -is first received are due to its disinfective property. It destroys -all the putrefactive germs which accompany the food, and many germs -which, if introduced into the blood, would give rise to disease. It -also destroys the germs which multiply in the stomach towards the end -of each interval between two meals. When withdrawn from the body, -gastric juice will keep an indefinite time, if evaporation of the acid -be prevented. - -=Pancreas.=—In structure the pancreas presents a marked resemblance -to the salivary glands. Probably this resemblance is merely -superficial. Minute examination reveals points, apparently of great -morphological importance, in which they differ. In the gland-tubes of -the salivary glands, and, indeed, in all glands with the exception -of the pancreas, secreting cells project into the lumen. The -secreting cells of the pancreas are invested internally by a layer of -flattened scales (intra-acinar cells). They lie, therefore, between -the basement membrane which invests them externally and this second -layer of flattened cells which separates them from the lumen of the -tube. At a very early date in embryonic life the gland-cells of the -pancreas are filled with highly refracting granules. As this occurs -long before any digestive action is called for, it may be taken as -indicating that the pancreas has functions which other glands—the -salivary, for example—do not possess. These granules do not, however, -appear in all parts of the tubes. Certain portions of the tubes -remain undeveloped—fail, that is to say, to acquire a secreting -function—even in adult life. Such patches of cells, not disposed in -gland-tubes, are known as islands of Langerhans. When the pancreas is -over-stimulated by artificial means, leading to its extreme exhaustion, -large portions of its glandular substance return to this primitive -condition. The gland-cells not only discharge their stores of granules, -but they lose the greater part of their cell-protoplasm. It would seem -that, in their effort to meet the demand for ferments, they use up -their own cell-substance in their manufacture. Having exhausted their -coal, they stoke the furnace with the looms and furniture of the mill. -It may be that other glands would do the same if it were possible to -stimulate them as strongly as the pancreas can be stimulated. The -result is probably due to the extreme susceptibility of the pancreas to -the action of secretin, a substance made in the intestine. Secretin can -be isolated and injected into the blood. We shall refer again to this -chemical stimulation of the pancreas when tracing the progress of food -through the alimentary canal. - -The secretion of the pancreas is a clear, colourless, alkaline -liquid of syrupy consistence. The quantity of juice secreted is -relatively small, but the organic substances which it contains are -in a concentrated form. They constitute as much as 10 per cent. of -the pancreatic juice. Proteins are present, if the juice be fresh. If -it has stood for any length of time, they are found as peptones. The -digestive ferments of pancreatic juice are the most powerful which are -secreted into the alimentary canal. - -=Bile.=—In its most important functions the liver has no relation -to digestion. It is a storehouse of absorbed food. This organ will -therefore be treated in a separate chapter. The bile which the liver -secretes into the alimentary canal has no chemical action on any of -the constituents of food, with the exception of a feeble tendency to -digest starch. Yet it is in some degree accessory to digestion. Poured -into the second portion of the duodenum through an orifice common to -the liver and the pancreas, it mingles with the semi-digested food, -or “chyme,” which, about two hours after a meal, passes through the -pyloric valve. Gastric digestion has converted the greater part of the -proteid constituents of the food into peptones or intermediate stages. -The proteoses or propeptones—a name is needed for the intermediate -products of proteid digestion which does not commit us to any theory -as to their chemical constitution—are quickly peptonized by the -pancreatic juice. But portions of the proteins have escaped the action -of gastric juice, or have at most been affected by its acid only; -these are precipitated by the bile-salts on the mucous membrane of -the small intestine, which is raised into projecting flanges for the -purpose of delaying the passage of the chyme, in order that it may -be thoroughly submitted to the digestive action of pancreatic juice. -Bile-salts also favour the digestion of fat, and its passage through -the intestinal wall. The action of bile-salts in spreading fats is well -known to artists. Ox-gall is smeared upon glass when it is desired -to apply oil-paints to its surface. When mixed with oil, it causes -its emulsification, or breaking up into microscopic globules. In the -absence of bile, but little fat passes into the lymph-vessels which -convey digested food from the intestine to the thoracic duct, and so to -the great veins of the neck. Its action is mechanical. It favours the -digestion of fats by rendering them easily amenable to hydrolysis by -pancreatic juice. - -Bile as secreted by the liver is a clear, limpid fluid of low specific -gravity; but during its stay in the gall-bladder it is concentrated by -absorption of water, and mucin is added to it. It contains “bile-salts” -of complex constitution. These salts favour the solution of certain -by-products of cell-metabolism, cholesterin and lecithin; substances -which are formed in many cells, both in animals and plants. Cholesterin -occurs most abundantly in nerve-tissue and in blood-corpuscles. -Lecithin also is a by-product of the metabolism of nerve-tissue. -Protoplasm appears to be incapable of oxidizing these substances, as -it does other products of metabolism. Other substances of equally -complex constitution are reduced to urea if they contain nitrogen; -to water and carbonic acid if nitrogen be absent. Cholesterin and -lecithin have to be eliminated without further change. Some of the -cholesterin is excreted by the sebaceous glands of the skin. It is the -chief constituent of “lanoline” prepared from sheep’s wool; an unguent -which owes its valuable properties to the resistance which cholesterin -offers to cell action, and therefore to the action of living ferments. -Bacteria cannot turn it rancid. The sebaceous glands have the power -of directing metabolism into a channel in which cholesterin is the -chief product, but apparently all cells make it in small quantity. The -bile-salts carry cholesterin and lecithin into the alimentary canal, -from which they are not reabsorbed. Some of the bile-salts are lost to -the body, but the remainder re-enter the circulation, and recommence -their work as vehicles for these inoxidizable and insoluble substances. -In the gall-bladder cholesterin is apt to separate out from the bile -in the form of gall-stones; but whether this is due to an excess of -cholesterin in the bile, or to an abnormal, inflammatory condition of -the lining membrane of the gall-bladder, is still an open question. - -Bile also contains bile-pigments. Their colour varies in different -animals, and changes according as the bile is exposed to the air, or -subject to the action of reducing agents. If oxidized, the colour -is green (biliverdin); if reduced, brownish-yellow (bilirubin). -Bile-pigment is formed from hæmoglobin, the colouring matter of the -blood, after the removal of its iron. Worn-out red blood-corpuscles -are destroyed in the spleen, in the manner already described, but it -is uncertain whether the conversion of the hæmoglobin thus set free -into bilirubin occurs in the spleen, or whether this chemical change -is reserved for the liver. Physiologists incline to the view that the -liver is the seat of the change. - -=Intestinal Juice.=—The mucous membrane of the alimentary tract, as -far down as the middle of the rectum, is, as previously stated (p. -102), studded with tubular glands. They secrete a light-yellow fluid, -alkaline in reaction, and opalescent. Its most important property is -due to a ferment which converts cane-sugar into a mixture of dextrose -and levulose, and changes maltose—the sugar produced by the action on -starch of saliva and pancreatic juice—into dextrose. It is in the form -of dextrose that sugar is carried about the body and assimilated by the -tissues. - -Intestinal juice also contains a ferment, erepsin, which shakes to -pieces the heavy molecules of peptones and partly formed peptones. -Under its influence they break up into comparatively simple bodies -containing the radicle of ammonia. Substances containing an NH₂ -group—one H of NH₃ (ammonia) having been given up, in order that -the group may have a “free arm” with which to link on to the other -component parts of the molecule—are termed “amides.” The amides -which are most characteristic of the action of erepsin are leucin, -an amidated fatty acid; and tyrosin, an amidated aromatic acid. The -tendency of proteins to break up along these two lines—the fatty acid -line and the aromatic acid line—is of considerable interest. The one -line is represented by acetic acid, CH₃COOH; the other contains the -hexone radicle, C₆H₆. Benzoic acid, C₆H₅COOH, is representative of the -latter. It used to be thought that proteins which were shaken into -simple bodies such as amides were lost to the economy. Their downward -career was a foregone conclusion. There could be no arresting it -before they brought up at the bottom—as urea, CO(NH₂)₂—the diamide -of carbonic acid. It was even supposed that this disintegration of -proteins was a provision for getting rid of the surplus animal food -which we consume. Physiological chemists now take quite a different -view. They believe that the epithelial wall of the intestine through -which these substances are absorbed, or the liver, to which they are -carried by the portal blood-stream, has the power of recombining these -fragments into the complex protein edifice. It is even thought that -disintegration is a necessary preliminary to the rearrangement of the -sub-groups. A large variety of proteins is ingested as food. Many of -them, especially the vegetable proteins, are quite foreign to the body. -By the activity of pancreatic juice and erepsin, they are broken into -small and relatively stable groups of atoms, which are again fitted -together into the particular forms of protein which are of use to the -economy. - -=The Story of a Meal.=—The chemistry of digestion will be understood -most readily if the constituents of a meal are traced from their -entrance into the mouth to their absorption through the wall of the -alimentary canal, or abandonment as indigestible. - -We may describe as a typical meal one consisting of bread, vegetables, -cane-sugar, meat, milk, fat, and cheese. In the mouth the various foods -are crushed and mixed with the alkaline secretions of the salivary -glands. A certain amount of the cooked starch contained in the bread -is changed into maltose. In the stomach the digestion of starch is -continued for a time, but a large part even of the cooked starch awaits -the action of pancreatic juice. A certain amount of cane-sugar is -converted into dextrose and levulose, which are rapidly absorbed into -the blood; but this action is due to hydrochloric acid, and probably -affects a comparatively small part of the cane-sugar swallowed. Fat is -quite unaltered in the stomach. All proteins are attacked by pepsin, -but some yield to digestion more readily than others. Gluten of bread, -like all vegetable proteins, is comparatively resistant; but since -it is presented to the action of pepsin in small quantities and in a -spongy form—very suitable for digestion—it is probable that most of -it is peptonized in the stomach. Chemists experimenting with gastric -juice taken from the stomach, and reproducing the conditions as to -temperature, removal of products of action, etc., as closely as it is -possible to reproduce them in the laboratory, find that the various -foods take different times to digest. The proteins of meat are more -quickly peptonized when raw than after coagulation by heat. The same -is true of white of egg. Amongst different varieties of cooked flesh, -beef is more quickly peptonized than fish. The casein of milk is more -quickly peptonized than any other protein; and it also is no exception -to the rule that digestibility is diminished by cooking. Similar data -may be obtained for all foods. They are no doubt useful indications of -the course of action which we may expect to occur within the stomach, -but we can never be sure that my lord will obey the ruling of the -chemist. Practice with a captive golf-ball is a useful preparation -for the game; but there are conditions on the links which cannot be -reproduced on the lawn. In an artificial stomach the clean fibre of -raw fish digests more slowly than raw beef. Even when the beef is -roasted and the fish fried or boiled in the ordinary way, the beef -disappears through the dialyser (the bag of membrane suspended in a -vessel of warm water in which experimental digestion is carried out) -more quickly than the fish. Nevertheless, the living stomach is better -disposed towards a mixed meal containing a certain weight of fish than -towards a meal in which, the other constituents remaining the same, -beef takes the place of fish. Important conclusions may, no doubt, -be drawn from observations of the time occupied in the peptonization -of pure food—_i.e._, fibrin, white of egg, clean meat, etc.—under -conditions simulating those which are present in the stomach; but -they must be accepted with many reservations. In the stomach it is -not pure substances, but mixtures, that the gastric juice has to deal -with. And here a most important factor comes into play, to which -further reference will be made later on. The amount and quality of the -secretion of the gastric glands depends upon the nature of the food. -Hence a food, or a combination of foods, which digest readily in the -laboratory may take a long time to disappear from the stomach, and -_vice versâ_. Digestibility depends upon the nature of the food. It -depends also upon its physical state. To take simple illustrations: -Cheese contains coagulated casein, one of the most easily digestible -of proteins, but the casein is intimately mixed with fat, upon which -gastric juice can make no impression. Even when finely divided, the -particles of casein are protected from the action of the juice by -fat. In the same way the meat of pork is as digestible as mutton, but -the fat of pork is quickly melted and very liquid. In the process of -cooking the muscle-fibres become saturated with fat. - -It is not the function of the stomach to complete digestion. Its -business is to initiate it. Food which reaches the stomach in fragments -is reduced to a condition in which its digestion will be readily -completed by pancreatic juice. Gastric digestion produces a much -larger proportion of intermediate products, proteoses or propeptones, -than does digestion in the duodenum. Such intermediate products are -quickly dealt with by pancreatic juice. Artificial tests of relative -digestibility do not, as a rule, take the amount of propeptones formed -in a given time into account. When considering the digestion of a -typical meal, we must bear in mind that it is not the duty of the -stomach to pass as much sugar, peptone, and fat as possible into the -blood. In fact, very few of the products of digestion are absorbed -by the bloodvessels of the stomach. The impermeability of its mucous -membrane is shown by the fact that hardly any of the water swallowed -passes through the stomach-wall. Practically all the water ingested -leaves the stomach through the pyloric valve. Various salts, some -sugar, and peptones are taken up by the vessels of the stomach; but the -bulk of all the different kinds of food passes into the duodenum in a -semi-digested state. The function of the stomach is to carry digestion -through a preliminary stage. The process will be completed in the small -intestine. It is to be noted that, although water is not absorbed by -the stomach-wall, alcohol passes through it with great rapidity. The -same is true of the various crystalline nitrogenous bodies found in -meat-extracts, and also of the essential principles of tea and coffee, -which chemically belong to the same class. All these substances are -degradation products of proteins produced by oxidation, far advanced -along the road to urea. In this selective absorption we see proof of -the activity of the cells of the mucous membrane. They take up the -substances which it is desirable to remove from the contents of the -stomach. Some may be wanted by the body for its immediate use; others -are better out of the way, because they are prejudicial to the progress -of digestion. - -When contemplating the activity of the cells of the gastric mucous -membrane, we feel the need of an adjective which shall express our -recognition of the fact that they have a power which we cannot confer -upon our clumsy mechanical imitation stomach. They can discriminate. -“Vital” is the only term available, though much abused. Using it -without prejudice, as lawyers say, we speak of the “vital activity” of -the cells when we wish to imply that things happen in a living stomach -for which we cannot make provision in a model. Of the many substances -which make their appearance as digestion proceeds, some are absorbed, -others left in the mixture. - -The mucous membrane shows its power of controlling digestion in yet -another way. In the neighbourhood of the pylorus its structure is -unlike that which it presents elsewhere. The gastric glands are short, -and tend to branch. Their lining cells are all of the same kind. -Over the greater part of the inner wall of the stomach the tubes -are long. They do not branch. The cells which line them are of two -kinds: small cubical cells (the term refers to their form as seen in -section), similar to those of the pyloric glands; large oval cells, -placed with their longest axes in the same direction as the axis of -the gland-tube. These oval cells do not project into the bore or -lumen of the tube, but are displaced from it by the cubical cells. -They rest on the investing, or basement, membrane. All parts of the -gastric mucous membrane secrete pepsin, although the pyloric portion -produces very little; the area which contains oval cells alone secretes -hydrochloric acid. If a short time after a meal an extract is made -from some of the mucous membrane near the pylorus, by pounding it -with salt-solution and sand to break up its cells, this extract, when -filtered and injected into the blood, stimulates the glands of the -cardiac end of the stomach. Under its influence they pour out both -pepsin and hydrochloric acid. The extract contains a substance which -acts as a chemical messenger. It is a representative of a class of -bodies which play a most important part in co-ordinating the activities -of the various organs. Hitherto physiologists have concerned themselves -with the visible or “external” secretions of glands. They have shown -how the production of these secretions is controlled by the nervous -system. Recently they have discovered that another set of influences -has to be taken into consideration. Glands, and possibly all other -tissues, take from the blood the materials out of which they make their -characteristic secretions, or, if they do not discharge secretions, the -substances which they require for the building of their own structures, -and return to the blood “internal secretions” which act as stimuli to -other tissues with which they are linked in harmonious co-operation. -The active principles of internal secretions have been termed -“hormones”—from ὁρμάω, I announce. The glands of the pyloric mucous -membrane secrete a hormone which calls upon the rest of the membrane to -pour out gastric juice (_cf._ p. 89). - -What induces the cells of the pyloric mucous membrane to produce the -gastric hormone? Their activity in this respect evidently depends upon -the presence in the stomach of partially digested proteid substances. -The cells judge, as it were, when these substances come into contact -with them, that there is more work for the great bag of the stomach to -do. They call upon the part which is most active in secreting gastric -juice to pour it out quickly and get the business of digestion over. -Meat-extracts, which contain the products of protein disintegration, -have a similar influence in promoting the formation of the hormone. -Hence, no doubt, the general custom, found from experience to be -beneficial, of commencing dinner with soup; although it must be -remembered that the rapid absorption of meat-extracts makes them -peculiarly valuable as restoratives. They afford very little energy, -but what they have to give is quickly placed at the disposal of the -economy. Persons whose stomachs are unduly irritable are advised to -avoid soup. It leads to undesirable activity on the part of the gastric -glands, and especially of the acid-secreting cells. Well chewed bread -also encourages the production of the hormone. - -Here it may be well to call attention to the evident division of the -stomach into two parts—the large bag, or cardiac portion, which hangs -down; and the smaller, funnel-shaped pyloric end, which is almost -vertical. The distinction between these two parts is faintly visible in -the resting stomach, but even opening the abdomen tends to obliterate -it. That it is much more evident during active digestion has been -shown by adding subnitrate of bismuth to the food, and throwing the -shadow of the stomach on a screen with Röntgen rays. When this is -done, it is seen that the two parts work in different ways. Food is -churned round and round in the cardiac portion, and pressed towards -the pylorus. Its fluid products, mixed with the abundant secretion -of the gastric mucous membrane, are wrung out of it by the pyloric -funnel. They are squeezed towards the pylorus, which opens at intervals -to let them through. If lumps of solid matter reach it, the pyloric -valve closes tightly, until the undigested food has fallen back into -the dependent bag. Dyspeptics are sometimes unpleasantly conscious of -the contractions of the pyloric funnel. In fact, putting aside pain -due to gastritis, all the discomfort of dyspepsia is felt on the right -side. Flatus accumulates beneath the pyloric valve. The valve will -not open to let it pass. The pyloric portion of the stomach contracts -strongly. Notwithstanding the general trend of movement in the opposite -direction, the gases are squeezed back into the larger bag, and escape -through the cardiac orifice. - -Tables have been prepared showing the length of time which various -articles of food take to digest. They are based in part upon -observations made upon the living stomach in cases in which it has -been possible to examine its contents through a fistulous opening; -in part upon the results of artificial digestions carried out in the -laboratory. It is hardly too much to say that such observations are -absolutely without value as tests of the relative digestibility of the -several articles of diet consumed as parts of an ordinary meal. The -fact that the commencement of the flow of gastric juice depends upon -mental stimuli, and its continuance upon hormones, shows how difficult -it must be to reproduce the conditions which obtain in a healthy living -body. The most wholesome of foods taken by itself may be longer in -digesting, or may produce more irritation, than many less desirable -things taken in judicious combination. Crushed chicken, hastily -swallowed, sometimes proves more difficult of digestion than meat so -cooked and served as to stimulate appetite and to demand mastication. - -Returning to the story of a meal, vegetables pass almost unaltered -through the stomach. Some of the scanty proteins which they contain -are peptonized, but unless they are very well masticated or cooked -until they are soft, and therefore easily pulped by the churning action -of the stomach, the gastric juice has to reach the proteins through -cell-walls. None of the digestive juices are able to dissolve the -cellulose of vegetable cell-walls. Blocks of vegetable tissue pass -down the whole length of the alimentary canal in the form in which -they were left by the teeth. Hence the extreme indigestibility of -ill-chewed cucumber or apple. The pyloric valve of the stomach is -forbidden to allow any lumps of food to pass until the very last stage -of gastric digestion. Pieces of ill-masticated vegetable tissue lie for -a long time in the stomach, irritating the ends of the gastric nerves, -until at last the time comes for them to be shot through the pylorus -into the duodenum. Many salts which vegetables contain, especially the -earthy carbonates and phosphates, are dissolved by the acid of the -gastric juice. - -Meat consists of muscle-fibres supported by connective tissue. In the -stomach the gelatiniferous connective tissue is dissolved, setting -the fibres free. Further, the fibres being surrounded by a membrane -of the same nature—sarcolemma—this is removed; and although it may -be hardly justifiable to speak of “Krause’s membranes” (_cf._ Fig. -10) as gelatiniferous septa, the fibres are certainly composed of -segments—Bowman’s discs, sarcous elements—into which they break up -under the action of gastric juice. As a result, meat-fibre is reduced -to a finely divided granular condition. The capacity of gastric juice -for dissolving collagen (the substance of which connective tissue -is composed) may be regarded as its most characteristic, as it is -one of its most valuable, properties. Collagen, when boiled or acted -on by acids, takes water into its molecule, becoming gelatin. Under -the influence of gastric juice gelatin is rapidly hydrolysed into -diffusible gelatin-peptone. Pancreatic juice is unable to act upon -collagen, unless it has been previously boiled, or swollen by the -action of dilute acids. - -Fat is composed of vesicles of oil supported by connective tissue. -Gastric juice, by dissolving the connective tissue and the collagenous -walls of the vesicles, sets the oil free. The oil, even though it be as -firm as suet when cold, is liquid, or almost liquid, at the temperature -of the body. - -Thus, with the exception of raw vegetables, the hard fibre of cooked -vegetables, elastic tissue of meat, and a few other indigestible -substances, the meal is reduced in the stomach to a cream-coloured, -fatty, strongly acid “chyme.” In this condition it enters the duodenum, -where it at once comes into contact with an alkaline secretion. The -passage of acid chyme down this portion of the canal provokes the -discharge of gushes of bile and pancreatic juice. By precipitating -partially digested proteins and “acid-albumin” bile renders the mixture -thicker and sticky. It colours it yellowish-brown. Under the influence -of pancreatic juice the remaining proteins and proteoses are rapidly -converted into peptones, some of which are shaken down by the violent -action of erepsin into simpler bodies, such as leucin and tyrosin, etc. -The chyme becomes alkaline, grey, and thin. All undigested starch is -changed into maltose, and this into dextrose. Cane-sugar is converted -into dextrose and levulose. These sugars are absorbed into the blood. -Milk-sugar, if not converted into lactic acid, remains as lactose -(C₁₂H₂₂O₁₁), in which condition it is absorbed without “inversion.” -Fats are split by a ferment of the pancreatic juice into fatty acid and -glycerin; some of the fatty acid combines with alkali to form soap, but -of this we shall have more to say later on. - -The duct common to the liver and the pancreas opens into the second -part of the duodenum. The organs which produce bile and pancreatic -juice are comparatively remote from the place where their secretions -come into contact with the food. By what mechanism are they thrown -into activity when the assistance of their secretions is required? -As in the case of the stomach, the agent is a hormone, a chemical -messenger. The hormone, termed “secretin,” is formed by the cells of -the mucous membrane of the duodenum when acid comes in contact with -them. It is absorbed by the blood, which carries it to the pancreas and -the liver. When it reaches the pancreas, it acts as a most powerful -stimulant to the discharge of accumulated ferments, and to the -production of an additional supply. It stimulates the liver to pour -forth bile. At present we are in ignorance as to the chemical nature -of this hormone. It is not a proteid substance, nor is it a ferment. -If scrapings from the mucous membrane of the duodenum be crushed with -sand and hydrochloric acid, the mixture boiled, neutralized with -carbonate of soda, and filtered, the clear, colourless liquid which -results has a powerful effect upon the pancreas, when injected, in -even small quantities, into the blood. Apparently, the cells of the -duodenal mucous membrane are constantly producing and accumulating -a substance which is converted into secretin when acted on by acid. -It is not necessary for the acid to stimulate the living cells. If -the mucous membrane is ground up with sand and salt-solution, the -filtrate is inactive but an active extract is obtained by treating the -crushed cells with HCl. It changes some substance which they contain -(provisionally termed “prosecretin”) into the efficient hormone. - -In the lower portion of the small intestine any maltose that remains -is converted into diffusible dextrose. A very large amount of water -has been poured into the canal in the various digestive juices. This, -together with water drunk, is absorbed in the large intestine. At the -lower end of the alimentary canal nothing remains but indigestible -substances taken with food, chiefly cellulose, and the pigments and -other bodies which, as already said, are eliminated in bile. - -The absorption of water is checked by the ingestion of extremely -soluble salts, such as sulphate of magnesia, the heavy molecule of -which diffuses with difficulty. We attribute the fact that sulphate of -magnesia remains in the intestine, and prevents water from diffusing -out of it, to its slowness in passing through a membrane, because -this is what would happen in dialysis;[2] but we must remember that -the living wall of the intestine is not a membrane. The cells which -line the intestine take up substances far less easily diffusible than -the sulphate of magnesia which they refuse. Nevertheless, speaking -generally, it is the less diffusible salts which act as aperients, the -more diffusible which are absorbed. The forward passage of the contents -of the alimentary canal is hastened by castor-oil. The peristalsis of -the intestines is stimulated by certain drugs, such as jalap or the -burnt products of tobacco. Another class of drugs, of which aloes is -an example, increases the secretion of the intestines, small or large. -Certain purgatives, such as calomel, podophyllin, etc., used to be -regarded as cholagogues. It was supposed that they increased the flow -of bile. This is an error. Their action is complicated, but it affects -chiefly the peristalsis of the intestine. The poor misunderstood liver -still suffers from the libels of primitive medical science. It is -the most innocent of organs, in no way responsible for derangements -of digestion. It carries out its functions without haste and without -delay. With the possible exception of salicylate of soda, no drug is -known which can stimulate it to a more rapid output of bile. - -=Absorption.=—All the cells which line the alimentary canal are -capable of absorbing food, if it is presented to them in a suitable -form. In a suitable form means, speaking generally, in a diffusible -condition, although it must not be supposed that the epithelial cells -are incapable, under certain circumstances, of taking up non-diffusible -substances, just as a unicellular organism—an amœba—can take in -food. If soluble proteins, such as white of egg or acid-albumin, are -injected into the large intestine, a very considerable proportion -of the substance so injected is absorbed. It is possible, indeed, -to supply in this way the whole of the nitrogenous food needed by -the system, none entering by the mouth. If milk is injected, a -certain amount of the fat also is retained. It can be shown that such -absorption takes place when no digestion of the food occurs in the -colon. The food is taken up by the epithelial cells in the form in -which it is injected. - -The organs specially devoted to absorption are the villi, which project -into the contents of the small intestine. Each is a conical process -about 0·5 millimetre long. The villi are longest in the upper half of -the small intestine. Below this level they decrease in number and size. -A villus is completely covered with epithelial cells of short, columnar -form. The free border of each cell is slightly hardened, forming a -disc or cap which appears striated in optical section—an indication, -as some think, that it is traversed by pores. Others hold that the -appearance of striation is due to minute cilia-like projections which -beset the free border of each cell. In worms and other invertebrates -the cells carry motile projections of not inconsiderable size, which -no doubt free their surfaces from the unassimilable matter which tends -to accumulate upon them. Possibly they help to fix particles which are -suitable for absorption. In mammals the presence of cilia has not been -demonstrated. The extreme minuteness of the striæ seems to point to -their being merely indications that the border is permeable to fluids, -including droplets of fat. - -The so-called basement membrane upon which the epithelial cells rest -must not be regarded as a membrane in the physical sense. Rather is -it a basket-work which supports the cells, without in any degree -limiting their power of disgorging into the lymph-spaces of the villi -the substances which they have absorbed. Within the villus, connective -tissue forms a sponge-work, the spaces of which are filled with lymph, -in which considerable number of leucocytes roam, on the look-out, no -doubt, for any germs which may make their way between the epithelial -cells. In the centre of the villus is a lymphatic radicle—_i.e._, a -fusiform cul-de-sac—which is the dilated end of a lymph-vessel. It, -like all other lymph-vessels, is walled by flattened endothelial -scales. It communicates with the lymph-plexus beneath the mucous -membrane, which, again, communicates with a coarser plexus outside the -muscular coat. From the peri-intestinal plexus vessels lying in the -mesentery converge to the receptaculum chyli, the bulbous commencement -of the thoracic duct, which lies at the back of the abdomen in front -of the bodies of the vertebræ. The thoracic duct runs up the front -of the vertebral column, through the thorax, and then hooks over to -pour the fluid which it conveys into the great veins shortly before -they join the heart. After a meal containing fat the fluid in the -lymphatic vessels of the mesentery, the lacteals, has, as already -stated (p. 43), the appearance of milk. The fat absorbed by the -epithelium covering a villus is passed on into its lymph-space. From -this into the central lacteal receptacle, thence to the submucous -and peri-intestinal plexuses, the lacteal vessels of the mesentery, -the thoracic duct. Absorbed fat does not pass through the liver, but -is carried into the heart; thence through the lungs, and back to the -heart, which pumps it to all parts of the body. In addition to lacteal -radicle; the villus contains long capillary bloodvessels, and the -arteriole and venule in which they commence and end. These traverse -the lymph-spaces of the connective tissue, which contains, not only -the fat which the epithelial cells have passed into it, but the other -products of digestion also. None of the fat traverses the walls of the -bloodvessels; but the other products diffuse from the lymph, through -the walls of the vessels, into the blood. Many nerve-fibres are found -in the core of the villus on their way to epithelial cells, or to one -or two plain muscle-fibres which are disposed in the direction of its -long axis. For each villus is a little pump. By the contraction of the -muscle-fibres it is shortened, and the fluid in its lacteal radicle is -forced into the submucous vessels. - -Two problems have to be considered: First, in what form and by what -mechanism are the several kinds of food absorbed? Secondly, what -becomes of them after they have been absorbed? - -Clearly, the epithelial cell is the absorbing mechanism. It is not -a membrane governed by the laws which regulate diffusion of fluids -through membranes, but a living cell. There is hardly any limit to -its power of selecting the food which it ingests. It could, and very -possibly it does, ingest albumin and fats as such. Still, the elaborate -provision which is made for converting albumin into diffusible peptone, -and cane-sugar and maltose into easily diffusible dextrose, suggests -that substances which will pass through membranes are more readily -absorbed than substances which will not. We are justified in looking -upon absorption as a physical problem up to a certain point. But we -must not dwell too much on the physical aspects of the problem. If the -absorption of food were merely a process of diffusion, an enormous -quantity of water would be required to carry the diffusible products -of digestion into the villi. The passage of the foods is aided by the -selective activity of the epithelial cells. Peptonization greatly -facilitates the work of the epithelial cells, but it is not a condition -essential to absorption, so far as soluble proteins are concerned. It -is, however, essential that the proteins should be presented to the -epithelial cells in a soluble form. They could do nothing with the -solid fibres of meat, however much they might have been disintegrated -by mastication and by the action of hydrochloric acid. It is only -after digestion by pepsin and by trypsin that all the proteins of food -are brought into solution. Digestion is needed to reduce them to a -condition in which the epithelial cells can take them up. - -Much thought has been devoted to the question of the form in which -fat is absorbed. Fat in the chemical sense—a pure fat, that is to -say—is a compound of a fatty acid and glycerin. Suet, lard, butter, -vegetable oils, etc., are mixtures of several fats. All consist of -glycerin united with fatty acids. The acids are stearic acid, palmitic -acid, oleic acid, and others of less importance. Fats are insoluble -in water; so also are the fatty acids. A fatty acid combined with an -alkali (in place of glycerin) is a soap. Soaps are soluble in water. -If milk is examined under the microscope, it is found to contain -droplets of fat, varying in size, but all minute. The larger droplets -tend to rise to the surface as cream, but the smaller droplets do -not run together. If milk from which the cream has been skimmed -is sterilized, it retains its normal appearance for an indefinite -time. Its fat remains in droplets. In technical language, milk is an -emulsion. Theoretically oil and water would make an emulsion, if the -droplets of oil were rendered sufficiently minute. Such a condition -has been almost obtained by agitating oil and water with powdered -glass. But the more viscous the medium through which oil globules -are distributed, the greater is the resistance to their fusion. If -oil which has become rancid—in which a certain quantity of fatty -acid has been liberated from the glycerin with which, in a neutral -fat, it is combined—is shaken with water containing carbonate of -soda, an emulsion is easily formed. The carbonate of soda and the -fatty acids form soaps. A solution of soap is sufficiently viscous to -keep the droplets of oil apart. Emulsification of fats occurs in the -intestine. It might be assumed that the epithelial cells ingest fat -in this finely divided state. But it must be remembered that, however -minute the droplets, they are enormously large as compared with the -molecules of peptones and sugar which the epithelial cells absorb. -It is unlikely that fat is absorbed in a manner so widely different -from that in which other foods enter the epithelial cells. Nor is -it necessary to make any such assumption. Pancreatic juice contains -a ferment which rapidly splits fats into their constituent fatty -acids and glycerin. In the presence of an alkali the fatty acids are -converted into soaps. In this soluble condition of soap and glycerin -the fats are probably absorbed. As soon as they have entered the cell, -the fatty acids and glycerin reunite to form fats, setting the alkali -free. The alkali is returned to the intestine, where it is available as -a solvent of further droplets of fat. The droplets of fat accumulate -in the epithelial cells. During active digestion they are also to be -seen in the connective-tissue cells, in the leucocytes, and in the -lymph inside the lacteal vessel. The epithelial cells extrude the oil -droplets, backwards, much in the same way as the cells of the mammary -glands extrude globules of milk. In herbivora, and in Man also so far -as we can judge, the contents of the small intestine are alkaline. -Conditions are therefore favourable for the formation of soap. But in -carnivora the contents are acid throughout the greater part of the -canal. Acid, it need hardly be stated, prevents saponification. Yet -carnivorous animals have an immense capacity for absorbing fat. Fatty -acids are soluble to a moderate extent in bile. It is possible that, -fats having been split into fatty acids and glycerin, the fatty acids -are carried into the cells in solution in bile. But if in carnivora -bile actively participates in the absorption of fat, there is no -reason opposed to its having the same function in Man; and, indeed, -all observations which have been made upon patients in whom the bile -was, for some reason, diverted from the intestine, and in animals in -which a fistula of the gall-bladder has been artificially produced, -show that in the absence of bile the absorption of fat is considerably -decreased. Yet there is no reason for thinking that bile is secreted -for the purpose of facilitating the absorption of fat. Just as much -bile is poured into the intestine of a cow which is feeding upon grass -as into the intestine of a pig or a dog when the animal is consuming -a very large quantity of fat. Nevertheless, it appears to be certain -that, not in carnivorous animals only, but also in herbivorous animals, -the assistance of bile is necessary for the satisfactory absorption of -fat. Doubtless the co-operation of bile and pancreatic juice is more -important to carnivora than it is to herbivorous animals, in which, -owing to the alkalinity of the contents of the intestine, all fatty -acids liberated by the action of pancreatic juice might be converted -into soluble soaps. - -The problem of the form in which foods enter the absorbing cells is -intimately associated with the further problem of the form in which -they leave them. In the villus, and even within the epithelial cells, -fat appears abundantly as such. If, as we have reason for believing -to be the case, it enters in the form of soap and glycerin, the -re-formation of fat is an illustration of the synthetic power of the -tissues. For the purposes of the economy it is needed as fat, and not -as the constituents of fat. There is no reason for thinking that at -any stage in its future progress it is again split into fatty acid and -glycerin. - -We cannot see absorbed proteins with the microscope, as we can see fat, -nor can we apply chemical tests which will distinguish between the -proteins which the cells contained before digestion commenced, and the -proteins which they have received as its result. Nevertheless, it is -certain that peptones are reconverted into proteins as soon as they are -absorbed. They are not to be found in blood or lymph. If the peptones -absorbed after a proteid meal remained as such after they passed -through the wall of the alimentary canal, they would produce various -undesirable results. - -There is some difficulty in following droplets of fat across the space -which intervenes between the epithelium of a villus and its lacteal -radicle. It has been asserted that leucocytes act as carriers, catching -the droplets as they are extruded by the epithelial cells, and bearing -them into the radicle, where they set them free. Undoubtedly, many -leucocytes are present in the lymph-spaces of a villus. After a meal -they are found charged with fat. But it is hardly in accord with what -we know of the character of a leucocyte to suppose that it will let go -fat which it has once ingested into its own body-substance. A leucocyte -is not a disinterested organism. If fat droplets are floating across -from the epithelium to the lacteal, leucocytes are pretty certain to -steal some of them. But we know of no other case in which they give up -what they have stolen, unless it be something which is injurious to -their own health. Even then they usually cling to it, whether it be a -germ or a particle of soot, until their own dissolution sets it free. - -Neither proteins nor sugar reach the lacteal radicle. Both these -substance are absorbed from the lymph in the tissue-spaces of the -villus by the blood-capillaries and venules which traverse them. The -veins of the intestine unite to form the portal vein, up which proteins -and sugar are carried to the liver, where they are stored, to be doled -out into the blood-stream as the tissues need them. - -=Bacteria of the Alimentary Canal.=—The enzymes (ferments) of the -several digestive juices are not the only agents which modify the -constitution of the foods within the alimentary canal. Throughout the -whole of the tract conditions are in many respects favourable for the -growth of putrefactive organisms. Mouth, stomach, small and large -intestine, has each its special bacterial flora. It is doubtful whether -any of these organisms, with the single exception of the bacteria which -in herbivorous animals break up cellulose, are favourable to digestion. -That they are not necessary has been shown by an ingenious experiment -on new-born animals. Guinea-pigs born in an aseptic chamber, through -which filtered air was drawn, and fed every hour on sterilized milk, -throve and put on weight. When killed at the end of eight days, no -germs were present in their alimentary tracts. Yet in all animals -under ordinary conditions bacteria are present in great numbers, at -any rate, after the nursing period, and, for good or ill, produce -important fermentations. Only a single bacillus (_B. bifidus_), and -that a friendly germ, is, it is asserted, present in the intestines of -an infant at the breast; whereas a bottle-fed baby houses a variety of -parasites. - -In the stomach, sugars are changed by the _Bacterium acidi lactici_ -into lactic acid, which is further split into butyric acid, carbonic -acid gas, and hydrogen. Succinic acid and other substances are also -formed. This occurs in the first stage of gastric digestion. When a -considerable quantity of hydrochloric acid has been poured out, lactic -fermentation is stopped. The small amount of gaseous products formed -normally is of little consequence; but flatulence is a most annoying -symptom of indigestion. “Put your trust in Providence, and you will -feel more cheerful after luncheon,” Dr. Jowett is alleged to have -remarked to a despondent friend. The presence of food stimulates the -stomach to contraction. Accumulated gases are expelled. Hydrochloric -acid is secreted, and puts a stop to fermentation for a time. But if -the meal be too heavy or the mucous membrane in an irritable condition, -the contents of the stomach become unduly acid in the later stages of -digestion. Other bacteria then develop, leading to fresh trouble; more -gases accumulate, and the dyspeptic’s distress is greater than it was -before. Unfortunately, antiseptics, such as creosote, and carminatives, -such as oil of lavender, oil of peppermint, or alcohol, which for the -moment give relief, increase irritability, and consequently in the -long-run make matters worse. It is the fermentation of the later stages -of digestion which causes most annoyance. Admirable as was the Master -of Balliol’s advice, it hardly took account of the fact that bacteria -which cause flatulence, with its resultant feeling of oppression, -are derived for the most part from the imperfectly digested, and -therefore actively fermenting, remnants of food which were present in -the stomach when the meal was taken. It would be far beyond the scope -of this book to consider the pathology of dyspepsia; but the study of -normal conditions reveals the fact that some amount of fermentation -invariably occurs. The _Bacterium acidi lactici_ is always present in -the stomach. Normally its activity is arrested by the hydrochloric -acid of the gastric juice about twenty minutes after a meal. After this -no further multiplication of bacteria should occur. The presence of -bacteria which grow in a strongly acid medium usually indicates that -the stomach was not completely emptied before fresh food reached it. It -may be that the last meal was too large or the interval too short. If -the mucous membrane is in an unhealthy condition, its own secretions -afford material on which bacteria thrive. Nothing short of washing it -out with a stomach-pump will clean it up. The presence, at the time -of feeding, of food left over from the previous meal is likely to -perpetuate the unsatisfactory state of affairs. All the glands of the -alimentary tract exhibit a tendency to periodicity. Their efficiency -is greatest when activity follows a period of rest. If the stomach is -not able to expel its contents, it has not the opportunity of preparing -for fresh duties. Fat undergoes a certain amount of rancid fermentation -in the stomach. Proteins are not attacked by bacteria in the stomach -unless the condition of the organ is very unsatisfactory. The odour of -the products of their decomposition is then recognizable in the breath. - -Bacteric fermentations in the small intestine are unimportant under -normal conditions, with the exception of the fermentation of cellulose. -Cellulose has the same empirical formula as starch. It is completely -insoluble, and is not affected by any of the digestive juices. The -greater part of the cellulose consumed by herbivora is, however, broken -up by bacteria into acetic and butyric acids, carbonic acid, and -marsh-gas. In Man also a small quantity is similarly destroyed. - -In the large intestine the bacteric fermentations are not unlike those -which occur in the stomach, with, in addition, the destruction of -proteins, or of products of proteid digestion. The greater the quantity -of undigested food which reaches the large intestine, the greater is -the development of bacteria. When the stomach is dilated, the ascending -colon, and especially its cæcum, is usually dilated also. Bacteric -fermentation in the large intestine, with resulting flatulence, is -evidence of imperfect digestion, due either to an excess of food or to -weakness of the alimentary organs, or, as is more commonly the case, -to the combination of these two factors. The relation of fermentation -to alimentation can be shown by counting the microbes in a specimen of -the contents of the large intestine. In a particular case it fell from -65,000 per milligramme upon a mixed diet to 2,000 per milligramme upon -a diet of milk. - -In the world at large bacteria perform many offices of the utmost -usefulness to other living things. They fix nitrogen in the soil, -sweeten polluted rivers, reduce animal and vegetable matter to a -condition in which it is available as plant-food. Their presence -within the alimentary canal is inevitable; but it is somewhat doubtful -whether, with the exception of the fermentation of cellulose, they do -the economy any service with which it could not dispense. As parasites -of the alimentary canal, some kinds are less desirable than others. -Recently a method of limiting their variety has been introduced -and advocated with much enthusiasm, as favourable to the hygiene -of the digestive tract. In countries in which the cows are driven, -in summer, to mountain pastures, the peasants of the plains live -during their absence largely upon milk brought down at intervals, and -allowed to turn sour. Sour milk, in Bulgaria, develops a bacterium of -extraordinary vigour. It can live in a medium containing as much as -10 per cent. of lactic acid, a concentration fatal to other forms of -_Bacterium acidi lactici_. It is easily cultivated, and when ingested -continues to multiply in the alimentary canal. So peculiarly lusty is -this bacterium that it makes life impossible for other germs. As it -dies out after two or three months, it seems unlikely that a man who -swallows the Bulgarian milk-germ runs a risk of inviting a repetition -of the tragedy which followed the acclimatization of the mongoose -in Jamaica. Its supremacy has been attributed to its capacity of -developing a concentration of lactic acid too high for the well-being -of other bacteria; but it is improbable that it has the opportunity of -doing this in the alimentary canal of a person living on a mixed diet. -The extinction of other bacteria (if they are extinguished) is more -likely to be due to an antagonism of a more subtle kind, at present -inexplicable, but not without parallel. The purifying influence of the -water of the Ganges has for ages been an article of faith. Pilgrims -from fever-stricken districts bathe in it, foul it, drink it, with the -corpses of their fellows floating down the stream. Recently it has been -shown that this belief is not without foundation. The water of the -Ganges at Benares contains bacteria which are as tigers among lesser -vermin. The germs of cholera and typhoid fever disappear from cultures -into which these overbearing microbes are introduced. - -=Conditions Requisite for Normal Digestion.=—When M. Chevreul, -Professor of Chemistry at the Jardins des Plantes of Paris, attained -his hundredth year, an interviewer very naturally inquired of him, -“Have you always had a good digestion?” To this the still vigorous -Professor answered: “I really cannot say, for I have never noticed.” -So long as it is well used, the stomach is an unobtrusive organ. It is -tyrannical when it deems itself the victim of inconsiderate treatment. -A study of its physiology serves to show that it will work contentedly -only upon certain clearly defined terms, of which the following are -perhaps the most important: The stomach exacts due warning that its -services are wanted. The nerves of smell and taste must announce the -approach of food and guarantee its quality. “What may I eat?” asked -a large-framed, strenuous, eager, overworked barrister of a great -physician. “Eat, sir? You may eat whatever you like. But be quite -sure that you do like it.” Wise advice. The human race would not have -developed its strong preferences for certain kinds of food if all foods -were equally suitable to satisfy its needs. Taste is not a matter of -fashion. It is the expression of the experience of mankind. Fanciful -as civilization has made us, and easily as appetite is perverted, if -we are sure that we really like, and want, a food, we may trust that -our liking will guide us as safely as it guides a buffalo or a deer. -“Eat what you like.” Eating with liking carries with it the idea -of obtaining the maximum of satisfaction from the exercise of this -necessary function. Most things which are reckoned unwholesome are full -in flavour or rich in consistency. They satisfy the palate when spread -out very thin. It is poor economy to help oneself to caviare with a -table-spoon. In the second place, the stomach must be assured that the -teeth are doing their proper share of work. Among the many half-truths -which every year are exalted to the level of a revelation or a rule of -conduct is the doctrine of the “chewers”—persons who take no meals, -but industriously and almost continuously masticate nuts and biscuits. -Thirdly, the meal must not be so large that the stomach cannot deal -with it “at a sitting.” In from two to three hours the last of the food -should have passed through the pylorus, allowing the stomach to rest -before it is called into activity again. As proteins are practically -the only foods which are digested in the stomach, the work required of -this organ depends upon the quantity of proteins present amongst the -constituents of a meal. Meat is the food richest in proteins, although -bread, vegetables, milk, cheese also yield them. Some people can digest -three meat meals every day; but others, probably the majority, find -that it is unwise to take any considerable quantity of meat more than -once in twenty-four hours. It is only when the cells of the gastric -glands have accumulated a store of pepsinogen-granules that proteid -digestion is vigorously carried on. Fourthly, the food must be in a -form in which it does not irritate the stomach, provoking an outflow of -acid out of proportion to the pepsin which accompanies it. Experience -alone can teach the foods which are to be avoided on this account. -But speaking generally, it may be said that the stomach resents the -presence of substances which cannot be amalgamated into chyme. Its -task is the reduction of the mixture of foods which compose a meal to -the consistence of a smooth cream. Hot buttered toast or pie-crust are -made of wholesome constituents enough, but, fat being melted into the -starch, the fragments are impermeable to the gastric juice. They act -mechanically as irritants of the mucous membrane. Again, it may be said -that “pure” foods are apt to provoke acidity. Nothing could be more -wholesome than eggs or pounded meat or custard pudding; but taken by -themselves these articles of diet over-stimulate the mucous membrane. -They need to be diluted with starch-foods, or even with cellulose. - -And this calls attention to the dietetic value of vegetables. -Vegetables, which consist chiefly of innutritious cellulose, distribute -the digestible constituents of a meal and increase its bulk, greatly -favouring its progress through the alimentary canal. Especially -in herbivora is it important that the bulk and looseness of the -food should be well maintained. Rabbits thrive on sugar, starch, -and albumin, mixed with such an absolutely indigestible substance -as horn-shavings. If the inert substance be omitted, they die of -intestinal inflammation, although fed on the same mixture of pure -foods. Other rules which govern digestion might be mentioned; and it -is needless to point out that, when the mechanism is deranged, steps -adapted to the particular malady must be taken to bring it back to a -normal condition. There is, however, one precaution upon which, in a -certain number of cases, it is impossible to lay too much stress. The -digestion of proteins is seldom carried out satisfactorily when much -sugar, and especially much cane-sugar, has been eaten at the same meal. -Excessive lactic fermentation prevents the proper peptonization of -meat. The chemistry of digestion is not sufficiently well understood -to enable the physiologist to say what is amiss; but probably -by-products of peptic digestion are produced. To many people this is of -little consequence; but to those who exhibit a gouty tendency it is, -unfortunately, a most serious matter. Civilized races are particularly -subject to the uric acid diathesis. In the course of nitrogenous -metabolism uric acid is formed in place of fully oxidized and easily -soluble neutral urea. Although the chemical sequence has not been -discovered as yet, there is no question but that imperfect gastric -digestion means the formation of uric acid, with all its lugubrious -results: malaise, neck-ache, emotional depression. Birds and reptiles -form uric acid as the end-product of nitrogenous metabolism, not urea. -So also do city-fathers, butchers, and others whose diet consists too -largely of meat. Many nervous, ill-nourished men and women tend to -do the same, however abstemious their meals. It is useless to tell -such persons to reduce the amount of proteins in their diet. Their -attempts at increasing the starch, sugar, and fat at the expense of -nitrogenous foods lead to dyspepsia, which makes matters worse. They -often find, however, that if they are careful to restrict to the -narrowest limits the amount of carbohydrates (especially sugar) which -they take in conjunction with meat, fish, eggs, or other proteid foods, -the formation of uric acid ceases. Sugar, bread, fruit, and other -carbohydrates, may be taken in abundance, and with great advantage, at -breakfast and lunch, without proteid food, if dinner consists of broth, -fish, meat, cheese, vegetables, with a minimum of bread. - -=The History of the Foods after Absorption.=—All foods, with the -exception of inorganic salts and salts of various vegetable acids, -fall into three classes: (1) Proteins—substances of complex chemical -constitution, containing nitrogen; (2) carbohydrates—so called because -hydrogen and oxygen, in the proportions in which they enter into the -formation of water, are united with carbon; (3) fats. Proteins of -various kinds are consumed as food. The peptones produced from them by -digestion also vary. Yet very little is known as to the differences in -physiological value which distinguish the various kinds of protein when -absorbed into the fluids of the body (_cf._ p. 134). All carbohydrates -after digestion and absorption appear as dextrose. The various fats -preserve their individuality until they are taken up by the tissues. -When fixed in the tissues, they assume, except under somewhat abnormal -conditions, the composition characteristic of the fat of the animal -which has eaten them. If a dog which has been severely starved is fed -upon mutton-fat, it puts on in the first instance fat which resembles -that of a sheep rather than the normal fat of a dog. As soon, however, -as it is well nourished (which would never occur unless some protein -and carbohydrate were added to the mutton-fat), its fat assumes the -usual form. - -For practical purposes we are obliged to speak of the three classes -of food—proteid, carbohydrate, and fatty—as if there were but one -member in each class. And we have abundant evidence that such a simple -classification is fully justified. The body has so large a power of -altering chemically the nature of the food which it absorbs that it -makes little difference in the further history of the food whether the -protein supplied to it be an albumin or a globulin; the fat, stearin, -palmitin, or olein; the carbohydrate, starch or sugar. - -In earlier days it was customary to regard the body as the receiver of -a variety of foods which it could break down into simpler substances -by oxidation, but could not reconstruct. Plants were regarded as the -manufacturers of organic compounds, animals as the destroyers of the -complex substances made by plants. The union of molecules, synthesis, -was looked upon as the function of the vegetable kingdom. Animals -built into their tissues the products elaborated by plants; some of -these products they shook to pieces for the purpose of setting their -energy free; others slowly disintegrated as the result of tissue -“wear and tear.” Gradually it was realized that many chemical changes -occur in the body which cannot be viewed as merely exhibitions of -its analytical capacity. The tissues were recognized as laboratories -in which reactions occur which consist in something more than the -splitting of complex into simpler molecules. The instances earliest -understood were connected with the history of carbohydrates and fats. -In the disease diabetes an enormous quantity of sugar is excreted, -amounting in extreme cases to between 1 and 2 pounds _per diem_. When -carbohydrates are present in the food, the amount of sugar excreted in -diabetes is greater than it is when they are withheld; on an almost -exclusively proteid diet the amount of sugar excreted far exceeds the -amount of carbohydrates in the food. Another illustration of the power -of making sugar possessed by the animal economy is afforded by a dog -fed upon lean meat, and nothing else. Sugar is found in its blood, -and a store of carbohydrate (glycogen) in its liver. The formation -of fat is an instance of constructive metabolism. There is abundant -evidence that the quantity of fat produced may greatly exceed the -quantity contained in the food. Animals are fattened for the market -on a diet which contains less fat than that which accumulates in -their bodies. When nursing her young, an animal may secrete in her -milk much more fat than she obtains as such in food. It was a great -mistake to suppose that the body is dependent upon its tradesmen for -fat and sugar. It can make either of these substances out of a mixed -diet in which it is relatively deficient. It must, however, be a mixed -diet. An animal cannot live exclusively on fat or exclusively on -carbohydrate. It is impossible, therefore, for us to determine whether, -if given the one alone, it can turn it into the other. Chemists were -very unwilling to credit the body with the power of performing even -the simpler of these transformations—the conversion of carbohydrate -into fat. Proteins are essential constituents of a fattening diet. -Their immensely complex molecule has always afforded a tempting field -for arithmetical ingenuity. It is easy to remove from it the atoms -needed for the composition of fat, and yet to leave such groups of -atoms as might reasonably be supposed to constitute its “nitrogenous -moiety.” The hypothesis that the metabolic capacity of the body is -limited to analytical processes justified the supposition that, when -more fat is laid on than the food contains, the balance comes from -proteid substances, which split into nitrogenous and fatty moieties. -It has been shown, however, that an animal during fattening may put -on more fat than is contained as such in the food, or obtainable from -its diet, even though all the atoms of carbon and hydrogen in its -proteid food were devoted to its formation. The balance must come -from carbohydrates. Perhaps a still more striking illustration of -constructive capacity is the power of making glycerin. If a dog receive -fatty acids in its diet, it accumulates normal fats. The glycerin -which, united with fatty acids, constitutes the fat, was not contained -in its food. Starch and sugar are sources of fat. As yet there is no -evidence that fat can be converted into sugar. - -The chemistry of the nitrogen-containing compounds appears to present -more difficult problems. Plants build up proteins. Is the animal’s -relation to these substances limited to their disintegration? Do -proteins inevitably descend from step to step until they reach -urea? There are reasons for thinking that, even when dealing with -nitrogenous substances, the metabolic power of the body is not -exclusively analytical. The liver can make urea from ammonia-salts, -such as lactate, or even carbonate, of ammonia—substances more stable, -and therefore in the chemical sense simpler, than urea. This is an -indication, though a faint one, that the body has a constructive -capacity, a power of producing more complex from simpler substances, -even in the case of nitrogenous compounds. Beef-tea, mutton broth, -meat-extracts have long been regarded as foods of value when the -power of assimilation is low. Chemists point out that the nitrogenous -substances which these decoctions contain are so near the bottom of the -ladder that the energy set free by their further oxidation to urea is -scarcely worth consideration. They admit that their ready availability -renders them useful as restoratives, but they deny them the status of -foods, on the assumption that their further progress must be downward. -As was stated when the conversion of peptones into leucin and tyrosin -was described, evidence is beginning to accumulate which shows that -within certain limits, at present impossible to define, the system can -reconstruct its proteins from amides and other simple products of their -degradation. - -The animal economy receives, and after due digestive preparation -absorbs, three classes of food—nitrogenous, fatty, and carbohydrate. -If either of the two latter kinds be deficient in the diet, the body -can to a certain extent produce it from the other two. What is the -special value of each kind of food? What use is made of it? Before -attempting to answer these questions, we must endeavour to trace the -further history of the foods after they have traversed the wall of the -alimentary canal. - -After leaving the stomach and intestines, the foods follow two -different routes. Proteins and carbohydrates are carried by the portal -vein to the liver. Fats are carried by the thoracic duct to the general -circulation. An excess of fat is found in the blood in all parts of the -body after a meal rich in fat. The eventual destination and fate of -fatty foods is unknown. Under certain circumstances they are added to -the fatty deposits in connective tissue; but if no additional fat is -being laid down, they go to other tissues, in which they are oxidized -into carbonic acid and water. When the amount absorbed is excessive, -a certain quantity of fat may be stored in the liver. In the cells of -this organ it is housed for a time, in order that it may be distributed -to the tissues after they have used up the supplies which first reach -them through the general blood-stream. - -Proteins are completely lost to sight after they are absorbed into the -blood. They take part, of course, in the formation of growing tissue, -blood-corpuscles, skin, hair, nails. It is also common to speak of them -as making good the wear and tear of active tissues, although it is -very doubtful whether we can legitimately speak of the wear and tear -of tissues. The protoplasm which does the work of the body is not worn -out in the same way as the materials of which a machine is made. There -is no friction to rub it down. Proteins, like other foods, are used up -as sources of muscular energy and heat. Eventually they are reduced to -urea, carbonic acid, and water. Chemists naturally seek for substances -intermediate in constitution between proteins and urea. They assume -that the degradation of proteins will occur in regular steps; complex, -partially oxidized, nitrogenous compounds being formed first—in -the muscles, for example—to be further oxidized in the glands. The -existence in all organs of nitrogenous “extractives,” which can be -separated out when the organ is subjected to chemical analysis, seems -to justify the search for stages; but hitherto this search has been -singularly unsuccessful. Urea is the final product. It is not found -in muscle, nor, indeed, in any tissue other than the liver, which, as -already said, has the power of making it, even from salts of ammonia. -It is therefore clear that if proteins are destroyed in muscle and -other tissues, and if all urea is made by the liver, the antecedents -of urea must be carried from the muscles to this organ. The substance -which is most characteristic of muscular metabolism is lactic acid. -It is not impossible that all the nitrogenous portion of the complex -proteid molecule is reduced to ammonia (NH₃), which may be regarded as -the simplest of all nitrogenous compounds, and that this, combined with -lactic acid (C₃H₆O₃) as lactate of ammonia (NH₄C₃H₅O₃), is carried by -the general circulation to the liver, where it is converted into urea. -A considerable amount of lactate of ammonia may be injected into a vein -without any of it overflowing through the kidneys. It is all reduced -to the condition of urea, water, and carbonic acid. If the liver is -so diseased as to be functionless, or if by operative measures it is -thrown out of action, salts of ammonia are excreted by the kidneys -instead of urea. In birds and reptiles uric acid takes the place of -urea. Their livers yield uric acid on analysis. If lactate of ammonia -be injected into their blood, it is converted into uric acid, so long -as the liver is intact. - -We know nothing of the forms assumed by the proteins absorbed into the -blood, of the organs in which they are stored, or of the higher terms -of the series of substances through which they pass before they are -finally excreted as urea, water, and carbonic acid. No nitrogenous -compounds are found in lymph or blood which can be pointed out with -confidence as the products of tissue wear and tear. When considering -the sources of muscular energy, we shall have something more to say -regarding the part that proteins play in the economy. - -If there is great difficulty in following fats and proteins after their -absorption, it is quite otherwise when we come to deal with sugar. -Carbohydrates are the great sources of energy. Muscular work may be -generated by the oxidation of either of the three classes of foods, -but undoubtedly the carbohydrate glycogen is its most constant source. -Provision is therefore made for the storing of glycogen in the liver, -and the distribution to the muscles of a regular supply. After a meal -the portal blood, on its way from the intestines to the liver, contains -a higher percentage of sugar than the blood in the hepatic vein or -in any other vessel. If sections of liver be examined after feeding, -and compared with those obtained after a period of starvation, it is -found that the cells of the well fed liver contain glancing masses -of a substance which takes a port-wine colour with iodine. This is -glycogen, or animal starch. It has the same empirical formula as starch -(C₆H₁₀O₅)ₙ. In the dry state it is a greyish powder, which, unlike -starch, forms an opalescent solution in cold water. Like starch, it is -non-diffusible. In the animal kingdom it stands to sugar in the same -relation as starch to sugar in plants. If a sheep be killed while it is -feeding in the paddock, and its liver removed and weighed, it will be -found that it is from one-third to one-half heavier than the liver of -a sheep of the same weight obtained from a butcher; for butchers have -the stupid practice of starving animals before they kill them. It was -long ago discovered that it is unnecessary to feed an animal for a day -or two before it is killed, and this option has been elevated into a -prohibition. A tradition has grown up that it is undesirable to give -food for some time before killing. Not only will the liver of a sheep -killed during active digestion be found to be heavier than that of a -starved sheep, but it will also prove more succulent; for it is loaded -with sugar (into which glycogen is rapidly converted after death), -as well as with proteins and fats, which are withdrawn from it when -the animal fasts. It appears that the liver cannot secure the whole -of the sugar which is absorbed after a full meal. Some of it passes -into the general circulation, and is stored in the muscles; but the -liver always maintains a considerable reserve. Even after prolonged -deprivation of food, it holds on to a certain quantity, especially in -carnivora. Glycogen is found in the liver of a dog after a long period -of starvation. The muscles lose during activity the glycogen which they -contain when at rest. - -It has already been pointed out that the body is not entirely dependent -upon external agencies for the production of the sugar which it needs. -When the supply is inadequate, it manufactures glycogen for itself -out of the other constituents of the diet. It can, indeed, make it at -the expense of its own proteins. If a dog which has been caused to do -muscular work, without a sufficiency of carbohydrate food, until (as -judged from a control experiment) all glycogen has disappeared from its -liver, be placed under the influence of a narcotic drug, which arrests -the activity of its muscles, glycogen reappears. - -=Dietetics.=—Even those who are most ignorant of the science of -physiology flatter themselves that they have one piece of information: -“The whole of the body is renewed once in every seven years.” I cannot -trace the origin of this sapient apothegm, which for generations -has passed current. If seven weeks or seventy years were the period -allowed for the renewal of the tissues, the statement would be equally -near the truth. Judging from the rate at which they are destroyed, -it is unlikely that blood-corpuscles live for more than five or six -weeks. Hairs are shed about two years after they first appear above -the surface. On attaining this age a hair drops off and a new one -takes its place. The superficial cells of the skin are shed in great -numbers every day, and their place taken by younger cells which come -up from the deeper layers. The cells of many glands would seem to -have a comparatively short term of life. On the other hand, some -tissue-elements are far more permanent. By the time a child is a year -old all its nerve-cells are in position. They last as long as the -individual lives. When the statement with regard to the renewal of the -tissues is understood as meaning, not that the cells are destroyed and -replaced by new ones, but that within a period of seven years all the -molecules which enter into their protoplasm are extruded from the body -and replaced by molecules received as food, the assertion verges on the -transcendental. It is unlikely that we shall ever obtain data against -which it can be checked. - -The essential part of every living cell is its sponge-work of -protoplasm. “Bioplasm” is perhaps a better term to use when we are -speaking of protoplasm as a structure, since it does not suggest any -prejudice with regard to its chemical constitution. Within the meshes -of the bioplasm are nutrient materials, as yet unused, and worked up -products in various stages. It has always been taken for granted that -when treating of nutrition, we have to consider the repair of the -bioplasm, as well as the provision of raw material which it can convert -into the specific products of the cell. Suppose that the cell belongs -to the class of supporting tissues; let it be a cell of cartilage, -for example. The bioplasm manufactures a collagenous substance which -remains in and around its meshwork. If it be an epidermal cell, it -forms horny substance. If a secreting cell, it accumulates secernable -products. If a muscle-cell, it develops a large quantity of material, -which by a change in form produces movement. In this last case we -suppose that the energy set free as muscular force is due to oxidation. -More stable bodies take the place of a less stable substance. After -contraction the relatively complex contractile material is renewed from -the foods stored in the muscle-cell; or if it be not, in the ordinary -sense of the word, destroyed, if it has merely parted with certain -oxidizable constituents, it obtains a fresh supply of such constituents -from the foods which the muscle-cell contains. Even in the case of -cartilage or epidermis, we imagine that, since the matrix is “alive,” -it is always undergoing molecular change, and consequently always -requiring food. The fact that every tissue, however inert, dies when, -owing to the blocking of the bloodvessels which irrigate the part, its -supply of nutriment is cut off, justifies this belief that all living -tissue is undergoing change. - -When we make up a balance-sheet of the body as a whole, placing to -the debit side the food which it receives, and to its credit side the -work done in external movement and in the production of heat, we again -find reason for believing that every part of every cell is constantly -undergoing change. - -The balance-sheet of the body can be drawn out in either of two ways. -We can estimate the quantities of nitrogen, carbon, hydrogen, and -oxygen supplied to it in the several foods, and compare them with the -amounts of each of these four elements given off in urea, carbonic -acid, and water, making, of course, a note of the body’s balance in -hand at the beginning and at the end of the period of observation. Or, -we may estimate the amount of potential energy contained in the food, -and ascertain the use to which this energy is put in doing external -work, in maintaining the temperature of the body, and in warming the -breath and other excreta. - -If we are making up the balance-sheet of a fully-grown man, we may -take for granted that he is not making fresh tissue. During the -period throughout which he is under observation, care is taken to -avoid altering the conditions of his life in such a manner as to -lead him to develop additional muscle. If he gains in weight while -under observation, he is putting on fat. If he loses in weight, he is -sacrificing fat. - -The whole of the nitrogen taken in leaves the body in urea, unless, -as we have said, growth of tissue is taking place. The body has not -the same temptation to store nitrogen as it has to store carbon. -Consequently, it is very sensitive to any deficiency of nitrogen in -the diet. If food does not contain as much protein as is needed, -the deficit is made up at the expense of the tissues. It does not -necessarily follow that under these circumstances a man loses in -weight. He may be putting on fat, although losing in strength owing to -waste of muscle. For observations upon the income and expenditure of -the body to be of any value, a condition of “nitrogenous equilibrium” -must be established. The nitrogen taken in must equal in amount the -nitrogen given out. - -Very exact determinations of income and expenditure may be made by -placing an animal, or even a man, in a box through which air is drawn. -A record is made of the volume of air drawn through the box. The -percentages of water vapour and carbonic acid which the air contains -are estimated before it enters and after it leaves. The solid food -consumed and the urea excreted are also measured. - -If it is desired to measure the amount of heat given off, an animal may -be placed in a calorimeter. - -Even when most passive, the subject under examination, whether an -animal or a man, is expending energy in keeping the body warm, in -movements of respiration, and in shifting position. If it is desired to -ascertain the relation of oxidation to external work, it is easy to -devise a form of resistance, such as the turning of a wheel, or the -lifting of a weight which can be measured. - -In testing diets, it suffices to make sure that nitrogenous equilibrium -is maintained, and then to estimate the gain or loss in weight and the -output of energy in external work. - -=The Relative Value of Foods.=—Dried proteins contain about 15 per -cent. nitrogen, 54 per cent. carbon, 7 per cent. hydrogen, 22 per cent. -oxygen, a little sulphur, and frequently some phosphorus. A large -proportion of their carbon and hydrogen is available for combustion. -Fats contain 75 per cent. of carbon, and a considerable quantity of -hydrogen available for combustion; carbohydrates, 40 per cent. of -carbon, with hydrogen and oxygen in the proportions in which they -occur in water. If 1 gramme of protein is oxidized to the condition -of urea, carbonic acid, and water, sufficient heat is liberated to -raise the temperature of 4,100 grammes of water 1 degree centigrade. -Its calorific value is therefore expressed as 4,100 calories, the unit -of measurement—a calorie—being the amount of heat needed to raise 1 -gramme of water 1°. The calorific value of 1 gramme of fat is 9,300 -calories; of 1 gramme of starch, 4,100 calories. Thus, the energy -potential in protein and in starch is the same; that in fat more than -twice as great as that in either of the other foods. - -=A Normal Diet.=—Nitrogenous equilibrium and body-weight can be -maintained and work done on diets which vary widely in percentage -composition. This is a question which we shall consider at greater -length later on. In the meantime, for the sake of illustration, it is -necessary to formulate a diet which is fairly representative of the -selection of foods made by a man of average weight—say 70 kilogrammes -(145 pounds)—who desires to do a moderate day’s work in comfort. It -has been found to amount to about 100 grammes of protein, 100 grammes -of fat, 240 grammes of carbohydrate, all measured dry and as pure -foods. If the several elements of such a diet be multiplied by the -figures which represent their calorific value, it will be found that -the man is supplied with 2,324,000 calories. The illustration that we -have chosen is the diet of a professional man who is not engaged in -hard physical work. The pure foods would be found to the amounts stated -in 17 ounces lean meat, 4 ounces butter, and 17 ounces bread. The -day’s diet would, of course, be much more varied than this, but it is -simpler to express it in these terms. - -Such a diet would hardly answer the requirements of a man doing hard -muscular work. Experience shows that he would expect to receive a more -liberal supply of energy, and that to obtain it he would increase -slightly his allowance of proteins, and very considerably increase -the quantity of carbohydrates that he consumed. The diet of European -workmen is remarkably constant in the relative amounts of its several -constituents, no matter what their nationality or the exact form of -their work may be: Proteins, about 135 grammes; fats, 80 grammes; -carbohydrates, 500 to 700 grammes—giving a supply of energy equal to -3,500 to 4,000 kilo-calories. - -Speaking generally, carbohydrates are the source of muscular force, -and fats of heat. In warm climates men work on carbohydrates. The -’rickshaw men of Japan are said to eat only rice on working days, and -to reserve fish for days of leisure. The Japanese, as is well known, -consume extremely little fat. The Esquimaux and other inhabitants of -high latitudes eat immense quantities of fat. Proteins constitute the -luxurious element of a diet. Not only are they more attractive to -most palates, and therefore preferred by persons whose dietary is not -severely regulated by price, but the body prefers them. It works with -greater alacrity when supplied with more protein than, in a strictly -physiological sense, it needs. - -The supply of food must exceed the apparent demand. The most efficient -of motors cannot convert more than 15 per cent. of the energy potential -in its fuel into work. If a man endeavours to obtain a better result -than this from his muscular system, if he tries to make his machine do -more than 15 units of work for every 100 units of energy with which he -supplies it, he does it at the expense of his own tissues. First he -loses in weight, owing to the consumption of fat; then the excess of -nitrogen discharged over nitrogen consumed shows that he is burning -up the proteins of his own tissues. It is needless to add that the -weakness which results puts a stop to excessive work. Muscles, as we -shall find when we consider the relation of their output of work to the -energy supplied to them, can produce a much better result than the best -of engines; but we are speaking of the body as a whole, which wastes -energy in the movements of respiration, masticating food, shifting -position, maintaining the body temperature, etc. - -Health may be maintained and work done on diets which depart widely -from the one which we have selected as a standard. Darwin found the -Gauchos of South America living exclusively on meat. Nansen and -Johannsen, when seeking the North Pole, lived for months on meat and -blubber. Millions of the inhabitants of India abstain from meat and -meat-fat, their diet consisting of rice, buttermilk, and a little -fruit. In the case of all persons with whom the price of food is an -important consideration, carbohydrates are preferred to proteins and -fats. Oatmeal is very much cheaper per unit of energy than meat. A -man may be a meat-eater or a vegetarian, although he is probably -unwise in overlooking the obvious teaching of his teeth and digestive -organs, which are those of an omnivorous animal. His prehistoric human -ancestors lived chiefly on the harvest of their spears and tomahawks. -If we insist upon looking back still farther, we discern a cleavage of -the race into the arboreal fruit-eaters, which still retain pre-human -characters, and the more enterprising and energetic troglodyte hunters -from whom the human race was evolved. - -A man may vary his diet within wide limits. Innumerable considerations -lead certain individuals to desire to depart from the diet which we -have termed “normal”—_i.e._, typical of inhabitants of the temperate -zone. One man rebels against the expense of living; he would fain -reduce the quantity and the cost of food. Another, having to traverse -regions in which food is scarce, wishes to ascertain the lightest, and -therefore the most portable, combination of its essential elements. -A third—and he belongs to a much larger class—tormented with -indigestion or harassed by gout, asks, “Why must I consume things which -give the stomach trouble, or produce disagreeable and incapacitating -after-effects?” Many circumstances prompt to experiments in diet. -Much latitude is undoubtedly allowed. But there are limits within -which alone health can be maintained and work done. It is of great -interest to ascertain exactly how wide these limits are; and especially -important is it to find out the lower limit, the minimum of food, and -the minimum of each particular kind of food, which will enable the -human machine to work. The problems involved are somewhat complicated. -If it were possible to live on a single food, it would be as easy to -ascertain the irreducible minimum as it is to find out with how much -coal or with how much petrol an engine can be made to turn a wheel. But -to support the body several different kinds of food are indispensable. -It is therefore necessary to determine, not only the minimum quantity -of the combined foods, but also the minimum amount of each kind of -food, and the effect upon the total of variations in the relative -amount of each of its several factors. The problem is complicated, but -certain limits are impassably defined. In the first place, with regard -to the total amount, the work which the body does cannot under any -circumstances be reduced below a certain level. The food consumed must -provide a supply of energy equal, at the least, to the performance of -the minimum of work. The body must receive each day food of due caloric -value. Then with regard to the amount of each several constituent. -Many considerations lead us to wish to increase one of them or to -diminish another. Some food is cheap, and other food is dear. Economic -reasons are in favour of the cheaper food. Even ethical considerations -are not without weight. We have, perhaps, a prejudice against -sacrificing life to supply the pot. We have doubts as to whether our -system can properly digest, metabolize, and excrete meat. We need an -unambiguous answer to the question, To what extent can nitrogen-foods -be replaced by carbon-foods, and _vice versa_? A cell, as already -said, consists of a framework of bioplasm bathed in cell-juice which -contains nutrient substances and manufactured products. The bioplasm -is alive; the proteins, carbohydrates, and fats of the cell-juice are -the materials with which it is nourished, and upon which it works. -Some physiologists incline to the view that non-living substances must -enter into the bioplasm before they undergo metabolism. They consider -that the molecules of the non-living substance must at the time when -they undergo a chemical change be physically and chemically a part of -the living substance. Others take the opposite view: that the living -substance does not undergo change, but brings about changes in the -non-living substance which is in contact with it, enclosed within its -meshes. This is a problem which is not likely to be solved, nor is -its solution of great importance in relation to the question which we -are discussing. Whichever of the two views be justified, we have to -distinguish between the bioplasm of the cell—the machine—and its raw -materials and manufactured products. The question to which we want an -answer is the following: Must the bioplasm undergo change? There seems -to be no reason in the nature of things why it should. It is not, as we -have already pointed out, subject to wear and tear. A perfect machine -would in the absence of friction, which rubs down its steel and brass, -continue to turn out its products so long as it was supplied with raw -materials and the energy needed to manufacture them. We could imagine -the bioplasm as indestructible, receiving energy from a portion of the -foods, and expending this energy in the production of chemical change -in the remainder. We could imagine that when once the tissues had -attained their full growth they would require no more protein for their -own nutrition; they would be occupied in producing heat and motion from -the non-nitrogenous foods. But observation shows clearly that this is -not the case. The force which energizes the bioplasm, enabling it to -evoke metabolism in non-living substance, is obtained at the cost of -its own destruction. The bioplasm wastes unless constantly supplied -with proteid food. - -Under ordinary circumstances the amount of urea excreted varies -directly as the quantity of nitrogen contained in the food. Since -urea contains 45 per cent. of nitrogen, and protein 15 per cent., -every gramme of urea excreted represents 3 grammes of dry protein -consumed; or, in terms of nitrogen, every gramme of nitrogen excreted -represents 6·25 grammes of protein consumed. If all food is withheld, -the excretion of nitrogen falls, but it never reaches zero. Many -observations have been made on fasting men. On the second day of -fasting the nitrogen excreted falls to about 13 grammes, representing -80 grammes of protein used up. It is generally thought that by the -second day all “floating proteins” are exhausted, and that therefore -nitrogenous metabolism is reduced, as it were, to a business basis. -So long as the supply of food is abundant, the body has a luxurious -habit of using proteins in preference to non-nitrogenous food. But -after a day’s starvation there is no longer any fancy metabolism, -no consumption of proteins as fuel when cheaper fats and sugar would -answer equally well. In the case of Succi, who fasted for thirty days, -the nitrogen excreted fell to 6·7 grammes on the tenth day, to 4·3 -grammes on the twentieth, and to 3·2 grammes on the last day. Clearly, -we have to make a distinction, when all food is cut off, between -the oxidation of the protein which, failing all other material, is -withdrawn from the tissues for the purpose of supplying the force -absolutely necessary to maintain respiration and such other movements -as are inevitable, and to keep up the temperature of the body—force -which under other circumstances might be supplied by non-nitrogenous -food—and the oxidation to which bioplasm is inevitably subject, -so long as it is alive. The oxidation of bioplasm under ordinary -circumstances of course supplies force; but it does not follow that -this is sufficient to maintain the respiratory movements and the -contraction of the heart. When a herbivorous animal is starved, it not -infrequently excretes more urea at the commencement of the starvation -period than it was excreting when well fed. Its activities did not come -to a standstill when carbohydrate food was cut off. For a time they -were maintained at the expense of its own tissues. On the other hand, -the results obtained from the observation of the man who went without -food for thirty days show that Nature is able to economize force by -reducing the metabolism of living substance below the normal. It might -be supposed that the irreducible metabolism could be ascertained by -giving a nitrogen-starved animal non-nitrogenous food, but it is found -that this scarcely affects the tissue-waste. Becoming more active, -the tissues, while saved from the necessity of supplying fuel for the -production of heat and motion, suffer more waste. Again, it might be -expected that if to an animal which had been starved for a few days, -until its urea had fallen to the starvation limit, exactly sufficient -protein were given to supply this amount, the tissues would be saved. -It is found, on the contrary, that nearly twice as much urea is -excreted as before. If the quantity of protein be steadily increased, -equilibrium is at last established, but not until the amount of -nitrogen in the protein given is two and a half times as great as the -amount excreted during the starvation period. Additional food at once -gives rise to additional waste. The tissues which during the period of -scarcity had reduced their oxidation to a minimum become more active at -the first hint of returning plenty. - -This last experiment illustrates a general law. An increase of -proteid food within certain limits increases the metabolic activity -of the tissues—provokes them to extravagance. It is possible, by -adding protein to a mixed diet which sufficed for the maintenance of -body-weight and nitrogenous equilibrium, to bring about a nitrogen -deficit and to reduce the body-weight. Or, if the body is gaining in -weight, owing to the accumulation of fat, the substitution of protein -for carbohydrate (weight for weight, since their caloric values are -the same) will lead to its reduction. It is difficult to avoid the -use of fanciful language in accounting for these results. The animal -economy is like an over-careful housekeeper, who, when meat is scarce, -doles out porridge also with a thrifty hand. When meat is plentiful -she is prodigal with every article of diet. Protein is the most costly -of foods. Any indication that it is scarce leads to a shutting-down -of activity. On the other hand, no other food is so readily absorbed -(unless the digestive organs be protein-sick); none is so quickly -incorporated in the bioplasm; none is so easy to decompose. When fed -with protein the machinery hums. The insatiable appetite for beef and -eggs which overtakes a man of sedentary habits after a long morning in -a boat or on a bicycle does not indicate that his muscular tissue is -suffering from wear and tear. It does not prove that he is setting free -energy by oxidizing proteid food. It shows that he is asking certain -tissues which are accustomed to a quiet life to exhibit prodigious -energy. They will not shake off their customary sloth unless he -stimulates them with sumptuous fare. At the end of a week he finds -that proteins are not the best fuel for steady work. If he consumes -sufficient to supply all the energy needed by his muscles, he is -hampered by a quantity of nitrogenous residues which have to be reduced -to urea and eliminated by the kidneys. He goes back approximately to -his old regimen, so far as proteins are concerned, and consumes more -carbohydrates for the supply of the force which his increased muscular -activity demands. - -It is possible to live on meat alone, but the quantity required is very -great, involving the digestive organs, the liver, and the kidneys in an -excessive amount of work. On the other hand, it is possible to reduce -the consumption of proteins to a minimum by substituting for them fats -and carbohydrates. But, again, after the proper balance is disturbed, -the substitution ceases to be a simple problem in arithmetic. The -carbon-food has to be increased out of all proportion to the protein -which it replaces. If a dog which is being fed on a diet natural -to it—chiefly meat—is in a condition of nitrogenous equilibrium, -carbohydrate may be substituted for some of the meat. But from the very -beginning it is found that, if nitrogenous equilibrium is not to be -disturbed (if the dog is not to be induced to consume its own tissues), -a weight of carbohydrate must be given considerably greater than the -weight of the protein withdrawn. The disproportion increases as the -experiment proceeds, until perhaps 12 to 15 grammes of carbohydrate -have to be substituted for every gramme of protein. The proteid food -has now come down to 1·5 gramme per kilogramme of the animal’s weight. -Owing to the increase of carbohydrate, the caloric value of the total -food, nitrogenous and non-nitrogenous, is several times as great as -the animal requires. The surplus is oxidized without any equivalent in -work. At about this point the experiment is brought to an end, owing -to the failure of the digestive organs to deal with so large a mass of -food. - -The value of gelatin as an article of diet is of interest in this -connection. Gelatin is not, strictly speaking, a protein, and it cannot -be built up into the tissues. It does not prevent, nor even delay, -starvation. Yet up to a certain point it can be used as a substitute -for proteid food. In the observation just referred to, protein might -be withdrawn at any stage, without disturbing nitrogenous equilibrium, -by substituting about 2 grammes of gelatin for every gramme of protein -withdrawn. It spares protein, although it does not take its place. -It is said that the minimum of protein necessary for the maintenance -of nitrogenous equilibrium may be reduced to about one-half by the -substitution of gelatin. This has been interpreted as indicating -that when we have reduced the oxidation of nitrogenous substance to -its smallest amount the nitrogen comes from two sources in about -equal proportions—(_a_) the bioplasm; (_b_) the food-proteins in -contact with it. It is inferred that gelatin, although it cannot be -built up into bioplasm, may take the place of proteins present in the -cell-juice. It appears to be impossible to starve the cell until it -consists of a bioplasm framework bathed in nitrogen-free cell-juice. -As the non-living proteins of cell-juice are removed, they are, if no -nitrogenous food be given, renewed by the breaking down of bioplasm. -When gelatin is absorbed, it takes its place in the cell-juice, and -the breaking down of bioplasm is no longer necessary. When digestion -is impaired, or vitality lowered, decoctions of meat which contain -extractives of low calorific value, useless, without synthesis (_cf._ -p. 144), for the purposes of tissue-repair, may to a certain extent -save tissue-waste. In the same way, gelatin, which is very rapidly -digested in the stomach, may cover the consumption of proteins, -although it cannot take their place. - -To sum up: The requisite daily income of energy must come from both -nitrogenous and non-nitrogenous food. It is impossible to reduce the -nitrogenous factor below a certain minimum. From this minimum upwards, -until a certain level is reached, every additional unit of nitrogenous -food enables the system to dispense with more than its equivalent of -non-nitrogenous food. When the proper balance of foods is attained, -there is no waste either of labour involved in digestion, or of labour -involved in metabolism and excretion. - -=The Liver.=—The liver weighs from 3 to 3½ pounds. It lies beneath -the diaphragm, more on the right side than on the left. Its posterior -border, which rests against the last three ribs (separated from them by -the diaphragm), is about 3 inches thick. Its anterior border is thin, -and keeps close along the line of the ribs. If the organ is neither -unduly enlarged nor squeezed out of its place owing to the use of a -tight corset, it does not project below the ribs, save where it crosses -the space between the rib-cartilages below the end of the breast-bone. - -[Illustration] - -[Illustration: FIG. 7.—DIAGRAM OF A LOBULE OF THE LIVER DIVIDED -VERTICALLY THROUGH ITS AXIS. - - In its centre is a space, the intralobular vein, - through which the blood falls into a branch of - the hepatic vein, on its way to the heart. An - interlobular branch of the portal vein, which - brings the blood from the digestive organs, pours - it by many smaller vessels over the surface of - the lobule. It filters into the lobule through - innumerable pseudo-capillary vessels, or spaces, - between the radiating columns of liver-cells. - Arterial blood is brought to the lobule by a - twig of the hepatic artery. Bile is drained away - from it by an affluent of the hepatic duct. In - the lower part of the diagram seven liver-cells - are shown, forming a divided column, magnified - about 300 diameters. The cells are loaded with - glycogen, and contain minute globules of fat. - Red blood-corpuscles and two leucocytes are seen - between the columns of liver-cells. One of the - leucocytes has ingested two blood-corpuscles.] - -The liver is supplied with blood by the hepatic artery. This vessel is -small for so large an organ. Although responsible for the nutrition -of the liver, it does not bring it the materials which are stored in -its cells. A much larger supply of blood is derived from the portal -vein, which breaks into capillaries, or, to speak more accurately, -into sinuses, or pseudo-capillaries, in the liver. The blood, whether -conveyed to the organ by the hepatic artery or by the portal vein, is -drained away by the hepatic veins. The plan of structure of the liver -is best understood when viewed with reference to the hepatic veins. -These, if traced backwards, are found to break up into fairly straight -vessels, each of which has a large number of lateral branches. Each -of the lateral branches is in the centre of a mass of cells, which -are packed round it in radiating columns. These masses, which have a -diameter of from 1 to 2 millimetres, are termed “lobules.” By mutual -pressure the lobules are squeezed into a pentagonal or hexagonal form. -The vein in the centre of the lobule is the intralobular vein. Turn now -to the portal vein; this is seen to break up into branches which run -between the lobules, and are therefore termed “interlobular veins.” -The branches of the hepatic artery also run between the lobules, as do -the radicles of the bile-duct. Each lobule is a liver in miniature. -The blood of the portal vein, which has come from the spleen, in -which red blood-corpuscles are destroyed, and from the stomach and -intestines, from which it has absorbed the products of digestion, is -poured over the surface of the lobule, to be filtered through into -its central intralobular vein. In its passage from the interlobular -veins (and branches of the hepatic artery) to the intralobular vein -the blood is confined to radiating capillary channels; but since these -merely prevent the escape of red blood-corpuscles without imposing any -restrictions upon the exudation of blood-plasma, the portal blood is to -all intents and purposes filtered through the columns of liver-cells. -The body-substance of the liver-cells is soft, destitute of envelope, -and capable, when free on the (warmed) stage of a microscope, of -changing in form, somewhat after the manner of a leucocyte. Such -cells have a great capacity for taking up the products of digestion. -Possibly they take up and store fats and proteins, but undoubtedly it -is their chief business to absorb sugar which accumulates as glycogen -in their substance. The glycogen is handed out to the hepatic blood -as required. The pigment which results from the disintegration of red -blood-corpuscles in the spleen is secreted, along with the bile-salts, -into minute channels, or canaliculi, which groove the flat surfaces of -adjacent liver-cells. These canaliculi converge to the bile-ducts. The -liver is therefore at the same time the storehouse of sugar which it -takes up from the blood when it is in excess, and passes out to the -blood when it is deficient, and an excretory organ which eliminates the -refuse of hæmoglobin. The iron derived from hæmoglobin it stores, and -returns to the blood. - -Another function of the liver has been referred to already. It is the -organ, and, as far as we know, the only organ, in which urea is made -in mammals, and uric acid in birds. If the liver of a freshly killed -animal be excised and a stream of blood passed through it, the blood -which leaves the organ contains urea. If a salt of ammonia, even the -carbonate, be added to the blood, it is converted by the liver into -urea. When a bird’s liver is made the subject of the same experiment, -uric acid appears instead of urea. The liver can convert many -nitrogenous substances into urea, but it seems probable that, normally, -the salt with which it has chiefly to deal is lactate of ammonia (_cf._ -p. 146). - -A few words must be added with regard to the functions of the liver -during prenatal life, obscure though these functions are. The liver -develops very early, and attains a relatively enormous size. At the -third month it weighs as much as the whole of the rest of the body -(_cf._ p. 34). Yet it cannot, one must suppose, have to do much of the -work which falls to its share in postnatal life. Food is reaching the -embryo in a constant stream, and not as the result of intermittent -meals. The embryo has no need to store glycogen; nor does its liver, -on analysis, yield much of this substance. In the embryo glycogen is -widely distributed throughout the tissues, not specially accumulated -in the liver. No digestion is occurring in the alimentary canal. Bile -is not needed to aid the hydrolysis and absorption of fats. A small -quantity of cholesterin and less lecithin is being eliminated, but not -much bile is needed to facilitate this process. - -A process which is proceeding at a great rate in the embryo, in -various situations, is the formation of red blood-corpuscles. In -this the liver takes part. But its duty in regard to blood-formation -is not sufficiently onerous to account for its size. The formation -of blood-corpuscles in the liver is observed with difficulty in -microscopic sections. It is therefore impossible to speak with -certainty as to the extent to which it is going on, but it may be -safely asserted that this function by itself cannot be held to account -for the great size of the organ in embryonic life. What other office it -fills at this period is a question which still awaits an answer. - -There is no more curious chapter in medical history than the story of -the views held at various periods with regard to the functions of the -liver. From being a mere mass of “parenchyma” serving as packing for -the abdominal viscera, it was elevated to the rank of Grand Purifier -of the “humours” of the body. Next, its excessive activity became -the cause of that form of dyspepsia known as “biliousness.” Still -later its want of activity was its chief vice. A “sluggish” liver was -held responsible for mental perversity and moral dulness. Calomel, -podophyllin, and other drugs were used as whips to stir it up; and the -increased secretions of the alimentary canal were mistaken for bile. -Poor patient organ! It is the still-room of the body, in which the -day’s supplies are stored, and from which they are served out, without -haste and without delay. And it makes urea. What else it does we have -yet to find out; and it is not impossible that when physiologists -have quite shaken themselves free from the explanations based upon -conjecture, which their predecessors have handed down, they may -discover that it has other duties which are not obvious, but of great -importance. - -FOOTNOTE: - -[2] Notice the distinction between filtration and dialysis. If water -containing soluble and insoluble substances is placed in a porous jar, -the water and the soluble substances pass through the pores of the jar. -The rate of flow varies as the pressure. If water containing soluble -substances is placed in a bladder, and the bladder is suspended in a -vessel of water, some of the substances which it may contain—white -of egg, for example—are non-diffusible; others will pass from the -water inside the bladder to the water which surrounds it. But every -diffusible substance has its own osmotic value. Some pass through the -membrane rapidly, soon establishing a condition of equilibrium in the -two fluids; others take a long time. Further, if the water on one side -of the membrane contains a certain percentage weight of a salt, the -molecules of which are large—say sulphate of magnesia—and the water -on the other side the same percentage weight of a salt of smaller -molecule—say chloride of sodium—water containing the salt of smaller -molecule will pass into the water containing the salt of heavier -molecule with a certain force. If, to start with, the two solutions -are at the same level, the level of the solution containing the less -diffusible salt, sulphate of magnesia, will at the commencement of -the experiment rise. It is therefore said to exert a greater osmotic -pressure than the more diffusible salt—chloride of sodium. Equilibrium -will not be established until the fluid on one side of the membrane -contains the same _number_ of molecules per unit volume as the fluid -on the other side. If the molecules of magnesic sulphate are pictured -as oranges, and the molecules of sodic chloride as nuts, it will be -understood that equilibrium is not established until the oranges and -nuts to the pint on one side equal in number the oranges and nuts -to the pint on the other. When these principles are applied to the -passage of water containing products of digestion through the wall of -the alimentary canal, it is evident that, if we understand all the -conditions, the process cannot be explained as merely an exhibition of -osmosis. Take the simplest illustration. When blood-serum is placed in -the intestine it is absorbed. If it were in a dialyser, there would -be equilibrium between the serum inside the intestine and the lymph -on the outside. There would be no osmosis. Or, again, supposing water -containing 2% of common salt is placed in the intestine, we find that -both salt and water pass through into the lymph. In a dialyser water -would pass from the lymph (which contains salts equal to about 0·9% -of sodic chloride) through the membrane into the stronger solution. -A salt-solution needs to be very concentrated to cause water to take -the reverse course through the intestinal wall, and so to act as a -purgative. When we study absorption from the alimentary canal, we find -that its wall, if it wants a salt or any other substance, sets the -laws of osmosis at defiance. If the salt is not wanted, the ordinary -phenomena of osmosis are exhibited. Sulphate of magnesia (Epsom salt) -would be deleterious if absorbed. The intestinal wall behaves towards -it like a dead membrane. The salt retains the water in which it is -dissolved: possibly water passes out of the lymph into the solution -of the salt. The contents of the intestines are in consequence unduly -liquid. The salt acts as a purge. - - - - -CHAPTER VII - -RESPIRATION - - -Life means change. We cannot imagine its continuance without -liberation of energy. Arrest of molecular activity is death. There -is no possibility of its revival. A watch that has stopped may be -started by shaking. On the cessation of molecular activity an animate -being becomes inanimate. Dead, it is liable to further chemical -changes. Bacteria invade it. They shake down its complex unstable -compounds into simple, stable, so-called “inorganic groups”; but the -ordered combination with oxygen, which constitutes living, can never -recommence. Putrefaction may be prevented by the exclusion of germs. -The inanimate mass of organic material may remain unchanged. Its return -to life would be a miracle. From time to time a frog is found enclosed -in old red sandstone, or some other rock which for countless ages has -lain beneath the surface. The cleft through which the frog entered -a few hours or days before it was discovered is overlooked. It is -supposed to have lived “in a state of suspended animation” for millions -of years. The fact that no frogs are to be found among the fossils of -the old red sandstone is an objection too casuistical to be seriously -entertained. The physiologist’s demand to know what has become of the -mountains of solid carbonic acid, water, and urea which the frog must -have produced during its unimaginable term of incarceration is regarded -as the natural expression of his prejudice—that life cannot continue -without molecular change. And he is bound to admit his inability to -prove that it cannot. Nevertheless, his experience that, whenever and -however he may, by experimental methods, arrest change, he loses the -power of causing it to recommence justifies him in his conviction that -life is change. Even a living seed is to his mind an organism whose -complex constituents are slowly—however slowly—setting free energy by -settling down the steps which lead to stability and ultimate, inanimate -rest; and the only source of this energy is combination with oxygen. In -the case of a seed the oxygen need not come from without. Seeds retain -their power of germination after long occlusion in nitrogen or other -neutral gases. But all the time some change is occurring, some internal -oxidation which resolves their less stable into more stable compounds. -Otherwise they would not be alive. A physiologist is willing to believe -that this may continue for ten years, fifteen years—for any period -that the botanist tells him that he has, under verifiable conditions, -observed that it does occur; but when he is told that peas taken from -the hand of an Egyptian mummy, or seeds set free by the spades of -navvies after a far longer burial, have been found to retain their -vitality, his credulity is stretched beyond breaking-point. He cannot -imagine a change so slow as to be spread over a geological period, -still without exhaustion of all changeable compounds. - -The term “respiration” has been extended until it is synonymous with -“oxidation.” At one time it was supposed that the combination of oxygen -with oxidizable substances occurred in the lungs. The lungs were the -hearth of the body, to which the blood brought fuel which burned -in the air drawn into them. When it was understood that the actual -combination of combustible material with oxygen occurs, not in the -lungs, but in the tissues, a somewhat illogical distinction was made -between “external respiration”—the combination of oxygen and blood -in the lungs—and “internal respiration”—the combination of oxygen -and tissue-substances. The terms are not comparable. The taking up of -oxygen by the hæmoglobin of blood is a different process to the union -of oxygen, after the hæmoglobin has parted with it, with the carbon, -hydrogen, and nitrogen of the tissue-substances. - -The blood-stream carries both fuel and oxygen to the tissues, but the -fuel while in the blood is not in an oxidizable condition. The foods -are taken up by the tissues. They enter into combination with their -protoplasm. Oxygen also combines with tissue-substances. In proportion -as the tissues are active oxidized compounds are split off. They fall -into the lymph, whence they are absorbed by the blood. If they are -nitrogenous compounds, they are carried to the liver, formed into urea, -and passed to the kidneys for elimination. If carbonic acid, it is -carried to the lungs for exhalation. The water formed by combination of -hydrogen and oxygen may escape from the lungs, the kidneys, or the skin. - -Two or three pounds of mixed foods are consumed every day. By the blood -they are carried to the tissues, whence an equivalent quantity of -waste—that is to say, oxidized—material is removed. About 1½ pounds -of oxygen is required to burn the day’s fuel. - -The problems of respiration are twofold. In the first place we have to -consider the physics and chemistry of the combination of hæmoglobin -with oxygen, and of the elimination of carbonic acid from the blood in -the lungs; secondly we have to explain the transference of oxygen from -hæmoglobin to the tissues, and the reception in the blood of carbonic -acid produced by the tissues. - -The apparatus by which air is brought into relation with the blood -consists of lungs and windpipe. At its upper end, where it joins the -portion of the alimentary tract common to deglutition and respiration, -the special respiratory tube is protected by the larynx. The nasal -chambers belong to the respiratory tract; the gullet, or pharynx, is -common to the two functions. - -The mucous membrane which lines the nose and windpipe is kept moist -in order that it may catch particles of dust drawn in with the air. -At the same time the nasal chambers serve to warm the air, and to add -moisture to it if it be too dry; for the lining epithelium of the lungs -would suffer if dry air came in contact with it. The wall-surface -of the nasal chambers is increased by the projection of folded and -chambered “turbinate bones.” The importance of warming the air before -it is admitted to the lungs is remarkably illustrated in the case of -certain sea-birds. The nasal chambers of the frigate-bird, and of some -other birds which resemble it, are exceptionally complicated. Since the -animal is devoid of any sense of smell, and the air which it breathes -must be nearly saturated with moisture, the only function which can be -assigned to these convoluted passages is that of warming inspired air. - -The larynx will be more minutely described when it is considered -as the organ of voice. In connection with respiration, it must be -regarded as primarily a valve which closes the entrance to the windpipe -during swallowing. It is overhung by a leaf-like appendage—the -epiglottis—formed of exceedingly elastic tissue. It was thought until -lately that the epiglottis drops over the aperture of the larynx when -food is passing down the gullet, and springs up again as soon as the -act of deglutition is over; but recent observations have shown that -during deglutition the epiglottis is pressed against the back of the -tongue, and that the closure of the larynx is effected by its own -sphincter muscles. The mucous membrane of the larynx is extremely -sensitive to stimulation by anything which would be prejudicial to the -tissue of the lungs. When its sensory nerve—the superior laryngeal—is -stimulated, the larynx closes. It is the agent in carrying out many -reflex actions, in which not the larynx only, but also the muscles of -the chest and diaphragm, take part. For example, it immediately stops -inspiration if an irritating vapour is present in the air. It stops -respiration if any foreign body, such as a crumb of bread or a drop -of water, touches the mucous membrane. When the trunk of the nerve is -stimulated by an electric current, respiration is inhibited. Further, -under suitable stimulation the nerve brings about respiratory movements -in which inspiration is gentle and expiration sudden, violent, -convulsive. Rib-muscles and diaphragm combine to produce a cough, which -ejects the noxious body. Again, its stimulation in a different way -probably helps to produce constriction of the smaller bronchi which -regulate the amount of air supplied to the air-cells of the lungs; -although this constriction may be largely due to a reflex which starts -in the air-cells. The epithelium of the air-cells has an immensely rich -supply of sensory nerves. In some persons this protective mechanism -is very prone to overact its part. A little dust or foul gas in the -air leads to such marked contraction of the bronchi that respiration -becomes very difficult. Such an exaggerated tendency to reflex action -constitutes the neurosis, asthma. In this malady the mechanism is -unduly sensitive. Very slight stimulation leads to a maximum discharge -of impulses to the muscular tissue of the bronchi. - -The trachea has a length of about 4 inches. It extends from the lower -edge of the cricoid cartilage, which is easily felt in the neck beneath -the thyroid cartilage (Adam’s apple), to the under side of the arch -of the aorta, where it divides into the right and left bronchi. The -epithelium which lines the trachea and bronchi is ciliated. The cilia -propel the secretion which accumulates on its surface upwards towards -the larynx. The wall of the windpipe is kept open by rings of cartilage -which are incomplete behind, where the trachea and œsophagus are in -contact. Rings and plates of cartilage also support the bronchi. The -bronchi divide and subdivide until their diameter is reduced to about -0·2 millimetre. Each bronchiole then breaks up into a bunch of very -thin-walled, elongated infundibula, club-shaped, and with a diameter -about five times that of the bronchiole with which they are connected. -They may be three or four times as long as they are broad. The wall -of an infundibulum is pitted like a piece of honeycomb into shallow -chambers—the air-cells or alveoli. - -The walls of the air-chambers, or alveoli, are formed of a membrane -upon which is spread a network of capillary bloodvessels. The -air-chambers are so closely packed together that a common wall -separates one chamber from the next adjoining. Minute bloodvessels -pierce the partitions which separate the chambers, appearing now on -one side of the wall, now on the other. The air-chambers are lined by -thin epithelial scales or tiles. The blood in the capillary vessels -is separated from the air in the air-chambers by the wall of the -capillary; by a lymph-space, probably rather potential than actual; and -by the epithelial tiles. This covering suffices to prevent the escape -both of red corpuscles and of plasma, yet offers very little resistance -to the passage of gases from the blood into the air, and from the air -into the blood. - -Leucocytes make their way between the tiles, and creep over their -internal surfaces, searching for cell débris or foreign matter. -Anything that they find they carry to the clumps of lymphoid tissue -which occur in the outer wall of the bronchi. In a town-dweller, -leucocytes are found in these lymph-thickets, charged with particles of -soot. They show droplets of fat and other evidences of degeneration. At -other spots are to be seen little collections of soot which have been -left behind after the dissolution of the leucocytes which brought them -there. - -It is not possible to make anything like an accurate estimate of the -number of alveoli in the lungs; 725,000,000 is a figure arrived at by -measuring the average cubic capacity of an alveolus, and comparing it -with the total cubic capacity of the lungs. Each alveolus supports some -forty or fifty capillary vessels. The superficial area of vascular -membrane exposed is placed at 90 square metres, or about 100 times the -area of the skin. Figures such as these convey very little meaning, but -they help one to realize the magnitude of the provision made for the -aeration of the blood. - -Pneumonia is a condition in which the lining of the air-chambers is -inflamed, usually, possibly always, owing to the entrance of bacteria. -Lymph exudes through the walls of the alveoli. Epithelial scales flake -off. Pus cells (dead leucocytes) accumulate in the air-chambers. -Respiration is curtailed, and dyspnœa results. After a time, if the -case progresses favourably, “resolution,” as it is technically termed, -begins to occur. The exuded substances are either expectorated or -absorbed, and the lung-tissue returns to a normal condition. - -Here a few words may be devoted to respiratory sounds. _Spirare_ -means to sigh. Breathing received the name by which it is known in -physiology from the sound which accompanies the exit of air from -the nostrils. Since the introduction of auscultation as a means of -ascertaining the condition of the lungs, other sounds, not heard until -the ear or a stethoscope is placed against the chest, have acquired -great importance. These sounds, termed “murmurs,” may be divided -into two classes. (_a_) When the ear is placed against the windpipe, -or in the middle of the back between the shoulder-blades, a murmur -is audible, due to the movement of air through the larynx. If the -larynx, the trachea, or the bronchi contain mucus, it is a harsh, -rough, bubbling, or crackling sound. It accompanies both inspiration -and expiration. (_b_) A softer, more delicate murmur is heard when -the ear is placed against the front or the side of the chest. This is -the vesicular or pulmonary murmur. It is heard during inspiration, -and is due to the passage of air out of the smallest bronchi into -the more spacious infundibula in which they end. These two kinds of -murmur must be rigidly distinguished—the laryngeal murmur, heard in -situations in which no lung-tissue intervenes between the ear and the -great tracheal or bronchial tubes; and the pulmonary murmur, heard -over all regions where the bronchi are buried in lung. Healthy lung is -as bad a conductor of sound as a sponge or a wad of cotton-wool. The -laryngeal murmur is inaudible in regions in which lung lies beneath the -chest-wall. It would be far beyond the scope of this book to attempt to -describe the very varied alterations in the chest-sounds which may be -produced by disease. The student would do well to familiarize himself -with the nature of the sounds which are heard in health, and the -situations in which they are heard, in order that he may be able, in -abnormal conditions, to recognize that something is wrong. - -The chief departures from the normal may be grouped under the following -heads: (1) The pulmonary murmur may lose its soft, smooth, sighing -character owing to inflammation of the alveoli and infundibula. It may -be as loud in expiration as in inspiration. Only a practised ear can -estimate the significance of these changes. (2) The laryngeal murmur -may be reinforced by “râles”—a convenient term for supplementary -sounds. The source of such râles may be a cold in the chest, -laryngitis, or bronchitis of various degrees. (3) The laryngeal murmur -may be heard in situations in which lung intervenes between the ear -and the larger bronchial tubes. This can be due only to the lung being -in an abnormal condition as a conductor of sound. Instead of being -as spongy as well-made Vienna bread, its air-spaces are filled with -solid or fluid deposit. It is as firm as dough. To such a condition -it attains at the height of pneumonia—a stage termed “hepatization” -because in section it looks like liver rather than lung. - -Breathing is the enlargement and diminution of the chest, which causes -air to be drawn into and expressed from the lungs. The windpipe being -open, the air inside the lungs is, of course, at the same pressure as -the atmosphere. Expansion of the chest results in the equal expansion -of the lungs. Since there is no air-space between the outer surface -of the lungs and the inner surface of the chest-wall, the lungs -cannot separate from the chest-wall when it expands. But the lungs -contain elastic tissue always slightly on the stretch. If the chest be -punctured, and air admitted between the chest-wall and the lungs, the -lungs collapse. The expiratory movement, the contraction of the chest, -is due to the elasticity of the lungs. This tendency on the part of the -lungs to contract is sufficient in quiet respiration to restore the -chest to its usual size after inspiration, and thus to expel air. The -lungs are held open owing to the negative pressure in the space which -separates them from the chest-wall. This negative pressure has a most -important relation to their permeability by air. Imagine the condition -reversed. Picture a lung into which air is forced by a muscular pump. -After each stroke of the pump the lung would collapse. Its finest -tubes and their dilated terminations could be maintained as open -spaces, between the strokes of the pump, only by giving a considerable -thickness and firmness to their walls. Such a substantial structure -would be unfavourable to an interchange of gases between the blood and -the air. The reverse of this condition is found in Nature. The lung is -stretched from without. Its tissue, delicate as crêpe, cannot collapse -even at the end of the deepest expiration. - -The ribs are united by intercostal muscles, disposed in two sheets. -The fibres of the external intercostals are directed downwards and -forwards, those of the internal intercostals downwards and backwards. -In tranquil respiration the chest is enlarged by the external -intercostal muscles, which raise the ribs, and the diaphragmatic -muscle, which renders peripheral portions of the diaphragm flat. -The rôle of the internal intercostal muscles is a subject still -under discussion. For the most part, physiologists regard them as -accessory to expiration, but some hold that they combine with the -external intercostals in raising the ribs and twisting them outwards -during inspiration. The diaphragm is a partition which separates the -thoracic from the abdominal cavity. It is in the form of a vault. The -central portion of the dome is membranous, its margins muscular. Its -membranous centre is in contact with the pericardium, which encloses -the heart. The level of this part is therefore fixed, except in -forced inspiration, when it descends slightly. It constitutes a fixed -plane for the muscles of the diaphragm, which are attached below -to the vertebral column and the ribs. When the muscles contract in -inspiration, the curvature of the marginal portions of the diaphragm -is diminished, and the chest-cavity consequently enlarged. During -expiration the space between the muscle of the diaphragm and the -chest-wall closes up, and the lower border of the lung slips out of it. - -There is a marked difference in the relative extent of the costal and -diaphragmatic movements in men and women. In women respiration is -chiefly costal; in men it is chiefly diaphragmatic. In men the abdomen -moves forwards, as the diaphragm descends in tranquil breathing; in -women the chest rises. Men who wish, for the purposes of athletics, or -singing, or public speaking, to retain the power of making the most of -their chest-capacity are wise in not allowing themselves to fall into -the habit of lazy, abdominal breathing. - -[Illustration: - - FIG. 8.—THE DIAPHRAGM AND ORGANS IN CONTACT WITH IT—A, - IN EXPIRATION; B, AT THE END OF A DEEP INSPIRATION. - TRANSVERSE VERTICAL SECTIONS IN THE LINE OF THE ARMPIT. - - A, At the end of an ordinary expiration the lung does - not extend below the upper border of the eighth - rib. From this level to the middle or lower border - of the tenth rib the two layers of the pleura - covering respectively the inner wall of the chest - and the upper surface of the diaphragm are in - contact. B, When the lung is distended with air it - occupies the whole of the pleural cavity.] - -When additional pressure is required, when respiration is forced, -various external muscles attached to the spinal column, the -shoulder-blades, and the clavicles, as well as the muscles of the -abdomen, come into play. - -The chest is lined and the lungs covered by a serous membrane—the -pleura. Normally there is only just sufficient lymph in the space -between the visceral layer of the pleura which invests the lungs and -the parietal layer which lines the chest-wall to prevent friction -during respiration. When the pleura is inflamed, one layer of the -membrane rubs against the other. In the early or “dry” stage of -pleurisy, the physician recognizes this condition by the friction-sound -which he hears on placing his stethoscope against the chest. In a -later stage lymph (pleuritic fluid) is poured out. It accumulates in -the lower part of the chest, and is recognized by the absence of the -resonant note which, under normal conditions, is given out by the chest -when percussed. - -The lungs are not compressed during expiration; they are not squeezed, -as a pair of bellows or a sponge may be squeezed, emptying it of its -contents. At the end of tranquil expiration the lungs still contain -about 3½ litres of air. At the top of tranquil inspiration the volume -of their contents does not exceed 4 litres. It is evident, therefore, -that air is not drawn into and driven out from the air-chambers by -the movements of respiration. The tide of air does not extend far -beyond the ends of the bronchi. The gases in the air-chambers are -exchanged with the fresh air drawn into the infundibula by diffusion. -The composition of the air which is in contact with the bloodvessels -is constant. It is about 4 per cent. poorer in oxygen and 3 per cent. -richer in carbonic acid than the outside air. - -Of the air drawn into the windpipe during an inspiration, about -one-third returns to the open with the following expiration; two-thirds -remains in the lungs. If, therefore, the air taken in at each tide -equals one-seventh of the quantity already in the lungs, and if of this -one-seventh two-thirds remains, each alveolus renews about one-tenth of -its air. Its contents are completely changed in ten respirations. - -Fresh air is composed of 21 per cent. oxygen, 79 per cent. nitrogen, -and a trace (0·04 per cent.) of carbonic acid. Forced by a syringe -through lime-water, fresh air does not produce any appreciable -milkiness, whereas air breathed through a tube into lime-water renders -it turbid owing to the formation of carbonate of lime. Carbonic acid -(CO₂) occupies the same volume as its oxygen (O₂) would occupy if free. -The oxygen which breathed air has lost slightly exceeds in amount the -carbonic acid which it has gained in exchange. The difference is due -to the retention of some of the oxygen for the purpose of uniting with -hydrogen to form water, and of forming urea. The proportion between -carbonic acid gained and oxygen lost, CO₂/O₂ is termed the “respiratory -quotient.” Its value varies, of course, with diet. In a herbivorous -animal, whose food consists of carbohydrates, it departs but little -from unity; in a carnivore, which eats fat and nitrogen-containing -food, it is about 0·8. - -The respiratory exchange is very much smaller in cold-blooded animals -than in animals which maintain the temperature of the body at a fixed -level. In warm-blooded animals it rises as the temperature falls, falls -as it rises, the increased oxidation warming the body, the diminished -oxidation allowing it to cool; whereas in cold-blooded animals it -increases as the temperature rises, owing to the greater activity -induced by warmth, and falls as the temperature falls. - -The respiratory exchange is increased by muscular activity. If the -amounts of oxygen absorbed and carbonic acid given out are measured -while a man is at rest, and again while he is doing hard physical work, -it is found that during work the respiratory exchange is twice as great -as during rest. During periods of starvation the respiratory exchange -remains unaltered, since heat has to be constantly produced if the -temperature of the body is to be kept from falling. - -Since the purpose of respiration is to give to the blood the -opportunity of renewing its supply of oxygen, and of getting rid of the -carbonic acid with which it is charged, it might be supposed that the -respiratory exchange would be increased, so far as the intake of oxygen -is concerned, by breathing oxygen gas instead of air; but it appears -that under normal conditions nothing is gained. When an animal is -breathing air, its blood takes up all the oxygen that it wants—all the -oxygen, that is to say, for which its tissues are asking. Offering it -pure oxygen in place of mixed oxygen and nitrogen does not induce it to -take up more. The hæmoglobin is almost saturated with oxygen when the -blood leaves the lungs under ordinary conditions. In certain diseases -of the lungs, however, in which the blood becomes unduly venous, the -respiration of oxygen may be beneficial; but even in these cases the -results are disappointing, because the system is suffering much less -from deficiency of oxygen than from accumulation of carbonic acid. -Substituting oxygen for air does not facilitate the escape of carbonic -acid. - -=The nervous mechanism of respiration= has been the subject of -much investigation and of many experiments, without, it must be -confessed, the development of a quite complete or satisfactory theory. -Respiration is a rhythmic process. About seventeen times in a minute -the intercostal and diaphragmatic muscles contract. Inspiration is -immediately followed by expiration, the falling movement being due, as -already explained, to the elasticity of the lungs, which are stretched -during inspiration. A slight pause intervenes between the end of -expiration and the commencement of the next inspiratory movement. -Tranquil respiration is a succession of reflex inspiratory movements, -the depth of which varies according to the needs of the body—that -is to say, according to the condition of the blood. If the need for -aeration of the blood becomes urgent, the depth of inspiration is -increased, and expiration also becomes an active movement, certain -muscles, especially those of the abdomen, being called into play. In -this condition two sets of reflex actions alternate. A large number -of nerves are concerned even in tranquil respiration. If a man in -falling “breaks his back” at the junction of the cervical and thoracic -regions, costal respiration ceases. The series of intercostal nerves -which arises from the dorsal spinal cord below the level at which -it is injured are thrown out of action. Diaphragmatic respiration -still continues, because the nerve of the diaphragm, the phrenic, -arises from cervical roots. The lungs are supplied by the vagus -nerve. This nerve joins the medulla oblongata as one of a group of -three—glosso-pharyngeal, vagus, and spinal accessory—which by a large -number of roots enter the groove between the olive and the restiform -body. The vagus is the channel along which afferent impulses from the -lungs enter the medulla. Such impulses call for respiratory movements. -Cutting both vagi, however, does not put an end to respiration. -Inspiratory movements continue, but they are much deeper and separated -by much longer pauses. Such a form of respiration is inefficient. The -blood is not properly aerated. The animal falls into a condition of -dyspnœa, which ends in death. When the central end of the cut vagus is -stimulated, the movements become more natural. Clearly, the respiratory -reflex is not dependent upon the vagus, since it continues after the -nerve is cut, although the impulses which pass up this nerve regulate -its rhythm. They govern the length of the inspiratory movements, cut -them short at the right moment, and secure their succession at proper -intervals. - -The transfer of afferent impulses into efferent channels occurs in -the medulla oblongata. Long ago it was found that if the brain above -this level be removed, part by part, respiration is not interfered -with until the medulla oblongata is injured. When a cut is made into -the floor of the fourth ventricle not far to one side of the middle -line, the respiratory movements on that side of the body cease. If the -injury be bilateral, even though very limited in extent, respiration -stops. This spot was therefore spoken of as the “respiratory centre.” -Flourens, who first discovered it, believed that it was a mere spot. -He gave to it the fanciful name of _nœud vital_. It is the place at -which the afferent nerves which call for respiration are brought -into connection with all the various motor nerves which bring about -the respiratory movements of nostrils, larynx, chest, and diaphragm. -Possibly the knife in Flourens’ incision divides the tract of fibres -which distributes afferent impulses, but whether the junction be a -defined tract or no, injury to this region of the medulla throws the -nervous mechanism of respiration out of gear. At this particular spot -lies the “centre” for respiration—the one part of the nervous system -which must be intact if the movements of respiration are to be carried -out. There is no reason for thinking that respiratory impulses are -generated at this spot. It is a centre in the same sense in which Crewe -is a centre for distributing the goods of Lancashire and other parts -of England to North Wales. The use of the term “nerve-centre” has been -very much abused. Centres were supposed to be collections of cells, -each group of which had some prerogative of initiation. Reasoning -from the analogy of human institutions, it was thought necessary that -the nervous system should be organized into departments severally -responsible for the administration of the activities of certain sets -of muscles: one centre controlled respiration, another the beat of -the heart, another deglutition. The centres were dependent one on -another; each regulated lower centres, and was governed by those above -it, in this bureaucratic scheme. We know nothing of any function of -nerve-cells other than that of transmitting impulses. All that we know -about nerve-cells is that they place afferent and efferent routes in -communication, and interpose resistance into nerve-circuits. Every -nerve-cell of the grey matter of the brain and spinal cord gives -off processes which ramify. The ultimate twigs into which a branch -divides are in connection with other sets of twigs derived from the -end-branchings of nerve-fibres or processes of other nerve-cells. A -nerve-fibre is but the axis-cylinder process of a nerve-cell. Impulses -encounter resistance in passing along the neuro-fibrillæ (_cf._ Fig. -22) contained in the twig-connections of the ramifying processes of -nerve-cells. There is no reason for supposing that anything like -the same resistance is offered to the passage of impulses along the -fibrillæ where they lie within the stout branches of the cell-processes -or within the body of the cell. It is easy to make a pictorial -representation of such a mechanism. Imagine a model of the stem of a -tree made by binding together a large number of wires; its branches as -containing small groups of wires; the ultimate twigs as separate wires. -Carry wires from the roots of one tree to the branches of another. -Trees so constructed might be taken as representing nerve-cells. We -have not as yet succeeded in demonstrating the isolated neuro-fibrillæ -as they pass over from the end-twigs of a nerve-fibre to the end-twigs -of a nerve-cell branch, but we have abundant reason for believing -that they do so pass, and that the resistance to the passage of a -nerve-impulse is interposed in this neutral or junctional zone. This -resistance has to be overcome. It is overcome by the summation of -impulses. All nerve-impulses are vibratory. The first vibrations may -fail to get through; but if the vibrations continue, they exert a -cumulative effect. After a time they overcome the resistance; sensory -impulses flow through the centre into motor channels. In this way we -endeavour to explain the rhythmic discharge through the respiratory and -other centres. It has not been found possible to determine the source -of all the afferent impulses which reach the centre. Respiration -continues after all accessible nerves have been cut, including even -the posterior roots of the cervical nerves. Probably it is a mistake -to look for definite afferent channels in the medulla and the rest of -the brain. All parts of the body need aerated blood. From all parts, -including nerve-tissue itself, arises the demand for respiration. -Possibly nerve-centres have the power, as it were, of storing impulses, -and discharging them after the stream of fresh arrivals has ceased to -flow. They may acquire a habit. - -The resistance in the centre is profoundly affected by the condition -of the blood. As the blood becomes more venous, impulses pass across -the nerve connections with ever-increasing force. Kept in the first -instance to definite channels, they spread as the centre becomes more -excitable farther and farther afield, reaching one group of muscles -after another, and pressing them into the service of respiration. -When, in dyspnœa, every muscle which can in any way help the movements -of the chest is doing its best, others which are useless for this -purpose receive the reflected impulses and join in, producing general -convulsions. The increased activity of the respiratory centre which -is produced by slight venosity of the blood is shown in the rapid -and deep inspirations which are caused by violent exercise. Perhaps -it is justifiable to go a step farther, and to assert that there is -something in blood which has been rendered venous by muscular activity -which is specially exciting to the respiratory centre. If the blood -from a limb be prevented from returning to the general circulation, -by compressing or tying its great veins, and if the muscles of the -limb be strongly stimulated by an electric current, their activity, -so long as the passage through the veins is blocked, has no influence -upon respiration. But, on relaxation of the pressure on the veins, -respiration may become twice as deep and twice as frequent as it was -before the muscles were stimulated, although the limb is now in a -condition of perfect rest. - -What is the special action of the vagus nerve? Its superior laryngeal -branch checks inspiration and induces expiration, as already said. The -impulses which pass up its main trunk bring about ordered movements. -They are not dependent for their generation upon the condition of the -blood in the lungs. When the chest is filled with nitrogen, inspiration -and expiration alternate in the usual way, although the blood is -growing steadily more venous. The failure of inspiration to bring -about aeration of the blood does not lead to a prolongation of the -inspiratory effort. Inspiration is cut off and expiration established -in regular sequence. In performing “artificial respiration” (_cf._ p. -184) for the purpose of saving life, in cases in which respiration -has ceased owing to the lungs being filled with water, or for other -reasons, the chest is enlarged by raising the arms above the head, -and diminished by pressing the elbows against the sides. Enlargement -promotes a tendency to expiration, compression a tendency to a natural -inspiratory effort. Evidently there is a connection between the -movements of the chest and the stimulation of the respiratory centre. -If respiration is being carried on artificially, by forcing air from -a bellows into the trachea, the nostrils dilate as the chest is -distended, and contract as it is emptied, so long as the vagus nerve -is intact, just as they do in normal respiration. This shows that, -when the chest is emptied, a message is sent through to the nucleus -of origin of the nerve which supplies the dilator muscles of the -nostril. When the lungs are full, a message calls upon the nostrils to -contract. The only factor which is common to pressing in and pulling -out the ribs, and filling and exhausting the lungs with a bellows, is -the alteration in the form of the lungs which is produced by the two -methods. It is impossible to resist the conclusion that the stretching -of the tissue of the lungs stimulates the nerve-endings of the vagus. -The impulses thus induced automatically stop inspiration, and lead to -an expiratory effort. - -There are many indications that the nervous mechanism of respiration -is a double one, certain stimuli inducing expiration, with inhibition -of inspiration, others inhibiting expiration and inducing inspiration. -There are, however, many difficulties in the way of formulating a -satisfactory theory of the relation of these antagonistic actions. -We may frequently observe indications of such an antagonism between -the two phases of the respiratory mechanism. Cold water dashed on the -back of the head (when the head is being shampooed) induces a long -inspiration with inhibition of expiration. A blow in the pit of the -stomach “knocks all the wind out of a man.” Expiration is prolonged -until the lungs are unusually empty, and yet the victim of the blow -feels as if he would never again be able to draw breath. - -=Modified Respiratory Movements.=—The object of coughing is to expel -foreign matter from the windpipe or larynx; of sneezing, to clear -the nose. The former action consists of a long deep inspiration; -the closure of the glottis; a forcible expiration. The blast of air -encountering a closed glottis acquires considerable pressure. When -the resistance of the glottis is overcome, the blast rushes through, -carrying with it mucus or bread-crumb, or whatever the substance -may be which irritated the endings of the superior laryngeal nerve. -In sneezing, the back of the tongue is thrust against the palate, -closing the aperture of the fauces. Inspiration is prolonged. A strong -expiration follows. The blast rushes through the nasal cavities. This -reflex is usually provoked by a tickling of the endings of the fifth -nerve in the nasal mucous membrane. It is also caused in many persons, -through the optic nerve, by a bright light; an apparently purposeless -reflex about which we shall have something more to say in a subsequent -chapter. Laughing and crying are modified respiratory movements as -useless, so far as any immediate purpose is accomplished, as sneezing -in response to a bright light. As means of expressing emotions they -have been cultivated by the human race. Possibly a case for crying -might be made out on physiological grounds. Under certain circumstances -it relieves a feeling of distress which, while it lasts, is detrimental -to the proper functions of the body. Laughing undoubtedly is -beneficial. The rapid movements of the chest quicken the circulation. -The shaking of the midriff favours the discharge of digestive -secretions, accelerates the movements of the alimentary canal, and -generally is beneficial to digestion. But “laugh and grow fat” is not -necessarily the order of cause and effect. An efficient digestion and -a good capacity for assimilation lead to a sense of _bien-être_ which -predisposes to a merry view of life. - -Yawning is a deep inspiration with open mouth and larynx. It commences -usually at the end of a normal inspiration, a slight pause being -followed by further inspiration, deep and prolonged. Its commencement -seems to be due to impulses generated by the relaxation of the tone of -the muscle which holds up the lower jaw. The masseter goes off duty for -a moment, allowing the jaw to fall. A reflex contraction of the muscles -which open the mouth immediately follows. Muscles of the neck and head -also come into play. Not improbably the yawn ends in a general stretch. -If the origin of this reflex is obscure, its usefulness is marked. The -circulation is quickened, the blood is changed, nervous system and -muscles again become alert. - -“Apnœa” is the condition of arrested respiration. If a man about to -dive into the water breathe deeply and rapidly half a dozen times, he -abolishes for a while the desire to breathe. One is naturally inclined -to explain this as due to a surplus of oxygen taken into the blood, but -a moment’s reflection shows that this cannot be the cause. In the first -place, as we have already pointed out, the blood which leaves the lungs -in tranquil respiration is very nearly saturated with oxygen. It can -take up but little more. Again, the deep inspirations do not change the -air in the air-chambers; time is required for the renewal by diffusion -of their gaseous contents. It is improbable that the constitution of -the air in the alveoli is sensibly altered by a few deep breaths. -Probably the explanation is to be found in the effect upon the -nerve-centre of distention of the chest. Stretching the nerve-endings -of the vagus in the lungs inhibits inspiration. If the stimulation be -excessive, inspiration is inhibited for a considerable time. That this -is the right theory of apnœa is proved by repeatedly inflating the -lungs of an anæsthetized animal with a pair of bellows. The same arrest -of inspiration is induced whether the lungs are inflated with air or -with a neutral gas, such as nitrogen, so long as the vagus nerve is -intact. If this be cut, inflation with a neutral gas no longer produces -apnœa. - -“Dyspnœa” is the term applied to the complex conditions and movements -which result from deficient aeration of the blood, or, rather, from -the distribution of insufficiently aerated blood to the centres in -the medulla oblongata. The blood of the rest of the body may be in a -satisfactory condition, but if, owing to ligature of the carotid and -vertebral arteries or other causes, the blood supplied to the brain be -inadequate to its proper nutrition, the phenomena of dyspnœa are as -marked as they are when air is prevented from entering the lungs. That -the excitability of the nerve-centres in the brain is greatly increased -when this organ is supplied with venous blood, and that their tendency -to transmit impulses which call for respiration is consequently -exaggerated, is remarkably shown by the following experiment: Two -rabbits—A. and B.—are placed under the influence of chloroform. Their -carotid arteries are cut, and a crossed circulation established by -connecting the proximal ends of A.’s arteries with the distal ends of -B.’s, and _vice versa_. The head of each rabbit is now supplied with -blood from the heart of the other, the rest of its body by blood from -its own heart. A.’s chest is now opened, so that its lungs collapse and -cease to take part in respiration. The animal continues to make the -movements of respiration in a tranquil manner, whereas B. is thrown -into violent dyspnœa. The animal whose brain is receiving aerated blood -remains normal, notwithstanding the fact that its lungs and the rest -of its body are poisoned with venous blood. The animal whose brain is -supplied with venous blood becomes dyspnœic, although its lungs and -body are receiving pure arterial blood. - -There is a regular sequence in the phenomena of dyspnœa leading up -to the final stage termed “asphyxia.” If the trachea be suddenly -blocked, so that no air can pass, the respiratory movements at once -become deeper and more rapid. This condition is termed “hyperpnœa.” -In a comparatively few seconds the system appears, as it were, to -find out that inspiration is not needed. Expiratory efforts begin to -preponderate. They increase in violence. All accessory muscles are -brought into play. The cry for air is heard even by muscles which -cannot help. Muscles of the limbs contract, although their contraction -has no effect upon the capacity of the chest. Every expiratory effort -is accompanied by convulsions of a flexor type. At the end of two -minutes there is usually a sudden change. Attempts at expiration cease. -Slow, deep, infrequent inspirations take their place, accompanied by -convulsions of extensor muscles. Pupils are widely dilated, mouth -open, head thrown back. The subject is absolutely insensitive to every -kind of stimulus. The pulse shows a high arterial tension. The beating -of the heart is slow and strong. In about four minutes from the time -at which the windpipe was blocked respiratory movements cease. The -arterial tension falls. The heart’s action grows rapidly weaker, -although for two or three minutes longer it may still continue to -flicker. Recovery is possible until it finally gives up. After death -the right side of the heart is found gorged with blood, the left side -empty, showing that the heart had been unable to force the blood -through the capillaries of the lungs. - -Under all ordinary conditions the sequence of phenomena of asphyxia -is the same—a stage of exaggerated breathing (hyperpnœa), a stage -marked by the co-operation of muscles which are not called into -action in tranquil breathing (dyspnœa), followed by the condition of -asphyxia properly so termed. An animal whose supply of fresh air is -cut off passes through these three stages, whether it be enclosed in -a small space or in a very large one. It must, however, be noted that -in asphyxia several factors combine in varying degrees. Carbonic acid -is in excess in the blood, oxygen deficient. The nervous mechanism -which regulates respiratory movements is thrown out of gear. Motor -and inhibitory impulses are in conflict. It is important, if these -complex phenomena are to be analysed, that one factor only should be -altered at any given time. For example, carbonic acid may be allowed -to increase in the air while a constant oxygen tension is maintained. -Under these circumstances the dyspnœic contractions are much less -marked. No convulsions follow. The paralysing action of carbonic -acid predominates. Anæsthesia passes into complete unconsciousness. -Death is tranquil. And this, speaking generally, is what happens in -disease of the lungs. Asphyxia comes on slowly. The supply of oxygen -is undiminished, but carbonic acid accumulates in the blood, acting as -a narcotic poison which lowers the excitability of the nervous system, -suspends consciousness, and slowly brings the vital activities to a -standstill. - -In cases of drowning, when the lungs are filled with water, the -resistance to the passage of blood through their capillary vessels is -greater than it is when they are still filled with air. The heart is -sooner beaten in its effort to drive the blood through them. Usually it -stops in about four minutes. Yet it is difficult to say for how long -after a person has been immersed in water it may be still possible to -resuscitate him. Reports vary, owing in large measure to uncertainty -as to the exact time at which the immersed person sank and his lungs -filled with water. It is a wise precept that artificial respiration -should be tried in every case, without waiting a single instant to -ascertain whether the heart still beats. The first thing to do is to -empty the chest of water. Then place the subject on his back. Kneel on -the ground behind his head. Grasp an arm just below the elbow, in each -hand. Draw the arms up above the patient’s head, so that the pectoral -and other muscles drag on the ribs, enlarging the chest; then lower -them, and press them into the sides. This must be done with the natural -rhythm of respiration, and not more frequently than twenty times in a -minute. It is well if an assistant draws the tongue forward, to give -free admission to air. Presumably the slight exchange of air brought -about by mechanical expansion and compression of the chest favours -the passage of blood through the capillaries of the lungs; but the -real object of artificial respiration is to stretch the endings of the -vagus nerve, and in this way to originate impulses which will call -the respiratory centre into action. Perhaps it may not be superfluous -to point out that the failure of the pulse must not be taken as an -indication that the heart has ceased to beat. Owing to the obstruction -to the circulation through the lungs, the left side of the heart is -almost empty. Very little blood is pumped into the aorta. None reaches -the wrist. - -=Exchange of Gases in the Lungs.=—In the lungs each red corpuscle -takes from the air a charge of oxygen which it carries to the tissues. -In the tissues the plasma of the blood receives carbonic acid, which -escapes from it when it reaches the lungs. Water dissolves oxygen and -carbonic acid. Towards animals and plants which live in it, water plays -the same rôle as the atmosphere towards dwellers on land. The quantity -of a gas which will dissolve in water is proportional to the pressure -to which it is subjected. If water were the circulating fluid, some -oxygen would enter it in the lungs; some carbonic acid would be taken -up in the tissues and liberated in the lungs. But it is clear that -the small quantity of fluid which the vascular system will hold would -be incapable of serving as an efficient medium of exchange between -the tissues and the lungs. When a given quantity of venous blood is -agitated with air, five times as much oxygen is taken up as the blood -could carry if the gas were simply dissolved. Both oxygen and carbonic -acid are held by the blood in chemical combination. - -The condition in which oxygen is carried was discovered in 1864 (_cf._ -p. 68). From all time it had been noticed that the blood which flows -from a vein is darker and of a more purple tint than the blood which -spurts out of a cut artery. Shortly before the date mentioned above, -the spectroscope had begun to be used to distinguish more accurately -than the eye can do the groups of rays which a coloured solution -transmits. The colour of a ray of light depends upon its wave-length. -The light of the sun, when its rays are sorted by a prism, according -to their wave-lengths, shows all colours from the long waves of red to -the short rays of violet, with certain gaps. At intervals where rays -are missing, the spectrum exhibits dark bands—Fraunhofer’s lines. -The colour of a solution is measured by placing a flat-sided vessel -containing it in the course of a beam of the sun’s light, on its way -to a prism. When the rays are spread out, it is observed that certain -groups have been absorbed by the coloured fluid. The colour of the -solution is due to the rays which it transmits. It had been pointed -out in 1862 that blood diluted with water absorbs parts of each end -of the spectrum, and also two groups of rays lying between the fixed -bands of Fraunhofer which spectroscopists had labelled D and E. Stokes -observed that this is true only of arterial blood. Venous blood absorbs -a broad band in this part of the spectrum in place of the two narrow -bands. He showed that, “like indigo, it is capable of existing in two -states of oxidation, distinguishable by a difference of colour and a -fundamental difference in the action on the spectrum. It may be made -to pass from the more to the less oxidized condition by the action of -suitable reducing agents, and recovers its oxygen by absorption from -the air.” The reducing agents of which Stokes made use were alkaline -solutions of ferrous sulphate or of stannous chloride containing some -citric or tartaric acid. These sub-salts of iron and tin very rapidly -absorb oxygen from the air or from any chemical substance which parts -with it readily. With these solutions Stokes replaced the tissues. -He abstracted the oxygen from the oxyhæmoglobin; then, shaking the -solution of reduced hæmoglobin with air, he reproduced the action which -occurs in the lungs. - -If the hand be held between a spectroscope and the source of light, -in such a position that the beam passes through the thin tissue of -two fingers where they are in contact, the spectrum of oxyhæmoglobin -is obtained. If now the circulation through the fingers be impeded by -putting strong indiarubber bands round them, the blood becomes venous, -and the two narrow bands of oxyhæmoglobin give place to the broad band -of reduced hæmoglobin. - -Although very soluble, hæmoglobin may be obtained in crystals, the form -of which varies in different animals. When obtained from human blood, -the crystals are rhombic prisms; from the guinea-pig, tetrahedra; from -the squirrel, hexagonal plates. Yet it is unlikely that the hæmoglobin -of one animal differs chemically from that of another in any proper -sense of the term. Probably the form of the crystals depends upon -the amount of water of crystallization. The apparent polymorphism of -hæmoglobin may be associated with the great size of its molecules -(_cf._ p. 66). - -Even when in the crystalline form, hæmoglobin can take up oxygen; but -the difficulties which attend its purification and crystallization -render somewhat uncertain the amount of oxygen which a gramme of -crystallized hæmoglobin can absorb. In solution, 1 gramme can take up -1·34 cubic centimetres. The whole of the hæmoglobin of the body would, -therefore, if it were all in the oxidized condition, hold about 4 -grammes of oxygen. - -It is not with oxygen alone that hæmoglobin can combine. It can -absorb the same volume of carbonic oxide or of nitric oxide gas. Both -of these gases it holds more firmly than oxygen. Neither carbonic -oxide-hæmoglobin nor nitric oxide-hæmoglobin is of any use to the -tissues. If the blood becomes charged with the fumes of carbonic oxide -(CO) given off by a coke-fire, this gas proves extremely poisonous. -The blood does not lose it in its circuit through the body, nor is it -exchanged for oxygen in the lungs. - -The instability of the compound of hæmoglobin and oxygen is shown under -the air-pump. The pressure of air in the open equals 760 millimetres of -mercury. When the pressure falls to about 250 millimetres, the oxygen -is rapidly given off. This is a matter of considerable interest in its -bearing upon the question of the height to which it is possible for a -human being to ascend. An animal placed in a chamber from which the air -is pumped dies when the pressure falls to 250 millimetres of mercury. -It has been ascertained that a man under the same circumstances can -bear with impunity a reduction to 300 millimetres. How much lower -must the pressure fall before it proves fatal? Of three aeronauts -who ascended in the balloon _Zenith_ to a height of 8,600 metres -(26,500 feet), two died. The third, Tissandier, became unconscious, -but recovered during the descent. The pressure of the atmosphere at -such a height is 260 millimetres. The greatest mountain heights yet -attained are 23,100 feet (Aconcagua, in the Southern Andes), reached by -Fitzgerald, and 23,400 feet (Trisul, in the Garhwal Himalayas), reached -by Dr. Longstaff and his companions. The pressure at this height was -320 millimetres. From these facts it is clear that mountaineers have -just about reached the limit; but since they have not as yet mounted to -a height at which the barometric pressure is less than 300 millimetres, -it is possible that slightly higher mountains are still waiting to -be conquered. At 23,000 feet the oxygen contained in arterial blood -does not exceed 10 volumes per cent. (_cf._ p. 190). It is therefore -about half the normal amount. Hence the breathlessness and sense of -feebleness experienced by climbers. The least exertion leads to the -consumption of all the circulating oxygen. But since the effects of -want of oxygen are felt at altitudes much lower than those to which -reference has been made, it is clear that the question cannot be -regarded as simply one of physics. The nervous system suffers when an -attempt is made to do work with a deficient oxygen-supply. Violent -headache and nausea attack most persons long before a level is reached -at which the combination of hæmoglobin with oxygen ceases to be -possible. The occurrence of this “mountain sickness” reminds us that -we must not take for granted that the nervous system will continue -to do its work right up to the altitude at which oxyhæmoglobin is -dissociated. Still, the figures show that, apart from these nervous -symptoms, which disappear after a time, no serious disturbance occurs -even though the atmospheric pressure be but little higher than the -absolute minimum at which hæmoglobin combines with oxygen. - -The capacity of the blood for rapidly absorbing oxygen in the lungs -and readily parting with it to the tissues is easily and completely -explained by the property which hæmoglobin possesses of forming an -unstable compound with this gas. - -It is quite otherwise with regard to the liberation of carbonic acid. -The problems presented by the solution of this gas in blood and its -elimination in the lungs are difficult to solve. Less than one-tenth -of the volume of carbonic acid which can be extracted from blood by -the air-pump is simply in solution. The remainder is in loose chemical -combination, the chief agents in holding it being the alkaline -carbonates which the plasma contains. With an excess of carbonic acid -they form acid carbonates, which give up carbonic acid and again become -normal carbonates in the lungs. About one-third of the carbonic acid -is, however, held by the blood-corpuscles—partly in virtue of their -alkaline carbonates and phosphates, partly in combination with their -globulin. The affinity of these several vehicles for carbonic acid is -sufficient to enable them to take it from the lymph, and to hold it -while the blood is in the veins. When they reach the capillaries of the -lungs, they part with their burden of carbonic acid to the air. It is -in connection with this renunciation that certain difficulties remain -to be explained. The carbonic acid is given up with greater readiness -than our knowledge of the chemistry of the compounds into which it -enters in the blood would lead us to expect. - -Why does oxygen enter blood as it circulates through the lungs, and -carbonic acid leave it? We have referred to the immense surface which -the lungs expose to air. If a soap-bubble be filled with a mixture of -oxygen, nitrogen, and carbonic acid, and if the oxygen be in smaller -proportion, and the carbonic acid be in greater proportion, than in -the air of the room, oxygen will enter the bubble, and carbonic acid -will leave it, by diffusion. If, instead of filling a bubble with gas, -we fill a bladder with water charged with carbonic acid, but destitute -of dissolved oxygen, a similar exchange with the gases of the air will -take place. It is merely a question of “gaseous tension.” The tension -of the gases in the lungs is measured by passing a small tube down -the trachea, and along one of the two chief bronchi until it becomes -blocked in a bronchus just large enough to admit it. Respiration is -carried on under normal conditions in the remainder of the lung; but -in the lobe which the catheter blocks diffusion from stationary air -to tidal is no longer allowed. At the same time, since the circulation -is not interfered with, the gases in the blood of the occluded lobe of -the lung are not in markedly different proportions from those in the -air-chambers of other parts. If at the end of a sufficient interval -the air of the occluded lobe is drawn off and its gases measured, -their tensions can be compared with the tensions of gases in specimens -of arterial and of venous blood. If from 10 c.c. of fluid 1 c.c. of -gas can be removed by the air-pump, the volume of gas dissolved is -10 per cent. of the volume of the fluid which dissolved it. Commonly -this is written “10 volumes per cent.” To ascertain experimentally -the tension of a particular gas in a particular fluid when dissolved -to the amount of 10 volumes per cent. at the ordinary pressure of the -atmosphere and at the temperature of the body, it would be necessary -to place it in an open vessel in air containing a sufficient admixture -of the gas to prevent its escape from the fluid. Suppose that it were -found that, when the fluid containing the dissolved gas was placed in -air mixed with the same gas to the extent of one-tenth of its volume, -the fluid neither gave up gas nor absorbed more gas, the tension -of the gas would be equal to one-tenth of an atmosphere. Since the -pressure of the atmosphere equals 760 millimetres of mercury, the -tension of the dissolved gas would be 76 millimetres. If more gas were -added to the air, more would dissolve in the fluid; if some of the -gas were removed from the air, gas would escape from the fluid. Gas -passes from the medium in which its tension is high to the medium in -which its tension is low. The tension of carbonic acid in tissues, -particularly in muscles and glands, is higher than in lymph; in lymph -higher than in blood; in blood higher than in air. Hence it passes by -these several stages from the tissues in which it is formed to the air -in the lungs. Much ingenuity has been devoted to perfecting methods -for the determination of the tension of carbonic acid in lymph and -in venous blood. Frequently results have been obtained which seemed -opposed to the doctrine that carbonic acid progresses from one medium -to another in accordance with the law of pressures; but such perplexing -results were probably due either to imperfections in method or to the -establishment of abnormal physiological conditions during the course -of the observations. When, for example, it was found that the tension -in lymph was less than the tension in blood, the specimen of lymph -examined was probably not in the same condition as the lymph in the -tissue-spaces where the exchange occurs. The experimenter in such -a case was in error in supposing that the specimen of lymph which -he examined contained as much carbonic acid as did the lymph in the -tissue-spaces from which the blood which he compared with it received -its supply of this gas. - -We have already given the figures for the composition of the air in the -air-chambers of the lungs. The figures commonly accepted as correct for -the percentages of the several gases in the blood are, at 0° C. and 760 -millimetres of mercury pressure: - - Carbonic - Oxygen. anhydride. Nitrogen. - In 100 vol. of arterial blood 20 39 1-2 - In 100 vol. of venous blood 8-12 46 1-2 - -This table shows the gain in oxygen and the loss in carbonic acid which -results from the passage of blood through the capillaries of the lungs. -The aerated blood returned to the heart by the pulmonary veins contains -8 to 12 volumes per cent. more oxygen, and about 7 volumes per cent. -less carbonic acid, than the blood which the pulmonary artery carries -to the lungs. - -As to the physics of this exchange, the air in the recesses of the -lungs contains about 16·36 per cent. of oxygen, and an amount of -carbonic acid variously estimated at from 2·57 per cent. to 3·84 per -cent. Of the 760 millimetres of mercury which the atmosphere holds up -in a barometric tube, the oxygen in the alveoli of the lungs supports -(760 × 16·36)/100 = 124·33 millimetres; the carbonic acid, at the lower -figure quoted (2·57 per cent.), 19·5 millimetres. - -The tension of gases in arterial blood is ascertained by opening an -artery into a closed vessel which contains nitrogen mixed with oxygen -and carbonic acid at about the tensions which it is computed that they -have in the blood. If the amounts of these gases are exactly right, -no exchange occurs between the blood and the mixture of gases. The -mean of many observations made in this way by various physiologists -is, for oxygen in the blood 72·2 millimetres mercury pressure, for -carbonic acid 20·5 millimetres mercury pressure. At a glance it is -seen that, since the tension of oxygen in the blood never exceeds 72 -millimetres, whereas its tension in pulmonary air never falls beneath -124 millimetres, there is no difficulty in accounting for its passage -from air to blood. The position is somewhat otherwise with regard to -carbonic acid. Aeration continues in the lungs until the tension of -this gas in the blood returning to the heart does not exceed 20·5 -millimetres; whereas the tension in pulmonary air, even accepting -the lowest figure obtained by experimental means, is as high as 19·5 -millimetres. This leaves a very small margin of pressure to account for -the escape—and it is undoubtedly a rapid escape—of carbonic acid from -blood as it circulates through the lungs. As was said regarding the -fixation of carbonic acid in the blood, it is somewhat doubtful whether -the problem has been completely solved. - -The carbonic acid exhaled contains all the carbon of the digestible -food, with the exception of a comparatively small quantity given off in -urea. It amounts to about 900 grammes per diem. - -How are we to determine the quantity of air which an individual -requires? We can but make the general statement that it must be -sufficient to dilute the carbonic acid exhaled to an extent which -precludes poisoning. It is impossible to fix a limit. Breathing becomes -embarrassed, and frontal headache and other symptoms make themselves -felt when 10 per cent. of pure carbonic acid is mixed with air. Even in -so large a proportion as this, carbonic acid is not fatal to life. Yet -an atmosphere in which there is present a hundredth part of this amount -of carbonic acid, produced by respiration, is extremely injurious to -health under the ordinary conditions in which people live. It may be -asserted, therefore, that under ordinary conditions 0·1 per cent. is -the extreme limit for wholesome living. But again we are obliged to add -that air contaminated to this extent is not under all circumstances -injurious to health. The explorers on the recent Antarctic Expedition -were obliged at times to sleep three men in one sleeping-bag, with -the aperture of the bag tightly closed. The atmosphere must have been -heavily laden with carbonic acid. Dr. Wilson assures us that it was -impossible to keep a pipe alight inside the bag. Not that any man so -placed would desire, one would imagine, to add the combustion-products -of tobacco to those given off from the lungs! The survival of the -explorers proves that it is impossible to fix a limit of safety even -for the carbonic acid in air vitiated by respiration. It is, however, -a matter of common observation that air which is moist and warm, -owing to respiration, and tainted with the odours of humanity, is -extremely prejudicial to those who live in it. Such an atmosphere -is a favourable medium for the conveyance of germs, whether of the -common cold or of a more virulent type. At one time it was supposed -that the volatile emanations which can be condensed, along with water, -by hanging a vessel of ice to the ceiling of a crowded room, were -actively poisonous; but this statement has not been confirmed by recent -research. It is unnecessary to call any such evidence in support of the -thesis that human beings thrive better in fresh air than in foul. The -admirable results achieved by the “fresh air cure” show that there is -no degree of vitiation which can be pronounced innocuous. Nevertheless, -public opinion demands that sanitarians should give some figure as a -guide. Commonly they fix the maximum of carbonic acid compatible with -health at 0·06 per cent., the quantity of carbonic acid being taken -as the measure of all impurities present. An adult exhales about 0·6 -cubic foot of CO₂ per hour. Fresh air already contains about 0·04 per -cent. If, therefore, the percentage is not to rise higher than 0·06 -per cent., each adult must be supplied with 3,000 cubic feet of air -per hour. With good ventilation air may be changed four times an hour, -and therefore 800 cubic feet is regarded as sufficient space for each -occupant of a room. The figure may pass. It is a reasonable basis from -which to calculate the packing capacity of a dormitory. So long as a -man has 800 cubic feet of air to himself, he may safely feel that he -has room to stretch his lungs. Dwelling on this figure may make him -feel uncomfortable when he finds himself in a railway carriage, seated -five on a side, with the windows closed. In the theatre or in church -he may doubt whether he has all the fresh air to which his humanity -entitles him. But, as a philosopher rather than as a physiologist, he -reflects that, whether on the Antarctic icecap in a sleeping-bag or -standing on a summit in the Alps, he takes all that he can get, for -fresh air is one of the few good things of which one can never have -enough. - -=Tissue Respiration.=—A frog will live for seventeen hours in an -atmosphere of nitrogen. Under these circumstances it is clearly -impossible for it to take up oxygen, yet for several hours it gives -off as much carbonic acid as it would do if it were living in air. -Such an observation as this proves that oxidation does not occur in -the lungs, but deeper in the body. At one time the blood was regarded -as the seat of oxidation; the products formed by the splitting up of -proteins in the tissues were supposed to be passed into the blood, -where they came in contact with the oxygen carried by hæmoglobin. A -certain amount of oxidation does take place in the blood, as in all -other tissues, for blood is a living tissue and needs to respire. But -the oxidation which occurs in the blood is small in amount as compared -with that in the organs which the vessels traverse. Muscle and other -tissues detached from the body and free from blood give off carbonic -acid. It is possible to wash the blood out of the vessels of a frog and -to replace it with a solution of salt. In an atmosphere of oxygen such -a “saline frog” lives for a day or two, taking in the same quantity of -oxygen and giving off the same quantity of carbonic acid as a normal -frog. The oxygen is chiefly absorbed through the skin, the carbonic -acid discharged from the lung. This experiment shows that blood is not -essential for oxidation. Oxidations do not occur in the salt solution -with which blood is replaced. Taking all the evidence together, it -seems to be safe to conclude that the tissues absorb the oxygen which -the oxyhæmoglobin brings into their neighbourhood, and that they have -some capacity of storing it. A piece of detached muscle which gives off -carbonic acid in an atmosphere of nitrogen would appear to be holding a -store of oxygen, much as hæmoglobin holds it. The proof is not quite so -definite as might be desired; but we are probably justified in holding -the belief that the main part of the respiratory exchange occurs in -the tissues. Lymph dissolves oxygen which it obtains from the blood. -The tissues take it from lymph. Tissues set free carbonic acid which -lymph dissolves. Its tension being higher than in blood, carbonic acid -diffuses from lymph, through the walls of the capillary vessels, into -blood, from which it passes into the air in the lungs. - - - - -CHAPTER VIII - -EXCRETION - - -Many things enter into the alimentary canal. If an analysis were -made of a day’s food and drink, from the cup of tea on waking to the -cocoa or other potion which is regarded as a necessary preliminary -to settling for the night, it would be found that a great variety of -substances were included in the food or taken as adjuvants to food. -All these things, differing widely in chemical constitution, must -leave the body. Some are not digested. They do not, properly speaking, -enter into the diet. Such are the cellulose of vegetables, especially -skins, husks, woody fibres; elastic fibres of meat; horny substances, -etc. The quantity varies greatly, according to the nature of the diet. -About 2 ounces (weighed dry) is the average. With this indigestible -refuse is included undigested food, if the diet be excessive, and a -variety of substances secreted by the liver, such as cholesterin and -bile-pigment, some residues of the secretions of the alimentary canal, -and products of bacteric fermentations. All food which is digested and -absorbed is oxidized. It leaves the body by the lungs, the kidneys, -or the skin. Foods, as already stated, are classified as proteins, -carbohydrates, and fats. The chief excreta are carbonic acid, water, -and urea. Carbonic acid makes its exit from the lungs; water from the -lungs, the kidneys, and the skin; urea from the kidneys. The three -great groups of foods and the three great groups of excreta overshadow -in amount all the other substances which pass through the system. A -balance-sheet in which proteins, carbohydrates, and fats appear on one -side, carbonic acid, water, and urea on the other, is substantially -correct. The energy which is set free by burning in a calorimeter -the items entered on the debit side, after deducting that yielded by -burning the urea (carbonic acid and water are incapable of further -oxidation), gives a day’s income. Other constituents of the diet are so -small in quantity as to be negligible in making up the body’s accounts. -The chemical changes which they undergo add practically nothing to its -capacity for work. Yet some of them are essential to the maintenance of -health. Of such are common salt (sodic chloride), alkaline and earthy -carbonates, sulphur, phosphorus, etc. These things, together with some -products of action of the bacteria in the alimentary canal, the final -stage of hæmoglobin, imperfectly oxidized nitrogenous substances, and -other soluble substances which enter with, or are formed from the -food, are removed by the kidneys. We speak of the elimination of waste -products, as excretion. Not that there is any physiological distinction -between excretion and secretion. Both terms refer to the selection or -production and the discharge of materials by cells. If the product -discharged has a useful function to perform—if it be a digestive -ferment, for example—it is said to be secreted. If it is of no further -use to the economy, we say that it is excreted—got rid of. In some -cases either term is equally appropriate. The sebum prepared by the -sebaceous glands is useful as a lubricant of the skin. It is thrown -off. We may speak of the glands as either secreting or as excreting -this fatty substance. - -=The Kidney.=—From worms upwards, all animals possess organs for the -removal of waste products in solution. This statement might, indeed, -be widened so as to include animals even lower than worms. All animals -which have a cœlomic cavity—a space between the alimentary canal -and the body-wall—have organs for the removal of soluble waste. The -segmental organs of worms are obviously the same organs as the kidneys -of mammals; the latter are distinguished from their prototypes by -greater concentration of structure and specialization of function. The -kidney is the oldest of organs, if its antiquity be estimated as the -length of time during which it has had a form practically identical -with that which it now presents. The lungs are of late appearance in -the animal scale. Alimentary canal, heart, brain, have passed through -many transformations. The kidney assumed its permanent form very far -back in the history of the animal kingdom. The most primitive animal -which has a digestive cavity, and vessels in which the products of -digestion circulate, needs an organ which provides for the overflow -from the body-fluids of all substances which are injurious or effete. - -The kidney is an aggregation of long urinary tubules. The head of -each tubule is dilated into a globular capsule, into which a tuft of -bloodvessels depends. This is the sink into which the waste-water -of the blood drips. The long urinary tubules are lined with cells -well qualified by form and constitution to search the blood in the -capillaries which border them, for substances which, not being easily -diffusible, have to be forcibly dragged from it and added to the water -trickling down the pipe which connects the rain-water head with the -sewer. The hydrostatic conditions of this apparatus—the provision for -greater or less flow of blood through the tufts (glomeruli) which hang -in the capsules, and for longer or shorter exposure of the blood to -the purifying activity of the epithelium of the renal tubules—will be -described after a very brief account has been given of the structure of -the organ. - -The outer border of the kidney is convex, its inner border concave. The -concavity is termed the “hilus.” The central depression of the hilus is -embraced by the expanded end of the ureter—the tube which carries the -secretion of the kidney to the bladder. The renal artery and the renal -nerves enter, and the renal vein leaves, the kidney at the hilus. - -If a kidney be split longitudinally, it will be noticed that its outer -part, the cortex, is darker in colour than its inner part, the medulla -(Fig. 9). The glomeruli already referred to occur in the cortex. The -medulla is occupied by radiating tubules, collected into groups. Those -of each group converge towards a common duct. From twelve to eighteen -ducts open into the expanded end of the ureter, each at the apex of -a pyramid. If the section of the kidney be examined with a lens, it -will be seen that narrow rays from the medulla extend into the cortex. -The cortex is therefore made up of interdigitating pyramids of dark -substance, consisting of glomeruli and the contorted tubules, about to -be described, and of lighter substance, consisting of straight tubules -continuous with those of the medulla. - -[Illustration: FIG. 9.—THE UPPER END OF THE LEFT KIDNEY, VERTICALLY -DIVIDED, AND MAGNIFIED. - - It is invested by a capsule with which, at the - hilus, the dilated end of the ureter blends. A - portion of a papilla (the end of a pyramid) is - shown projecting into one of the calices into - which the ureter dilates. The peripheral portion - of the kidney containing glomeruli and contorted - tubes is termed its cortex, the central portion - medulla. At A is shown a single urinary tubule. - Commencing at the third glomerulus, it winds in - the cortex, descends into the medulla, turns in - a loop of Henle, again winds in the cortex, and - ends in a collecting tube, which joins a duct. The - arrangement of the bloodvessels is shown at B. A - straight artery and a straight vein lie side by - side. The artery gives branches to the glomeruli. - The venules from the glomeruli again divide into - capillaries, which supply the contorted tubes - and loops of Henle. The ducts are supplied by - long arterial capillaries. C shows the structure - (magnified) of a glomerular tuft of capillary - vessels, invested by a capsule which closes into - a contorted tube, _ct_; _dH_, a descending limb; - _aH_, an ascending limb of a loop of Henle; _d_, - a duct.] - -The urinary tubules are the separate pieces of apparatus of which -the kidney consists. The problems connected with a single tubule are -therefore the problems of the kidney as a whole. These structures are -all exactly alike. The description of any one of them applies to all. -Each begins as a capsule containing a glomerulus. The wall of the -bulb—which is merely a thin basement membrane covered by epithelial -scales—is involuted by the tuft of bloodvessels. The vessels do not -penetrate its capsule. Between the tessellated epithelium which covers -the tuft and the similar epithelium which lines the capsule there is a -space communicating by a narrow aperture with the next portion of the -tubule—termed its “contorted” part, because it is twisted about like a -tangled thread in the cortex of the kidney. The contorted tubule is of -relatively large calibre. The cells which line it are irregular in form -and indistinct in outline. The basal half of each cell, between its -nucleus and the basement membrane, is vertically striated, or “rodded,” -as it is usually termed. Such an arrangement of the protoplasm of a -cell is commonly associated with a habit of absorbing fluid. It would -seem to indicate in this case that the cells take water and various -substances dissolved in water from the direction of the basement -membrane. After a time the contorted portion of the tubule, although -still sinuous, becomes more nearly straight—the “spiral portion”—and -assumes a radial direction. In the zone between the cortex and the -medulla, the spiral portion tapers into an exceedingly slender tubule -which, after running some distance in the direction of the hilus, turns -back again towards the cortex, making a loop, known as the “loop of -Henle.” The ascending limb of this loop is of larger calibre than the -descending limb. The descending limb is lined by flattened epithelium, -each cell so thin that (in microscopic sections as ordinarily -prepared) its nucleus bulges into the lumen of the tube. The cells -of the ascending limb are more nearly cubical in form. On reaching -the cortex, the tubule again becomes contorted. The second contorted -portion narrows into a “collecting portion,” which joins a ductule. The -ductules unite together, until at last a single duct is formed which -opens at the apex of a pyramid. The cells of the ductules are cubical -or columnar. Their cell-substance is clear, whereas that of the cells -lining other parts of the tubule is cloudy in appearance. - -Such a tubule, viewed as a hydrostatic mechanism, presents three -portions, evidently fitted for different functions: (1) The glomerulus -is an apparatus which allows of the rapid exudation of water from -blood. (2) The contorted portions of the tubule present the appearance -of a secreting mechanism. The large soft, cloudy cells which line them -are eminently fitted to take from the blood, or rather from the lymph -which fills the tissue-spaces which intervene between the walls of the -capillary bloodvessels and tubules, the various substances which they -excrete. (3) The loop of Henle is a remarkable piece of apparatus, the -purpose of which has been a subject of much controversy. Looking at it -from the point of view of hydrostatics, it seems safe to conclude, from -its extremely narrow bore, that it raises the pressure of the fluid -in the glomerulus and first contorted portion; but it may have other -functions also. - -A consideration of the arrangement of the bloodvessels of the kidney -bears out the conclusion that the secreting apparatus is divisible into -at least two separate portions, possibly into three. The glomeruli -are supplied by short and relatively wide arterioles. Each arteriole -breaks up, as soon as it enters the capsule, into a bunch of capillary -vessels, which, in the same abrupt manner, reunite to form a venule. -On leaving the capsule, this little vein behaves in a fashion for -which the only parallel is to be found in the portal system of the -liver. Instead of uniting with a larger vein, it again breaks up -into capillary vessels, which supply the contorted tubules and loops -of Henle. The medulla of the kidney is supplied by long arterial -capillaries of the usual type. The short arterioles of the glomeruli -are controlled by nerves which, constricting them, or allowing them -to dilate—possibly by actively causing them to dilate—rapidly -diminish or increase the amount of blood passing through their tufts -of capillary vessels. Here, therefore, is a mechanism by which the -glomeruli can be suddenly flushed with blood—a condition favourable -to exudation into the urinary tubules. The interposition of a second -set of capillaries prevents this sudden flushing from unduly disturbing -the pressure in the vascular system as a whole. In the renal-portal -capillaries of the kidney the blood-pressure is fairly constant and, -presumably, low. The use of the term “renal-portal” is justifiable, not -only on the ground that the vessels of the kidney behave like those of -the portal system of the liver, but also owing to the very significant -fact that in fishes and amphibia the kidney actually has a double -blood-supply. In such an animal as the frog the glomeruli are supplied -with arterial, the tubules with venous, blood. The glomeruli receive -branches from the renal artery, the tubules from a portal system -derived from veins of the abdomen and hind-legs. - -Sir William Bowman, who in 1842 gave the first detailed description -of the microscopic structure of the kidney, concluded that, whereas -“the tubes and their plexus of capillaries are probably the parts -concerned in the secretion of that portion of the urine to which its -characteristic properties are due (the urea, lithic acid, etc.), the -Malpighian bodies [_i.e._, the glomeruli] may be an apparatus destined -to separate from the blood the watery portion.” - -All physiologists are in accord in regarding the glomeruli as the -principal seat of exudation. There is great diversity of view as to the -function of the tubules. In 1844 Ludwig advanced the opinion that all -the constituents of the urine pass through the glomeruli in a large -excess of water, and that in the course of the tubules this excess of -water is reabsorbed. This theory was based, among other considerations, -upon the extreme thinness of the epithelium which covers the glomerular -tufts; he judged that water would filter through it very readily. A -large amount of experimental work has been directed to the solution -of these two problems—viz., (1) Do urea and other similar substances -pass through the glomeruli? (2) Is water returned from the tubules to -the venous system? Our views as to the functions of the kidney as a -whole will not be greatly influenced by the answers that may eventually -be given to these questions; yet their discussion is of very great -interest, owing to the nature of the evidence which may be marshalled -on either side. - -There is, perhaps, no other organ in the body the problems with regard -to which seem to be so nearly plain questions of hydrostatics. It is -easy to make a model of a urinary tubule and its blood-supply. If -such a model were shown to a sanitary engineer, and he were asked to -explain the working of the drainage system of the body, and especially -to answer the two questions which we have propounded, he would say -that there could be no doubt as to the part of it through which most -water enters the tube, the glomerulus. He could give no opinion as -to whether urea, uric acid, and other substances of a like nature, -accompany the water until he had tried the experiment of separating -blood from water containing the inorganic salts of urine by a permeable -membrane—the blood being at such a pressure as the physiologist told -him he might expect it to have in renal arterioles, the water at such -a pressure as he might expect it to have at the upper end of a urinary -tubule. He would find that urea, and still more uric acid, is very -reluctant to pass through the membrane. Again, when asked whether -water, in which urea and other things were dissolved, would leave the -tubule—say from the loop of Henle—to pass back into the blood, he -would repeat his experiment with a membrane. This time he would allow -the urine and the blood to be at the same pressure (or, possibly, would -assign a higher pressure to the former), and he would dilute the urine -to make the conditions agree with those which Ludwig supposed to exist; -but his experiment would prove to him that, unless the urine were very -dilute indeed, water would still tend to pass into it from the blood, -and not _vice versa_. And here it may be remarked that the results of -these experiments might have been predicted by calculation. When Ludwig -advanced his theory, osmosis was a mysterious phenomenon. Its laws -have since been accurately ascertained. Given the molecular weights of -bodies in solution and their degree of concentration, the direction -in which they will pass through a membrane can be predicted. The -force with which water will tend to pass from one solution to another -can be calculated. Urine as secreted contains far more urea, sodic -chloride, and other salts than blood. It has a much higher degree of -concentration. The concentration of blood is 0·55; that of urine, 1·85. -Water passes from a less concentrated to a more concentrated solution, -not _vice versa_. As a solution of a problem in hydrostatics Ludwig’s -hypothesis is untenable. - -=Osmosis.=—Cells of all kinds, both vegetable and animal, are -limited, or surrounded by a layer of cell-substance which is firmer -than, and probably different in constitution from, the substance in -the interior of the cell. This outer layer is a living membrane. The -nutrition and growth of the cell are dependent upon the capacity of its -limiting membrane for regulating the ingress and egress of water and -of substances dissolved in water. The phenomena of osmosis—that is to -say, of the passage of water and of solutions through membranes—are -of such high importance in relation to the life of the tissues that -it may be permissible to make a further digression for the purpose -of describing them (_cf._ pp. 40, 128). A very simple apparatus will -suffice to exhibit a phenomenon which will give an idea of the meaning -of osmosis. If the top of a glass funnel, covered with a piece of -bladder, so fastened to its edge as to make it water-tight, be fixed -in an inverted position in a glass vessel, the glass vessel filled -with water, and the funnel filled to the same level with a solution -of sugar, it will soon be evident that water is passing through the -membrane into the funnel. The level of the sugar-solution will rise in -the tube of the funnel. If, instead of water outside the funnel and -sugar-solution inside it, a strong solution of sugar be placed in the -funnel and a weaker solution outside it, water will leave the weaker -for the stronger solution, and sugar the stronger solution for the -weaker. If some of the solution in the funnel be removed from time to -time so that the pressure in it is kept down to the same level as that -outside it, water will continue to enter through the membrane and sugar -to leave the contents of the funnel until the concentration of sugar -is the same on the two sides. The fluids will then be of identical -composition, and therefore isosmotic. In the further consideration of -the phenomena of osmosis, a distinction must be made between permeable -and hemipermeable membranes. Suppose in the first instance that a -permeable membrane is used. Let it be so placed as to separate two -watery solutions of different constitution, yet of the same osmotic -pressure. By their being of the same osmotic pressure is meant that -they are of the same molecular concentration. The liquid A contains -certain salts in solution; but the liquid B may contain the same salts -in quite different proportions. It so happens, however, that the salts -are so balanced that the total tension of the salts in A is equal -to the total tension of the salts in B. At first there may be some -change in level in the two liquids, owing to differences in rates of -diffusion through the membrane of the various salts which they contain; -but after a time the levels of the two liquids will be the same. To -outward appearance, nothing will have happened. Nevertheless, if the -experiment has been continued for a sufficient length of time, it will -be found that great changes have occurred in the constitution of the -two liquids. At the commencement, although their total tensions were -equal, the proportions in which the various salts were distributed in -A, and therefore their partial tensions, were very different to their -proportions and partial tensions in B. At the end of the experiment -each of the several salts is equally divided between A and B, supposing -the volume of A to equal that of B. This experiment shows that the -molecules of substances in solution are free to move. They behave like -gases. Gases diffuse through a membrane until their partial tensions -are the same in the two spaces which the membrane separates. The æther -in which physicists picture gases as dissolved offers no resistance to -the migration of their molecules; neither does the solvent—water, for -example—prevent the movement of salts which are distributed through it. - -One other illustration of the phenomena of osmosis will suffice to -give an idea of the laws by which they are governed. In the case just -cited the membrane was permeable to all the salts in solution. When the -phenomena of osmosis were first investigated, a distinction was drawn -between substances which will pass through membranes—crystalloids—and -substances which cannot pass—colloids. We have already had occasion -to note that, whereas albumin is a colloid which does not diffuse, its -hydrate, peptone, is a crystalloid which does. The term “crystalloid” -indicates that substances which can be crystallized are diffusible. -Substances which are diffusible are therefore allied to those which -crystallize. The nature of the membrane used to test diffusibility was -not at first taken into account. Now a distinction is drawn between -membranes which are permeable to all diffusible substances, and -membranes which are permeable to the solvent, but impermeable to the -substances which it dissolves. The latter are termed “hemipermeable.” -Imagine now that water is separated from a solution of sugar by a -membrane which stops sugar, but is permeable to water. Water will pass -through the membrane into the solution of sugar. The level of the -solution will rise. Pressure will be needed, and a very considerable -pressure, to prevent its rising—to prevent endosmosis, that is to -say. The force needed to resist osmosis is directly proportional to -the degree of concentration of the solution. If the solution contain 1 -per cent. of sugar, a pressure of 500 millimetres of mercury is needed; -if it contain 2 per cent., a pressure of 1,000 millimetres; if 6 per -cent., of 3,000 millimetres. - -In the next experiment separate two solutions, A and B, by a -hemipermeable membrane. Let A contain one salt only—X; let B contain -several salts—X, Y, Z. Water will pass from A to B, or _vice versa_, -unless the osmotic pressure of the salts which the solutions contain -is the same. The osmotic pressure will be found to be the same if -the total number of molecules dissolved in A equals the total number -of molecules dissolved in B. If in A there be N molecules of X (per -unit volume), and if in B there be nX, n′Y, n″Z, the osmotic pressure -will be the same provided n + n′ + n″ = N. This, it will be seen, -is a very different matter from equality of percentage composition. -Some molecules are light; others are heavy. The percentage weight of -X + Y + Z in B may be very different from the percentage weight of -X in A. To estimate the osmotic pressure of a mixed solution, it is -not sufficient to add together the percentages of the various salts -which it contains. “Concentration,” in the sense in which it was used -in regard to blood and urine, refers to the number of molecules of -dissolved substances in a given volume, not to their weight. - -It would be undesirable to attempt in this place to enter upon the -theory of osmosis. Enough has been said to suggest to the reader that -he should, when endeavouring to apply its laws to the explanation -of physiological phenomena, bear the following facts in mind: Some -membranes are permeable to water and to the crystalloids which it -dissolves; others, although permeable to water, are impermeable -to substances in solution. Some substances are diffusible through -permeable membranes; others are not. Osmosis of water occurs from the -solution of lower to the solution of higher concentration. Diffusion of -crystalloids is their escape, owing to their own molecular movements, -from a situation in which they are denser to a situation in which they -are less dense. It must be added, however, that various circumstances -prevent the reduction of the laws of osmosis to simple terms—the -tendency of salts to dissociate when in solution, their bases and -acids acting as independent “ions,” is an example of the complications -which produce apparent departures from these laws. It must further be -added, and with emphasis, that, important though it be that anyone who -attempts to explain the interchanges which occur between the various -fluids of the body should be conversant with the laws of osmosis, -it is impracticable, and in some cases misleading, to rigidly apply -them. Living membranes and dead membranes do not necessarily control -diffusion in the same manner. Still less do the laws which govern -diffusion through dead membranes hold good, without qualification, to -living cells. - -To return to the sanitary engineer whose opinion we asked regarding -the mode of working of the drainage system of the kidney. Probably he -would deny that the problems came within his province. “They are not -physical, but vital,” he would say. “I know nothing about the vital -action of the cells which line the tubule.” Objection may be taken to -the form of expression, albeit he was fully justified in declining to -discuss the question any further. He does not know enough about the -internal structure of a cell to be able to predict the phenomena of -osmosis which will occur within it. No one can say what capacity living -cells may have of taking substances from the blood, returning some of -them, and excreting others. This unknown capacity leads to results -which, when they do not appear to be in accordance with the laws of -physics, are commonly termed “vital.” The term is a stumbling-block -which has tripped up generations of physiologists. The expressions -“vital action” and “physical phenomena” have been used as if they -were antithetical, whereas all vital actions are physical phenomena. -“Vital” in this sense connotes “as yet unknown.” Yet, in truth, there -is abundant excuse for the use of a term which covers ignorance, so -long as its connotation is not extended until it assumes a positive, -antiphysical sense. “Physical” and “vital” are expressions which -point a contrast constantly present to a physiologist’s mind. He -knows perfectly well that the passage of water and salts through a -membrane, and their passage into and out of a living cell, are equally -phenomena of osmosis. But the former process he can test and measure -in his laboratory; the latter he can but observe in much obscurity -in the living body. He cannot make a model of a living cell. In the -case of the salivary gland, as we have already seen, living cells take -water from lymph, and discharge it as saliva in apparent opposition -to osmotic force. They reverse the direction of the flow which would -occur were lymph and saliva separated by a membrane. But a cell is not -a membrane. It is an extremely complicated structure with an elaborate -architecture of its own. As well might we compare the distribution of -water by a County Council water-cart and its passage through a brewery. -According to all the laws of hydrostatics, the water which flows into -a brewery should leave it through its drains. Its exit in barrels on -drays is antiphysical. When the physiologist can explore the living -cell, he will discover that the imbibition and extrusion of water, the -selection, retention, and discharge of salts, are phenomena as strictly -physical as their passage through a dialyser in his laboratory. In -the meantime he can but contemplate the cell with a certain degree -of awe. His best devised model of a urinary tubule may lead him into -error, for the simple reason that he cannot line it with living cells. -A living cell has a power which upsets all calculations, falsifies all -experimental findings. Its protoplasm can isolate and place out of -action any of the substances which enter it. If observations eventually -prove to us that water passes from the urinary tubules into the blood, -“in the face of osmotic force,” we shall be constrained to explain -this antiphysical phenomenon as due to the action of living cells. The -cells, we shall say, take up fluid from the urinary tubules, fix its -urea and other salts in their protoplasm, discharge its water into the -venous blood, return the urea and other salts to the urine. Given this -property of protoplasm, such a process is strictly in accordance with -physical laws. - - * * * * * - -Enough has been said regarding the theory, or want of theory, of the -action of the kidney. Turning now to matters of observation, it can -easily be shown that the epithelium of the tubules has the power -of excreting into the urine highly complex materials which diffuse -with difficulty. If a substance soluble in blood, but insoluble in -urine, an alkaline salt of indigo, for example, be injected into the -vascular system, it is rapidly excreted by the kidney. The indigo is -precipitated even before it comes in contact with the acid urine. -If the animal be killed a short time after the administration of -the indigo, the contorted portions of its tubules and the ascending -limbs of the loops of Henle are strongly coloured blue. An ammoniacal -solution of carmine may be used for a similar experiment; but the -results are not nearly so sharply limited to the large-celled portions -of the tubules. Even the glomerulus is coloured red, a fact which has -been interpreted as showing that, although the greater part of the -carmine is excreted into the tubules, some of it accompanies the water -which exudes from the blood through the glomerular tufts. - -The practical identity in structure of the kidney in birds and -reptiles and mammals would seem to have an important bearing on this -controversy. The urinary excretion of birds consists almost exclusively -of uric acid. As seen under the microscope, it is a semi-solid white -deposit, made up of crystals, supposing no special precautions have -been taken to obtain it fresh. The water, pigment, and salts which are -essential elements of the excretion of mammals are practically absent. -Yet the kidney of a bird presents the same arrangement of glomeruli -and tubules as the kidney of a mammal, although the glomeruli are -relatively smaller. Uric acid diffuses with great difficulty. If it -is, so to speak, washed through the glomeruli, and the water which -dissolved it reabsorbed by the tubules, an enormous quantity of water -must pass through the kidney in order that it may carry the uric acid -in its stream. If uric acid be excreted by the epithelium of the -tubules, it is difficult to account for the presence of glomeruli, -since no water leaves the kidney. Crystals of uric acid are to be seen -in a section of the kidney, not only in the cells of the tubules, but -also in the glomeruli; but it may well be that in both situations -crystallization has been induced during the preparation of the section. -It jars an histologist’s conception of the constitution of a secreting -cell to contemplate the formation within its network of protoplasm, -and the extrusion from it, of sharp-angled crystals. As a matter of -fact, it is not in its crystalline form that uric acid is excreted by -birds, but as quadri-urates—_i.e._, salts containing only one-fourth -of their “normal” complement of base; crystalline spheres or amorphous -deposit, not angular crystals. These quadri-urates decompose very -quickly, setting free crystals of uric acid. It must be confessed -that, in whatever way one attempts to account for the excretion of -uric acid by birds, the similarity of structure of their kidneys and -those of mammals is difficult to reconcile with the wide difference in -consistency and in chemical composition of the excrement. - -Reflecting upon all the evidence bearing upon the mechanism of the -mammalian kidney, the majority of physiologists come to the following -conclusions: The greatest outflow of water occurs in the glomeruli. The -water is accompanied by salts, including a small quantity of urea. The -contorted and spiral portions of the tubule and the ascending limbs of -Henle’s loops add to the urine the remainder of the urea, together with -various bodies still less readily diffusible. - -It may be that the chief function of the loops of Henle is to oppose -resistance to the passage of fluids, thus heading up the secretion, and -favouring the osmosis of water into it from the blood of the glomerular -capillaries. It is possible that the calibre of the slender descending -limbs is influenced by external pressure, their partial occlusion being -increased, and the pressure in them raised, when the organ is very -active and its intermediate zone turgid with blood. - -Various drugs influence the secretion of the kidney. In some cases -their action seems to be mainly hydrostatic. They change the rate of -flow by altering blood-pressure. Digitalis increases the force of -the heart. The heart beating more strongly, blood-pressure rises. -Higher blood-pressure is accompanied by a more copious secretion. -This action of digitalis is far more marked when the heart is out of -order than when it is healthy. In heart-disease the blood-pressure is -unduly low, and the tissues become water-logged in consequence. When -the blood-pressure is restored and a brisker capillary circulation -established, water and waste products, which have accumulated in lymph, -pass, as they ought to do, into the veins. Carried into the general -circulation, they overflow from the kidney. - -It is a little difficult to realize the abundance of the body-fluids. -From one-quarter to one-third of the whole body-weight is due to lymph, -using this term in its most general sense. The waste products of -tissues collect in the lymph. The blood circulating through capillary -vessels which traverse lymph-spaces takes up water and waste products. -Its just composition is maintained by the eliminating activity of the -kidneys. - -Even in the diuretic action of digitalis we see indications of -something more than an alteration of the hydrostatics of the -blood-supply of the kidney. The brisker circulation carries waste -products to the liver; the liver transforms nitrogenous refuse into -urea; urea stimulates the renal epithelium. It would be a mistake -to lay too much stress upon the direct effect of the drug upon the -blood-pressure in the kidney. Other illustrations throw the mere -hydrostatics of the problem into the background. Adrenalin (extract of -suprarenal capsule) causes a severe contraction of the small arteries, -which raises the general blood-pressure considerably; but the increased -blood-pressure is not accompanied by diuresis, because the glomerular -arterioles share to a full extent, perhaps to a disproportionate -extent, in the general constriction. In migraine and certain other -disorders it frequently happens that the blood-pressure in the aorta -is unduly high, yet very little fluid enters the renal tubules. If -a “saline diuretic,” potassic nitrate, sodic acetate, or some other -drug of the same kind, be administered, a copious flow is established, -the blood-pressure is relieved, the distressing symptoms disappear. -Then, again, certain diuretics, such as “sweet spirits of nitre,” -tea, gin, etc., may bring about a flow out of all proportion to the -alteration they produce in the hydrostatics of the circulation. The -diuretic action of these various drugs is clearly due to increase -in permeability of the renal epithelium. And, of all stimulants to -secretion, urea, the natural stimulant, is the most effective. If a -kidney be removed from the body, a cannula inserted into its artery, -and defibrinated blood caused to circulate under pressure through the -organ, water may or may not drip from the ureter. On addition of urea -to the blood, a copious excretion is set up. In explaining the mode of -working of the kidney, as, indeed, in explaining that of every other -organ of the body, the mechanical aspects of the problem must be kept -in the background. When we are contemplating the plan of construction -of the kidney, the hydrostatics of the circulation attract attention; -but alterations in hydrostatic conditions are not the initiating cause -of a greater or less flow of urine. The chemical condition of the -blood circulating through the kidney is the initiating cause. When the -presence in it of urea demands a more copious flow, the hydrostatic -conditions are adjusted to this need. In the case just cited of the -isolated kidney, it might be urged that the flow caused by urea is a -mechanical effect. The cells of the contorted portions of the urinary -tubules remove urea from the blood. They secrete it into the tubules. -The solution of urea, being headed up towards the glomeruli, owing to -the resistance offered to its passage down the tubules by the narrow, -descending limbs of Henle’s loops, surrounds the capillary tuft. Urea -rapidly attracts water from the blood. A copious flow is the result. -But it is just this contrast between the capacity of removing urea -possessed by living cells, and the passage of urea in solution from -one side to the other of a membrane, which justifies the retention of -the expression “vital.” Mechanical conditions are those which we can -imitate in a model; vital conditions, those which at present we are -unable to reproduce. - -=Nitrogenous Waste.=—Meat, fish, eggs, milk, vegetable-albumins, are -the sources of nitrogen. The kidney is the organ which eliminates -it from the body. Since all nitrogenous food which is digested is -eventually reduced to simple, soluble compounds which appear in the -urine (the quantity thrown off in perspiration is so small as to be -negligible), the proportion which the nitrogen of the urine bears to -the nitrogen in the food is a measure of the efficiency of digestion. -A certain quantity of the nitrogen eliminated is in the form of uric -acid, creatinin, and other compounds of a like order; but these less -oxidized substances, though always present in some degree, are not, -in Man and other mammals, the normal end-products of nitrogenous -metabolism. Urea is the final and simplest product. It is therefore -sufficient to estimate the quantity of urea excreted, and to compare -the nitrogen which it contains with the nitrogen ingested in the form -of “animal food.” About nine-tenths of the nitrogen ingested should -be accounted for by urea. When alimentation is excessive or digestion -imperfect, the proportion is less than this; some nitrogenous food is -not absorbed; some that is absorbed is imperfectly oxidized. - -=Urea= is characteristically an animal product. Inorganic chemistry -deals with stable, organic chemistry with unstable, compounds. Not -that there is any boundary between inorganic and organic chemistry. -They are merely terms which it is convenient to use to indicate the -groups of atoms which occupy the chemist’s attention at the time. Nor -is stability an attribute of certain groups, instability an attribute -of others. Stability is relative, not absolute. But admitting these -terms as convenient indications of degree, it may be said that -inorganic chemistry has to do with such substances as carbonates, -nitrates, ammonia; organic chemistry, with compounds in which carbon -is not satisfied with oxygen, as it is in carbonic acid; nitrogen -not satisfied with oxygen, as in nitric acid, or with hydrogen, as -in ammonia. Carbonic acid (anhye)drid has the formula CO₂; ammonia, -the formula NH₃. Urea is a combination of the two compounds. It is -carbonic acid in which one (divalent) atom of oxygen is replaced by two -(monovalent) atoms of ammonia. It is ammonia in which two (monovalent) -atoms of hydrogen are replaced by one (divalent) atom of carbonic acid. - - Carbonic Urea Ammonia - anhydride - NH₂ - / - CO₂ CO NH₃ - \ - NH₂ - -Urea is an amide—carbonic diamide. It very readily takes -water into its molecule, changing into carbonate of ammonia. -N₂H₄CO + 2H₂O = (NH₄)₂CO₃. This change is rapidly brought about by the -influence of bacteria in urine exposed to the air. - -In thinking of the transformations which proteid substances undergo -in the system, it is legitimate to regard their nitrogen as from -the first united with hydrogen in the form of ammonia. Not that the -grouping is so simple as this. An albumin is not an amide. But in the -dance of atoms of its great molecule as it progresses through the -system—forming part of the blood, taken up by the cells as floating -protein, incorporated in the protoplasm of the cells, shaken into -smaller aggregates in the muscles—nitrogen and hydrogen are partners. -They leave the body hand in hand. Gusts of oxygen atoms enter through -the lungs; use blood-corpuscles as carriages; dismounting, they -traverse lymph, forcing their way into the interior of the cells; they -join in the dance. With their strong arms they detach carbon atoms and -hydrogen atoms from the huge albumin chain. As carbonic acid and water -they bear them to the lungs. But nitrogen clings to hydrogen. Oxygen -cannot detach its grasp. Out of the molecule of albumin this firmly -united couple slips, without contributing anything to the energy which -moves the body and keeps it warm. Nitrogen is not a source of energy. -It even saves a portion of the hydrogen of albumin from combustion. -Urea burnt in a calorimeter has a balance of energy to give up. - -Many attempts have been made to ascertain the stages through which -proteins pass on their road to urea. The search for intermediate -compounds is probably futile, since there is no sufficient reason -for supposing that proteins disintegrate in stages, each a step less -complex than the food and a step nearer to urea. Every nitrogenous -extractive found in the tissues is, of course, on its road to urea. It -will be removed as urea, unless indeed, like uric acid or creatinin, -it has to be excreted without further change. But it appears to be -impossible to discover in the tissues any nitrogenous compounds which -occur in sufficient quantity to justify us in regarding them as -inevitable halting-places on the downward road (_cf._ p. 146). - -The metabolism of albuminous substances, like other oxidations, takes -place chiefly in muscles. Very little is known regarding the nature of -the products. Urea is not amongst them. Whatever they may be (_cf._ p. -267), they are carried to the liver, in which they are turned into urea. - -The metabolism of the body is not normally derived from the oxidation -of nitrogenous foods. Failing a sufficient supply of other kinds of -food, they may be used as sources of energy; but we must picture them -as splitting into carbonaceous and nitrogenous portions. If, after the -reserve of glycogen in the liver has been brought low by abstention -from carbohydrates and fats, nitrogenous food is consumed, and the -muscles are then called upon to do severe work, the amount of carbonic -acid and water given off rises at once. The excess of urea derived from -the nitrogenous food which was destroyed for the purpose of liberating -the energy which the muscles expended makes its appearance some time -later. If the diet contains a sufficiency of carbohydrates, muscular -work does not increase urea. The output of urea is exceedingly steady. -It is not increased by muscular work, nor diminished, beyond a certain -limit, by absence of food. The tissues are constantly throwing off -nitrogen-containing molecules, which, if the body is not to waste, must -be as constantly renewed. - -=Uric Acid.=—When nitrogenous metabolism has reached the bottom, when -albuminous substances have been shaken into the simplest and most -stable compound or compounds which the muscles are capable of making -(we know not whether the end-products be one or many), they are carried -to the liver by the blood. The mammalian liver converts them into urea; -the liver of birds and reptiles changes them into uric acid. Uric -acid is not, however, completely absent from the urine of carnivorous -animals. In Man the amount excreted is about 0·8 gramme per diem, but -subject, even in perfect health, to considerable variations (0·2 gramme -to 1·4 gramme). There is no reason for thinking that uric acid is made -in the liver of mammals. On the contrary, it seems to be either an -end-product of the disintegration and oxidation of leucocytes (_cf._ p. -53), or, like certain other more complex nitrogenous compounds which -appear in very small quantities in the urine, the relic of albuminous -food which has missed the broad down-path, via muscles and liver, -to the kidney. It is a troublesome burden for lymph and blood, and, -unfortunately, the kidney finds difficulty in throwing it out. Uric -acid has a pernicious way of accumulating in tissue-spaces, producing -all the malevolent symptoms of gout. During an acute attack of gout the -quantity of uric acid in the system may be largely increased. It may be -so abundant in the blood that, when a sample is allowed to cool, uric -acid begins almost immediately to crystallize out. Speaking generally, -it is right to ascribe gout to an over-production of uric acid; -but it must be remembered that the balance between elimination and -production is very delicately adjusted. During an attack of gout the -amount excreted in the urine is not increased; frequently it is less -than usual. The clearing up of the attack is accompanied by abundant -excretion of urates, or lithates (λίθο, stone), as they used to be -called, because the “stones” which are found in the bladder consist -largely of uric acid. From this it appears that faulty distribution -and inadequate excretion have more to do with the development of the -symptoms of gout than over-production. In a previous chapter (p. 140) -we gave as the predominant cause of gout acid fermentations in the -stomach. It does not, by any means, follow, however, that we were right -in correlating imperfect digestion with an excessive formation of uric -acid. It may well be that the gouty symptoms to which hampered peptic -digestion gives rise are due in larger measure to a disturbance of the -composition of the body-fluids which renders them unfit to carry uric -acid to the kidneys in such a form, or in such relation to the fluid -in which it is dissolved, as will insure its escape into the urinary -tubules. The interference with the efficient working of the system -caused by accumulation in it of uric acid gives a particular interest -to all that is known regarding the nature and origin of this substance. - -Uric acid has the formula C₅H₄N₄O₃. It is a more complicated and a more -stable body than urea. The deposits of guano in Peru contain uric acid -(the excrement of birds) which has remained practically unchanged for -years—for centuries, perhaps. Its chemical nature is not completely -understood. It can be readily made to yield urea; and it can be formed -by conjugating urea with a nucleus derived from lactic acid (_cf._ -p. 13). Its formula is therefore commonly represented as that of a -diureide—a substance containing two urea radicles: - - { HN——CO - { | - CO { C——NH } - { | } CO - { HN——C——NH } - -But notwithstanding this inclusion in its molecule of two radicles of -urea, it is safe, when one thinks of the contrast between urea and -uric acid, to lay stress, in the case of the former, on the binding -of nitrogen to hydrogen; in the case of the latter, on the binding of -nitrogen to carbon. - -Uric acid is soluble with difficulty; it crystallizes in rhombs. It -forms salts, normal and acid. Those which appear in the urine are -always acid salts. As a treatment for “stone,” lithia water has long -had a reputation which it probably deserves, the acid urate of lithium -being the most soluble salt of uric acid which the kidney can secrete. -When uric acid is in excess in urine, brown crystals of uric acid are -deposited as “gravel” soon after it is passed. Even when not in excess, -uric acid crystals appear after a sufficient time. In other cases uric -acid, when in excess, is thrown down in the form of a cloud of acid -urates of sodium and other bases, which renders the urine turbid. These -urates are redissolved when the water is warmed. - -The more fortunate of human beings need never concern themselves -with the chemical history of uric acid. It is always present in -their body-fluids. It is excreted by the kidney. Its formation is -of no greater interest than that of creatinin and other nitrogenous -compounds which escape the almost universal reduction to urea. Persons -who have a uric acid diathesis are in a very different plight. Every -scrap of evidence bearing upon its origin is of supreme importance. -Unfortunately, the evidence collected as yet is scanty, and its -application for remedial purposes impracticable. - -The only disease in which uric acid is invariably in excess is -leucocythæmia. This is a condition or habit marked by the presence -in the blood of a very great number of white blood-corpuscles and -a paucity of red ones. The connection between this disease and the -production of uric acid is made plain by certain experiments in diet. -If flesh which contains relatively a large proportion of cell-nuclei -is eaten, the uric acid excreted is markedly increased. Sweetbread, -especially “neck sweetbread”—_i.e._, thymus gland—is a mass of -comparatively small cells with large nuclei. If thymus gland be -substituted for all other meaty foods, the quantity of uric acid -appearing in the urine is doubled. A large increase in the quantity of -ordinary meat or fish consumed also increases uric acid, because all -meat-fibres contain nuclei. If egg-albumin be taken instead of meat, -uric acid is not increased. A sudden excess of muscular work leads to -an increase in uric acid, owing presumably to the unusual activity -of the tissues. This used to be very noticeable in the case of young -men during the first few days of “training” under the old system; -but it may have been due to the generous consumption of chops and -steaks, rather than to the increase in physical work, and consequent -destruction of tissue. Nuclei contain nucleo-proteins, which split -into proteins and nuclein. Chemically, it is reasonable to attribute -to nuclein the parentage of uric acid; a plausible line of descent can -be traced. The association of leucocythæmia with the production of uric -acid is probably due to the destruction of leucocytes which are present -in abnormal numbers (_cf._ p. 53). - -Such is the evidence at present in the hands of physiologists. -Naturally, physicians have endeavoured to turn it to account. -Patients have been recommended to avoid animal foods which contain -nucleo-proteins—to take, instead of meat and fish, eggs, milk, -cheese, vegetable-albumins. Certain physicians contend that such a -diet is followed by the happiest results; others, equally competent, -and perhaps less biassed by “medical theory”—the most dangerous of -handicaps for anyone who practises an art which must ever remain -empirical—are satisfied that equally good results are obtained -by excluding from the diet eggs, milk, and cheese. Physiological -discoveries suggest treatment. Modern medicine is in the fullest sense -applied physiology. But treatment based upon theory must be controlled -by unprejudiced observation. It is possible that the gouty diathesis -may be held in check in certain cases by the exclusion from the diet -of certain kinds of nitrogenous food. The experience of generations -has taught us that the injudicious use of such articles of diet as -fruit, pastry, sugar, which do not contain nitrogen, is the main factor -in inducing an attack of gout; that imperfect digestion, sluggish -circulation, insufficient activity on the part of the kidneys, lead to -the accumulation in tissue-spaces of the _fons et origo malorum_. Even -sweetbread, which with the precision of a chemical experiment increases -the production of uric acid by a healthy person, is not necessarily -found unwholesome by those who are inclined to gout. It is amongst -the most digestible of all meat foods, and easy digestion covers a -multitude of metabolic sins. - - - - -CHAPTER IX - -THE CIRCULATION - - -The blood circulates in a closed system of tubes, continuous from the -heart back to the heart. The walls of these vessels separate the blood -from the tissues. Nowhere, except in the spleen, does it come into -contact with any cells other than the lining cells of the vessels in -which it flows, and the exception made by the spleen is more apparent -than real. The spleen (p. 79) is a kind of sponge invested with a firm -capsule. Small arteries discharge their blood into its spaces; small -veins collect it. But the organ is essentially a part of the vascular -system. Its spaces take the place of the capillary vessels which -connect arteries with veins in other situations. - -[Illustration: FIG. 10.—THE HEART CUT IN THE PLANE OF ITS LONG AXIS, -AND THE VESSELS WHICH OPEN INTO AND OUT OF IT. - -Chordæ tendineæ attach the margins of the auriculo-ventricular valves -to musculi papillares which project from the inner aspect of each -ventricle.] - -The blood makes a double circuit. From the right heart it passes -through the vessels of the lungs. Returning to the left heart, it is -driven through the body. Although the heart consists of two separate -pumps, it makes but a single organ. Its division into right auricle and -ventricle and left auricle and ventricle is but slightly indicated on -the surface. In most invertebrate animals the two pumps are distinct. -In some the lung-heart and the body-heart are on opposite aspects of -the body. But one must not, when thinking of the morphology of the -vertebrate heart, picture it as formed by the juxtaposition of two, -originally separate, pumps. Truly, in its very earliest stage of -growth, it is represented by two tubes which lie, in the embryo, far -apart. But these, before we can speak of the existence of a heart, fuse -into a single tube, with four contractile bulbs in series. As the heart -develops, the dilatation at its hinder or venous end and the dilatation -at its anterior or arterial end disappear. A partition is formed which -divides the two middle bulbs into right and left auricle and right and -left ventricle respectively. Immediately after birth the lungs are, -for the first time, distended with air. Up to that particular minute -they have had no functional use. Nothing would be gained by compelling -all the blood of the body to traverse the vessels of the embryo’s -lungs. Until birth, therefore, the inter-auricular septum is perforate. -The blood takes a short-cut, through the foramen ovale, from right -auricle to left. But by birth-time a curtain has grown down on the -left side of the foramen. When the lungs are expanded by the forcible -enlargement of the chest-cavity which contains them, their bloodvessels -are distended by the same extensile force. Blood is sucked into them -from the right side of the heart. A difference in pressure on the two -sides is established. A condition is set up which is favourable to what -may almost be termed the adherence of the flap which hangs down on the -left side of the foramen ovale. The growth of its margin very rapidly -obliterates the hole. Occasionally the closure of the foramen is not -complete. A child grows up with a perforate inter-auricular septum. If -the aperture be very small it causes little inconvenience. Shortness -of breath and blueness of lips indicate its existence if it be large -enough to lead to deficient aeration of the blood. - -The two sides of the heart being quite separate, it is clear that all -blood ejected by the right ventricle into the lungs must return to the -left auricle, to be driven by it round the body. Yet it does not follow -that the heart must at each stroke drive exactly the same quantity of -blood into the pulmonary artery and into the aorta. On the average, -each of the two sides ejects the same amount—about 3 ounces. Nor does -it follow that as much blood is lodged in the lungs as in the whole of -the rest of the body. The amount varies, but on the average the lungs -contain not more than one-fifteenth of the whole blood. The heart may -be likened to two cogwheels; the blood-stream to a chain, folded into -a figure eight, against which the cogwheels work. Synchronously each -cogwheel lifts a link, the right one of the smaller, the left one of -the larger loop. Any given link returns to its starting-place in half -a minute. Such an illustration gives an idea of the arrangement of -the circulation as a whole, although the motion of a fluid is widely -different from the motion of a chain. - -If, the jugular vein of the neck being cut, a colouring matter—such, -for example, as ferrocyanide of sodium or methylene-blue—is injected -into its central end towards the heart, it begins to appear in the -blood which issues from its distal end in half a minute. In this -short space of time it has passed through the right heart, through -the lungs, through the left heart, and through the vessels, arteries, -capillaries, and veins, of the head. Half a minute is therefore the -“circulation-time.” Not that all the blood-corpuscles of the body -make the circuit as rapidly as this. The time taken depends upon the -particular route they follow in the greater or systemic circulation. -Some traverse the vessels which supply the walls of the heart itself—a -short journey; others go down to the foot and up again. But the -average circulation time does not exceed a minute or a minute and a -half. It is particularly in the veins of the liver and other abdominal -viscera that blood tends to linger. Usually half the blood of the -body, or even more, is lodged in these capacious reservoirs. It is -thanks to their capacity for storing blood that a supply is provided -adequate to meet any special demand. If a man runs a few hundred -yards, two-thirds of the whole blood of the body is transferred to his -limbs. It is quickly withdrawn from the abdominal vessels when it is -wanted elsewhere; but, failing an efficient cause for removing it, its -accumulation induces lethargy. Even tight-lacing has been defended by -an eminent physiologist on the ground that it prevents accumulation -of blood in the abdomen. But tight-lacing diminishes the capacity of -the chest, hampers the action of the heart, checks the circulation, -distorts the abdominal viscera, and generally deforms and jams the -domestic machinery, even though the professor be right in his view that -visceral compression may favour alertness of mind. More by token, it -interferes with this admirable adjustment by which blood is distributed -to the various parts of the body in proportion to their needs. The -brain is the only organ which has any difficulty in securing all it -wants, and its claim to so much blood might be disputed. Nature has -not provided for long-continued passivity of the body associated with -strained activity of mind. When the stimulus to mental activity is not -unreasonable, most “nervous” people are apt to discover that their -brains are better supplied with blood than is good for their health. - -The effect upon the distribution of blood throughout the body of -squeezing the viscera is experienced after taking a deep breath and -contracting the muscles of the abdomen. The contents of the abdomen are -compressed between the depressed diaphragm and its muscular wall. - -Certain other forces co-operate with the beat of the heart in causing -blood to circulate. Two such factors are especially deserving of -attention. In the first place, the movement of blood in veins is -largely dependent upon external pressure. The veins are valved at -frequent intervals, the folds in their interior being of course -directed towards the heart. Any external force which empties a section -of a vein drives blood forward. A “good stretch” brings the lateral -pressure of contracting muscles to bear upon the walls of the veins -which lie between them, or beneath. More blood is delivered to the -heart. The exercise of standing erect in the attitude of attention, -and then slowly raising the arms until the thumbs meet above the head, -and slowly lowering them again, has a remarkable effect in quickening -the circulation—increasing the blood-supply of the brain. Changes -of posture, by relieving pressure on subcutaneous veins, removes an -impediment to the flow of blood. - -The second of the factors to which we have referred as adjuvant -to the heart’s action is the negative pressure of inspiration. In -explaining the effect of this force upon the circulation, the relation -of the lungs to the thorax must be taken into account. The box in -which the lungs are enclosed is too big for them; nevertheless, -being extensible and elastic, they always fill it. They follow its -movements when in inspiration the muscles between the ribs enlarge -it, and when in expiration it diminishes again. No air or fluid, save -the moisture which lubricates the surface of the pleura, reducing -friction, occupies the (potential) space between the lungs and the -chest. But the moment the chest is punctured the lungs collapse. -Air is sucked into the pleural cavity. The lungs fill the chest -only so long as there is neither air nor fluid between it and them. -Lung-tissue is extremely delicate. Each air-cell is a cup of thin -membrane holding together a basket-work of capillary vessels. So long -as the chest-wall is stationary the negative pressure in the pleural -cavity has no effect upon these slender tubes. But when the chest -expands, the capillaries are between two minus pressures, the pull of -the chest-wall and the resistance offered to the entrance of air into -the lungs by the passages through which it has to pass. The calibre of -the lung-capillaries is increased, just as it would be increased were -they hanging in an air-pump while the piston was drawn out. More blood -passes to the left heart through the wider capillaries. Ejected into -the aorta, it raises the pressure in the arterial system. A record of -the pressure in an artery shows a rhythmic rise for each heart-beat. It -shows also a rise with inspiration and a fall with expiration. These -larger undulations correspond with the movements of the chest, although -they are necessarily somewhat late on respiration, for the first effect -of the dilatation of the capillaries is to cause them to hold more -blood and to deliver less. The first effect of expiration, on the other -hand, is to urge on the blood which the dilated vessels contain. In -any case a single beat is needed to throw into the aorta the blood -which has been received by the right auricle. - -The expansion of the chest influences the flow of blood in yet another -way. The heart and the great vessels which join and leave it are -themselves enclosed within the chest, subject to the negative pressure -produced within that cavity by the elasticity of the lungs. The lungs -pull upon the pericardium, the membranous covering of the heart. When -this pull is increased owing to the forcible expansion of the chest, -blood is sucked into the great veins, just as air is sucked into the -windpipe. The thick-walled aorta, containing blood at high pressure, -does not feel the effect of slight variations in the pressure round -it. The soft-walled veins are expanded during inspiration to a not -inconsiderable degree. What relief a deep yawn gives by hastening -a languid circulation! Leaning over an account-book late in the -afternoon, every condition is unfavourable to the flow of blood. It -accumulates in the legs and in the abdomen. The head is thrown back -and the mouth opened wide, while the chest expands in a long deep -inspiration. Down on the liver, stomach, and intestines presses the -flattened diaphragm, squeezing their blood towards the heart. The -negative pressure within the chest sucks this up, and draws down the -blood contained in the great veins of the neck. The capillaries of the -lungs are widened, allowing blood to pass more quickly from the right -side to the left side of the heart. The heart responds to the call upon -it, throwing all that it receives into the aorta. Only a great effort -of the will had kept the pale brain at work; in the attic it suffers -more than organs on the lower storeys from insufficient pressure. For -a short time after the yawn it finds itself nourished with an adequate -supply of blood. - -The negative pressure in the thorax is considerable at all times. If -a manometer—a =U=-shaped tube with mercury in its loop—be connected -with a cannula passed through the wall of the chest, the difference -of level of the mercury in the two limbs of the =U= is a measure of -the force with which the lungs are endeavouring to shrink away from -the chest-wall. Even at the end of expiration the mercury in the limb -next the chest stands about 6 millimetres higher than the mercury in -the outer limb. During a deep inspiration the pressure in the chest -falls 30 millimetres below the atmospheric pressure. Hence a problem -is presented of which no completely satisfactory solution has yet been -given. How comes it that lymph is not sucked into the pleural cavity? -In health there is no more pleural fluid than just suffices to keep -the membrane moist. The endothelial cells which cover the surface of -the pleura resist further exudation. Valves in the lymphatic vessels -prevent backward flow. Yet in disease, when the pleura is inflamed, -lymph pours out quickly, often to be reabsorbed with equal rapidity -when the pleurisy subsides. This flow uphill, from a lower to a higher -pressure, can be explained only as a phenomenon due to the “secretory” -capacity of endothelium. As an answer to the hydrostatic problem this -is hardly satisfactory. - -The circulation of the blood is the result of the difference between -the pressure in the vessels through which it leaves the heart, and that -in the vessels through which it is returned. The pressure in the aorta -amounts to about 200 millimetres of mercury. In the venæ cavæ it is -nil, or, owing to the aspiration of the thorax, less than nil. - -=The Heart.=—Inspection of the liver, the spleen, or the kidney helps -but little to the comprehension of the mechanism of these organs. -It is quite otherwise in the case of the heart. Its mechanics being -comparatively simple, physiology is concerned with measurements, with -the conditions under which it can and cannot work, and with the action -upon it of the nervous system and of drugs. The heart of any mammal -will suffice for anatomical study. A sheep’s heart is about the same -size as that of a man, and exactly similar, save in minute particulars, -which do not appreciably affect its mode of working. - -The heart is a hollow muscle, composed of minute contractile cells. -Each cell is a cylinder, about twice as long as it is broad, with an -oval nucleus in its centre. There is no impropriety in speaking of the -heart as a single muscle. Muscles which we can move at will, “voluntary -muscles,” consist of fibres, each from 1 inch to 2 inches long, and -of about the thickness of a piece of thread (Fig. 16). Every fibre is -surrounded by a membranous sheath, its sarcolemma, which completely -isolates it from the others. Each has its separate nerve-supply. A -voluntary muscle-fibre is a cell-complex. The single embryonic cell -which grew into the fibre underwent nuclear division until hundreds -of nuclei were formed, but its cell-substance was not divided into -territories appertaining to the several nuclei. In heart-muscle, on -the other hand, nuclear division has been followed by cell division; -but minute protoplasmic bridges are left between the cells. The whole -of the heart-substance is thus in structural continuity. The cells are -not invested with sarcolemma. As the result of this arrangement, an -impulse started in one part of the heart spreads over the whole, with -certain limitations as to the directions in which it is able to travel, -whereas in voluntary muscle a separate impulse must be delivered to -each fibre. The wave of contraction commences in the great veins, the -venæ cavæ and pulmonary veins, near their junction with the heart, -spreads from cell to cell throughout the auricles, and onwards down -the ventricles to the apex of the heart. The substance of the heart -has not, however, a homogeneous appearance. Its cells are collected -into fascicles, which lie in various planes and cross the axis of -the heart at various angles. In a boiled sheep’s heart it is easy to -separate one fascicle from another, and to distinguish the sheets into -which the fascicles are collected. The four valves of the heart lie in -almost the same plane. They are supported by a fibrous plate divided -into four rings (Fig. 11). Most of the fascicles are attached to this -plate, though some which encircle the auricles are independent of it. -With one or with both ends attached to the plate, fascicles loop over -the auricles. They run down the ventricles with a twist from right to -left. Those on the surface turn in at the apex of the heart, and run -up the inner surface of the ventricles. Some of them go to form the -free columns which are found on the inner surface of the ventricles, -pointing towards the valves—musculi papillares. The fibrous plate -which supports the valves cuts off almost all of the muscle which makes -the walls of the auricles from that which constitutes the ventricular -walls; but a thin sheet is continued from the inner surface of the -auricles down the interventricular septum. To a considerable extent the -walls of the two auricles and of the two ventricles are respectively -continuous, insuring synchronous contraction. - -The arrangement of the fascicles accounts for the changes in form -which the heart undergoes when it contracts. Systole commences in the -cardiac ends of the venæ cavæ and pulmonary veins. They empty the last -of their blood into the auricles, and close to prevent regurgitation, -their mouths not being valved. Then the auricles quickly shrink in -all dimensions, and as soon as their contraction is at its height -the ventricles contract, while the auricles relax. The ventricular -wave runs from base to apex too rapidly to be followed with the eye, -and ends, owing to the involution of the fascicles, in the musculi -papillares. As soon as ventricular systole has commenced, the auricles -relax. After emptying their contents into the aorta and pulmonary -artery, the ventricles relax, their contraction giving way first at -the apex, and being longest held at the base. Then follows a pause -(diastole), during which both auricles and ventricles are flaccid. If -the pericardium is open, the heart is seen to become round instead of -oval in transverse outline during systole. It shortens. Its apex twists -a little to the right, and projects forward. But if it is within its -pericardium the shortening is not accompanied with any displacement of -the apex. Instead of the apex mounting, the base descends. The front -of the right ventricle, at some little distance from the apex, presses -the chest-wall forwards in the fifth intercostal space, about an inch -to the inner side of a line falling vertically through the nipple. This -pressing forwards is felt as the “impulse of the heart.” - -[Illustration: FIG. 11.—A SECTION APPROXIMATELY AT RIGHT ANGLES TO THE -LONG AXIS OF THE HEART, EXPOSING THE FOUR VALVES WHICH LIE VERY NEARLY -IN THE SAME PLANE. - -The semilunar valve which guards the aperture of the pulmonary artery -is the nearest to the breast-bone.] - -The contraction of the heart is not a see-saw of auricles and -ventricles. During diastole blood is falling from the veins through -the auricles into the ventricles. In a sense, the auricles are not -necessary parts of the double pump. They collect blood while the -ventricle is contracting, thus preventing it from heading up in the -veins. They save time. Their contraction completes the filling of the -ventricle, so that the instant the ventricular contraction begins blood -enters the aorta and pulmonary artery. - -=The Valves.=—If ever expressions of admiration were appropriate in -a treatise on the animal body, such preface might be permitted to a -description of the cardiac valves. Which means no more than this: -Men make pumps. Therefore they are in a position to appreciate the -mechanism of the heart. We cannot admire what we do not understand. -If we made secreting organs or self-contracting springs, glands and -muscles would evoke our commendation. We should recognize that Nature’s -apparatus is even better adapted to its work than any that men can -make. This is the admission which is forced from us when we study the -heart. - -The apertures connecting auricles and ventricles are extremely wide, -allowing the contents of the former to be emptied into the latter -almost instantaneously. If we attempted to make a pump fulfilling this -condition, we should find that it failed in several respects. In the -first place, the rush of fluid from the one chamber into the other -would press the flaps of the valves back against the wall of the second -chamber. They would cling to the wall, and would not float up quickly -into place when the second chamber was squeezed. Let us call the two -chambers A and V for brevity’s sake. When V contracted, some of the -fluid would be thrown back into A, because, the resistance in that -direction being lower than the resistance offered by the column of -fluid above the pump (the resistance in the aorta is very high), the -contents of V would rush past the margins of the A-V valve. This would -happen even though its flaps were not pressed back against the wall. -Further, at the height of contraction the membranous valve would bulge -backwards into A, making a cup towards V which V could not empty. In -the heart these difficulties have been overcome. - -The tricuspid valve, which separates the right auricle from the right -ventricle, has three flaps. The mitral valve, on the left side of the -heart, has but two. The flaps are composed of tough membrane, but are -comparatively thin. The following direction for deciding at an autopsy -whether or not they were healthy at the time of death was given many -years ago by a surgeon of repute: “You ought to be able to see the -dirt under your thumbnail when you place it beneath one of the flaps.” -Surgery has improved in cleanliness as well as in other ways; indeed, -the possibility of advance has been due to the recognition of the need -for transcendental cleanliness. But this is a digression. The margins -of the flaps are crenulated. Threads—chordæ tendineæ—are attached to -them like the stay-ropes of a tent. At their other end these tendons -are attached to the musculi papillares already mentioned. The bunch -of tendons from each papillary muscle spreads, to be inserted into -the contiguous margins of two flaps. We have mentioned some of the -difficulties which have been overcome in the construction of the pump. -(1) The flaps do not flatten back against the wall of the ventricle -during systole of the auricle. It must be remembered that during -diastole of both chambers blood is flowing through the auricle into -the ventricle. The latter being partly filled before systole of the -auricle commences, the flaps are floated up. This is greatly favoured -by the form of the inner wall of the ventricle. It is not flat, but -raised in pillars—columnæ carneæ. The spaces between these pillars -cause backwash currents, which lift the flaps and help to bring them -into apposition as soon as systole of the ventricle commences. (2) No -blood which has entered the ventricle is thrown back into the auricle. -The valve “balloons” over the blood in the ventricle before the -contraction of the auricle has ceased. The thin margins of its flaps -come together with great rapidity. The tendinous cords holding their -edges on the ventricular side, they meet, not edge to edge, but folded -flap to folded flap. (3) The valve does not bulge into the auricle. On -the contrary, at the height of systole it is pulled into the ventricle -by the contracting musculi papillares. As the ring to which the valve -is attached is diminished in size, by the contraction of the base of -the heart, which continues, it will be remembered, until after the apex -has begun to relax, the edges of the flaps are folded farther and still -farther over by the pull of the musculi papillares, and the blood is -squeezed out from between the wall of the ventricle and the indrawn -valve. - -The “semilunar valves,” which close the apertures into the aorta and -pulmonary artery, have each three flaps. The aortic semilunar valve, -which has the higher pressure to bear, shows its characteristic -features in a rather more marked degree than the other. Each of its -three flaps is a half-cup. At the centre of the margin of the half-cup -is a small fibrous nodule. The edge of the cup on either side of this -is very thin. Fine elastic fibres radiate from the nodule to all parts -of the flap. The wall of the aorta shows three bays, or “sinuses,” -one behind each flap. Hence, when the valve is forced by the rise of -pressure in the ventricle, the flap is not flattened back against the -wall of the aorta. There is always a certain amount of backwash in the -pocket behind it. The instant the pressure in the ventricle begins -to fall, the three flaps come together with a click, so smart as to -be plainly audible over most of the front of the chest. The click is -the “second sound” of the heart. The auriculo-ventricular valves also -make a sound when they close; but this “first sound of the heart” -has a different character. It is prolonged, soft, low-pitched. It is -customary to represent the sounds by the syllables “lūbb dŭp—lūbb -dŭp,” the pause during diastole being of about the same length as -the sounds when the heart is beating with its normal rhythm. The -duration of systole is little affected by variations in the rate of -beat. It is diastole that is shortened or prolonged. The second sound -is due entirely to the closure of the semilunar valves. It is heard -most clearly when the stethoscope is placed over the region where -the aorta comes nearest to the wall of the chest—at the second rib -cartilage on the right side of the breast-bone. The first sound is -loudest near the apex of the heart. It is generally agreed that it is -not wholly due to the closure of the auriculo-ventricular valves, but -possesses a second constituent. Some persons assert that they can with -the ear distinguish the clearer valvular sound at the commencement -from the general rumble which overtakes it. The main part of the -sound, if it have two constituents, or the whole sound, if there be -no distinguishable valvular constituent—observers differ—is just -the noise of a distant cab (_bruit du cab_) or the waves on a far-off -beach; it is the sound which the ear picks up from any irregular -mixture of tones which it cannot analyse. It is the resonance-tone of -the ear. That the membranous valves play the leading part in producing -the first sound cannot be doubted, whether by their first closure or -by their subsequent vibration. We should be inclined to attribute to -them the whole performance, were it not that the first sound, or at -any rate a sound, is heard during the beating of a bloodless heart. -If an animal be killed and the heart removed from its thorax with the -utmost despatch, it will beat for about a minute while lying in the -palm of one’s hand. When a stethoscope is applied to the ventricle, -a “first sound” is heard. This was described as a muscular sound, -owing to a misconception. It is similar to the sound which is heard -when a stethoscope rests upon a contracting biceps. Until recently -the voluntary contraction of a muscle was believed to be vibratory—a -tetanus. The sound corresponds to a rate of about thirty-six vibrations -to the second. There being reasons for thinking that muscle contracting -naturally does not vibrate as fast as this, the sound was interpreted -as the first overtone of the muscle-note. Muscle was said to vibrate -eighteen times a second. The similarity of the first sound of the -heart and the ordinary muscle-sound led physiologists to infer that -the contraction of the heart also was a tetanus. But this was a -mistake. Neither voluntary muscular action nor the contraction of -the heart is an interrupted contraction in this sense. In the case -of the musculature of the heart especially, contraction is a steady -shrinking, followed by a steady relaxation. The sound produced by the -bloodless heart is due to the various displacements which occur when it -contracts. Its interior is very irregular, with its columns, papillary -muscles, tendinous cords, valves. The displacement of these various -structures is responsible for the noise. - -The sounds of the heart afford to the physician a means of ascertaining -with the utmost nicety the condition of the valves. If the sounds -are altered from the normal in the least degree, the valves are not -healthy. Alteration of the structure of a valve is in ordinary parlance -heart-disease. It is usually indicated by an addition to the normal -sound. Such addition is termed a “murmur”; in French, _un bruit de -souffle_. Either term is somewhat misleading to the tyro. We remember -a fellow-student to whom our chief had in vain expounded the nature -of a murmur. “Surely, Mr. S., you can hear the murmur in this case.” -We others could hear it as we stood around the bed. After listening -for a minute, S. replied: “I think I could hear it, sir, if the heart -wasn’t making such a thundering noise.” The thundering noise was the -murmur. It is the business of the physician to recognize that there is -a departure from the normal, to analyse its character, to determine -the time at which it is heard in relation to the cardiac cycle, and to -locate the place on the chest where it is heard most loudly. He is then -in a position to state which of the valves is affected and what is the -nature of its lesion. Is it a lesion obstructing an orifice, or is it -causing regurgitation of blood? Or is one of the valves, as is commonly -the case in heart-disease, imperfect in both respects? - -A murmur, in the strictest sense, is a sound added to a heart-sound. -It is due in all cases to vibration of a fluid column (“fluid vein” is -the term in physics). When fluid passing under pressure along a tube -of a certain calibre enters a tube of smaller calibre, no vibration -occurs. When it passes from a tube of smaller calibre into a larger -tube or space, it is thrown into vibration. Under normal conditions no -vibration occurs in the heart. The auriculo-ventricular orifices are so -large that auricle and ventricle form a single cavity when the valve is -open. The ventricles drive the blood into tubes of smaller dimensions -than themselves. These are not the conditions which set up vibration -in a fluid column. But if one of the orifices is constricted, owing to -thickening or partial adhesion of its valve, the fluid column vibrates -on entering the space beyond it. The sound is propagated forwards, -beyond the constriction, not behind it, and transmitted to the wall -of the ventricle, aorta, or pulmonary artery, as the case may be. -When either of the auriculo-ventricular orifices is constricted, the -vibration of the fluid column can be felt as well as heard. The finger -placed against the chest-wall at the spot where the impulse of the -heart occurs is sensible of a thrill. The vibration may occur whilst -blood is flowing _through_ an auricle into a ventricle, before the -auricle contracts. In time, it is presystolic. The murmur produced by -regurgitation into an auricle is synchronous with systole. The murmur -due to regurgitation into a ventricle past an incompetent semilunar -valve is postsystolic. - -We have said that the heart is so formed that no vibrating fluid -vein is produced when it is functioning normally. Murmurs are due to -alterations in the valves which are visible after death. This statement -needs modification. Not infrequently functional murmurs are heard, -which disappear again after a time—in a few weeks, or even days, -perhaps. The explanation of murmurs of this class is very difficult. -They are heard most frequently in anæmic persons, and appear in these -cases to be due to the heart having shrunk, owing to the blood in -circulation being deficient in quantity, until the cavities of the -ventricles have a smaller diameter than that of the great arteries into -which they expel their contents. - -Such is the explanation of the physical cause of murmurs given by -Chauveau and Marey, the physiologists who have paid most attention to -this subject. But it must be remembered that the valves which, when -diseased, are the sources of the murmurs are membranous structures. It -may be that fluid veins would be produced by them if they were rigid -ledges which jutted into the blood-stream; but, being membranous, they -are capable of vibration. Certain physicists are of opinion that a -murmur is caused, not by the vibration of a fluid vein, as such, but by -the vibration of the membranous structure which impedes the passage of -the fluid. The physics of the problem is of little consequence to the -physician. The murmur is produced at the spot where a diseased valve is -situated, and is propagated forwards. It enables him to ascertain with -accuracy what is amiss with the heart. - -=Bloodvessels.=—The greater circulation occurs through a closed -system of vessels which unite the left ventricle with the right -auricle. The aorta gives off lateral branches. Its branches branch. -Subdivision continues until the vessels are just wide enough to allow -blood-corpuscles to pass in single file, or but little wider. When -a bough of a tree divides, the united cross-sections of its twigs, -their soft bark being stripped off, may be a little larger than the -cross-section of the bough; but the disparity is usually small. -The united cross-sections of the smaller arteries is considerably -greater than that of the trunks which give origin to them. By the -time the capillaries are reached, their total bed—their united -cross-section—is about 640 times as great as that of the aorta. This -estimate is based upon the diminution in the rate at which blood -flows through the vessels. The velocity with which a stream flows -through a channel varies as the cross-section of the channel. In a -capillary vessel the blood flows at the rate of from 0·5 millimetre -to 1 millimetre per second. In the aorta the velocity is about 320 -millimetres per second. In the re-formation of the venous system a -converse process of reduction occurs, but not with anything like the -same rapidity. The united calibre of the two venæ cavæ, in which the -reduction is complete, is about twice that of the aorta. From this it -follows that the veins hold much more blood than the arteries; and -since veins are more easily distended, the amount that they can hold -varies within wide limits. They constitute to some extent a reservoir -for blood. - -The capillary vessels are the tubes of the circulatory system in which -blood comes into use. On the average they are about 0·5 millimetre -long. Through them the blood flows slowly. Through their walls alone -is there any exchange worth mentioning between the blood within the -vascular system and the lymph by which it is surrounded. Interest -therefore centres in these vessels. Their walls are formed of -endothelial tiles. In the centre of each thin transparent tile is a -boss, where its lens-shaped nucleus is situate. The outline of the tile -is sinuous. Its margin dovetails with the margins of those adjacent to -it. Oxygen and carbonic acid, nutrient substances and waste products, -pass rapidly through the endothelial cells. Leucocytes have the power -of pushing the cells aside, in order that they may make their way out -of the blood into the lymph which fills the tissue-spaces. With the -exception of the lens and cornea of the eye, cartilage, and the various -epidermal structures, all tissues are traversed by capillary vessels. -It is not difficult to calculate the number of such vessels in the -body exclusive of the liver and the lungs. The diameter of the aorta -is 28 millimetres, that of a capillary about 0·008 millimetre. The -cross-section of all the capillaries added together is 640 times that -of the aorta, as already stated. - -Many schemata have been devised to illustrate the vascular system; -but all are misleading, inasmuch as they fail to give any idea of the -extent to which the subdivision of its vessels is carried. If the -water-pipes supplying a town branched until the original conduit was -represented by five to six thousand million little pipes, the friction -which the pumping-station would have to overcome would be very great. -But little force would remain in the water when it reached the smallest -pipe. Still greater is the resistance to the flow of blood, which -is slightly viscous, and contains solid corpuscles, which increase -friction. Two thousand miles of capillary tubing in the body of a man, -without reckoning the vessels of his liver and lungs! - -[Illustration: FIG. 12.—A PORTION OF THE WALL OF A SMALL ARTERY CUT -TRANSVERSELY AND HIGHLY MAGNIFIED. - - Its inner coat consists of a lining sheet of - epithelial scales supported by connective tissue - and a strong elastic membrane. This membrane is - perforated with holes which place the lymph-spaces - on its two sides in continuity. The middle coat - is composed of plain muscle-fibres and patches of - elastic membrane; the outer coat of elastic fibres, - mostly longitudinal, and connective tissue.] - -Water is supplied to houses in rigid tubes. Arteries are elastic, -and their elasticity is self-regulating. The cause of this will be -apparent if a section of an artery is examined. It contains much -elastic tissue. It also contains plain muscle-fibres. The smaller the -artery, the greater is the amount of muscle relatively to the other -constituents of its wall. The wall of a vein contains very little -muscle, and not much elastic tissue. The muscle of all arterial walls -is in a chronic state of tone. To some extent the degree of tone is -varied automatically. Pressure within an artery acts as a stimulus -to the muscle-fibres of its wall. Any increase leads the fibres to -contract more strongly. Any diminution induces them to relax. The -arteries resist distension; they do not narrow to any great extent -when pressure falls. But more important than this automatic mechanism -for maintaining a uniform pressure in the capillaries in general are -the changes of pressure in particular localities, brought about by the -mediation of vaso-constrictor and vaso-dilator nerves. In almost all -organs and parts of the body the automatic tone of arteries is enhanced -by impulses which flow continuously down vaso-constrictor nerves. These -impulses start from, or, to speak more accurately, pass through, the -vaso-motor centre in the medulla oblongata. From every part of the body -impulses ascend to this centre, urging it to keep up the blood-pressure -by universal constriction. Yet no separate organ would be interested -in sending such a message if it were not open to it to ask at the -same time that the constriction of its own vessels might be relaxed. -Hence it may be said that every individual in the community is crying -out for universal economy, with more generous treatment of himself. -The response made by the State to the latter part of his demand is in -proportion to the vehemence with which it is presented. - -If the spinal cord of an animal be cut across near the medulla -oblongata, respiration being maintained by pumping air into and out of -the lungs, the heart continues to beat with undiminished force, but -the pressure in the large arteries falls to one-third of its normal -height. Constricting impulses no longer pass down the spinal cord from -the vaso-motor centre. This experiment also illustrates the truth of -the statement that models of the vascular system—arrangements of pumps -and indiarubber tubes—are more likely to mislead than to inform. In an -artificial schema the relaxation of the constriction of the small tubes -on the proximal side of the capillary vessels would reduce friction. -Fluid would reach the capillaries in larger quantity, and pass through -them more quickly. The pressure in the tubes which represented veins -would consequently approach more nearly to that on the arterial side. -But when the spinal cord is divided the pressure falls in the veins, -as well as in the arteries. This is due to another factor, and one of -very great importance in the regulation of the circulation. The blood -from the digestive organs is collected by the “portal system” of veins. -These do not join the inferior vena cava; they go to the liver, where -they again break up into capillaries. It is not until after this second -distribution through minute vessels that the blood is re-collected by -the hepatic veins and forwarded to the heart. As in the case of the -arteries, the portal system of vessels is controlled by the nervous -system. When the spinal cord is divided they also dilate. The whole -vascular system becoming more capacious, blood-pressure falls in veins -as well as in arteries. - -When the digestive organs are active, other parts of the body are kept -short of blood. It chanced to the writer, in his student days, to -spend the early summer in Paris, with a big healthy Yorkshireman as -companion. We dined together each night at one of the restaurants of -the Palais Royal _à prix fixe_. After dinner, with British regularity, -my friend called for the _Times_. Then followed a short period of -placid reading, interrupted by the remark: “How cold it is!” Half an -hour later, giving himself a shake: “Suppose we go and dine somewhere -else?” His well-ordered digestive organs had made short work of the -two-franc dinner. They had been ably supported by the vaso-motor system -of nerves which provided them with the bulk of the blood, while limbs -and skin ran short. - -Vaso-constrictor nerves leave the spinal cord by the roots (called -“rami communicantes”) of sympathetic ganglia. Beyond the ganglia they -apply themselves to the large arteries whose course they follow. The -constrictor nerves for the face and neck leave the spinal cord within -the chest by the roots of the first four thoracic nerves. They do not -at once apply themselves to the great artery of the head. Until the -upper part of the neck is reached, they traverse the ganglionated -sympathetic cord, which lies behind the carotid artery and internal -jugular vein. If in a rabbit this cord be cut, the vessels of its -ear dilate, as evidenced by the rosy blush which is observed when a -light is held behind it. If the upper part of the sympathetic cord be -stimulated, the ear grows pale. The redness of the ear remains for -many days after section of the nerve; but gradually the engorgement -diminishes, and the vessels acquire the power of automatically -regulating the flow. - -The classical experiment with the rabbit’s ear suffices to show the -relation of bloodvessels and nerves which holds good for all areas of -the skin. The condition of the skin is the chief factor in regulating -the temperature of the body. In a cold atmosphere its vessels are -severely constricted to limit loss of heat. When one passes into a -warm room the constriction is relaxed. The skin is flushed; heat is -thrown off by radiation. The sweat-glands secrete water, which is -evaporated by the heat of the skin. Constriction and remission of -constriction are the processes which diminish or increase loss of heat. - -This mechanism is different in the case of glands and some other -structures which, when active, require an abundant supply of -blood. Such organs are provided with vaso-dilator in addition to -vaso-constrictor nerves. The most conspicuous example of this is to be -seen in the case of the submaxillary gland. The nerve to this gland -runs for some distance as an isolated thread—the chorda tympani. -Stimulation of the chorda tympani has the double effect of dilating -the arteries of the gland and of causing it to secrete. But the -administration of atropin prevents secretion. Vaso-dilation is then the -only visible effect. Stimulation may increase sixfold the outflow of -blood from the veins of the gland. It rushes through with such rapidity -that it retains its bright arterial hue. The gland also receives a -twig from the sympathetic cord in the neck, which, as already stated, -controls the vessels of the face. By stimulating the one nerve or the -other the physiologist can at will increase or diminish the amount of -blood flowing through the submaxillary gland. Stimulating any sensory -nerve causes in a reflex manner an increased outflow of constrictor -impulses from the centre in the medulla oblongata to all parts of -the body, with the exception of the part to which the sensory nerve -appertains. Its own constituency receives an increased supply of blood. -It is not difficult to appreciate the importance of this double action. -A part is injured. The restrictions placed upon its supply of blood are -suspended. Lest its increased consumption should lead to a general fall -in pressure, all other parts have their supply curtailed. The effect -is even more pronounced than this. The whole blood-pressure is raised -above its ordinary level. The flow of blood to the injured part is -therefore greater than it would be were relaxation of its arteries the -only change. - -The most important of all constrictor nerves are the splanchnics which -control the supply to the stomach and intestines. When these nerves -are cut, the digestive organs become engorged to such an extent that -a pronounced fall of the general blood-pressure is the result. Their -stimulation renders the digestive organs anæmic. We have already -shown that the relaxation of vaso-constriction occurs in a reflex -manner. The reflex relaxation of the splanchnic area is a matter of -great importance, because it can be brought about by stimulation of -one of the sensory nerves of the heart. The higher the blood-pressure, -the harder the heart would work if left to itself. It is an impetuous -organ, always trying to quicken its pace and to increase the force -of its beat. Excessive zeal would get it into trouble if severe -precautions were not taken to hold it in check. True, it is encouraged -by certain “accelerator nerves”—sympathetic filaments which leave the -spinal cord by the anterior roots of the second and third thoracic -nerves; but the influence which the accelerators exert under normal -conditions is not, it would seem, very pronounced. The nerves which -restrain the heart are much more in evidence than those which urge -it on. The arrangements for diminishing the work of the heart are of -two kinds. In the first place, branches derived from the vagus act -as a continuous check. From a certain spot in the medulla oblongata, -the cardio-inhibitory centre, impulses are always descending to slow -the heart. They are of reflex origin, but a high blood-pressure in -the centre increases the facility with which they are transmitted. -Some of these stimuli originate in the heart itself, ascending and -descending the vagus nerve. The remainder come from various sources. -A severe injury to any part of the body slows the heart. Injury -to the intestines, such as occurs in peritonitis, is particularly -effective in increasing vagus inhibition. Slowing of the heart lowers -blood-pressure. When both vagi are cut, the heart begins to gallop -whatever may be the pressure against which it has to work. - -A sensory nerve of the heart, termed the “depressor,” is the chief -agent in lowering blood-pressure. Its course is not the same in all -animals, but it runs more or less in conjunction with the vagus. -Usually it joins its superior laryngeal branch. Impulses which ascend -this nerve inhibit the constriction of the splanchnic vessels. They -open a floodgate which brings down the general pressure. The severe -pain and extreme distress of angina pectoris are the cry of the heart -when blood-pressure is too high—when it feels unable to work against -it. This was recognized by physiologists long before a remedy was -known. A systematic search was instituted for a drug which could be -used with safety to lower blood-pressure. The discovery that the -inhalation of amyl nitrite answers this purpose and fulfils this -condition was the result. - -[Illustration: FIG. 13.—MANOMETER FOR MEASURING BLOOD-PRESSURE. - - A =U=-tube contains mercury, on which floats - a rod supporting a scratching point, which makes - a “tracing” on blackened paper wrapped round a - revolving drum. Between the manometer and the - cannula which is introduced into the central end - of a cut artery is a three-way cock, which leads - to a pressure-bottle containing a half saturated - solution of sodic sulphate. This solution prevents - blood from clotting. Before it is connected with - the artery the apparatus is filled from the - pressure-bottle. The cock is then turned into - the second position, and the bottle raised until - the mercury in the manometer stands at a level - somewhat higher than that which it may be expected - to attain under the influence of blood-pressure. - The cannula being then inserted into an artery, - the cock is turned into the third position, which - places the manometer in connection with the - blood, and excludes the pressure-bottle. As the - mercury is a little higher than blood-pressure, - some of the sodic sulphate solution enters the - artery, but no blood enters the cannula. The - scratching point, rising and falling with every - variation in blood-pressure, makes a record on the - soot-blackened paper, which is subsequently removed - from the drum, and varnished.] - -When we consider the hydrostatics of the circulation, it becomes -evident that changes in the force with which the heart beats, -and changes in the calibre of the bloodvessels, work together -in determining blood-pressure. Both vessels and heart contract -automatically—the former continuously, the latter rhythmically. The -heart of a frog, if it is enclosed in a moist chamber, beats for a -long time after its removal from the animal. Even when cut in pieces, -in certain ways, the separate pieces beat. A strip from the ventricle -of a tortoise’s heart, kept gently stretched by the weight of a light -lever attached to one of its ends, continued to contract rhythmically -for forty-eight hours. When the heart has come to a pause, it cannot -be started again by stimulating any nerve. It has in the most marked -degree its own views as to the rapidity and force with which it ought -to beat. But within certain limits it is under nervous control. The -accelerators hasten it, to its own detriment. They belong to the -division of katabolic nerves—a name given them to indicate that they -waste the tissues, impoverishing their condition. The vagus nerve slows -the heart. It protects it from itself. Its action is anabolic. The -condition of the heart is improved under its influence. If it has been -kept in check for a time by stimulation of the vagus, the heart beats -more strongly when this nerve ceases to act than it did before it was -induced to rest. - -The arteries also are under the influence of two antagonistic sets -of nerves. Those which increase their tonic contraction are almost -universal in their distribution. It may be that those which actively -check it are equally widespread, but the evidence is not altogether -free from ambiguity. On certain organs—such as the salivary glands, -already instanced—which require great variations in the amount of -blood supplied to them, the influence of dilator nerves is very marked. -The simplest hypothesis as to the mode of action of vaso-constrictor -and vaso-dilator nerves leaves the initiative with the muscle-ibres -of the vessel-wall. The distending internal pressure of blood is the -stimulus which induces the muscle to contract. In some invertebrate -animals—the snail, for example—if blood be prevented from entering -the heart, so that there is no distending pressure, the heart stops. -In higher animals the heart has acquired a habit of contracting, which -keeps it going in the absence of its proper stimulus. The two classes -of nerves exercise opposing influences on the muscle. Vaso-constrictor -nerves increase the excitability of its fibres; vaso-dilator nerves -diminish it. Only thus can we explain their action on a common -basis. A good deal might be said as to the reasonableness of such -an explanation. Our views as to the relation of nerve-influence and -muscle-contraction are apt to go astray, owing to the fact that -generations of physiologists have observed the phenomenon of a spasm -of a muscle following on a sudden stimulus to a nerve. The two events -are evidently related. The stimulus appears to set up a new condition -in the nerve—to initiate a process which was not occurring before -the electric current was passed through it. The muscular spasm equally -appears to be an isolated event. As usual, we are misled by the analogy -of human inventions. We compare the nerve-impulse to the fall of a -hammer, the muscle-spasm to the explosion of gunpowder. We forget that -nerve and muscle are in permanent connection; that the impulse is a -sudden exaggeration of an influence which the nerve is continuously -exerting, the contraction an exaggeration of metabolic changes which -are constantly occurring in muscle. (See in this connection the -explanation of muscle-tone, p. 273.) In the case of plain muscle, nerve -stimuli do not cause contraction; they merely increase the excitability -of the muscle. It may be more difficult for us to figure to ourselves -the way in which dilator nerves diminish excitability; but the -existence of such an anabolic influence is beyond the reach of doubt. -Heart and bloodvessels are part of the same system. The heart has its -accelerator and inhibitory nerves, the bloodvessels their constrictor -and dilator nerves. For both vessel-wall and heart the stimulus to -contraction is the distending pressure of blood—although it is not -altogether necessary that this stimulus should be acting at the time. -Sympathetic and vagus nerves can to a certain extent control the -beating of a bloodless heart. The heart-tissue has acquired the habit -of beating, and the habit of listening to advice conveyed to it through -these nerves. - -The self-adjustment of the blood-tubes to the pressure to which -they are exposed is exhibited in the adaptation of their degree of -contraction to the position of the body—to the weight, that is to say, -of the column of fluid which they have to support. Everyone has played -the game of “right hand or left.” When the hand is held above the head -the blood leaves it, and the hand becomes cold; but if there be need -for adjustment, and time is given for the mechanism to come into play, -it works to perfection. When we are standing erect, there is neither -too much blood in the feet nor too little in the head. But after a -fortnight in bed a convalescent finds, the first time that he stands -upright, that his legs are quickly engorged—his slippers after a few -minutes feel too tight for him—whereas the brain becomes so anæmic -that he turns giddy, or even faints. - -Numberless illustrations of vaso-motor action are met with in daily -experience. It is a curious fact that the nerves which control the -calibre of the bloodvessels tend to overact their part. When an organ -demands more blood, it is supplied at the expense of the rest of the -body, and especially of the parts most nearly adjacent. This is partly -a mechanical effect. If all the houses in a terrace are supplied with -water from a common main, the bursting of a water-pipe in one of them -will reduce the supply of its neighbours more than it will reduce the -supply of houses in distant parts of the town. But vaso-motor nerves, -in their compensating adjustment, go farther than this. A thimbleful of -blood removed by a leech produces an effect upon an underlying engorged -organ altogether out of proportion to the hydrostatic requirements -of the case. “Cupping” the loins diminishes the congestion of the -kidneys. This is the explanation of the curative efficacy of various -agents which, with improvements in surgery and the introduction of -more reliable drugs, have almost disappeared from the surgeon’s -armamentarium—scarification, blisters, setons, and the like. Such -methods have been relegated to veterinary practice. - -There is a marked tendency to see-saw between the skin and the mucous -membrane of the alimentary canal. During active digestion, when the -“splanchnic area” is full of blood, the skin is cold. Hot fomentations, -by dilating the vessels of the skin, diminish congestion of the -alimentary tract. An inflamed throat is relieved by a compress round -the neck. Conversely, it must be admitted that, in certain persons, -slight constriction of the vessels of the skin induces inflammation -of the mucous membrane. This is one reason for the almost universal -dread of draughts. A draught cools a limited area of the skin. Some -of us cultivate a love of draughts. They are the sensible evidence -of the entrance of fresh air. Yet we admit reluctantly that certain -fragile mortals are not altogether fanciful in supposing that a draught -may give them a catarrh or a toothache. If asked why they object to -draughts, many persons answer that they “are afraid of catching a -chill”—carrying us back to the time before the clinical thermometer -was invented; to days when the shivering fit, or “rigor,” which first -calls attention to the fact that the temperature is already two or -three degrees above the normal, was supposed to be the commencement -of the illness. The patient imagined that the “chill” caused him to -shiver, and that if he had not “caught” it he would not have been ill. -The substitution of the term “cold” for “rheum,” naming the malady -after one of its prominent symptoms, has done much to perpetuate this -superstition. “Chill” is a word we scarcely dare to mention. When -doctors could no longer attribute to witchcraft the occurrence of -disorders for which they had no other explanation, they invented the -luminous theory that inflammatory diseases—especially those of the -stomach, liver, and lungs—were produced by “a chill.” At one time all -diseases which were not evidently infectious were caused by chill. The -discovery of germs and the recognition of their maleficent activity -has stripped this cloak of ignorance off almost every case of abnormal -tissue-metabolism. It is recognized now that the germ _is_ the disease, -not the effects which the germ produces. Pneumonia is impossible in -the absence of the pneumococcus, however severe the chill to which the -patient was exposed when out in the cold and wet. Consumption is the -effect produced by the tubercle bacillus. If there are no bacilli, -there can be no consumption. Yet these two diseases illustrate the -possibility of the use of the term “chill” without impropriety. The -coccus of pneumonia may frequently be found in the mouth of a healthy -person. If everyone with whom the tubercle bacillus has at some time -come in contact were inevitably its victim, no human being would be -free from phthisis, if any still survived. There are conditions of -health, or rather of unhealth, in which the economy is less resistant -than usual to the germs. Apparently the vaso-motor disturbances of -internal organs caused by the cooling of the surface of the body, if it -occur when health is otherwise depressed, contributes to the production -of such a state. - -The vaso-motor system is influenced by emotions. It is a little -difficult to express accurately the relation between emotion and -vaso-motor change. Some psychologists regard the vaso-motor change -as the emotion. “All emotions,” says a prominent exponent of this -view, “are wholly due to excitation of a particular kind of the -vaso-motor centre.” The person about to be subject to an emotion -of shame, anger, fear, disgust, recognizes a fact or circumstance, -or conjunction of circumstances, which justifies the emotion. (We -are assuming that emotions may be justified; that the intellectual -appreciation of a situation and reasoned decision regarding the -action which it demands is not sufficient.) This recognition as an -intellectual act of the higher brain is accompanied by certain forms -of enhanced activity or inhibition of activity of the vaso-motor -centre in the medulla oblongata which cause changes in the degree of -contraction of the bloodvessels of certain organs. The vascular changes -produce an alteration in the state of the organ which is reflected -in nerve-currents sent back to the brain, providing the background -of feeling which constitutes emotional tone. We are not prepared to -endorse this extreme view of the nature of an emotion. A maiden’s -blush is not an emotion of embarrassment or shame. It is its harmony. -Her mind plays the air. The sensations which originate in the flushed -skin of the face sustain it with their accompaniment. The emotional -tone keeps attention fixed on the fact or circumstance which led her -to conclude, by the exercise of her reason, that she was placed in an -awkward situation. This fixing of attention is frequently so pronounced -as to inhibit all other intellectual action. The maiden is less quick -than she would have been, had the emotion not glued her thoughts -together, in recognizing the readiest means of extricating herself -from embarrassment. All nerves found within the chest and abdomen -were in very early times termed “sympathetic.” The cord in the neck -was the “little sympathetic.” The name explains itself; but it will -be understood that it implied much more in the days when the liver, -spleen and heart were supposed to pour out emotions than it does now. -The vagus nerve was termed the “middle sympathetic.” Shame inhibits the -activity of the vaso-constrictor nerves of the face; dilation of the -vessels which they supply is accompanied with constriction of other -cutaneous nerves. Kipling must, we think, have embellished Nature -when he represents the very unimpressionable hero of Lungtungpen as -admitting “I niver blushed before or since; but I blushed all over -my carkiss thin.” Usually the carmine of the face contrasts with the -pallor and coldness of the hands. Still, we are not prepared to assert -that it is impossible, under circumstances as trying as those in which -Private Mulvaney and his companions were placed, for all the cutaneous -constrictor nerves to let go their grip at the same time. Terror -heightens the control of the vaso-motor centre over the vessels of the -skin; it increases vagus inhibition of the heart. Even disgust evoked -by a revolting sight or a foul smell may call the vagus so forcibly -into action as to bring the heart to a standstill. - -=The Pulse.=—The arterial system is always distended. The pressure in -the largest arteries amounts to about 140 millimetres of mercury. The -source of pressure is the beat of the heart pushing the blood forward -against the resistance offered to its flow by the smallest vessels. -At every stroke another 3 ounces is added to the already overfull -vessels. In the aorta, therefore, the blood moves forward with jerks, -but by the time it reaches the capillaries the intermittent accessions -of force have been taken up by the elastic walls of the vessels and -returned to the stream in the form of constant pressure. In the very -smallest arteries the blood flows in a steady stream. If the corpuscles -in a capillary vessel are watched under the microscope, they show no -variations in rapidity synchronous with the beat of the heart. The -“pulse” in the larger arteries is the push given to the column of blood -by the sudden contraction of the left ventricle. Its propagation along -the arteries will be understood if it is remembered that the blood is -contained within elastic tubes. The first effect of the ejection into -the aorta of an additional quantity of blood is the distension of its -wall. The wave of distension travels down all the arteries of the body -with gradually decreasing force. - -[Illustration: FIG. 14.—SPHYGMOGRAPH. - - A, An ivory button which is pressed on the skin - over the radial artery by a metal spring. B, - A continuous screw which works against the - cogwheel C. By rotating B, the lever D is raised - to a position in which its point scratches the - travelling-plate E (covered with blackened paper). - F, A box containing clockwork which moves E. G, A - screw by means of which the pressure of the spring - is adjusted to the force of the pulse.] - -[Illustration: FIG. 15. - - A, A cardiogram, or tracing of the impulse of the - heart, recorded on a blackened plate borne on - the end of a vibrating tuning-fork; _a-b_, the - systole of the auricles; _b-e_, the ventricular - systole. From _c-e_ the heart is shrinking as blood - leaves it by the aorta and pulmonary artery. B, - C, D, E, F, Sphygmograms. B is shaded to show the - portion of the pulse-wave which corresponds to the - systole of the heart. C, High-tension pulse of - vigorous health. D, Low-tension pulse. E, Dicrotic - pulse of fever. F, Hog-backed pulse of hardened - (atheromatous) arteries.] - -Much may be learned from the pulse with regard to the condition of -the vascular system, although it is impossible to balance the effects -of the several factors which go to the production of its various -modifications. The character of the pulse depends upon the vigour -with which the heart is beating, the efficiency or otherwise of the -cardiac valves, the quantity of blood in circulation, the suppleness -of the arterial walls, the degree to which they are contracted, the -resistance offered by the smaller vessels. Departures from the normal -may take the direction of unduly high tension or of unduly low tension. -In place of the sudden rise and more or less gradual fall, with the -slightest possible roughness due to secondary waves, which constitutes -a healthy pulse, the rise may be shorter, its subsidence prolonged. -This is a high-tension or hard pulse. The pressure in the arteries -is unduly high, or the walls of the vessels are not as elastic as -they should be. Considerable pressure is needed to obliterate such a -pulse—_i.e._, to prevent it from passing on beneath the finger. As -the converse of this condition, the difference between the beginning -of the pulse and its end may be very marked, the vessel suddenly -dilating and as suddenly collapsing. But little pressure is needed to -stop such a low-tension pulse from passing beneath the finger. Usually -it has a distinct secondary or dicrotic wave. Some tactile education -is needed by the finger that aspires to read the pulse. It was hoped -that the personal equation would be of less importance if mechanical -records were substituted for statements as to the impression produced -upon the observer. Various forms of sphygmograph (σφυγμός, pulse) -have been invented for this purpose. The form commonly used (Fig. 14) -consists of a metal spring which is adjusted so that a button beneath -its free end presses on the radial artery at the wrist. The force -with which it presses is regulated by a screw. At each pulsation its -free end is lifted by the distension and rounding of the artery. Its -movement is transmitted by means of a continuous screw, attached to -it vertically, to a cogged wheel, which in its turn raises a lever. -The end of the lever scratches blackened paper fastened on a plate -moved by clockwork. Records made in this way are useful for future -reference. They are not, however, so valuable as it was anticipated -that they would be. The form of the tracing depends to so large an -extent upon the amount of pressure exerted by the spring, and the -amount of pressure must be adapted to the vascular tone in every case. -Some of the most interesting tracings are obtained from old people -affected with atheroma of the arteries. This is a condition in which, -owing to old-standing inflammation of the subepithelial coat of the -vessels, the arteries have lost their suppleness. They are hard and -inelastic. Instead of showing the normal steep face of the pulse-wave -rising abruptly to its highest point, the tracing rises vertically for -a short distance, and then slopes upwards. The wave is flat-topped or -hog-backed. - -All pulses are dicrotic, although the dicrotism may not be sufficiently -pronounced to be felt with the finger. The notch which divides the -primary from the secondary wave is produced by the closure—that is -to say, by the falling down of the aortic valve. The wave from the -commencement of its ascent to the dicrotic notch corresponds to the -period during which blood is passing from the heart into the aorta. -This part of the tracing represents systole of the ventricle after the -semilunar valve has been forced. It is the push given to the bottom of -the column by the additional 3 ounces of blood thrust into the aorta. -The effort of the ventricle then comes to an end. The pressure beneath -the semilunar valve is less than that above it. The valve closes. If -the blood were contained in an open tube, the wave would now end, -save for secondary oscillations, due to inertia of the fluid. But the -arterial system is practically closed owing to the fineness of the -tubes into which it ultimately divides. Its walls are elastic. They -distend, taking up the pressure and returning it again in the second -half of the wave. In fever, after the consumption of alcohol, and in -other conditions in which the finest bloodvessels are dilated, the -division between the two parts of the wave is very marked. Dicrotism -is plainly felt. We have used the expression “finest vessels” rather -than “capillaries,” because the ascription to the capillary vessels of -all peripheral resistance has led to misunderstanding. Resistance is -offered throughout the whole vascular system, with the exception of the -largest veins. It is greatest in the small arteries, capillaries, and -small veins. It is so adjusted as to fall to zero just before the blood -reaches the heart. - - - - -CHAPTER X - -MUSCLE - - -Living matter, protoplasm, is irritable. It responds to influences -impressed upon it by its environment. An effective influence, termed -a “stimulus,” produces a change in protoplasm at the spot at which -it acts. From this spot the change spreads outwards as an “impulse.” -Protoplasm is said to “conduct.” A stimulus may be likened to a -blow given to a fixed but elastic mass; an impulse to the vibration -which travels outwards from the spot struck. Unfortunately, the term -“stimulus” is used both for the stick that strikes, the stimulator, -or stimulant, and for the blow that is struck; but breaches of logic -seldom lead to confusion in an experimental science. The context -indicates the particular application of the term. The manifestation of -stimulation is a physical or chemical change—most obvious when it is -one of form. This change of form may occur at the spot stimulated, or -may be deferred to a distant part to which the impulse is conducted. - -In opening the study of muscle and nerve we need to form a conception -of the nature of these three functions—irritability, conductivity, and -changeableness of form. Not that the functions are as distinct as the -ideas to which the three terms give rise. They are three aspects of a -common function; although this is a reflection which will carry more -weight when the ways in which protoplasm reacts to external forces have -been considered. - -A stimulus may be mechanical, something in the nature of a blow which -displaces the particles of protoplasm; or it may be chemical or -thermal, disintegrating a portion of its substance; or electrical, -divorcing the ions of its molecules. Only the last in any way -resembles a natural stimulus; since electrical stimulation alone can -be repeated without the substance stimulated showing any evidence of -injury in the process. Mechanical, thermal, chemical stimuli destroy -a portion of the protoplasm upon which they act. Yet even the weakest -of electric currents is a gross disturbance as compared with natural -stimuli, such as touch, warmth, sound, light. The essential and most -distinguishing quality of living matter is its return to its original -state immediately after stimulation. It does not even wait until the -stimulator has ceased to act. An effective influence is a sudden change -in the environment. It is answered by a sudden response, followed by a -return of protoplasm to the state in which it was before the impact of -the external force. The change progresses through the protoplasm as a -transitory alteration of state, the particles concerned in conducting -it returning to their original condition the moment it has passed. -No non-living matter responds to force in this way. If a stone is -dropped into a pond, a wave circles outwards from the spot it strikes; -but this is a wave of displacement, not a change of state. Suppose -the pond contained a solution of sugar which the impact of the stone -changed into vinegar, and that the zone of vinegar spread outwards, -the liquor returning to the condition of sugar and water as it passed. -Here we should see some analogy to the progress of an impulse. But no -non-living matter behaves like this. A product of the laboratory may -be so unstable as to explode when shaken, passing on the slightest -provocation into a more stable state. It does not return after the -explosion to its previous strained condition. Having thrown away its -energy, it continues on a lower plane. Protoplasm parts with energy -to recover it again. It returns to instability after assuming a more -stable form. - -If we are to form a conception of the cause of the irritability of -living matter, we must have a mental picture of the physical conditions -which distinguish life from death. All matter is in a state of motion. -It consists of separate molecules, each moving in its orbit with vast -rapidity. A molecule is a cluster of atoms. The dimensions of its -orbit depend upon the number and weight of the atoms in its cluster. -If we could watch the dance of the molecules of proteins and other -substances into which protoplasm breaks up on dying, we should see -each separate cluster executing the figure appropriate to its mass, -indifferent to the movements of neighbouring groups. But if living -protoplasm were of the company, the scene would be one of vastly -greater animation; for now it is the ambition of our dancers to form -a single group. To this they can never attain. There is a physical -limit to the number of dancers who can hold together while the music -carries them in wide sweeps backwards and forwards across the floor. -At every gust of wind which bursts through an open doorway a group -breaks, to clasp hands again as the wind subsides. Protoplasm is -always on the verge of instability; always snatching at additional -atoms which it draws within its ring; always shaking off other groups -of atoms because the ring is too large to hold together. Touch it, -and it falls into simpler combinations. Kill it, and it becomes a -mixture of organic and inorganic compounds which we know and can -name. But as long as it is alive—as long as it is protoplasm, that -is to say—integration and disintegration are occurring. Simultaneous -complication and simplification _is_ life. The protoplasm-molecule, if -we dare to think of it as a molecule, in the sense in which a chemist -uses the term, is always changing. It is its variability which makes -stimulation possible. Irritability is a tendency to dissociation under -the influence of an external force, with reassociation when the force -ceases to act. - -The molecules which protoplasm gathers into itself may be classified -under the headings oxygen, foods, water, and inorganic salts. It is the -two latter which most affect its state, conferring upon it the capacity -of exhibiting the phenomena of life. Water and the ions of salts -dissolved in water, electrolytes, are linked to the other elements of -its groups. Striving to find room for more molecules of water and more -ions, protoplasm expands. It becomes more mobile and more irritable; -for irritability and mobility vary as the number of these extraneous -groups of atoms which protoplasm is in a position to let drop. As -an impulse travels through it they lose their hold, recovering it -as they pass the impulse on. This progress towards expansion is the -lifeward tendency; the quickening of activity which leads also to the -incorporation of additional atoms of nitrogen-containing substances, -and consequent growth. - -The opposite tendency is deathwards. Protoplasm drops extraneous groups -of atoms; retires into itself; loses irritability; settles down to rest. - -The molecules of proteins exhibit a property which appears to pertain -in some degree to living matter also. When their relation to the water -in which they are dissolved and the electrolytes which it contains is -disturbed, they appear to go out of solution, they coagulate. This -disturbance is brought about in all proteins by heat; in some it is -the result of altering the amount of salt in the water in which they -are dissolved. Coagulation in protoplasm is the prelude to death; but -it would appear that a step in this downward path is taken whenever -an impulse is conducted. Coagulation is due to the clustering of the -molecules of a protein. When protoplasm drops electrolytes and water -its molecules cluster in some degree, regaining their independence and -reattaching their accessory groups of atoms as the cause which drove -them to make for safety passes by. - -Our conception—not of life, but of “the physical basis of life,” may -be very wide of the mark. The account given above is intended as little -more than a hint of the lines along which thought is travelling at the -present time. The reader must not regard it as a serious attempt to -present in detail the views of any of the workers who are endeavouring -to apply the results of recent discoveries in molecular physics to the -solution of problems in the chemistry of living matter. There can, -however, be little doubt but that we are on the eve of further advances -which will secure data upon which it will be legitimate to construct -hypotheses. At present it would be unreasonable to do more than -indicate the direction from which it may be hoped that light will shine. - -A stimulus is a change of circumstance rather than a transient -disturbance. When an electric current is thrown into it, protoplasm -dissociates—parts with something. It instantly reassociates. The -continued passage of the electric current does not maintain it in a -dissociated condition. When the current is cut off, the sudden change -again acts as a stimulus. Within limits, the efficiency of an electric -stimulus varies as its suddenness. Similarly with all other stimuli to -which protoplasm responds: crushing, burning, chemical decomposition, -are effective at the moment of their occurrence. When they generate a -succession of responses, it is because they continue to produce changes -in the protoplasm. Their continued action does not, under ordinary -circumstances, prolong the response. - -Response to stimulation travels as an _impulse_ through protoplasm. -An impulse is commonly likened to a wave, but enough has been said -already to prove that the simile is misleading. It is not of the same -nature as the wave which a stone starts on the surface of a pond, a -pulsation of sound through air or water, an undulation of light or heat -in the æther. These various kinds of waves are waves of displacement, -a swing first to one side and then to the other. An impulse traverses -protoplasm, whether it be the apparently diffuse protoplasm of a -leucocyte or the severely oriented protoplasm of a nerve or muscle, as -a change which may be described as chemical, with reservations as to -the meaning allowed to this term. We may without impropriety represent -the fall (dissociation) and subsequent rise (association) graphically -as a wave; but even then it is but a half-wave, and inverted. It is -a very different thing to the onward progression of an accession of -force, with which it is not infrequently confused. - -All protoplasm is not equally susceptible of stimulation. Probably it -is safer to put this in a different form. Protoplasm is not everywhere -equally exposed to stimulation, nor is it when especially exposed -to stimulation in one way equally accessible to all other effective -forces. A sense-organ is a collection of cells in which protoplasm is -so disposed as to be susceptible to a certain kind of stimulus. It is -a “receptor” for a particular force. At the same time it is essential -to its efficiency that it should be insusceptible to other forces. The -protoplasm in certain of the sense-organs of the skin dissociates when -compressed, in others when warmed. The cells of these receptors have -a certain structure which exposes their protoplasm in such a manner -that it cannot escape dissociation when, in the one case, the cells are -squeezed, or when, in the other case, they are heated. The ear contains -sensory cells so constructed that the protoplasm which they contain -dissociates when affected by pulsations of sound. In the receptors of -the tongue and the nose protoplasm is exposed to the influence of -chemical stimuli; in the eye it is exposed to the dissociating action -of light. - -Protoplasm is responsive to external force. It conducts the impulses -to which stimulation gives rise. Eventually the impulses, which -travel along strands of tissue highly specialized for the purpose -of conduction—nerves—reach collections of protoplasm which are so -disposed that when they dissociate energy is set free. A comprehensive -term is much needed for the connotation of this third essential -property of protoplasm, the capacity of liberating energy which -characterizes “effectors.” An external force, so small in intensity -as to be negligible when we are dealing with the body’s accounts, -acts upon the protoplasm of a receptor. A change in state results. -The change is conducted to an energy-liberating organ. This organ is -supplied with blood which brings it food. Food is its store of energy, -the raw material from which it manufactures its ammunition. When an -impulse reaches an energy-liberating organ its protoplasm dissociates. -But here the protoplasm is so disposed—the cells which contain it have -such a form—that when it dissociates a change in the cell follows; -it alters in shape, or it discharges into its environment heat, or -electricity, or light. The dissociation and reassociation of the -protoplasm of an effector involves chemical change. Molecules of water -and of carbonic acid are cast off. The energy sacrificed in letting -matter fall into these very stable forms is the energy made visible, as -it were, in lifting a weight or dispersing heat. It must be replaced if -the organ is to retain its power of acting when next an impulse reaches -it. To replace it, protoplasm takes up food and oxygen from the blood. - -The liberation of energy which occurs when a muscle contracts is not -a special phenomenon—something which does not occur when the muscle -is at rest. It is an intensification of a process which is always -taking place. The substance of muscle, like that of nerve and every -other tissue, is always combining with oxygen and giving off water and -carbonic acid. When we are auditing the body’s accounts, we enter so -and so much food and oxygen on the debit side, we credit it with the -same weight of water and carbonic acid; or we debit it with the energy -potential in the food, and enter to its credit the mechanical work done -and the heat set free by the oxidation of this food. Food is the petrol -the combustion of which causes the movement of the car. The external -force which stimulates a receptor is too insignificant in amount to be -carried to account. Physiologists neglect it, just as engineers neglect -the energy liberated by the sparking-plug which ignites the petrol, -when they are estimating the efficiency of a motor. - -Compared with the amount of energy actually received from the -environment when a sensory cell of the eye or ear is excited, the -energy needed to start an artificial impulse in a nerve is relatively -enormous; yet a well-known comparison of the energy conveyed to the -nerve in a certain experiment with a nerve-muscle preparation from a -frog, and the energy expended by the muscle in contracting, brings home -to our minds the fact that it is impossible to carry even this item to -account. The energy furnished to the nerve from an electric condenser -measured 0·001 erg; the energy expended by the muscle reached 100,000 -ergs. - -It is easy to determine the amount of mechanical work which results -from a given expenditure of energy. By alternately flexing and -extending the joints of his legs, a man lifts his own weight up a -hill of a certain height. The work can be measured in foot-pounds or -in kilogrammetres. But this by no means accounts for all the energy -potential in his food. A still larger amount is expended for the -purpose of keeping the body warm, or, not improbably, making it too -warm; in either case generating heat which is dissipated into the -atmosphere. When a machine is being planned, attention is concentrated -upon the problem of how to get the largest result in work for a given -quantity of fuel. Fuel costs money. All energy dissipated as heat -is wasted. Every ounce saved makes for economy. Engineers therefore -speak of the “efficiency” of an engine as the relation between the -work actually done and the work which would have been done if no -energy had been wasted. In the best steam-engines it stands at about -1 to 10. Since the chief function of muscle is to do mechanical work, -physiologists are apt to adopt the engineer’s point of view. But in the -case of muscle this is justifiable only in a limited degree. The body -of a warm-blooded animal is maintained at a temperature higher than -that of the surrounding air. Muscles are the chief producers of heat. -If they turned all the energy which they receive into work, they would -be inefficient as regards this very important function. Yet even from -the engineer’s point of view muscles are more efficient than the best -of engines. - -It is almost impossible to determine with accuracy, in regard to -isolated muscles, the amount of food taken up from the blood, and the -return in work by the muscles of the energy potential in the food. -Calculations have to be based upon observations of food consumed, gain -or loss of body-weight, work done by a man or an animal during a period -lasting for several days. We shall consider the evidence obtained -in this way in a subsequent section (p. 149). But whether we study -isolated muscles or the body as a whole, the relation between work and -heat varies within wide limits. So wide, indeed, are the variations as -to justify the conclusion that there is no necessary relation between -the two phenomena. Muscles develop heat when they are quiescent. -Activity is accompanied with an increased evolution of heat; but, if it -be desirable, the evolution of heat is reduced until it is, relatively -to the output of work, much smaller than in the case of any engine -which has yet been made. It is sufficient in this connection to state -that, under certain conditions, the return in work may amount to about -one-half. The comparison with an artificial motor, of whatever kind, -breaks down. In an engine combustion develops heat, heat causes steam -or gas to expand, the expanding gas pushes a piston. In muscle certain -of the carbon, hydrogen, and oxygen atoms contained in protoplasm -combine to form water and carbonic acid—compounds too stable to be -reassociated with the remaining atoms of the protoplasm-molecule. -They are replaced by complex, energy-yielding substances—foods—and -by oxygen, carried in the blood. Their displacement brings about a -change in the form of the molecules which involves, owing to their -peculiar orientation, a change in shape of the muscle as a whole. -Such an explanation is, perhaps, more exact than our knowledge at -present warrants; or rather let us say, since we do not know what the -expression “the form of a molecule” means, it has an appearance of an -exactitude which does not characterize it. It is merely intended to -help the reader to realize the hopelessness of attempting to compare -muscle with any mechanical contrivance. In the boiler of a steam-engine -heat is applied to water until its molecules cannot remain in so -close a state of aggregation. Their orbits are greatly increased. The -cause of the thrust given to the piston of an engine is the increased -amplitude of movement of the molecules of steam behind it. In a -combustion engine a mixture of petrol and air is ignited. Energy is -set free by the resolution of unstable petrol into stable water and -carbonic acid. This energy heats the gases, causing them to expand. -Waste of energy as heat is inevitable in a machine which depends for -its motive-power upon the translation of molecules. The source of -muscular force (if it be not intramolecular change) is certainly not, -directly, increased amplitude of molecular swing. - -But we must not conclude, as we are tempted to do, that muscle is -capable of liberating as mechanical work the whole of the energy -supplied to it in food, seeing that its activity is always accompanied -by evolution of more heat than can be attributed to friction. If -the bulb of a thermometer be inserted into a group of muscles, the -instrument shows a marked rise of temperature when the muscles -contract. Even though the temperature of the chamber in which an animal -is placed is equal to its own, the animal makes more heat if compelled -to work, notwithstanding the fact that the consequent rise of its body -temperature may prove fatal. - -Nor can muscles dispense unlimited heat without doing mechanical work. -If I am too cold, the obvious means of getting warm is jumping about. -There appears to be a level of heat-production which cannot be exceeded -without movement. When more heat is called for than quiescent muscles -can produce, they exhibit flickering contractions, shivering, without -moving the limbs. The signal for increased production is given by -the skin. The skin is sensible of the amount of heat which is being -lost. Exposure to cold air makes one shiver, by suddenly withdrawing -heat. But an increase of temperature in the blood behind the skin -has an exactly similar effect. In the first stage of fever, when the -temperature of the body has risen two or three degrees, and before -the system has become accustomed to this state of affairs, the skin -announces to the muscles that heat is being rapidly lost. A severe -shiver, termed a “rigor,” is the result. At the same time loss of -heat by evaporation is checked, just as it is when the skin is cold. -The sweat-glands are rendered inactive. A phenomenon which marks the -nightly fall of temperature in consumptive patients is the sudden -return of activity in these glands. - -Muscle when most highly developed has an extraordinarily definite -structure. It is minutely subdivided into units which appear, looked -at separately, simple in design. We are tempted to believe that the -explanation of the way in which each of these units works is not far to -seek. It is disappointing to be obliged to admit that, notwithstanding -all the thought which has been devoted to the problem, we are as far -as ever from a definitive solution. We understand the principles on -which steam-engines, combustion-engines, electric motors are planned. -We compare muscle with each of these mechanical contrivances in -turn, expecting to discover the principle of its construction. Many -ingenious hypotheses have been formulated; but the fact that some of -these are mutually destructive shows clearly enough that as yet no -approach to certainty has been made. Probably the fundamental error -lies in attempting to compare muscle with a mechanical contrivance. The -apparent simplicity and regularity of structure of “striped muscle” -misleads us. We ought to have commenced our investigations at the other -end of the scale of mobile tissue—to have begun with semifluid and -apparently homogeneous animal matter, working upwards to the tissue -which, being limited to the one function of movement, and movement in -one direction only, has, as it were, crystallized along the lines of -force. - -All protoplasm is mobile. Its particles move one on another. Hence -follows either circulation of the living matter within the cell or -change in shape of the cell. The two phenomena are identical in nature. -Circulation is best studied in a large-celled, transparent part of a -plant. A filamentous water-weed is suitable for the purpose. If this -be examined with a microscope while still alive, its cells are seen to -contain a watery juice enclosed in spaces of denser cell-substance. -Bridges of cell-substance span the spaces. The particles of which -these bridges consist are in a state of constant streaming motion, -which has, it is needless to say, no effect upon the shape of the cell -(_cf._ p. 9). - -The unicellular animal amœba, leucocytes, and certain spores of plants, -are devoid of cell-wall (_cf._ p. 28). Their soft protoplasm is not -limited by a rigid case. When it streams, the form of the cell is -changed. True, we must not think of the body-substance of an amœba as -homogeneous. It exhibits an internal structure. Yet its architecture -is not, so far as we can see, sufficiently fixed to restrict the -directions in which it can stream. Any change of shape is possible. -We cannot find in Nature an isolated clump of living protoplasm; nor -do we suppose that, if we found it, it would prove to be homogeneous. -It appears to be necessary that protoplasm and metaplasm—the terms -have no chemical significance; “primary” and “secondary,” or “chief” -and “subsidiary” would be equally distinctive—should be intermixed. -Streaming is apparently due to alterations in the surface relations of -the two substances. - -In multicellular animals certain elongated cells are arranged in -groups, with their long axes all pointing in the same direction. They -can change in shape, diminishing in length, with equivalent increase in -breadth. Since all the cells of a group undergo this change of form at -the same time, the result is an alteration in the shape of the animal -of which they are a part. Applying the experience which we have gained -in studying the movements of unicellular organisms, we conclude that -these elongated cells are composed of two substances—protoplasm and -metaplasm. The restriction of their capacity for altering their shape -to one direction indicates that their protoplasms and metaplasm are not -indifferently mixed. The two substances set in lines in the direction -of the long axis of the cell. Hence, when streaming occurs—when the -force which keeps the molecules of protoplasm and of metaplasm in their -respective rows is relaxed—the lines thicken. The cell broadens, with -an equivalent diminution of length. - -Muscle-fibres exhibit all degrees of specialization. The simplest, -“plain muscle-fibres,” are found in the wall of the alimentary canal, -of bloodvessels, of ducts, in the tissue of the spleen, in the skin, -and elsewhere. Each fibre is a fusiform cell. Save for its central -nucleus and a little granular protoplasm in which the nucleus is -embedded, the cell may show no architectural features. But in most -varieties of plain muscle, and especially in that of the alimentary -canal, the substance of the fibres is striated longitudinally. This is -visible evidence of the orientation of the molecules of protoplasm and -metaplasm in the direction of the long axis of the fibre. It shows that -the streaming of particles occurs along these lines. It is, as it were, -a diagram of the lines of force. - -Heart-muscle has been described already (p. 224). Its striation, which -is both transverse and longitudinal, is so delicate as almost to defy -microscopical analysis. The transverse striæ are the darker and more -distinct. But close examination shows that the transverse striæ do not -indicate the direction in which the particles of cell-substance are -oriented. They are oriented longitudinally. The cell is a bundle of -rods of substance A, embedded in substance B. The transverse markings -are very thin lines which cross the bundles at right angles. - -The third variety of muscle is the kind by which locomotion is -effected. It is present in large masses—all the red tissue to which -the term “meat” is commonly applied. It accounts for about 35 per cent. -of the body-weight. This kind of muscle is not composed of single -cells, but of compound cells, or cell-complexes, termed “fibres.” A -fibre may attain a length of upwards of 2 inches, with a breadth of -about ¹/₅₀₀ inch. In most cases the fibres are attached by one end to -a bone, by the other to a tendon; and since they are shorter than the -muscle as a whole, the tendon commences as a membrane which covers the -surface of the muscle, sloping to it from the bone to which by their -other ends the fibres are attached. A fibre is developed from a single -cell. The cell elongates, its nucleus divides, and the daughter-nuclei -divide until several hundred have been formed; but cell division -does not follow. The result is a cylindrical mass enclosed within a -delicate membranous sheath, the sarcolemma. In the early stages of its -development its nuclei are in the axis of the fibre, but subsequently -they are displaced outwards. In the most highly specialized muscle, -known as the “white” variety, they lie just beneath the sarcolemma -(_cf._ Fig. 16, B). - -The feature of this type of muscle is its transverse striation, almost -mathematically regular. Commonly striated muscle is spoken of as -“voluntary,” because, for the most part, it is under the control of -the Will; but the term, in so far as it implies a connection between -structure and mode of actuation, is misleading. Transverse striation -is evidence of capacity for rapid action. The muscles which the Will -directs exhibit promptitude; but striated muscle, which is not under -the direction of the Will, is found in certain situations—_e.g._, the -upper part of the œsophagus. Conversely, many animals can voluntarily -call into action muscle which is not striped. A turkey erects its -feathers by setting in motion little groups of “plain” fibres, which -pull on elastic tendons attached to the tips of the buried ends of -their shafts. Plain muscle contracts less promptly and relaxes more -slowly than the striped variety. Cardiac muscle is quicker in acting -than plain, but does not hold the contraction so long. - -All striped muscle is not equally rapid. Two varieties are -distinguishable: “white fibres,” which respond suddenly to a single -stimulus and quickly relax; “red fibres,” which respond in a more -leisurely way, but remain contracted longer. In some muscles these two -types of fibre are intermixed. Others are wholly red or wholly white. -Everyone is familiar with the contrast which the white flesh of a -turkey or of the domestic fowl presents to the red flesh of game-birds -and birds of prey. In the breast of a blackcock a sheet of white -muscle overlies a mass of red. When the bird is cooked the difference -in colour is strongly marked. Of the two muscles which, in a rabbit, -correspond to our muscles of the calf, the superficial, gastrocnemius, -is white; the deeper, soleus, red. The former acts over both knee -and ankle joints; the latter over the ankle only. The muscle which, -acting over a longer range, has to contract more quickly is white; the -shorter, more slowly acting muscle is red. Experiment shows that red -and white muscles are distinguished by a difference in the promptitude -with which they respond to an electric current. It shows, too, that -the white muscle is exhausted sooner than the red. It cannot give so -many successive responses to stimulation without a rest. We shall find, -when we are considering the minute structure of striped muscle, a -difference between its two varieties which we can correlate with their -different modes of action. All human muscles belong to the red kind. - -The most efficient muscle-fibres in the animal kingdom are found in -insects. This will not surprise anyone who thinks of an insect’s power -of movement. If a man could jump as many times his own height as a flea -can, he would clear the dome of St. Paul’s. An ant can drag an object -sixty times as heavy as itself, with no wheels beneath it to diminish -friction. Under the same conditions a horse cannot drag much more -than its own weight. A dragon-fly, it is asserted—although we have -not met a man who guarantees that he has made the observation—will -support its heavy body in the air by the rapid vibration of its wings -for four-and-twenty hours without alighting. The chirp of a cricket is -produced by the rubbing together of its hind-legs. A mosquito sounds -its war-cry much in the same way. The pitch of the note proves that -the insect’s muscles are contracting and relaxing at least 300 times a -second. None of these figures must be applied without qualifications in -estimating the relative strength of insect and human muscle. Weight for -weight, the muscle of a flea is not so much stronger than ours as the -figures might lead one to infer. To ascertain the numerical relation, -it is necessary to compare the total cross-section of the two chief -segments of a flea’s leg with the cross-section of the extensor muscles -of a man’s thigh and calf, and a man’s weight with the weight of a -flea. Nevertheless, after all deductions have been made, a considerable -balance of superiority lies with the insect as regards the strength -of its muscles, their rapidity of contraction, and power of repeating -contraction without fatigue. An insect’s muscle is the most suitable -that can be obtained for microscopic examination. Its pattern is larger -and more distinct than that of other animals. That the pattern should -be larger is not quite what might have been expected. It would not have -surprised us had we found the pattern finer in the more effective type. - -Nothing is easier than to mount a specimen of insect-muscle. The large -water-beetle (_Dytiscus marginalis_) is an excellent subject. It is so -easily handled. Having cut off the animal’s head, a leg is pulled out -from the thorax. It is split open with a penknife, and a little of the -muscle is dug out from within its hard case, placed on a clean slide, -and covered with a cover-slip. If the preparation has been made quickly -and cleanly, the muscle remains alive for five or ten minutes. Not only -can it be studied, with the microscope, unaltered by reagents, but -under the most favourable circumstances the progress along its fibres -of waves of contraction can be watched. The structure of the fibres is -more easily made out if a little salt-solution or white of egg is added -to the preparation. - -[Illustration: FIG. 16.—A, A MINUTE PORTION OF AN INSECT’S -MUSCLE-FIBRE, HIGHLY MAGNIFIED. B, WHITE FIBRE OF MAMMALIAN MUSCLE. - - A, The nuclei are in the core of the fibre. B, The - nuclei lie immediately beneath the sarcolemma. The - disc on the left of this fibre, and the fibril on - its right, show the two ways in which striated - muscle-fibres tend to cleave. The dark line, or - row of dots, is known as Dobie’s line, or Krause’s - membrane. The figures are severely diagrammatic.] - -Striped muscle is crossed by bands, dim, bright, and dark. The sequence -is as follows: Starting with the very thin dark line, which often -appears as a row of dots, the next band is bright; then comes a dim -band about twice as broad as the bright one; then another bright -band. This sequence is repeated with extreme regularity from end to -end of the fibre. Usually the bands cross the whole breadth of the -fibre, although occasionally it is divided by longitudinal lines into -parts in which the stratification is shifted a little backwards or -forwards. A segment of a fibre comprises the substance between two -dark lines—_i.e._, two bright bands with a dim one between them. If -the muscle has been hardened in one of the fluids commonly used for -the purpose of preparing tissues for the microscope, with its two ends -fixed, say, by binding them to a piece of a match, so that it could -not shrink, a thin clear line appears crossing the middle of the dim -band. This seems to show that the fibre is not made up of single dim -discs between two bright discs, but of couples, comprising half a dim -disc and a bright disc. The thin dark lines indicate that the fibre -is divided into compartments by transverse septa, which are probably -reticulated. The appearance of a transverse line of dots, in place of -a continuous line, is due to the existence of very fine longitudinal -markings (it is unsafe to give them a name which connotes structure). -Where the longitudinal lines cross the transverse lines, the optical -effect is the appearance of a dot. - -If pieces of muscle are placed in a solution of osmic acid, they become -hard and brittle, and their markings are accentuated. Muscle from -the claw of a crab or a lobster is very suitable for this purpose, -owing to its exceptional freedom from connective tissue. After this -hardening the fibres are easily separated with the aid of needles into -fibrils immeasurably slender. An isolated fibril shows with extreme -distinctness the alternation of dark, bright, dim, bright, dark -markings already described. The appearance of a cross-section of a -fibre also proves that it is a bundle of fibrils. The cut ends of the -fibrils appear as dots surrounded by homogeneous substance. In this -respect there is an important difference between red muscle and white. -In the red fibres the fibrils are fewer and thicker than they are in -white, and the embedding substance is more abundant. It is generally -assumed that the homogeneous substance, sarcoplasm, is the nutrient -protoplasm of the fibre, the fibrils the contractile elements. The -more complete the differentiation of the fibre into fibrils, the more -rapid is its action; the more abundant the sarcoplasm, the greater its -capacity for continued work. - -If a living muscle-fibre is observed while a wave of contraction -is passing down it, the ends of the fibre being free, so that its -shortening is not prevented, it is noticed that the widening of the -fibre is accompanied by the thinning, even to obliteration, of the -bright bands. The dim discs extend laterally, without any noticeable -diminution of their thickness. It looks as if the bright discs, or -something contained in the bright discs, were absorbed into the -dim discs. The fibre is, as we have already pointed out, striated -longitudinally. The striation is more clearly visible in the dim discs -than it is in the bright ones. That the dim disc has an architectural -structure absent from the bright disc is placed beyond doubt when a -muscle-fibre is illuminated with polarized light. The dim disc is then -found to be doubly refracting; the bright disc is not. When the prism -in the tube of the microscope is placed with its axis at right angles -to the axis of the prism which intervenes between the source of light -and the stage of the microscope, a succession of bright bands is seen -corresponding to the dim bands seen with unpolarized light. The rest -of the fibre is invisible, because it has not the property of twisting -the undulations of light which the lower prism has set all in the same -plane. Various hypotheses as to the cause of contraction, or, to speak -more correctly, as to what happens during contraction, have been based -upon the thinning of the bright discs. It is assumed that the dim discs -have a definiteness of structure which the bright discs do not possess. -They are thought of as being traversed by pores, or as consisting of -short rods. Microscopists who take the latter view believe that during -contraction the more fluid substance, sarcoplasm, which occupies the -bright bands is drawn into the dim bands between the short rods, or -sarcostyles, which are consequently separated more widely. - -No tissue could be more unsuitable than muscle for microscopic -examination; for none other offers the same optical difficulties. -This will be evident to anyone who considers the description already -given of the markings which it exhibits. Whatever may be the true -interpretation of these markings, it is clear that they point to -an almost infinite multiplication of minute elements adjusted with -absolute accuracy side by side and end to end. A cylinder filled with -these transparent objects has to be viewed by transmitted light. The -elements, whatever may be their nature, refract light in different -degrees. It is impossible to eliminate the effects of internal -reflection, refraction, and interference of waves of light. The most -alluring hypothesis must be accepted with a considerable amount of -reserve. Any fact which seems to militate against it must be taken -into consideration. The view set forth above, in general terms, is -very attractive to everyone who wishes to bring muscle within the -category of machines. Suppose we accept the hypothesis that the dim -band is a plate made of sarcostyles surrounded by sarcoplasm then the -impulse which reaches a fibre causes an alteration in the surface -relations of the rods to the substance in which they are embedded. -Molecules of fluid from the bright bands are drawn in amongst -them; the rods are pushed farther apart; the fibre broadens with a -corresponding diminution in length. This brings muscular contraction -into the category of the phenomena which play the most important rôle -in bringing about the varied activities of the animal mechanism. -Contraction is due to osmosis. - -The separation of muscle into fibrils after hardening does not seem -to bear out either the rod or the pore hypothesis of the structure of -the dim disc. It must be remembered, however, that before the fibrils -are teased apart the substance of the fibre has been coagulated. The -fluid in the bright disc may thus have become as much a part of the -fibril as the rod in the dim disc. The longitudinal striation of plain -muscle and the appearance of continuous fibrillation in heart-muscle is -more difficult to reconcile with the hypothesis that striped muscle is -composed of interrupted rods. - -Muscle transforms the energy supplied to it by the blood into -mechanical work. It is doubtful whether any hypothesis as to -structure will help us to an understanding of the way in which this -transformation is effected. Explanations are seductive, but all -attempts at explaining the connection between molecular change and -change in shape must be viewed with suspicion. It is quite clear that -muscle as a motor is not to be compared with any form of motor with -which we are acquainted. It is also clear that the theory of muscle -must be applicable to all its varieties—striped, cardiac, and plain. -It must cover the alterations in form of an amœba and the streaming -movements of protoplasm within a vegetable cell. Probably it must -extend farther, and cover the discharge of electricity by an electric -organ and the emission of light by the lamp of a firefly. We are on -ground so treacherous that we are not sure whether, in crossing it, we -may lean with confidence on the laws of thermodynamics; and doubt as to -the applicability of these laws to living tissue almost upsets one’s -mental balance. Until we have evidence to the contrary, we are bound to -exclude such a misgiving from our minds. If we allow it to influence -us at all, it is merely to the extent of causing us to hesitate to -assume that the explanation of muscular contraction can be based upon -an analogy between muscle and any known mechanical contrivances for -generating power, not even excluding apparatus designed for the purpose -of measuring osmotic force. - -If living muscle is frozen, pounded with snow containing 0·6 per cent. -of sodic chloride, and placed upon a filter, a fluid plasma passes -through the filter as the mixture thaws. Like blood-plasma, it clots -spontaneously—without, however, so far as is known the intervention of -a ferment. - -All muscles become rigid after death, owing to the coagulation of their -plasma. It used to be thought that contraction was a stage towards -rigidity—a stage from which muscle, so long as it is alive, recovers. -This view was based upon the fact that exhausted muscle—such, for -example, as that of a hare which has been coursed—becomes rigid -much sooner than rested muscle. But this phenomenon has a different -explanation. The setting of muscle in rigor mortis is due to the -development of lactic acid (one of the waste products of active -muscle). The more there is of this ready formed at the time of death, -the more quickly does coagulation of muscle-plasma occur. The formation -of lactic acid is due to deficiency of oxygen. So long as muscle -obtains as much oxygen as it wants, its metabolism is complete. The -oxidized products which it loses are water and carbonic acid. This is -true also of the changes which occur after death. If a strip of muscle -is hung in an atmosphere of oxygen, it forms no lactic acid, and it -does not become rigid. If, on the other hand, the supply of oxygen has -run short before death occurred, rigor mortis sets in very quickly. -A dead frog takes a long while in becoming rigid, and its rigidity -is transient. Until the moment of death the frog is taking up oxygen -through its lungs, and even after death it probably takes it, as it -does when it is alive, through the skin. A fish becomes rigid very -quickly. For some time after it is caught it continues to live; but, -being unable to breathe in air, every molecule of oxygen which was in -its body when it left the water is used up before it dies. - -In the human body rigor mortis usually sets in from two to four hours -after death, and lasts about two days; but both the rapidity of -its appearance and its duration depend upon various circumstances. -As muscles become rigid they contract, moving the limbs, and the -shortening is more extensive than mere coagulation of muscle-plasma -would account for. It is evident that a process similar to functional -contraction precedes coagulation. Many a watcher in the chamber of -death has been startled by the shaking of the bed. Even a sound -resembling a sigh may be caused by contraction of the muscles of the -chest. Placing his hand over the region of the heart, the attendant -finds the body warmer than it was when life became extinct, for much -oxidation has since taken place. - -What chemical changes occur in muscle when it contracts? What is the -chemical source of its power? Carbonic acid is given off. This is -the only product which we can collect and measure; but it is taken -for granted that hydrogen atoms also combine with oxygen, forming -water. There is no reason for thinking that nitrogen is removed from -the molecules of its protoplasm with any greater rapidity during the -activity of muscle than when it is quiescent (_cf._ p. 212). Is the -oxidation immediate and complete, or does it occur in stages? For -many years attention has been directed to lactic acid, partly because -this substance is found in muscle which has been made to contract -under experimental conditions, partly because, on theoretical grounds, -glycogen (animal starch) is looked upon as the most important of -muscle-foods. Lactic acid—C₃H₆O₃—has the same percentage composition -as glycogen—C₆H₁₂O₆. Its formation from glycogen merely involves -a rearrangement of atoms. It has been supposed that lactic acid is -formed in the first instance, and then, if the supply of oxygen be -sufficient, oxidized to carbonic acid and water. But this hypothesis -may be resisted on various grounds. Undoubtedly, lactic acid appears -when oxygen is deficient. Under all circumstances and in all tissues a -certain amount of it is formed. There are reasons for thinking that it -carries away the nitrogen which is wasted, as lactamide. But it does -not follow that under normal conditions, when muscle is abundantly -supplied with blood, lactic acid appears in any greater quantity during -activity than during rest. The hypothesis is due to the misconception -which we have already endeavoured to correct. It is difficult to get -away from the steam-engine analogy. A steam-engine is made of iron and -brass. These materials are subject to wear and tear; but they are not -the source of its power. Its power is due to the combustion of fuel. -Muscle, physiologists formerly said, is made of protoplasm. This wears -down when it works, setting free creatin and other nitrogenous débris. -Its fuel is glycogen. This is not the way, however, in which the matter -is now regarded. Protoplasm is not the machine only, but also the -source of power. Glycogen is not burnt in a framework of protoplasm. -When muscle contracts, protoplasm casts out CO₂ and H₂O. Glycogen is -the food readiest to restore to it the atoms which it has lost. - -Another consideration opposed to the hypothesis of the conversion of -glycogen into lactic acid is the uselessness of such a transformation -from a physical point of view. The stability of the atoms of C₃H₆O₃ is -so little greater than that of the atoms of C₆H₁₂O₆ that practically no -energy is set free when the one substance changes into the other. We -cannot, however, overlook the fact that the formation of acid may be -a means of profoundly altering the state of the colloid substances -dissolved in cell-juice. The casein of milk coagulates when milk turns -sour. The neutralization of a faintly alkaline solution of a protein -(and muscle is faintly alkaline) will throw it out of solution. The -appearance of lactic acid may be intimately associated with movement of -protoplasm, and yet the change of glycogen into lactic acid not be the -source of the energy which muscle expends. - -=Fatigue.=—For its continued activity muscle needs an adequate supply -of food and oxygen. If the blood which distributes food is circulating -properly, and the liver, the great depot of food, is well stored, -fresh supplies are brought to the muscles as they are needed. There -are muscles—those of the eye and of the heart, for example—which -never become exhausted. However continuous their activity, they -take food from the blood as rapidly as they waste it; a statement -which, perhaps, needs qualifying by the addition, “so long as the -work exacted of them is such as may be reasonably expected.” If, in -a picture-gallery, one keeps the eyes elevated for an hour or more a -headache follows. Our eye-muscles have taken over their duties on the -understanding that we look down or straight forwards far more often -than we look up. If a long-sighted child is required to focus his -eyes upon a printed page without the aid of spectacles, not headache -merely, but actual disease of the brain, may be the result. The -ciliary muscle within the eyeball, which effects accommodation of the -eye for near objects, is unduly strained. Even the use of our modern -type, with its vertical height greater than its breadth, which has -taken the place of square Roman letters, is probably related to the -development of astigmatism of the lens, and thus indirectly a cause of -headache. It is asserted on high authority that vertical astigmatism, -the commonest form, is not present in the eyes of children before they -learn to read. Headache is an exaggeration of the feeling of fatigue. -It may be interpreted as the brain’s expression of unwillingness to -be made to work; a protest always to be listened to, notwithstanding -that it does not necessarily follow that unwillingness to work is the -result of overwork. Constipation, irritation of the sensory nerves -of the stomach, overdosing of the brain with alcohol, and many other -causes, may, through the vaso-motor system, set up the conditions -which normally result from activity unduly prolonged. The fact that -a central disturbance, headache, results from undue muscular work -calls our attention to the double nature of the mechanism concerned -in movement. Muscles are set in motion through the intervention of -the nervous system. After they have worked to an unusual extent the -nerve-centres connected with them grow tired. This, at least, is a -legitimate inference from the fact that headache occurs when certain -muscles of the eyeball have been subjected to an improper strain. But -it must be remembered that the muscles of the eyeball never tire. They -do not, like other voluntary muscles, give notice that they are in need -of rest. It is not so clear that the central mechanism is in any way -involved in the fatigue which is produced by excessive use of arms or -legs. The muscles of the limbs (and the central nervous system) are -protected by the sensations which originate in muscles when they are -overworked. The fact that a weary man can, if a great emergency demands -activity, use his muscles with as much vigour as if he were fresh from -bed, has been cited as an argument in favour of the view that fatigue -is of central origin; but it is an argument which works both ways. A -strong emotion causes a fervent response from the nervous system. Tired -muscles contract energetically when the impulses which reach them are -sufficiently urgent. - -Nothing so definitely removes muscle from the category of machines -as its liability to fatigue. To speak of a muscle as tired is, of -course, to transfer to an object a term which is applicable only to a -phenomenon of consciousness; but it is necessary, unless a cumbrous -expression is to be used, to designate thus the effect upon the muscle -of prolonged activity. The petrol may be low in the tank, but the -quantity burnt in the cylinder at each stroke is not reduced. If an -isolated muscle is repeatedly stimulated by an electric current of a -certain strength, the response which it makes improves for the first -two or three induction shocks; then it begins to weaken. At each -succeeding spasm the muscle shortens a trifle less than before. More -remarkable than the diminution in the amount of work done by a muscle -which is growing tired is the prolongation of the time taken both -in contracting and in relaxing. Further, it has been shown that the -fatigue which accompanies the contraction of an isolated muscle is -not a condition dependent upon the shrinking of the store of energy -which it possessed when it was first thrown into activity. Muscles -undisturbed as to blood-supply, and contracting under the direction -of the Will, also exhibit it. Speaking generally, it may be said that -the tiring of muscle is not so much due to the exhaustion of its store -of food as to accumulation of products of action. Vigour is restored -to a tired muscle by passing through its bloodvessels a stream of -salt-solution, which brings it no food, but washes away some of its -waste. But the problem is far more complex than this. The machinery -is not simply clogged with the products of its own activity. If the -blood of a tired animal is injected into the vessels of one that is -rested, the muscles of the latter exhibit the phenomena of fatigue. -Evidently muscle is self-protective. During activity it prepares -a “fatigue-substance” which poisons its own nerve-endings, making -them worse conductors from nerve to muscle of the commands which -descend from the brain. Not only does the fatigue-substance dull the -nerve-endings in the particular muscle which has contracted, but, being -distributed by the blood to the whole body, it produces a general -effect. If the legs have been severely worked, they exhibit fatigue in -the highest degree; but after a long walk the arms also are less ready -and less capable than the state of their nutrition warrants. - -The condition of stiffness experienced for a day or two after excessive -exercise is due to various causes in combination. The fact that it may -be remedied by encouraging the circulation through the muscles most -affected, as by hot baths and massage, tempts us to assign it also in -large measure to accumulation of products of action; but the means -taken to reduce stiffness favour the nutrition of the muscles both by -giving them more food and by carrying off their waste. - -Equally remarkable with the self-protective disposition of muscle, -which forbids it to give, except at the instance of increasingly urgent -messages from the central nervous system, more than a part of the work -of which it is capable, is its preparation for meeting an increased -demand. It grows with use. Running increases the girth of the leg by -developing especially the muscles of the calf. Raising weights enlarges -the muscles of the shoulder and arm. Use-growth may reach inconvenient -proportions. Nothing is more noticeable during the training of young -athletes, whose nutritive responsiveness is at its height, than their -liability to pass through a stage in which they are “muscle-bound.” -Their legs grow bigger, but their pace falls off. - -The development by means of exercises of a strong muscular system has -received much attention during recent years. Our ancestors cultivated -strength and agility in certain movements without paying much attention -to the muscles by which the movements were performed. It is fashionable -nowadays to lay stress upon the importance of maintaining an abundant -musculature, because of its relation to general “fitness.” The balance -between muscular activity and the organic functions which is observed -by everyone who takes an active holiday proves beyond doubt that -the nutritive condition of the various glands and of the heart and -bloodvessels is in some degree dependent upon the condition of the -muscles. Possibly they secrete into the blood other “messengers” in -addition to fatigue-substance—messengers whose call wakes up the -organs of digestion. The man who is so fortunate as to be able to use -his muscles in the open air has no need of exercises in his bathroom. -Failing out-of-door opportunities, much can be done by the systematic -use of the various muscles working against resistance. It is alleged, -and we are not disposed to dispute the justice of the contention, that -movements made with the fullest degree of mental concurrence have -a more rapid effect upon the growth of muscle than actions more or -less unconscious. Muscle and nerve are parts of a single mechanism. -It may be that fixing the attention on an exercise, and watching its -performance in a looking-glass, aids the nutrition of muscles by -increasing the influence of their nerves, possibly by improving the -nutrition of their nerve-centre. Unfortunately, this is one of many -theories which hardly come within the reach of a control experiment. -Could one concentrate attention on the movements of the right arm, -then absent-mindedly repeat them with equal vigour with the left, -it might be possible to ascertain whether there is anything in this -idea. Two other contentions with regard to the best way of performing -movements, with a view to the promotion of muscular growth, appear to -be justified by their results. Working against a moderate or light load -is said to be more effective than putting muscles to a severe strain. -A small number of maximal contractions, it is said, induce more rapid -growth than many partial shortenings. According to this scheme, when a -particular muscle needs strengthening, because in a certain action it -is to be the chief performer, it is made to bring its two ends as near -together as the plan of its attachments allows. Maximal shortening is -apparently favourable to blood-supply and otherwise promotes nutrition. - -=Tone.=—Hitherto we have spoken of quiescence and activity, as if -muscle were doing nothing when not visibly contracting. A wrong -impression may be engendered by these terms. Muscle is never idle. -During sleep, and still more when a person is under the influence -of anæsthetics, the muscles approach the condition of machines at -rest. But again the language of the workshop is inapplicable. When a -headless frog is hanging from a hook its legs are slightly bent. All -its muscles are weakly contracted, if we understand by contraction a -condition in which the length of muscle is less than it would be were -it not alive. But the flexors are tenser than the extensors, hence the -crooking of hip, knee, and ankle. If the sensory roots of the sciatic -nerve are cut, the leg straightens out. So long as the nerve was intact -the weight of the limbs acted as a stimulus to sensory nerve-endings, -causing a reflex “tone” of the flexor muscles via the spinal cord. The -tone of the extensor muscles was less because they were not stretched -by the weight of the limbs. Every joint is under the influence of -antagonistic muscles which are perpetually watching one another. When -the limb is extended the flexors become anxious. When it is flexed the -extensors get ready for a spring. Only when it is half flexed is there -anything approaching to a truce. And this in most cases is the position -of greatest comfort. But even when most at rest, muscles still possess -a certain degree of tone. The tendency to shortening in one set causes -it to pull against, and thereby increases the tone of, its opponents. -When a muscle contracts it does not lift a loose bone. It has to -overcome the tone of the muscles which would cause a movement in the -opposite direction. And here another adjustment comes into play. The -same gross stimulus which leads to the contraction of A starts impulses -of a finer kind for B, directing it to relax its tone. We have seen -how the heart and bloodvessels are under the influence of two sets of -nerves of opposite sign—anabolic, diminishing irritability; katabolic, -increasing it. All muscles are under similar management; but we can -rarely detect the influence of the anabolic, inhibitory nerves, the -brakes, because the katabolic display is overwhelmingly conspicuous. -We must be content with two experimental demonstrations. An animal’s -hamstrings have been cut; the flexor muscles of its thigh are therefore -severed from their attachments below the knee. The tone of the extensor -muscle keeps this joint extended. If now the pad of the foot be -tickled, the flexor muscles contract, just as they would do if they -were still able to carry out the reflex action of raising the foot. -They cannot do this, because their tendons are divided; nevertheless, -the knee bends owing to reflex relaxation of the extensor muscles. -Still more striking evidence of reciprocal contraction and relaxation -is afforded by the claw-muscles of a crayfish. A weak stimulus to its -nerve causes the claw to set open; a stronger stimulus causes it to -close. Both these movements are due, not to “contraction,” but to -change of tone. Under certain conditions, a current passed through -its abductor muscle, the claw being open at the time, causes closure -by inhibiting the tone of this muscle. In this case the stimulus acts -directly on the muscle, producing an effect which is opposite to the -one we are accustomed to associate with stimulation; in place of -contraction, relaxation. - -Contraction of muscle, moving something, impresses one as a positive -phenomenon. Relaxation seems negative—the undoing of contraction—and -to a very large extent this attitude of mind is justified. Return of a -muscle to its full length is due either to stretching by the weight it -has lifted, or to the antagonism of other muscles. An isolated muscle -lying on a pool of quicksilver does not return to its full length -after it has contracted. But it is necessary to banish the machine -idea. A machine gives out all the energy it has in store. Muscle is -extremely parsimonious. No stimulus can induce it to part with more -than a fraction of its energy. Recovery is as definite a function -as disturbance. A machine starts when a crank is moved, stops when -it is replaced. Muscle has a certain degree of automatism, although -its tendency to act on its own account has been almost completely -transferred to the governing nervous system. Muscle and nerve work -together, and the efficiency of muscle depends upon the maintenance -of its relations with its nerve. If the nerve is cut, the muscle -atrophies. We will not stop to consider whether wasting may be properly -attributed to disuse, or to vaso-motor changes. In its lowest form -nervous influence shows itself in the regulation of the nutrition of -muscle. A somewhat more forcible exhibition of control is seen in the -regulation of tone. The maximum is reached when a wave of undoing which -has passed down a nerve infects the protoplasm of muscle with the same -tendency to disintegration. The muscle-substance explodes. The muscle -shortens. - -Remarkable evidence of the existence of muscle-tone is afforded by -the =knee-jerk=. Place a person on an upright chair, with his legs -crossed, muscles lax, foot hanging free. With a paper-knife or the end -of a stethoscope, or even the hand used edgewise, tap the ligament -which connects his knee-cap with his shin. The tap is instantly -followed by a jerking forward of the foot. The deep muscles of the -thigh, vastus, and crureus, have contracted. This phenomenon is easy -to account for. When we are standing upright, the trunk is supported -on three joints, of which one—the hip—is a perfect ball and socket, -and the other two—knee and ankle—are of the same order so far as -the absence of any provision for locking them is concerned. If the -muscles on the front and the back of the leg did not constantly adjust -our balance, by swaying the trunk forward when it falls back, and -pulling it back when it sways forward, the joints of the leg would -double up beneath us. A photographer knows how little confidence is to -be placed in a man’s assertion that he is able to stand still. This -see-saw of alternate contraction and relaxation is kept up by means -of nerve-impulses which ascend from the nerve-endings surrounding the -separate bundles of tendons, or from the Pacinian bodies which are -found in abundance in the neighbourhood of tendons and ligaments, or -from the elaborately twisted nerve-fibres found in muscle-spindles, or -possibly from all three classes. Muscle and tendon are richly supplied -with sense-organs susceptible to pressure and stretching. There is an -abundance of nerve-endings to choose from. The slightest change in -their tension, whether due to the muscle’s own contraction or to the -action upon it of other muscles or weights, is recorded not only in -the spinal cord, but also in the cortex of the cerebellum, and, if the -contraction is an act of volition, in the cortex of the great brain. -Although it was skin which was tapped, skin-nerves have nothing to -do with the jerk. It was the result of the slight sudden stretching. -In short, the tone-mechanism has been fooled. Notice the position of -the leg. The knee is semiflexed; the foot is hanging free. There is -nothing for the extensor muscles of the thigh to do. Now, if ever, they -are justified in dozing. It is not to be wondered at that the sudden -stretching of the ligament takes them off their guard, or that on -waking they give a quite unreasonable start. The phenomenon is, as we -asserted, easy to account for. It would also be easy to explain, if it -were not for the extreme rapidity with which the jerk follows the tap. -The interval is about one-hundredth of a second. This is thought to be -too short to allow an impulse to ascend a sensory nerve, pass through -the cord, and descend a motor nerve. It is true that these reflexes -of adjustment must stand on a different level to other reflexes. The -tone-impulses which cause them are incessantly patrolling to and fro -from sense-organs to nerve-endings. The paths they follow must be the -most open in the nervous system. Receptors and effectors must, in an -electrician’s phrase, be incessantly switched on; or, to express the -analogy more accurately, the flexor and extensor tone mechanisms are -incessantly and reciprocally switching each other on and off. It must -be confessed that it is very difficult to explain the knee-jerk if it -be not a reflex action, but, as has been supposed, a direct response of -the thigh muscles to their own stretching. The latter hypothesis does -not appear to be reconcilable with its dependence upon the maintenance -of the nervous connection of the muscles with the spinal cord. It -cannot be elicited unless the “spinal arc” is intact. It ceases after -the severance of either sensory or motor roots. Nor will it occur if -the supply of blood to the lower end of the spinal cord has been cut -off. Still more difficult is it to explain its extraordinary sympathy -with everything that happens in the whole nervous system, if the -impulses which cause it do not pass through the spinal cord. By a very -simple mechanical arrangement it is possible to record the amplitude -of the knee-jerk. The foot moves a lever which writes on a travelling -surface. The jerk is elicited by the hammer of a clock strapped to the -shin. In this way it is possible to extend the period of observation -over several consecutive hours, the subject becoming completely -oblivious of the movement his foot is making once a second, if it be -screened from his view. In deep sleep the jerks stop; but the subject -may doze, and still jerk follows tap. And the record made by his foot -mirrors all the changes in his nervous system. If he clench his fist, -the movement is reinforced, as it is when a child cries, a lamp is -lighted, his ear itches. There is music in an adjoining room. His foot -is the baton which beats _fortissimo_ to Wagner, and is lulled to -_piano_ by the “Lieder ohne Wörte.” On a bright day this spinal pulse -throbs gaily. It is indolent in dull, depressing weather. The knee-jerk -is the physician’s guide to the condition of the nervous system. - -=Elasticity of Muscles.=—Muscles are very extensible, and after -stretching return to their original length. Their elasticity is a -quality of great practical importance. It enables them to meet sudden -resistance without rupture, as when a man alights from a height. At -the moment when the feet touch ground elasticity dissipates the shock. -The stretching of the muscles then leads reflexly to the increase of -their tone. Here we see an advantage in the short reaction-time of -the knee-jerk. Tone comes into play long before impulses generated by -contact of the sole of the foot with the ground have had time to reach -the brain, or even to induce reflex contraction through the spinal -cord. The elasticity of the muscles is also of use in the performance -of certain sudden actions. A pea is flicked across the room by pressing -the thumbnail against the pad of a finger, or a finger against the -thumb, and releasing it with a jerk. - -An electrical change accompanies an impulse in its passage down a -nerve, and a wave of contraction in its passage along a muscle. In -1788 Galvani observed that the hind-limbs of a frog, suspended by -a metal hook to metal railings, twitched when the wind blew them -against the bars. The hook passed through the lumbar plexus of -nerves. He recognized that the cause of the twitch was the closing of -a circuit. The birth of dynamic or galvanic electricity dates from -this observation; and ever since this phenomenon was first observed -the electric changes in nerve-muscle preparations made from frogs’ -legs have been favourite subjects of research. Many observations with -regard to nerve-conduction and muscle-contraction may be made, and many -experiments performed, without special apparatus. A frog having been -killed by cutting off its head, or by placing it beneath a tumbler -with a wad of cotton-wool soaked in chloroform, the skin of the leg -is removed, displaying the khaki-coloured muscles, bluish tendons, -and bright white threads of nerve. A stretch of the largest nerve of -the back of the thigh, the sciatic, is isolated. All the muscles of -the thigh are then cut away and the bone nipped across just above the -knee. The bones below the knee are removed, the superficial muscle of -the calf, the gastrocnemius, being allowed to hang free, its bifid end -attached to the fragment of thigh-bone. Its lower end terminating in -the tendo Achillis, with its insertion into the prominence of the heel, -is left intact. The bone is fixed in a clamp. A light lever made from -a wooden spill is suspended from the tendo Achillis. The nerve may -then be stimulated in various ways: by crushing in a pair of forceps, -burning with a heated needle, touching with a drop of glycerin or -a strong solution of salt. But of all methods of stimulation, the -best is the current from an induction coil. Since it does not injure -the nerve, it can be applied as often as may be desired. The amateur -provided with an induction coil is in a position to study the relation -between stimulus and response. He can vary the strength of the stimulus -and vary the weight which the muscle has to lift. He can observe the -progressive onset of fatigue, and otherwise gain much information -regarding the behaviour of muscle as an isolated piece of apparatus. - -It is the ambition of the expert to obtain absolutely correct records -of the time-phases and of the changes in electric potential of nerve -and muscle under varied experimental conditions. For this purpose he -needs the finest apparatus which instrument-makers can furnish, and the -knowledge and dexterity requisite for its employment. Consider, for -example, the record of the change of form. A nerve-muscle preparation, -obtained by the method already described, is arranged so that the point -of the lever scratches on a rapidly travelling blackened surface. As -the muscle contracts it makes a “tracing.” A tuning-fork vibrating -at the rate of, say, 400 times a second also scratches a tracing on -the same travelling-plate. It is easy to time the several phases of -contraction and relaxation by comparing them with the undulations made -by the tuning-fork. By means of an induction shock a single impulse -is generated in the nerve and a single spasm evoked in the muscle. -Our tracing shows that the spasm lasts about one-tenth of a second, -and that about half this time is occupied by contraction, and half by -relaxation. But the ascending curve is usually a little steeper than -the descending curve, and the apex a little nearer to the commencement -of ascent than to the termination of descent. An electric signal marked -the instant at which the current was sent into the nerve. The time -taken by the impulse in travelling from the spot where the electric -current entered the nerve to its junction with the muscle can therefore -be estimated. The contraction begins so much sooner or later, according -as the shock is delivered nearer to, or farther from, the muscle. By -shifting the electrodes up and down the nerve, the rate at which the -impulse travels is directly measured. After the time that the impulse -took in reaching the muscle has been allowed for, there still seems to -be an interval before the muscle begins to shorten. This was termed -the “latent period,” under the impression that some time is actually -lost in turning the nerve-impulse into a muscle impulse. The impulse -was supposed to be latent in the end-plates of the nerve. Various -hypotheses were formulated as to the nature of the transformation. -The progressive improvements in apparatus and methods is testified by -the diminution in this latent period as given in the text-books of -successive decades. It is now put at ¹/₄₀₀ second, and is regarded by -most physiologists as a delay due to the inertia of the muscle. Owing -to its elasticity, the molecular change in muscle does not immediately -affect its shape. When the latent period appears to be longer—say -¹/₁₀₀ second—the balance is due to the inertia of the recording -apparatus. Usually the curve shows a rise lasting ⁴/₁₀₀ second and -a fall occupying ⁵/₁₀₀, due to the fact that inertia of muscle and -apparatus delays the commencement of the rise, but does not hasten the -termination of the fall. - -When an impulse is generated artificially by an induction shock, a -single spasm or twitch is the result; but in Nature contraction is -never limited to a single twitch. Impulses descending from a motor -nerve-cell to a muscle are always rhythmic. They follow at the rate -of eight or ten a second in human nerves; and since in our muscles -contraction and relaxation take longer than in a frog, a second impulse -reaches the muscle before the effect of the first has passed away. -The muscle has not had time to relax, when it is again called upon to -contract. Hence a summation of contractions. The muscle continues to -shorten until the maximum of contraction is reached. This condition is -termed “tetanus,” to distinguish it from a single spasm. In fullest -contraction the length of a muscle may be diminished by one-half, or -even by two-thirds. - -It would be impossible to treat of the =electrical phenomena= displayed -by nerves and muscles without presupposing some acquaintance with the -methods and laws of physics. As this is contrary to our understanding -with our readers, we must be content with the statement of a few -salient facts. At the moment when an impulse is passing along a nerve, -or a wave of contraction along a muscle, the electric potential of the -active part of the structure, whether nerve or muscle, is different -from that of the not-acting parts on either side of it. A battery in -its commonest form is a glass vessel containing sulphuric acid in -which a plate of zinc and a plate of copper are immersed. The zinc is -electro-positive as regards the copper. In a muscle the contracted -portion is electro-positive as regards the parts uncontracted. The -degree of positivity can be measured by connecting the muscle at two -spots with the two wires of a galvanometer. When one wire makes contact -with the contracted portion, and the other with a part which is not -contracted, a current passes through the galvanometer, causing its -needle to swing; and since the wave of contraction is not stationary, -but passes down the muscle, the current is subsequently reversed. -The wave, as it were, first tilts up one end, and then, passing on, -tilts up the other, letting down the first. The contracted spot is -electro-positive to the spot not contracted, and then the latter, -contracting, becomes electro-positive to the former, which has relaxed. -The needle of the galvanometer swings first to the left, then to the -right. The importance of this method of investigation lies in the fact -that the electric variation exactly represents, both in time and in -intensity, the change which is occurring in nerve and in muscle. By -following it, we can ascertain the rate at which an impulse travels -down a nerve. We can determine its length and its “form.” Represented -on paper, it is a wave. This wave travels in warm-blooded animals -with the rapidity of 35 metres in a second. When it reaches a muscle, -its rate—that is to say, the rate at which the wave of contraction -invades the muscle—is 6 metres in a second. The time during which any -particular level in the muscle remains contracted in a single spasm, -under the influence of an artificial stimulus, is about 0·05 second. -The length of the wave is 300 to 400 millimetres. These measurements -give us a very clear idea of the events which occur in a nerve-muscle. -An impulse picked up by a motor cell in the spinal cord runs down its -axon—termed later a nerve-fibre—with great rapidity. Even the most -distant muscle is reached in less than one-thirtieth of a second. -From the end-plate of the nerve it travels in both directions along -the muscle-fibre—or group of fibres, since each nerve divides into -branchets for thirty to forty muscle-fibres—with reduced velocity. -Every particle of each fibre rises and falls; but, seeing that the wave -of contraction is much longer than the fibre, the whole fibre is in a -state of contraction at the same time, although not with equal vigour -throughout its whole length. - -We cannot dismiss the further consideration of the electric phenomena -of nerves and muscles without some inquiry into their meaning. It is -evident that they are intimately related to the molecular changes which -constitute an impulse. But at present the physics of the phenomena are -beyond our grasp. We may speak in a general way of dissociation of -ions; but we do not really know what is happening at the spot which is -in a state of impulse. We cannot bring the transformation which it is -undergoing into line with chemical and physical transformations which -we understand. Probably the electrical phenomena which mark it are not -peculiar to muscle and nerve. All living changes of state are of the -same nature. Cellular activity, or protoplasmic activity, to use a -better term, wherever it occurs, is accompanied by electrical change. -But it so happens that nerve-substance and muscle-substance have a -definite orientation which gives to the electric force a cumulative -effect. In a liver-cell it is dispersed in all directions. In a muscle -the change of potential at one particle is added to the change at -the next, until the sum of all these changes, transmitted along the -length of the fibre, is sufficiently large to deflect the needle of a -galvanometer. Owing to its summation it attracts our attention. - -Although they cannot tell the true significance of the electromotive -change which marks the passage of an impulse, physiologists are in a -much better position now than formerly to controvert certain popular -misconceptions. There is no such thing as “nerve-force” in the vulgar -sense. A nerve does not transmit energy to a muscle. The muscle -obtains the energy which it dispenses when contracting from the foods -with which the blood supplies it. The nerve transmits an excitation. -Over-excitability is not a sign of strength, but of weakness. Nor is -an impulse in a nerve an electric current. It may be generated by an -electric shock, but a chemical stimulus is equally as effective. The -slow rate at which it travels, as compared with electricity, puts it -altogether out of comparison with an electric current. Its relatively -rapid progress, on the other hand, equally excludes the hypothesis that -it is a movement of ions, as that phenomenon is observed in solutions -of salts. - -What is the nature of the process by which energy is conveyed along -a nerve? When speaking of the passage of impulses from receptors to -the central nervous system, and through this to effectors, we have -used the vague expression “molecular change,” to avoid the necessity -of being more precise. But the problem is of such profound interest -that we look with eagerness for any hint of the direction from which -light will eventually be thrown upon it. Recent discoveries regarding -the nature of electricity, combined with investigations at present in -progress as to the physical constitution of proteid substances, give -more than a hint. Hitherto the choice has lain between a chemical -and a physical explanation; now the border-line between chemistry -and physics, always wavering, has disappeared. The hypothesis that -an impulse is a progression of chemical change has meant in the past -that the “wave” was due to the oxidation of substances contained in -nerve, with liberation of CO₂ and H₂O. Various considerations render -such metabolism of the substance of which nerve-fibres are composed -improbable. In the first place, nerve-cell bodies contain a store of -material, tigroids (p. 320), which is recognizably drawn upon during -nervous activity. It would appear, therefore, to be the tigroids, -and not the substance of the nerve-fibre, which supply the energy -transmitted along a nerve. Then, again, the axon of a nerve-fibre, -enclosed as it is in a tube of fat, is peculiarly ill-placed for the -reception of the nourishment which would be needed to make up for -waste, if its metabolism be fluctuating and at times excessive. Nor -have nerves more than a very meagre blood-supply. Secondly, observation -does not give any support to the hypothesis of fluctuating metabolism. -A nerve does not give off more CO₂ when active than when passive. Nor -does it become acid. Thirdly, nerves, or, to be quite accurate, -medullated nerves, are indefatigable. Their capacity for conduction -is not diminished by previous use, as it would be were it dependent -upon their reserve of nutriment. These various considerations rule out -a “chemical” explanation of the old-fashioned type. It is premature -to do more than outline the “physical” theory which seems destined -to take its place; and the reader will perhaps forgive if, for the -sake of clearness, the case is put with unjustifiable definiteness -and simplicity. Proteid substances are constituted of clusters of -molecules. The form of the clusters depends upon the salts (or, -more precisely, the ions) with which they are associated, and the -associations depend upon the electric charges which the ions carry. -In resting nerve-protoplasm the clusters are small, and, since the -total surface-area of a number of small spheres is greater than the -surface-area of the same weight of matter when condensed into large -spheres, there is, so to speak, more surface for the ions to cling to. -Conversely, when the ions leave the small clusters, the latter are -not protected from the influence of mutual attraction. They fuse into -larger clusters. Fusion is carried to its extreme limits when a protein -coagulates. A nerve-impulse is a “wave” of partial coagulation. The -positive electricity generated in a cell-body by the metabolism of its -tigroids repels the positively charged ions which cling to the nearest -protoplasm-clusters in the axon. Like acrobats swinging from trapeze to -trapeze, each flight of ions dispossesses the ions from the clusters in -front of it; and in this way the disturbance progresses down the axon -as an electric wave. - -Thus we interpret the shadow cast by a theory of which either of -several pioneers who are diligently climbing may at any time obtain -a view. The conductivity of protoplasm (and what is true of its -conductivity will be found to hold good equally for its irritability -and changeableness of form) is due to the readiness with which its -molecules enter into unstable associations with electrolytes. The -instability of these associations is related to the tendency of -the molecules to cluster. An impulse is passed along a nerve as a -displacement of ions; the ions being transferred from one molecule, -or group of molecules, to the next. Such an explanation of an impulse -involves no chemical breakdown of nerve-substance during its passage -along a nerve. It transfers the metabolism which liberates energy -(reinforcing the impulses which have originated in sense-organs) to the -nerve-cell bodies. It is based upon certain experimental data which -appear to have been established; but, like all other hypotheses which -are intended to account for physiological phenomena, this one must be -brought to the test by varying the conditions under which impulses pass -along nerves, and ascertaining whether the consequent alteration in the -force, rate, and other attributes of the phenomena are in accordance -with physical laws. In applying these tests to the activities of -protoplasm, we are, however, met by an insuperable difficulty. The -matter which transmits nerve-impulses is alive. We have no laboratory -standards by which to judge whether the changes in conduction which -are produced by changes in the conditions of the conductor are, or -are not, consonant with physical theory. It is with protoplasm that -we are dealing, and not with a mixture of proteins in solution. If -we surround a nerve with nitrogen, it loses its conductivity in five -hours, to recover it when oxygen replaces the neutral gas. This has -been regarded as proving that metabolism of the nerve is necessary for -the transmission of impulses. But conductivity is a phenomenon of life. -Deprivation of oxygen for five hours must bring the nerve-substance -to the verge of death. It might be argued that the retention by the -nerve for so long a time of its power of conducting impulses shows -that its metabolism is not a cause of the phenomenon. Again, it has -been shown that warming the nerves of cold-blooded animals greatly -increases the rapidity of conduction. It is more than doubled in the -nerves of the “foot” of a slug-and a similar increase has been proved -for the nerves of a frog-by a rise of temperature of 10° C. Reflecting -on the results of this experiment, a physicist would exclaim: “Then an -impulse is a wave of chemical change. A rise of 10° C. increases the -rate of chemical processes from two to three times; whereas no known -physical process is accelerated by more than 5 to 15 per cent.” But the -physiologist remembers that a rise of temperature of 10° C. increases -all the activities of a frog. He is hardly prepared to say that its -greater vivacity may not be the expression of more rapid oxidation; -but he sees no fore-ordained balance of vital enterprise and chemical -change. He is, or ought to be, extremely suspicious of any explanation -which appears to over-ride physical laws; yet, at the same time, he -is aware that until he has more accurate knowledge regarding the -constitution of protoplasm he will not be in a position to understand -how physical laws apply. The protoplasmicity of protoplasm is increased -by warmth. What change of molecular constitution does this imply? - -The view that in a muscle molecular change gives rise to an electrical -change, which in turn produces the change in form, has been very -widely held. The hypothesis was based on observations which seemed -to show that the electric variation travels a little ahead of the -wave of contraction; but every improvement in recording apparatus has -diminished this apparent want of synchronism. There can be little doubt -but that the lagging behind of the wave of contraction is due to the -inertia of the muscle and of the recording apparatus. Molecular change -and electric variation are simultaneous. If this be true, the electric -change cannot be regarded as the cause of the molecular change, in the -sense, at any rate, in which they used to be considered as cause and -effect. - -The =power of muscle= varies as its cross-section. For human muscles -the maximum lift amounts to from 7 to 10 kilogrammes for each square -centimetre. This is a large figure, but it must be remembered that, -owing to the arrangement of the bones as levers, most muscles act -at a great mechanical disadvantage. The greater the difference in -distance from the fulcrum between the point of application of the -force and the point of incidence of the weight, when the force acts -nearer to the fulcrum than the weight, the greater is the mechanical -disadvantage. The greater also is the rapidity with which the weight -is lifted. What is lost in strength is gained in swiftness. Contrast -the slow steps of a negro, whose long heel separates the point of -application of the power (tendo Achillis) from the fulcrum (the -ankle-joint), with the springy movements of a European. A European -needs, and as a rule has, a better developed calf, which allows him -his more sprightly gait, without sacrificing his carrying power. Our -preference for slender wrists and ankles is not purely æsthetic, unless -we admit, as may be maintained, that all natural canons of taste rest -upon utility. Slimness of joints means nimbleness. A few muscles act -directly, without loss of power—as, for example, the masseter, which -lifts the lower jaw (hence a grand capacity for cracking nuts)—but -most muscles move levers of considerable length. Compare with the -masseter the biceps and brachialis which lift the forearm. Their -tendons are inserted into the radius and the ulna at a distance from -the elbow-joint which is about one-tenth as great as the distance from -it of a weight held in the hand. Their united cross-section is about -16 square centimetres: (16 × 10) / 10 = 16. One cannot hold out in the -hand, the elbow being pressed against the side, so that these muscles -alone are acting, a greater weight than 16 kilogrammes (34 pounds), -although the muscles are exerting a traction ten times as great as -this. The strength of muscle when pulling straight is well illustrated -by the thick white mass in the centre of an oyster. It keeps the shell -closed until a force equal to 1,300 times the animal’s weight has been -applied. This muscle also affords a good illustration of the part -played by reflex contraction in opposing stretching—the reaction by -which tone is maintained. Anyone who inserts an instrument, such as the -end of a screwdriver, between the slightly open valves of an oyster -lying under water will find that he needs to give it an exceedingly -smart twist if he would catch the muscle asleep. Stretching it causes a -reaction proportional to the stretching force. - -[Illustration: FIG. 17.—BICEPS MUSCLE IN ACTION.] - -The fact that the output of energy by muscle is proportional, within -certain limits, to the work to be done, is brought out even in -laboratory experiments. A nerve-muscle preparation teaches that the -amount of work is not a function of the stimulus. Within certain limits -a stronger stimulus evokes a higher and stronger lift; but the stimulus -remaining the same, the work done by muscle (_i.e._, the product of -weight multiplied by height) is, up to a certain optimum, increased -by increasing the weight. Often a very light load is not lifted as -high by a nerve-muscle preparation as a slightly heavier one. No -satisfactory theory of this reaction to load has yet been formulated. -Explanations have been put forward, but they merely substitute one -unknown for another, a not uncommon drawback to explanations. - -Muscles are strongest when at their full physiological length. As he -dips an oar into the water a man exerts the greatest force of which he -is capable, provided that he is not guilty of “missing the beginning.” -Hands over the stretcher, body between the knees, ankle, knee, hip, -fully flexed, arms straight—all his strongest muscles are at their -greatest physiological length. Rowing is an exercise which has no -rival. Every muscle in the body, from little toe to little finger, -comes into play under the conditions which suit it best. And not less -admirable is the effect upon the abdominal muscles during recovery at -the end of the stroke; and the rhythmic movement which encourages deep -and measured respiration. - -The greatest output of work is obtained when muscles contract against -a progressively diminishing load. Towards the end of the lift the -load must be small, if contraction is to be carried to its extreme -limit. The provision for this is well seen in the case of the muscles -of the arm when lifting a weight up to a position above the head. A -portmanteau is held in the hand. Its handle is gripped by flexing the -fingers. And here it may be noted that, since the range of movement of -a muscle varies as its length, the thumb and fingers are not worked -only by muscles contained in the palm of the hand. Fingers are bent -and wrist flexed by muscles of which the origin is carried up even to -the lower end of the humerus. As the portmanteau hangs by the side, -biceps and brachialis are at their fullest length. Suppose it to be -necessary to place it on a cab. These muscles begin the work under the -best conditions. They could not, however, lift the portmanteau far did -not the muscles of the shoulder displace the elbow from the side, so -that at the end of their pull, the forearm being almost vertical, the -muscles of the arm have little more to do than to move the hand inwards -towards the head, in preparation for the extensor thrust. The secret of -getting the greatest amount of work out of any particular muscle lies -in securing for it the due co-operation of other muscles. - - -ELECTRIC ORGANS. - -Muscle disperses energy in the forms of mechanical work, heat and -electricity. Its structure, as already pointed out, is peculiarly -favourable for the display of electromotive force. In certain fishes -muscle is so modified as to give an electric discharge without -developing mechanical work. The production of an electric change is a -by-phenomenon of muscular activity. It becomes the sole function of -an electric organ. If the skin be removed from the tail of a skate, a -cylindrical column of brawny tissue about the size of a finger will -be found embedded amongst the muscles near its root on either side. -These are electric organs, although so weak that it is barely possible -to feel the shock which they give in a live fish. The nearly allied -Torpedo of the Mediterranean has far more powerful batteries. They are -situate near its gills, occupying the whole thickness of the fish from -skin to skin. When the back of a torpedo is pressed, it discharges a -current of 30 volts, or even more. Still more violent are the shocks -given by an eel—Gymnotus—which haunts the tributaries of the Amazon, -a terror to all who have to cross their fords on foot; or the African -fish, Malapterurus. The current which these animals develop attains an -intensity of 200 volts. With the exception of those of Malapterurus, -all electric organs are modified muscle, and closely similar in -structure. The organs of Malapterurus appear to be modified glands. -The skate’s electric organ may be taken as typical of the rest. When -sliced with a knife, it is seen to be divided by firm connective tissue -into minute chambers. These chambers are piled into hexagonal columns, -which lie lengthwise in the organ (they are set dorso-ventrally in -Torpedo). Each chamber contains a jelly-like substance which embeds an -electric disc. The disc divides the chamber into a smaller anterior and -a larger posterior compartment. Each chamber is supplied with several -nerves which ramify into innumerable twigs on the front surface of -the disc. The development of the disc must be considered for a moment -if its structure is to be understood. It starts life looking as if it -would grow into a voluntary muscle-fibre. A nerve joins it, forming an -end-organ in the usual way. Then the end-organ increases its spread -unduly, while the rest of the fibre fails to grow. The structure -becomes toadstool-shaped, with the nerve arborizing on the seat of -the stool. The front aspect of the disc, therefore, corresponds to -a nerve-ending in a muscle. Its middle layer indicates clearly that -the fibre makes an abortive attempt to develop cross-striation. It is -laminated, the laminæ strangely contorted; in section they appear, -not as plain lines, but as rows of dots, evidently a suggestion of -longitudinal striation. The posterior layer of the disc consists -of granular protoplasm drawn out as a number of short backwardly -directed tongues, and one long process, the stem of the stool. No -structure could be more suggestive of the function of the organ; but -no one has as yet succeeded in catching the suggestion and pressing -it into a definite explanation of the way in which it works. Certain -physiologists, laying great stress on the fact that the functional -connections between an electric organ and its nerves are not easily -interrupted by the administration of curari, atropin, and other drugs, -which block the passage of impulses from nerves to muscles, look upon -the nerve-layer of the disc as the generator of electricity, and the -rest as an accumulator or resonator, which stores, or exaggerates, -the electric charge. Others consider that the portion of the disc -which is altered muscle-fibre—the middle, or middle and posterior -layers—generates the electromotive force, the nerve simply calling it -into activity. All agree that a brief interval (about 0·003 second) -elapses between the arrival of the nerve-impulse and the discharge of -an electric shock. This “latent period” may be used as an argument in -favour of either view. It would be in harmony with the general account -which we have already given of protoplasm as a liberator of energy -to suppose that a nerve-impulse, having reached a disc, immediately -infects the protoplasm of the disc, inducing molecular commotion, -and that the ions move in such directions as to disturb the electric -equilibrium of the disc, its front surface becoming in relation to the -back as zinc to copper in a battery. The current generated in the fish -is in the direction from head to tail. It is certain that the change -does not occur until an impulse reaches the organ. The organ is not -charged by the nervous system during a period of inactivity, and then -discharged by a releasing impulse. This is sufficiently evident from -the fact that when a piece of the organ, with its nerve, is removed -from the fish, although much sooner exhausted, it responds like a -nerve-muscle preparation to repeated stimulation. - -[Illustration: FIG. 18.—ELECTRIC ORGAN OF A SKATE IN LONGITUDINAL -SECTION—A, SLIGHTLY, B, HIGHLY MAGNIFIED. - - A shows the compartments into which septa of fibrous - tissue divide the organ. In the centre of each - compartment is a disc formed from a modified - muscle-fibre. Nerves ramify in abundance on its - anterior surface. B, a minute portion of a disc. At - the top are seen nerve-fibres in delicate nucleated - sheaths; then follow the nucleated layer with which - they come in contact, the contorted laminæ which - represent the striations of the muscle-fibre, the - granular nucleated substance of its posterior - layer, some connective tissue, a capillary - bloodvessel containing oval nucleated corpuscles. - In a tissue space, a single coarsely granular - leucocyte is to be seen.] - -The usefulness of a torpedo’s electric organs is unmistakable. They are -powerful enough to paralyse every animal that touches its back, whether -foe or little fish suitable for food. But of what service is its feeble -battery to a skate? This and the allied question as to the advantages -which can have accrued to the ancestors of the torpedo who first -began to change innocent muscle into a weapon of offence are usually -answered by pointing to the liability of flat fish lying on the bottom -of the sea to become resting-places of parasites, corallines, and other -fixed growths. Very mild shocks would suffice to disturb the peace of -would-be settlers. In the same way, the electric organs of fresh-water -fish may, when rudimentary, have protected the skin from invasion by -moulds. - - -LUMINOUS GLANDS. - -If it be difficult, when considering the dispersal of energy as -mechanical work, heat, or electricity, by living tissues, to bring the -phenomena into line with those of which physics takes experimental -cognizance, how are we to approach the problems involved in the -generation of light? Yet the photogenic property of protoplasm is -widely distributed. Protozoans and various other invertebrate animals -cause the so-called phosphorescence of the sea. The abysmal depths -of ocean are lighted by forests of luminous polyps, and traversed by -fishes whose heads are furnished with lamps. By her own light the -female glow-worm enables her winged mate to keep his tryst. Fireflies -(Lampyrus) flash amongst the orange-trees of Italy, and blaze -(Pyrophorus) beneath the mangoes of Ceylon. - -Luminous organs vary too widely in structure to allow us to pick out, -as in the case of electric organs, the features which are common to -them all. In Pyrophorus the organ is a double mass of cylindrical -cells near the tip of the abdomen. The cells are set vertically to -the surface, and are supported by a tubular membrane. Their substance -contains a kind of fat. Beneath them there is a layer of cells, not -luminous, but evidently a part of the photogenic apparatus, containing -chalky granules. The organ is well supplied with nerves and with -respiratory tubes (tracheæ). - -More interesting than its structure is the study of the peculiar -character of the light which the organ emits. It gives a spectrum which -extends from the red (beyond Fraunhofer’s line B of the solar spectrum) -to the first blue rays (F). It shows no lines. Green rays appear only -when the light is bright, and then they are the brightest of all the -rays. The light is practically destitute of actinic or chemical rays. -A photographic plate may be exposed for several minutes, almost -without changing, to the light of a firefly bright enough to enable one -to read with ease in a dark room; whereas light of equal brilliance -from any other source would change it in the fraction of a second. Nor -are heat-rays mixed with the light. Measurements show that the activity -of the photogenic organs does not give rise to any greater rise of -temperature than would occur in the case of any other gland. - -The contrast between the emission of light by an animal and its -production in any other manner is very striking when the physical -evidence, or want of evidence, of what happens in the protoplasm which -produces it is considered. The fact that no heat accompanies the light -precludes us from attributing it to oxidation. If a firefly is enclosed -in a vessel of oxygen, its lamp burns no brighter—clear evidence that -its luminosity has nothing in common with the burning of a match or -the glowing of a stick of phosphorus. Nor is the lamp put out when -the insect is suddenly exposed to great cold (-100° C.). It continues -to shine until the cold kills it. There is no relation between the -luminosity of a firefly and the phenomenon termed “phosphorescence” -by physicists. Sulphide of calcium—the substance used for rendering -matchboxes visible in the dark—returns light which it has absorbed. A -firefly’s power of emitting light is in no wise affected by keeping it -for a long while in the dark. - -Like all other events in vital chemistry, the generation of light by -protoplasm is due to a process of fermentation. The luminous organs -may be crushed, and the mixture of fermentable substance and ferment -extracted with water. The extract is luminous. If an extract is -prepared rapidly, and evaporated to dryness _in vacuo_, the residue -glows when moistened with water. That two substances are present in the -extract, one (luciferin) fermentable, the other (luciferase) a ferment, -is proved by the following experiment: A certain quantity of extract is -divided into two portions. One part (A) is allowed to glow until its -capacity for emitting light is exhausted. The other portion (B), as -soon as it is separated, is heated to 55° to kill the ferment. B still -contains luciferin; A contains luciferase, although all its luciferin -has been used up. Recombined, the extract is luminiferous. - - - - -CHAPTER XI - -THE NERVOUS SYSTEM - - -Twenty-five years ago a new process was introduced for colouring the -elements which by their combination make up the nervous system. With -its aid anatomists discovered the inadequacy of their conceptions -of nerve-cells. It was already known that a nerve-fibre—that is to -say, its essential part, its core—is a part of a cell, the body and -other parts of which are situate within the brain or spinal cord, or -in one of their dependents, a ganglion. But the new method showed the -nerve-cell as more elaborate in form than anything which had been -imagined hitherto; and since the word “cell” was often loosely used -when the cell-body alone was referred to, it seemed worth while to give -the unit of structure a new name. The term “neurone” was introduced -to emphasize its functional individuality. The nervous system is an -association of neurones. - -By the extremely simple expedient of placing a small block of -nerve-tissue in bichromate of potassium, and then transferring it to -nitrate of silver, jet-black pictures of nerve-cells are obtained -showing with amazing completeness all the details of contour of their -bodies and all the intricacies of branching of their limbs. The most -surprising feature of the process is the absence of confusion in its -results. Dyes were in use which stained one kind of cell better than -another, or picked out a particular part—usually the nucleus—of every -cell. If the chrome-silver process had acted in the same way, a dense -black preparation in which no details could be distinguished would have -been the result. But instead of treating all cells alike, the process -blackens one cell here and another there, leaving hundreds or thousands -untouched. It shows no preference for any particular kind of cell. In -one section large cells are picked out, in another small ones; in a -third no nerve-cells are blackened, but connective tissue is brought -into view. When the block of tissue soaked with bichromate of potassium -is immersed in a solution of nitrate of silver, the chromate escapes -from it into the surrounding liquor much more quickly than the nitrate -gets in; and when at last the nitrate of silver enters, it finds that -some of the cells have fixed the chromate in their substance. This -retained chromate combines with silver. The product is rapidly reduced -to a black subchromate. No explanation of the fixing of the chromate -by individual cells has yet been offered. It is a remarkable fact that -another process which similarly makes choice amongst the elements -has since been introduced, giving even more valuable results. Pieces -of fresh tissue are placed in a very dilute solution of methylene -blue. When staining is satisfactory, nerve-cells alone take up the -dye. The selection of individual nerve-cells is not carried so far as -it is by the chrome-silver method, but it is exhibited to a certain -extent. It is probable that nerve-cells live (in a physiological -sense) longer than other tissue-elements. Methylene-blue contains some -easily removable oxygen of which the oxygen-starved nerve-cells take -advantage. The reduced methylene-blue remains in their substance, so -that when the preparation is reoxidized by exposure to air the pattern -of the nerve-cells is rendered conspicuous. When a few cells are -selected, it is, presumably, because they were the only ones alive at -the time when the dye entered the tissue. Preparations made from the -wall of the alimentary canal seem to justify this simple explanation. -They show patches in which muscle-fibres are stained, patches in which -there is no staining, and intermediate zones in which nerve-cells are -coloured and muscle-fibres are not. But the hypothesis is inadequate -to meet all cases. When first employed, the blue was injected into -the animal in successive doses until it killed it. The staining was -believed to occur _intra vitam_. Subsequently it was found that its -application to fresh tissue, or, for certain results, to tissue which -has been kept for some hours, is equally effective. - -Without an understanding of the nature of the two new processes, and of -the character of the results which they yield, it would be impossible -for the reader to realize the extraordinary advance in our knowledge of -the finer structure of the nervous system which has marked the period -during which they have been employed. - -The chrome-silver process is the more useful for the central nervous -system. Methylene-blue gives better results with tissues containing -minute nerve-cells and the branches of nerves. The latter method has -revealed such a profusion of nerve-twigs as would never have been -suspected but for its use. Consider, for example, the lining epithelium -of the lungs (p. 168). Every one of its flattened cells has its own -nerve twig or twigs. They lie between the cells. They give branchlets -which enter them. A similar statement might be made regarding the -richness of the nerve-supply of the muscle-fibres of the alimentary -canal, or of the cells of glands, and possibly of other tissues. -Each fresh success achieved in the application of the method makes a -further revelation of the abundance in which nerves are distributed, -increasing our sense of the dependence of all vital processes upon -nervous control, and our appreciation of the unifying and integrating -importance of the nervous system. - -The term “neurone” is used by certain writers with a view to -emphasizing their belief, not in the functional individuality alone -of the unit of structure, but also in its anatomical isolation. The -peculiarity of the methods of coloration which we have described lies, -as already pointed out, in their selecting the cells which happen to -be in a particular nutritive condition, and ignoring their neighbours. -Hence pictures of separate and discrete units are obtained. This proves -the nutritive autonomy of the cells, but it does not necessarily follow -that A is not structurally connected with B, and B with C. Impulses are -passed along the chain from A to C. Functionally, therefore, they are -linked together; but until the question as to the way in which contact -is established is settled, it is as well to think of the neurones as -anatomically discrete. - -It would be impossible in this book to describe all the varieties of -neurone, for nothing is so characteristic of these elements as their -enormous range both in size and form. It may be truly described as -having no limits. Each of the two electric organs of Malapterurus is -governed by a single neurone. Its cell-body is a fifth of a millimetre -or more in diameter—large enough to be seen with the naked eye—and -traversed by capillary bloodvessels. The axon of this nerve-cell—its -single nerve-fibre—ramifies to supply a separate branch to each of the -2,000,000 chambers of the electric organ, and each branch breaks up -into a bunch of twigs within the chamber. Contrast with such a giant -cell as this one of the granules of the retina or cerebellum, the -smallest cells to be found in the body, yet each a perfect neurone, -exquisitely elaborate in form. - -[Illustration: FIG. 19.—A NERVE-FIBRE CONSISTING OF A, THE UNDIVIDED, -FIBRILLATED AXON OF A NERVE-CELL, WITH ITS VARIOUS WRAPPINGS. - - In segment 1 the wrappings comprise B, a tube of - phosphatic fat (myelin), interrupted at H, a node - of Ranvier; C, a delicate membrane (sarcolemma); - D, connective tissue; E, the rind of the axon; - F, a tubular space containing lymph, between the - axon and its sheath of myelin; G, nucleus of - an enwrapping cell. At I the myelin is seen to - be divided into overlapping conical rings. 2, - The medullated nerve-fibre, running an isolated - course, is merely enclosed in a tube of connective - tissue containing lymph. 3, As a “grey” or - “non-medullated” fibre, the axon has lost its - myelin sheath.] - -As types for description we may take one of the motor cells of the -spinal cord and a granule of the cerebellum. Every nerve-fibre which -supplies a group of voluntary muscle-fibres is a thread drawn out from -a large cell-body which lies in the grey matter of the spinal cord or -of the axis of the brain. The fibres pass out in the anterior root of a -spinal nerve or in a cranial nerve. The cell-body may have a diameter -of as much as 100 µ (1 µ = 0·001 millimetre). In shape it is like a -very irregular starfish, owing to its being continued into several, -usually four or five, thick tapering branching limbs or processes, -known as dendrites, in addition to its slender thread-like axon. From -its origin in a cell-body to its destination in a muscle—it may be a -few inches, or it may be a yard away—the axon is an unbroken thread. -A short distance from the cell-body it enters a tubular sheath, which -protects and insulates it, recalling the covering of gutta-percha -in which the wires of a telegraph cable are enclosed. The sheath -is of a phosphatic fat, invested and held in place by a delicate -transparent membrane, neurilemma. Beneath this membrane nuclei occur -at regular intervals, and midway between each two nuclei the sheath -is cut across by a septum. Such interruptions or nodes show that the -sheath is not a part of the nerve, if the term is used in the most -restricted sense. Each internode is a cell which has been wrapped round -the nerve for its protection. The axon with its sheath is spoken of -as a nerve-fibre. A large number of nerve-fibres bound together by -connective tissue constitute a nerve. In some cases the axon before it -leaves the spinal cord, but after it has entered its myelin sheath, -gives off one or two lateral branches (“collaterals”), which return to -arborize in the grey matter of the cord. It does not appear that they -are always present in the case of the motor neurones of the spinal or -cranial nerves—probably they are usually omitted—but collaterals -are important features of the large neurones of the cortex of the -cerebrum and cerebellum (Figs. 23, 24). Usually two, three, or four -such branches start off at right angles from the axon, and after a time -turn back towards the surface, dividing into a few extremely slender -branches. Their purpose is an enigma. Possibly they bind a group of -cells together in functional unison. Such an explanation would seem -reasonable in the case of an arrangement of collaterals on the plan we -have just described; but in various situations in the brain cells are -seen of which the axons, instead of becoming nerve-fibres, break up -completely into collaterals, which branch repeatedly. - -[Illustration: FIG. 20.—A GANGLION OF A LEECH. - - Pear-shaped cells are set round a felt-work of - nerve-fibrils (neuropil). A neuro-sensory cell - is shown with one fibre directed peripherally, - branching on the surface; and one directed - centrally, ramifying in the neuropil. Several very - slender fibrils from the neuropil pass up the stalk - of each ganglion-cell. They join a network near its - surface. This net is connected by radiating fibrils - with a coarser net which surrounds the nucleus. - From the central net a relatively stout fibril - passes to muscle-fibres.] - -By various methods it may be shown that dendrites, cell-body, and -axon contain fibrils (Fig. 22). These neuro-fibrillæ lie parallel -to one another in the axon. Where it divides they are distributed -amongst its branches. Possibly they also branch. In the neurones of -Malapterurus, already referred to, this would appear to be inevitable. -The discovery of neuro-fibrillæ seemed to carry us a step nearer to -a comprehension of the physics of nervous conduction. They clearly -indicate that particles of the substance of a nerve-fibre are -oriented in the direction in which impulses pass. It is a structural -differentiation similar to the fibrillation of muscle, and probably -of the same order—a response to the same demand. But when we examine -the arrangement of the fibrils in a cell-body and its dendrites, the -appearances which we discover serve to perplex us. They complicate -instead of simplifying our mental picture of the conduction of -nervous impulses. The coarsest and most distinct neuro-fibrillæ are -to be found in annelids, the ganglion-cells of a leech, for example, -affording excellent preparations. These cells are pear-shaped, with -a single stalk. As is usual in invertebrate animals, they do not -exhibit separate dendrites and axon, but dendrites and axon pass out -from the cell in the common stalk. The bodies of the cells are set -round a felted mass of nerve-filaments, into which their stalks break -up. Just beneath the surface of the stalk of one of these cells two -or three very fine neuro-fibrillæ are to be seen. A single, much -coarser fibril occupies its axis. The fine fibrils join a network at -the periphery of the cell-body. The thick fibril is connected with a -coarser network which surrounds the nucleus. Radiating threads unite -the finer with the coarser net. It has been suggested that afferent -impulses ascend the fine fibrils, pass from the finer to the coarser -net, and take their exit by the thick fibril, which can be traced into -a motor nerve. Such a transit could not, so far as one can imagine, -have any effect upon the distribution of the impulses which pass -through the neurone; besides, there are reasons for believing that the -course taken by impulses which are delivered to the ganglion by sensory -nerves is determined by the felt-work in its centre, the neuropil. It -is probable that during their passage through the cell-body impulses -acquire the energy requisite to discharge the muscles to which the -motor-fibre carries them. In vertebrate animals, sensory nerves are -branches of neurones of which the cell-bodies lie in cranial or spinal -ganglia. They resemble the ganglion-cells of the leech in as much as -they are unipolar; both branches, the one which collects impulses from -sense-organs, and the one which distributes them to the spinal cord, -come off from the cell in a common trunk which afterwards divides, -although the unipolar condition of the cell of the spinal ganglion is -not primitive, but acquired. In the earliest stages of its growth the -cell is bipolar. Its two ends subsequently grow together for a certain -distance, the common portion being the vertical limb of the =T= (_cf._ -Fig. 21, which shows the growth of a granule of the cerebellum). The -body of the cell contains a network not unlike the network of the -leech. It is probably related to what may be termed the charge of the -neurone, the development of a suitable degree of force in the impulses -which pass through it. - -The neuro-fibrillæ of a large nerve-cell, such as a motor cell of -the spinal cord, are exceedingly slender (Fig. 22). They branch and -reunite. A certain number gather towards the axon; but the majority -pass through the cell from one dendrite to another, or from one branch -of a dendrite to another branch. It is very tempting to suppose that -neuro-fibrillæ are connected with conduction. When first discovered -they were regarded as conducting strands; but it is evident that -they are not comparable with telephone wires or other isolated or -separate conductors. There are good reasons for regarding dendrites -as collecting processes, taking up impulses from the end-twigs of -the nerves which branch in the grey matter around them, passing -them through the cell-body into the axon. The continuation of -neuro-fibrillæ from dendrite to dendrite seems to be irreconcilable -with the hypothesis that they are disposed in the lines of conduction. - -In common with those of various other types of neurone, the dendrites -of spinal motor cells are beset with “thorns.” These projections are -not rugosities or serrations, but short, delicate threads which stand -out at right angles from the dendrites (_cf._ Fig. 1). About a dozen -years ago, the writer made a careful investigation of these structures; -at a time when most anatomists regarded them as artifacts. He found -that their claim to be regarded as parts of the neurone is as good -as that of its axon or its dendrites; although never seen on certain -types of cell, the thorns, of cells which carry them, are perfectly -definite in arrangement and spacing. In some kinds of cell they are -more numerous, in others less. Neuro-fibrillæ, as we now know them, had -not been discovered at the date when this investigation was undertaken; -but on various grounds the conclusion was arrived at that thorns are -the cell-ends of fibrils which pass from the end-twigs of arborizing -axons into dendrites. Upon this conclusion was based an hypothesis -of conduction which is here submitted, not because there is not much -to be said against it—or, at any rate, many a hiatus in knowledge -to be filled—but because it happens to be the writer’s own. The -chrome-silver and methylene-blue methods which reveal the existence of -thorns do not stain neuro-fibrillæ. They colour the soft protoplasm -in which fibrils are embedded. By modifying the chrome-silver method -in every way which still allows a result to be obtained, it was found -that thorns sometimes appear as comparatively long slender filaments, -at others as shorter filaments ending in minute knobs, or as filaments -bearing two or three dots; or finally no filaments are visible, but -the dots are in the position which they would occupy if fibrils were -present, but not stained. From this it was argued that the soft -protoplasm which during life surrounds the filament as a continuous -film, either falls back towards the cell after death or is made to -shrink into the cell by reagents. This accounts for the appearance -of rod and knob. What is supposed to happen may be illustrated by -dipping a wire in treacle. At first, when the wire is withdrawn, it -is surrounded with a film. Then the film gathers into droplets. It -was suggested that the entrance of impulses into dendrites, their -conduction across the space which separates the end-twigs of axons -from the dendrites into which their impulses pass, is by means of -the thorns, although the thorns are not in themselves conductors. -Conduction occurs only when films of cytoplasm surround the thorns. The -first effect of impulses is to call out the films, in the same kind of -way that a current of electricity converts a row of falling drops into -a continuous stream. A succession of impulses, by adding to the number -of the filaments which are enveloped in cytoplasm, or by increasing the -amount of cytoplasm investing certain groups of filaments, increases -the openness of the path. Sleep is a condition in which all paths are -open. Hence no impulses are effective. Wakefulness, alertness, depends -upon the closing of all paths save those which are actually in use. -We may go further. The power of concentrating attention is the power -of limiting the spread of nerve-impulses in the brain. Alcohol opens -extra paths; the concentrated effort which was making progress with -a problem becomes more diffuse. The first effect appears in greater -brilliance of thought, gained at some sacrifice of cogency. Unexpected -analogies are discovered. Imagination takes a wider range. But as the -dose is increased, a condition akin to sleep is set up. Nerve-impulses -become ineffective because, many paths being open, they do not attain a -sufficient intensity in any set of paths. These few illustrations are -given for the sake of showing the need of a theory of the opening and -closing of paths. It is not suggested that they favour the particular -hypothesis here set forth as to the structural arrangement which -provides the paths and regulates their accessibility. - -Recent discoveries in the finer structure of the central nervous -system have provided many problems which at present appear insoluble. -One of the discoveries most difficult to make use of in constructing -theory is the existence of extracellular or pericellular nets, which -have the appearance of extraordinarily delicate cases of wire-netting -immediately surrounding the nerve-cells. It is somewhat remarkable -that the spacing of the nets is often very similar to, if not -identical with, the spacing of thorns. While some anatomists look -upon the nets as nervous, others regard them as pertaining to the -connective tissue of the nervous system. At present it is not known -how impulses get across from the finest visible twigs of arborizing -axons to the dendrites of the neurones which they influence. The -wealth of structural detail which recent research has revealed is an -embarrassment to anyone who tries to devise a scheme. Not improbably, -pericellular nets are intermediate factors in the exchange; or, if -not the nets, the structures whose existence is indicated by the -appearance of the nets. In the case of many of the finer markings which -staining methods bring into view, it is impossible to say whether they -indicate the presence during life of the structure as it appears to -be, or whether the markings are due to coagulation of plasma or to -strain caused by shrinkage in coagulating agents. In a sense this is -not of much consequence. Coagulation in a uniform pattern would mean -the existence of an architectural substructure which determines the -pattern. We may be looking at the cake or at the tin the cake was baked -in. - -There is a danger of seeing too much in a nerve-cell when examining -it under the highest powers of the microscope, and of endeavouring to -picture in too much detail the arrangements which regulate the flow -of impulses. Its markings are so complicated as to suggest to the -mind of the observer that it is itself a microcosm—a nervous system -in miniature. Neuro-fibrillæ appear to offer many alternative paths -within the cell. It is unlikely that such a way of looking at the unit -of structure is the right one. A certain motor cell of the spinal cord -is connected by its axon with thirty or forty separate muscle-fibres; -but there is no reason for thinking that the fibres ever contract save -as a single group. The axon consists of parallel fibrillæ, but these -do not appear to be needed as separate conductors; an impulse travels -down the fascicle. It does not appear to be necessary in the case of -a motor cell, and presumably the statement holds good for the large -cells of the cerebellum and cerebrum to picture any arrangement for the -simultaneous conduction in its axon of several impulses, or for the -conduction of one impulse along one of its fibrillæ and a different -one along another. What is necessary is that this particular efferent -path Z should be accessible from every other part of the nervous -system—from A to Y. If, merely for the sake of filling the space -which would otherwise be blank in the mental picture, we imagine a -pericellular net connected by thorns with the body and dendrites of -the nerve-cell Z, then the net is the meeting-ground of all the routes -through which Z is called into action. A nerve-wave from any of the -neurones A to Y, breaking upon this net, passes along the thorns into -the protoplasm of Z. - -In size a granule of the cerebellum presents a marked contrast to a -motor cell of the spinal cord; yet it is formed on essentially the -same plan. From its minute round body (about 8 µ in diameter) four or -five slender dendritic processes are drawn out. Each dendrite ends in -a little bunch of twigs, resembling fingers curved over the palm. Its -single slender axon runs towards the surface of the cortex. As the -granules lie at a considerable depth, this course is, for those which -distribute to the most superficial layers, a long one. They pass from -the granular to the molecular layer between the big cells of Purkinje. -When the axon has reached a certain level in the molecular layer, it -divides into two threads which run for a great distance, right and left. - -The granules of the cerebellum have a curious developmental history. -Every neurone in the body has a lifelong existence. Except for the -rare accident of its destruction by disease it occupies its station to -the hour of death. But at the time of birth many neurones are still -immature. Not all the granules of the cerebellum have yet assumed -their permanent form or situation. Beneath the pia mater there is -still a layer of minute undifferentiated cells. These, as they grow -into granules, elongate, in the first instance, into long spindles. -Subsequently they sink down through the molecular layer and between -the cells of Purkinje, leaving the poles of the spindle as the right -and left divisions of the axon (Fig. 21). It is interesting to learn -that such a migration is possible. It is also of interest to find -that a tiny granule of the cerebellum goes through the same stages in -attaining its adult form as one of the large cells of a spinal ganglion. - -[Illustration: FIG. 21.—THE GROWTH AND MIGRATION OF GRANULES OF THE -CEREBELLUM. - - Half a dozen nuclei of as yet undeveloped granules - are seen lying beneath the pia mater. From this - level to the bottom of the drawing granules are - shown in successive stages of growth. These - developing granules, selected from various - preparations of the cortex of the cerebellum, were - drawn from nature.] - -There are many different types of neurone. Any attempt to describe -them, or to give an account of the various details of structure which -recent improvements in technique have enabled anatomists to observe, -would fill a lengthy treatise; and would, moreover, be beside our aim, -which is limited to obtaining such an idea of the unit of the nervous -system as will enable us to form a conception, however crude, of the -way in which it works. From the brief account that has been given, it -will be evident that anatomists are approaching to an understanding -of the mechanism. It will also be evident that they have already more -information than they can apply. They are cognizant of many details of -structure which they cannot interpret in terms of function; and at the -same time are aware of wide gaps in their knowledge regarding facts -which are essential to the construction of any scheme. This much is -clear: A sense-cell on the surface or beneath it is touched (probably -entered) by the ultimate twig of the outer limb of a neurone whose -cell-body lies in a spinal ganglion, while its inner limb, as a fibre -of a posterior root, enters the spinal cord. In the spinal cord the -root-fibre splits into an ascending and a descending division which -rain branches into the grey matter over a considerable area above -its point of entrance, and a smaller area below it. The finest twigs -of these branches are to be seen in the vicinity of the cell-bodies -and dendrites of certain other neurones. The axons of these second -links arborize in a similar way in the vicinity of large motor cells, -whose axons in turn become fibres of anterior roots. (For simplicity’s -sake no reference is made to hosts of other neurones which link the -ganglion-cell and the motor cell to other cells higher in the cord or -brain.) An impulse generated in the sense-cell on the surface of the -body runs up the root neurone into the cord, where the ultimate twigs -of the posterior root-fibre offer it a wide choice of distribution. -Following the path of least resistance, it passes into neurone No. -2. Again, the arborization of No. 2 offers it alternative paths. It -makes a choice which lands it in No. 3. No. 3 passes the impulse on -to the muscle-fibres with which it is connected. Three points are -especially worthy of attention: (1) The impulse has a wide (literally, -an unlimited) choice of routes. The skin of the finger is touched. Any -muscle may respond, although resistance is so graded as to cause the -impulse to seek in the first instance the group of muscles which is -most often required to act in consequence of stimulation of the finger. -This means, we may suppose, that it follows the chain which, having the -smallest number of links, offers least resistance. If it cannot get -through to these muscles, owing to the fact that other impulses, acting -simultaneously, either increase the resistance in this particular path, -blocking its way, or reduce the resistance in an alternative path, it -spreads farther afield. (2) Owing to the ramification of the root-fibre -which conveys it to the cord, an impulse is not limited to a single -line of distribution. It reaches many secondary links. It may therefore -influence various effector neurones simultaneously. For example, a -stimulus which calls extensor muscles into action, at the same time -inhibits their flexor antagonists. (3) The path which it finally takes -is accessible to all other impulses. Its root neurone was peculiar to -itself. Link No. 2 was more or less a common path. Neurone No. 3 is -open to every impulse which traverses the nervous system. - -Anatomy justifies the construction of the scheme just outlined. But -there are many points regarding structure upon which a physiologist -desires information, many details that he wants to see filled in. -How is the impulse passed from the arborization of axon No. 1 to -the dendrites of neurone No. 2? By what structural arrangement is -resistance introduced, and how is it regulated, if it varies? Supposing -the resistance to be higher in one path than in another, or supposing -that more force is needed to enable an impulse to invade a wider field, -how is additional energy supplied? To the first question no answer can -be given at present—the mechanism by which impulses are transferred -from one neurone to another is unknown; yet it is convenient to find -a name for the junction of axon-endings and dendrites. It is termed a -“synapse,” on the understanding that the word involves no hypothesis -as to its structural nature. It is generally held that resistance is -introduced into nerve-circuits at synapses; although this again is -a provisional statement. The phenomena for the explanation of which -the idea of synaptic resistance was introduced, may be accounted for -on a purely anatomical basis of distribution. The extent to which -one neurone influences another may depend upon the size of the brush -of fibrils with which its axon touches it. If a certain force is -needed to discharge a neurone, a nerve-current must either have a -sufficiently high potential when it reaches it, or it must act upon it -for a sufficient length of time. There is little to choose between the -arguments which place the resistance at the synapse and those which -transfer it to the nerve-cell body. - -As a mechanism the nervous system is unthinkable, unless we picture -its units as independent, yet capable of forming associations; as -functionally discrete, yet entering into functional continuity. When -acting, they act as chains. Impulses run from link to link, from -the end-twigs of an axon of one cell to the dendrites of the next. -Neurones are so arranged as to make it impossible for impulses to -escape backwards out of dendrites into axon-twigs. In this respect the -system is valved. But there is no reason for thinking of the substance -of the neurone as polarized in any way. The physical accompaniment of -an impulse—the electric variation—travels with equal facility up and -down its axon. - -There is no evidence of any specificity of neurones; on the contrary, -it is clear that impulses of every kind—that is to say, from every -source, for we recognize no specificity of impulses—can travel -equally well through neurones of all forms. At every junction, in -passing through each synapse, they are delayed. It takes at least 0·01 -second (less if the knee-jerk be a true reflex action) for a message -delivered to the cord by a sensory root to reach a motor root. This -hundredth of a second—the sum of the delays entailed in fording two -or three synapses—is regarded as the minimum reflex time. To it -must be added, in considering any particular reflex action, the time -taken in travelling up sensory and down motor nerves. Delay indicates -resistance. If a sensory stimulus be not sufficiently pronounced to -provoke a reflex action, the reflex may be obtained on intensifying -it. Prolonging or repeating the stimulus—really the same thing, since -sensory impulses are rhythmic, not continuous—has a far more potent -effect than increasing its force. The resistance of synapses gives way -after a number of impulses have bombarded them. The desire of brushing -a fly from the skin, if resisted, becomes intolerably urgent after a -time. A persistent outflow of impulses produced by the irritation of -a spot in the cortex of the brain overwhelms the nerve-muscle system -in an epileptic fit. The following is an experiment illustrating the -spread of impulses from their customary path to another less often -used: A piece of blotting-paper, wet with vinegar, is placed on the -inner side of the thigh of a brainless frog. There is no use in trying -the experiment on a frog which retains its brain; the substitution -of one action for another would be an exhibition of the adaptation -of means to end—a demonstration of the animal’s right of choice. -Besides, the frog might choose not to act, and so the experiment would -fail. The brainless frog wipes off the blotting-paper with the foot of -the same side. This foot is then fixed so that the action cannot be -performed, and the blotting-paper replaced. After a longer interval the -frog removes it with its other foot. Evidently it is more difficult -for the impulses generated by the irritation which the vinegar causes -to get across the cord than it is for them to reach motor neurones on -the same side. Evidently, too, the continued irritation of the vinegar -adds to the travelling power of the impulses. They are strengthened -until they are capable of overcoming the resistance in the longer -path. “Resistance in conductors” and “potential of current” are terms -with which the study of electricity has rendered us familiar; but it -must be evident from the experiment just described that these terms -are not really applicable to nervous phenomena, convenient though they -may be for use in an allegorical sense. Holding the foot does not, by -any mechanism which we can recognize, switch off the shorter circuit, -yet the impulses abandon it for the longer path. There is no evidence -of a struggle to free the foot that has been fixed, coincident with -the spread of impulses, as they gather sufficient strength to reach -the nervous mechanism of the other leg. The right foot not being -available, the impulses _choose_ the route to the left foot. Any -attempt to explain this in terms of resistance and potential involves -the formulation of a number of subsidiary hypotheses; easy to devise, -no doubt, but stultifying to the explanation exactly in proportion as -they complicate it. Yet the hypothesis of lines of greater and of less -resistance (keeping as far away from electrical analogies as possible) -is essential to any explanation of nervous phenomena, and is, moreover, -justified by the evidence available. There are two causes in chief -upon which it depends: (1) The greater the number of neurones in a -linear chain, the greater is the number of synapses to be traversed. -If A, B, C are in the same circuit, the sum of their resistance has to -be overcome. (2) The greater the number of neurones amongst which a -nerve-current has to be subdivided, the smaller the charge available -for each of them. Imagine - - B - A - C - -so placed as to divide B and C, the charge delivered by A between. This -arrangement has, probably, an anatomical expression which accounts for -the relative ease or difficulty of a path, even on the supposition -that impulses do not open out as they advance—do not spread along -all the branches into which an axon divides—but keep to a given -line. The axon of neurone A divides, to branch about B, C, and D; but -its representation in the several pericellular nets (the expression -may pass for the sake of the simplicity which it introduces into the -picture) is unequal. In the vinegar experiment the impulses delivered -to the spinal cord by the root-ganglion neurone A pass to neurone B of -the posterior horn. B’s axon arborizes more freely about the cell-body -of neurone C in the anterior horn of the same side than it does about -neurone D in the anterior horn of the opposite side. Hence the impulses -generated by the vinegar stimulate C, sufficiently to discharge it, so -long as that road is open, more quickly than they stimulate D. That C -should be dischargeable only so long as the foot is free implies that -the activity of the neurone is in some way conditioned by its relation -with the muscles which it innervates. When the foot is held this -relation is interfered with, giving to the impulses generated by the -continued action of the vinegar time to overcome the resistance of D. - -The simile of the opening up of paths is fairly applicable to the -results which follow the use of artificial stimuli. Neurones seem to -link up in series under the influence of the impulses which bombard -them, popping like fireworks united by a common fuse. - -Experimental evidence points to the following conclusions: (1) -Resistance is offered at a synapse. This resistance must be overcome -before an impulse can get through from neurone 1 to neurone 2. (2) -The impulse does not, properly speaking, pass from 1, through 2. It -infects 2, causing it to discharge a fresh impulse. (3) Time is of the -essence of this process. Either the impulses head up at the synapse, -or, passing through into the neurone, they produce a cumulative effect -within it, which provokes it to discharge. (The latter hypothesis, -which is the less likely of the two, transfers the resistance from the -synapse to the neurone to be infected.) These conclusions are based -upon experiments of the following kind: The minimal stimulus which -will evoke a reflex action is determined. A stronger stimulus is then -applied. The reflex occurs more promptly, and is more pronounced. But -on further increasing the stimulus, it is found that the limit of -effectiveness is soon reached. The proportional relation of response -to stimulus is much less evident than it is when the experiment is -tried with a nerve-muscle. Choosing a reflex action easily provoked, -the afferent path is stimulated with an electric current interrupted -fifty times a second. The impulses which flow down the efferent path -to the muscle follow one another at the rate of about ten a second. A -column of nerve-fibres within the spinal cord is stimulated fifty times -a second. Again, the discharge into anterior roots has the natural -rhythm of about ten. The cortex of the “motor area” of the great brain -is stimulated with a rapidly interrupted current. The muscles which -it governs contract with their natural rhythm. The cortex is sliced -away, and the stimulus applied to the white matter beneath. A similar -result is obtained. Evidence such as this points to an independence of -action on the part of the neurones which one can express only in terms -of resistance and explosion. But there is another line of thought which -leads to the development of a picture of the working nervous system -which seems at first sight incompatible with the one that we have -sketched. The phenomenon of the knee-jerk (p. 274) reveals a nervous -system so intimately linked together, so homogeneous, so mobile, -that no event, however trivial, occurs in any part without sending a -vibration throughout the rest. Instead of a multitude of batteries -enveloped in a labyrinth of wires interrupted by myriads of switches -which are crackling on and off, the image of a sheet of water better -figures our conception—a material so frictionless that it is a-ripple -from side to side and end to end, from the most distant rivulet which -feeds it to the farthest trickle in which it drains away. It is a -fluid in a state of infinite commotion, the movements of its particles -varying in amplitude from tremulous quiverings which scarcely frost -the silver of its surface to waves which, breaking on the muscular -system, throw it up in heaps. The vinegar experiment seems to demand -a scheme of batteries and wires. The knee-jerk points to a continuous -conducting medium. Other phenomena suggest the superposition of the two -pictures; the conception of a nervous system consisting of a uniform -medium conducting, not indifferently in all directions, but with such -freedom that from our point of view the paths are infinite in number; -and within this conducting medium nerve-cell bodies and their processes -which collect and distribute groups of vibrations sufficiently strong -in combination to produce visible effects. In order that one of these -neurones may be stimulated to discharging-point, the medium by which it -is surrounded must be thrown into such a state of agitation as suffices -to infect it. The considerations which point to the formulation of -this double or superposed scheme are such as follow: The passage of -tone-impulses does not appear compatible with the ideas we have formed -on other evidence of synaptic resistance and neuronic discharge. They -are too feeble for such a mechanism. The short “reflex time” of the -knee-jerk points to the passage of the agitation up a sensory root to -the spinal cord, and through a non-resistant medium to the environment -of the motor cells which it discharges, missing the neurone or neurones -which intervene in the case of ordinary reflex actions. This is an -illustration of the way in which tone-impulses, which we imagine as -conducted by the non-resistant medium, pass over into discharges which -produce visible effects. Again, the phenomena of =inhibition= appear to -require the supposition of extra-neuronic conduction. Whenever a reflex -path is in use, all other paths in its neighbourhood are closed. The -passage of impulses leading to a particular reflex action is favoured -by the suppression of conduction in its vicinity. When A is talking -to D through the nerve-telephone, B and C are compelled to hold their -peace. Inhibition is a phenomenon of universal occurrence. In relation -to various actions, it is sufficiently pronounced to be visible in -the effects which it produces. A simple experiment will illustrate -this. Holding water in the mouth has no effect upon respiration, but -during the act of swallowing respiratory movements are suspended. -Whilst the swallowing reflex is occurring the respiratory reflex is -inhibited. This might be attributed to the volitional control of -respiration, and certainly when attention is being directed to the -process volition plays a large part. But if a finger is placed on the -pulse, it is possible to detect that, during the act of swallowing, -the pulse quickens, owing to the suppression of the slowing action of -the vagus upon the heart. Here is a case in which inhibition is in no -degree a voluntary action. Nor is it of any value as an adjunct to the -particular reflex with which it is associated. It is an illustration -of the universal rule that activity of any one spot in the nervous -system is the cause of the quieting of the surrounding area. Impulses -which reflexly check the heart cannot get through the medulla oblongata -whilst the swallowing impulses are traversing it. Inhibition has been -described as a drainage of nerve-force into the active area. On the -structural side it seems to require the conception of an extra-neuronic -substance which, agitated in the vicinity of the cells which are to -be discharged, is brought to rest around neighbouring cells. The -promulgation through the nervous system of the state which, when it -reaches the centres of consciousness, produces pain also seems to call -for an hypothesis of extra-neuronic conduction. - -Any reference to =pain= in a work on physiology needs a few words -of preface, since popularly the term “pain” is used in various -senses. When I see pink geranium and nasturtiums growing in the same -flower-bed, I may exclaim: “It is positively painful.” The want of -harmony, and at the same time the insufficiency of contrast, of chalky -pink and translucent orange, jars my æsthetic sense. Dislikes, however -well founded, are ruled out in thinking of the physiology of pain. -Further, in defining pain, we must be careful to isolate the real -thing, and not to confuse it with sensations which seem to lead up to -it. If, putting my finger in a pair of pincers, I touch it as lightly -as possible, the first sensation is one of contact; a little harder, -and it becomes a sense of pressure; harder still, and all sense of -contact or pressure is lost in pain. It is usual to regard pain as -sensation carried to excess. But neither is this physiological. An -excessively bright light or an excessively loud sound is disagreeable. -It causes a sudden movement for the purpose of avoiding it—just such -a movement as one would make if one touched a red-hot poker—but it is -not, strictly speaking, painful. Not uncommonly in cases of accident -or disease of the spinal cord a sharp distinction is drawn between the -sense of touch and the capacity for experiencing pain. Below the injury -the patient retains his sense of touch undiminished in acuteness, but -no blow, or cut, or burn, causes him any pain. The pain caused by -squeezing the finger in a pair of pincers is not, therefore, an excess -of touch sensation. Pain begins to be experienced in the skin just when -the object applied to it is affecting it to an extent which might do -harm. If the point of a needle touches it, it causes pain as soon as -the pressure is a trifle less than that needed to pierce its surface. A -hot object begins to hurt when the temperature reaches 48° C.—almost -enough to coagulate the tissue fluids. Pain is not a discriminative -sensation. If I hold my arm out at right angles, I am conscious for -the first few minutes of its weight, and have, besides, some sense -of the traction exerted by the muscle of the shoulder. At the end of -ten minutes these sensations are merged in pain, and for some time -after lowering the arm the shoulder-muscle aches, much as it does -in rheumatism. Pain is an effect upon consciousness, which absorbs, -engulfs, and therefore obliterates sensation. To use an ancient phrase, -“It is less that I feel pain than that I am pain.” If we speak of the -capacity for pain as a sense, we may call it for the purpose of our -present argument the “sense of damage.” The nerves of the skin are -acutely affected by any agent which is likely to do harm. It is their -business to convey to the central nervous system an influence which -so affects it as to set up in consciousness the condition of pain. -Sensations of damage evoke reflex movements by means of which the part -of the body likely to be injured, or the whole body, is removed to a -safe distance. It being the duty of the skin to give this warning, a -service of nerves sensitive to noxious agents has been developed which -scouts in co-operation with the services devoted to the recognition -of physical contact and heat and cold (_cf._ p. 425). If, imagining -that the fire has not been lighted, I touch an almost red-hot stove, I -acquire quite a considerable amount of information of which I am able -to make use. I gain an accurate notion of the situation of the stove, -and I put the right part of my finger in my mouth. The skin sends to -the brain the ordinary sensations of touch and pressure before the -condition of pain is established. In seeking for a definition of pain, -we must eliminate the two attributes which have characterized all the -forms of stimulation which we have considered up to the present time: -(1) The tendency to provoke movement; (2) the supply of information. -If I am suffering from a whitlow, the last thing that I am disposed -to do is to jerk my finger about. Although it enhances the urgency -of skin-reflexes, pain, in general, inhibits movement instead of -provoking it. This is well illustrated in pleurisy. So long as a man is -healthy he is quite unconscious of the fact that at each respiration -the lower part of the lung slides on the lining of the chest-wall; -but commencing inflammation on the surface of one of the lungs causes -intense susceptibility to friction, and the pain produces an effect -which the man is quite unable to produce by an effort of will; it stops -the movements of the chest on the damaged side. Pain is inhibitory, not -stimulant. It is not, properly speaking, a sensation. Frequently being -mixed with sensational elements, it conveys topographical information; -but pure pain approaches in quality the nebulous sense of distress of a -patient who, when asked where he felt it, replied: “Nowhere; but there -is a deal of it in the room.” - -Sufferers describe pain in figurative language, as “burning,” -“stabbing,” “throbbing,” “aching,” and so forth. Two persons afflicted -with the same lesion, the same source of pain, use approximately the -same terms. Hence we cannot say that pains do not differ in character. -But this is not a sufficient reason for assigning any specific quality -to pain. It varies in severity, in continuity or intermittence, in -suddenness of onset, in the sensations which accompany it, in the -emotional tone to which the disturbance of the organ from which it -proceeds gives rise, in the tenseness of the part affected and its -consequent sensitiveness to a throbbing pulse. All these things make -a complex of pain plus sensation, which causes toothache to differ -from headache, and both from the pain of burned skin. But they do -not give specific qualities to different varieties of pain. This -being the case, there is no need to presume the existence of special -nerve-endings for the reception of pain, or of a special region of the -cortex of the brain for its reception. On the contrary, the evidence -is conclusive that the nerve-fibres which serve the more highly -specialized senses, which have well-defined connections in the cortex -of the brain, do not convey the influence which enters consciousness as -pain. It is the innumerable nerves which have no specialized receptors -that take up pain. The afferent nerves of the viscera—the vagus and -sympathetic—convey no impulses which enter consciousness, so long as -the tissues which they supply are healthy. They have no representation -in the cortex. The organs with which they are connected (with trivial -exceptions, easily accounted for) are absolutely insensitive to -injury. Before the virtues of chloroform were known—in the days when, -however severe the operation, the patient had to nerve himself to -bear it without an anæsthetic—surgeons proved that the liver or the -intestines, or practically any other viscus, may be cut or cauterized -without the patient being aware that it is being touched. The same is -equally true of the brain itself. But if damage in a viscus is set up -gradually, its nerves convey to the central system an agitation which -has the most pronounced results upon consciousness, and on the way -profoundly affects the reflex actions which the spinal cord can carry -out, and also its capacity as a conductor. Once in his life, perhaps, -a man passes a gall-stone; for generations such a thing may not have -happened in his family. Yet the man finds that he is provided with a -nervous apparatus which conveys to consciousness intensest pain. - -It is difficult to think of pain as travelling along nerves in the form -of rhythmic impulses, similar to those which produce in consciousness -the effects which we have distinguished as sensations. A few lines -above we stated that no impulses which affect consciousness normally -travel up the vagus or the sympathetic nerve, limiting the term -“impulse,” perhaps unjustifiably. The vagus conveys an influence which -enters our experience, as hunger. Probably other states of feeling -for which we have no names, which resemble pain and hunger and their -opposites, are set up through the agency of visceral nerves. - -Fifty years ago attention was called to the difficulty of finding -pain-paths amongst the white tracts (nerve-fibres) of the spinal cord. -It is as difficult to point them out now as it was then; but the -inference that pain travels up the grey matter has given way to the -“neurone theory”; under a misapprehension as the writer holds. Pain -travels slowly. If one happens to notice a person who unsuspiciously -touches a hot surface, one observes that an interval elapses between -contact of his finger with the iron and the exclamation with which he -“relieves his feelings.” It amounts to more than a second—if the iron -is not very hot, to several seconds—whereas the “reaction-time” for -touch is only one-seventh of a second. The slowness of movement of -pain through the nervous system can on the neurone theory be explained -only on the hypothesis that it travels from link to link along a very -long chain of very short neurones. That pain is a state of the grey -matter rather than a succession of impulses, and that (within the -cerebro-spinal axis) the state is transmitted through an extra-neuronic -medium, seems a simpler explanation. - -The state set up in the segment of the cord in which afferent fibres, -conveying pain from viscera, embouch affects its conductivity. It -subdues reflex action through the segment, and at the same time -facilitates or reinforces the transmission of sensory impulses towards -the seat of consciousness. This shows itself in the apparent increased -sensitiveness of the skin of the area of the surface supplied by -the posterior root which joins the segment of the spinal cord into -which the pain influence is also being poured. For example, afferent -sympathetic nerves from the cardiac end of the stomach join the sixth -and seventh thoracic spinal nerves. Other afferent fibres run up the -vagus to the medulla oblongata. When the cardiac end of the stomach -is diseased, pain is referred to the skin area supplied by the sixth -and seventh dorsal roots. The ordinary inevitable stimuli acting upon -this area cause pain. Experimental stimuli which elsewhere would be -felt as touch or warmth are painful. The impulses to which they give -rise pass through pain-agitated segments of the spinal cord. The vagus -nerve carries its pain influence to the medulla oblongata. Now, it -happens that the sensory nerve of the face—the fifth—spreads for a -considerable distance up and down the axis of the brain. The fifth -nerve in consequence pours its sensory impulses into a region which is -pain-agitated by those fibres of the vagus which come from the cardiac -end of the stomach. Hence disease of that organ gives rise also to an -“illusion” of pain—pains and illusions of pain are philosophically -indistinguishable—on the surface of the head. The viscera, having no -direct access to consciousness, appear by deputy. When the stomach -is distressed, it makes its appeal to the whole body politic for -considerate treatment through certain nerves which have the privilege -of appearing at Court. The message is misread as coming from the front -of the chest—“heart-burn”—or from the shoulder, or from the scalp, or -from the other skin areas which these nerves serve. When the liver is -in trouble, consciousness, having no knowledge of its whereabouts—is -it the business of hand and eye to explore another man’s liver, or -incumbent upon the mind to accept their findings?—infers that the cry -comes from the shoulder. Nor have the tissues beneath the root of the -nail, or the muscle of the shoulder, or the pulp of a tooth, any direct -representation in consciousness; but since the pain-condition in the -grey matter converts it into a microphone, messages from neighbouring -structures which otherwise would fail to arouse attention, after -traversing the pain-segments of the nervous system, ring out clearly, -and hence the mind locates approximately the “pain” of the whitlow, the -muscle-ache, the decayed tooth. Sufferers from toothache are familiar -with the phenomenon of the spread of pain from a definite spot to the -whole jaw or the whole side of the head, dependent upon the spread -of the pain-agitation from the segment of the axis of the brain in -which the dental nerve ends to neighbouring segments. Our ability or -inability to localize a pain does not depend upon the presence or -absence of pain-nerves, but upon the existence or non-existence of -nerves coming from the same organ, or from its neighbourhood, and -capable of conveying impulses to the seat of consciousness. In passing -through the part of the spinal cord or of the axis of the brain which -is disturbed by the influence exercised by a damaged organ, silent -impulses acquire force sufficient to render them audible, and combine -with the pain to produce a feeling which consciousness can analyse, to -a certain extent. Informed as to its whereabouts by these accentuated -sensations, consciousness recognizes a sense of pain limited in its -topographical extension. - -Sneezing when a bright light falls upon the eye is a curious -illustration of the exaggeration of the effectiveness of sensory -impulses when they happen to be poured into an agitated segment of -grey matter. About one person in every three is affected in this way. -A friend of the writer, who was particularly sensitive, rising in the -night because he heard his child cry, three times lighted a candle -and three times sneezed it out before he could watch the application -of match to wick without suffering from a nerve-storm. Some nervous -dogs—especially fox-terriers—are very liable to this neurosis. Many -persons who do not sneeze feel, when the sunshine stimulates their -retinæ, a tickling in the nose. Again the illusion is to be traced to -the door of the fifth nerve—the sensory nerve of the whole of the -face. The nose is the true tip of the body. Morphologically it is -anterior to the eyes. Just as the fifth nerve extends its distribution -to the nose, so also its root-fibres extend their connection within -the axis of the brain forwards, until they traverse the mid-brain, -the primary centre of the optic nerve. A bright light, by stimulating -the optic nerve, sets up a commotion in the mid-brain. The ordinary -every-moment impulses from the nose, carried by the fifth nerve to -this region, ought not to appear in consciousness at all; but owing -to the excited condition in which they find the grey matter they -assume an importance which does not belong to them, and discharge -the reflex action of sneezing, just as they would do had one taken -snuff. Several lessons are to be learned from this phenomenon—as, for -example, one which cannot be too often impressed, that the impulses -which appear in consciousness (or, more accurately, the impulses to -which attention is directed) are but a most insignificant fraction of -those delivered by sense-organs to the central nervous system. The -impulses which give rise to the sensation of tickling in the nose -are not exceptional impulses which happened to be started when the -light fell on the eye. They were reaching the brain in a steady flow -before the agitation of the mid-brain gave to them exceptional force. -No consideration regarding the working of the nervous system has a -more important bearing than this. We cannot picture to ourselves the -activity of the sensory nervous system. Our experience is limited -to the scattered sensations which we _perceive_. Are the sensory -nerve-endings incessantly responding to external forces, throwing -an almost continuous procession of impulses up each of the millions -of nerve-fibres which connect them with the central system? Such a -conception is probably nearer to the truth than the conception which -we should develop if we trusted to experience. Yet even experience -tells us that an infinity of messages is delivered to the brain, of -which consciousness takes no account. Changing trains at a roadside -station in France, my attention was attracted by an electric bell -on the platform, which was ringing continuously. “Why does the bell -ring?” I asked the station-master. “To make known that everything -goes well,” was the response. “If it stops, something is wrong.” “But -do you not become so accustomed to it that you cease to hear it?” -“Yes, truly; it rings day and night. One does not pay attention to it -until it has stopped.” Sensory impulses generated by the contact of -my skin with the chair that I am sitting on are incessantly ringing -the bell of consciousness. I should notice them immediately if they -stopped. As it is, they do not attract my attention until they ring a -little louder than usual, or until some particular group, owing to -unrelieved pressure, produces a cumulative effect. Another lesson; that -the condition of the nervous system, and therefore its conductivity, -is determined at any given moment by the sensory impulses which are -reaching it. We cannot describe the effect of a bright light as pain, -yet it agitates the grey matter, altering its state, in the same way as -the nerve-inflow which we recognize as pain. A wet rag on the forehead -does not assuage a headache by cooling the brain (_cf._ p. 106). The -headache is “in the scalp.” The cool wet rag diminishes the dilation -of the bloodvessels of the forehead, and quiets the impulses from the -skin which are pouring into a tract of grey matter pain-agitated by the -influences ascending a visceral nerve—usually the vagus. - -It is necessary to warn the reader that a reversion to the old idea of -“conduction through grey matter”—_i.e._, otherwise than by a chain -of neurones—is unorthodox. It is set forth here because it seems to -the writer that the various phenomena which have to be accounted for -fit in best with the hypothesis of a double path. If evidence of the -anatomical possibility of extra-neuronic conduction is asked for, it -may be pointed out that the chrome-silver and methylene-blue methods, -upon which our knowledge of neurones is based, do not, in the very -nature of the case, show that grey matter consists only of neurones -and their obvious branches. As they select particular elements of -structure, we can never by their use alone know what they fail to -show. Attention may also be called to the fact that the same staining -process which reveals pericellular nets (p. 301) shows also a structure -resembling a network in the substance which intervenes between them. -Truly the method is a rough one. It may well be thought that the -nitric acid used to fix the tissue may cause strange coagulations with -solution of uncoagulated substance; but, as was remarked with regard -to the pericellular nets, regular patterns indicate architectural -differentiation. But whether these nets do or do not give hints as -to the nature of the conducting medium, there is no difficulty in -finding sufficient material, after all the substance entering into the -formation of the conducting neurones, as we imagine them, has been -accounted for. _Ex hypothesi_, the conducting material is provided -by the fibrils of the sensory nerves in their extensions beyond -the limits to which the deposit of subchromate of silver extends, -when the chrome-silver method of displaying neurones has been used. -Sensation-impulses enter neuronic chains. The condition which, when it -affects the seat of consciousness, is known as pain, progresses up the -vertebrate neuropil. - -Energy is developed within the nervous system. The =force of impulses= -is adjusted to the resistance which they have to overcome. Stimulation -of the millions of twigs of the vagus nerve in the lungs brings about -the gentle movements of ribs and diaphragm which constitute peaceful -respiration. A crumb of bread touching the mucous membrane of the -larynx stimulates a few of the endings of the same vagus nerve. Like -an avalanche, the impulses gather head as they advance, causing, not -the diaphragm and intercostal muscles alone to do their utmost, but -calling into action half a dozen accessory muscles of respiration. It -is difficult to account for this reverberation of the messages which -clamour for the ejection of the crumb of bread without figuring them as -spreading from neurone to neurone, urging each in turn to deliver its -maximal discharge. - -Neurones are provided with material which serves as a store of energy. -In their cell-bodies, including their dendrites, are to be seen coarse -granules of nucleo-protein, which, being fitted in between groups of -neuro-fibrillæ, assume an angular form. They are known as Nissl’s -corpuscles, or are termed “tigroids,” owing to the spotted appearance -which they give to the substance of a cell. If the nerve-cells of birds -be examined just after they have alighted from a migratory flight, the -granules are found to be few and small. In a bee returning to the hive -at evening with its last load of pollen, they are smaller than they -were when it commenced its morning’s work. They disappear in certain -pathological conditions, and under the influence of various drugs; and -since their presence is revealed by staining, their disappearance is -spoken of as “chromatolysis.” - -[Illustration: FIG. 22.—THE BODY OF A MOTOR NEURONE. - - In its centre is a large clear spherical nucleus, - with a nucleolus. The body-substance is prolonged - into five dendrites and an axon. Neuro-fibrillæ - are seen in dendrites and axon. They traverse the - body of the cell in all directions, in little - bundles which are separated by angular granules of - stainable substance (tigroids).] - -The wasting of tigroids during functional activity proves clearly that -nerve-cells do work, in the physical sense. Energy is expended in -transmitting messages from receptor to effector, from sensory cell to -muscles, from recipient nerve-ending to glands. Have nerve-cells any -privileges or duties? Their functions, so far as we have considered -them hitherto, are automatic, from a mechanician’s point of view. -Their situation and connections determine the direction in which they -conduct, and the degree in which they reinforce stimuli impressed upon -the nervous system by the environment, including what may be termed -the internal environment, food in the alimentary canal, secretions -in ducts, and so forth. Have the cells any directive or executive -functions? There is no evidence that they have; nor, it must be added, -is there any line of reasoning which leads inevitably to the conclusion -that they have not. Remembering that, until recently, it was the -custom to solve all obscure problems and to shelve all difficulties -by conferring human attributes upon nerve-cells and collections of -nerve-cells, termed “centres,” a physiologist admits the negative with -reluctance. The unconscious argument in the past used to run somewhat -thus: “I decide to act or to abstain from action. The nerve-cell is -the mechanism by means of which I decide. Therefore the nerve-cell -decides.” (In the past a distinction was drawn between the cell-body -and its processes, but that, we now see, was absurd.) It is very -difficult to relinquish completely this attitude of mind. I feel, I -remember, I will. There must be a _something_ which feels, remembers, -wills. But a physiologist finds in the nervous system no evidence -of a capacity for any function other than that of conduction, with -adjustment of the force of current. He can no more discover feeling, -memory, or will in a chain of neurones than he can find music in a -violin. He hears the strings singing in the breeze. He can twang them -with an electric shock. But he has no vision of ghostly performers, -no glimpse of the conductor’s baton. Yet he knows, as every sane man -knows, that the neurones are the instruments played in the orchestra of -mind. He knows that, while all are sounding, some are muted, in order -that the others may produce a dominant effect. He knows, too, whenever -he decides to continue writing or to close his notebook, that the -conductor is raising the baton or allowing it to sink by his side. - -A neurone or nerve-cell is a transmitting link. It is scarce a thing -to wonder at that physiologists, having wrestled successfully with the -superstition of the “pontifical nerve-cell,” are unwilling to reinstate -it even as doorkeeper in a free church. It may be that it exercises -some discretion in admitting impulses, but until its authority as a -guardian of the path which stretches behind it has been established, -it is better to regard it merely as a door which swings open whenever -pressed with sufficient force. - -Is it possible to classify neurones according to their function? -They can be classified according to size, and, with some degree of -completeness, according to form. But if, as we believe to be the -case, size and form are governed by purely physical requirements, the -divisions into which the cells fall have no physiological significance. -The motor cells of the spinal cord and axis of the brain are large -and irregular in shape. Their dimensions are clearly dependent upon -the size, thickness rather than length, of the nerve-fibres which are -drawn out from them. They discharge impulses to groups of voluntary -muscle-fibres at a considerable distance. Small cells could not do the -work. Precisely similar reasons can be given for the large size of the -cells of Purkinje in the cerebellum, which transmit the elaborated -product, as we may term it, of this organ to the great brain; and -for the dimensions of the large pyramids of the great brain, which -convey its decisions to the spinal cord. The small pyramids of the -cortex of the great brain distribute the first crude impressions of -sensations to neighbouring (association) areas of the cortex. A cell of -Purkinje (Fig. 23) has a more complicated, and at the same time a more -regular, form than any other nerve-cell. It resembles an exceedingly -richly branched espalier pear-tree, set at right angles to the narrow -convolutions of the cerebellum; a disposition easily accounted for, -when the structure of the cortex of this organ is considered. Its -outer layer in which the espalier processes ramify is traversed -longitudinally by an infinity of nerve-threads, the bifurcated axons of -granules. These granules are small neurones which take up impulses from -afferent (“mossy”) fibres, and distribute them to the dendrites of the -Purkinje cells—each collecting from a few fibrils only of the sensory -channels. (The word “sensory” is used to indicate that sense-organs -are their provenance, and not that their messages become sensations.) -The numerous spreading branches of a Purkinje cell, disposed in a -transverse plane, are obviously arranged to hold up and keep apart -these myriads of longitudinal threads. A cerebral pyramid is shaped -like a fir-tree. It is placed in a definitely stratified layer. By -its branches it collects impulses from the superficial strata, which -it transmits through its stem to the white matter beneath the cortex. -The various parts of the central nervous system have work of different -kinds to do, and we find interposed in the circuits which compose the -several parts cells of various types. We speak of the large cells as -“motor,” the granules as “sensory,” the small pyramids as “association” -cells—such terms indicating the positions which they occupy in the -arcs, but not defining their functions. Of specialization of function -the physiologist cannot obtain a hint. He cannot classify nerve-cells -in groups concerned in reflex action, in feeling, in remembering, in -willing, in thought. On the contrary, he can assert with confidence -that such distinctions are not to be drawn. - -In various situations in the central nervous system a certain type -of cell is found for which, in the present state of knowledge, it -is impossible to account. We mention these cells lest it should be -inferred, from what has been said above, that all neurones can be -fitted into a simple scheme of conducting arcs. In the spinal ganglia -there are neurones whose axons divide to form “baskets” around other -ganglion-cells. In the cerebellum there are similar cells, the axons of -which divide into branches, which break up to encase Purkinje-cells. -Cells of the same kind are found in a few other situations. In some -cases the end-branches which enter into the formation of the baskets -are few in number, and thick and clumsy. They grasp the body of the -cell which they surround, with gouty fingers, as it were. In other -cases the basket is a tangle of fine threads. It is difficult to see -what rôle cells of this kind can play in conduction. From the olfactory -and optic centres nerve-fibres extend outwards to the olfactory bulb -and retina. Here again is an arrangement which does not fit in with any -scheme. We might multiply examples. But enough has been said, perhaps, -to convey the impression which we wish to leave, that, although -experiment abundantly proves that the nervous system consists of an -association of sensori-motor conducting arcs, and although anatomical -investigation demonstrates the existence of chains of neurones which -take part in the formation of such arcs, it is impossible to reduce -the system to schemata or to prepare diagrams in which all structural -elements are, even hypothetically, fitted into place. - -It may be convenient at this point to call attention to the differences -which distinguish the =sympathetic system=—the ganglia and nerves -of the viscera and bloodvessels—from the system devoted to bringing -sense-organs into connection with the skeletal musculature which we -have chiefly considered hitherto. The fibres of the posterior root -of a spinal nerve which convey impulses from the skin and muscular -sense-organs, and the fibres of its anterior root which convey impulses -to skeletal muscles, have a similar diameter of about 15 µ. In addition -to these, the roots contain fibres which carry impulses from and take -them to the viscera. Those which bring impulses from the viscera vary -greatly in thickness, some being as large as the other sensory nerves -of the posterior root. The diameter of the fibres which go to the -viscera is not more than one-fifth as great as that of the other fibres -of an anterior root. Similar slender fibres are found in the vagus -nerve. If all organs are removed from an animal’s chest and abdomen, a -string of small pearl-like ganglia, united by a longitudinal cord, is -seen lying on either side of the bodies of the vertebræ, one ganglion -for each segment. This string of ganglia is termed the “sympathetic -chain” (_cf._ p. 243). The small medullated fibres of the anterior -spinal roots join these ganglia. Some of them arborize about their -cells; some pass by them to arborize in ganglia which lie farther -afield, on the course of the great bloodvessels and within the viscera. -The axons of neurones whose cell-bodies are within a ganglion break up -into bunches of non-medullated fibres. In this way the fibres of the -sympathetic system are increased in number. Each of its neurones is a -multiplying and distributing station. There is no evidence that it in -any way serves as a “centre,” takes part in reflex action, or otherwise -usurps the functions of the grey matter of the spinal cord. Nerve-cells -are thickly strewn between the mucous membrane and the muscular coat, -and again between the two layers of the muscular coat of the alimentary -canal. It is not so certain that this system has no “central” -functions. The remarkable degree in which the wall of the intestines -retains its capacity for co-ordinated movement, after all nerves which -reach it from the ganglia and through the vagus have been cut, suggests -that the plexus of nerves within it does act to some extent as a reflex -centre. If we leave the case of the intrinsic nervous system of the -alimentary canal open, awaiting further proof, there is no reason for -looking upon the sympathetic system as in any degree independent of -the spinal cord and brain. It does its work on a large scale, and its -work is of a low order. Nature does not need to connect up the viscera -and bloodvessels with the central nervous system by means of fibres -as thick as those used for skeletal muscles. It is more convenient to -provide for the multiplication of the nerves—which must be extremely -numerous, owing to the relatively minute size of the muscle-fibres for -which they are destined—outside the central system than it would be to -include the necessary distributive cells within it. Again, we find that -a nerve-cell, when we see it at close quarters, shows no evidence of -administrative capacity. Although of a different shape, a ganglion-cell -of the sympathetic system is as large and as complex in form and -structure as a pyramidal cell of the cortex of the brain; yet the work -which it does is of a purely mechanical order. It receives, reinforces, -transmits impulses which reach it from the central nervous system. - -The often-repeated statement that a nerve-fibre is a drawn out process -of a nerve-cell body has prepared the reader to anticipate that it -dies when cut off from its central connection. When the axon is dead, -the sheath which invests it rapidly loses its tubular character. If -the situation of the cell-bodies of a nerve be known, it can be at -once foretold on which side of the cut =degeneration= will occur. -Suppose that the median nerve has been severed at the wrist. All -nerve-fibres on the distal side of the wound must atrophy, whereas none -of the fibres on the proximal side will be affected. The motor fibres -have their cell-bodies in the spinal cord, the sensory in the spinal -ganglia. Degenerations following lesions in the central nervous system -have taught pathologists more about the course of the fibres in the -white matter than any other class of observations. Degeneration above -the lesion is spoken of as ascending, below as descending—not that it -progresses upwards or downwards. It occurs throughout all the stretch -of the fibre which has been isolated from its cell-body at the same -time, or nearly so. The thought that impulses can no longer ascend or -can no longer descend, as the case may be, has given sanction to the -expressions “ascending” and “descending” degeneration. - -Restoration to functional activity of tracts of fibres which have -degenerated in the brain or spinal cord never occurs, but severed -peripheral nerves =regenerate=. Not that fibres join cut end to cut -end, however clean the wound. A wound in the wrist which has divided -the median nerve may heal in a few days “by first intention,” so far -as other tissues are concerned; but the patient does not for two or -three months recover the power of using the muscles of the hand which -the nerve supplied or the sense of touch in the area of skin to which -it was distributed. The ends of the axons on the proximal side of the -wound have to grow downwards to establish new connections in the -muscles and in the skin. The interval which elapses between the healing -of the wound in the wrist and the restoration of sensation and power of -movement is occupied in their downgrowth. - -The re-connection of regenerated nerves with their terminal apparatus -presents to the mind a curious problem. There is no evidence that as -function is re-established the brain has to re-learn the situation -of the sensory spots on the skin, or to re-acquire skill in using -the muscles which again come under its control. From the moment that -the outgrowing nerves have recovered their terminal connections skin -and muscles have their right representation in the brain, however -much the two cut ends may have been twisted in their relation one to -another. It seems inconceivable that each nerve-fibre can find its -way to its original station; but if it does not, our conception of -the mode of working of the nervous system still needs much refining -from the telephone-exchange analogy by which we naturally help out our -explanations. If a telephone cable has been severed, it can be made -useful again only in one of two ways. Either the two segments of every -wire that has been cut must be reunited, or the subscribers’ numbers -must be redistributed. - -The experiment of uniting the proximal segment of one nerve with the -distal segment of another of a quite different function gives results -which have an even more disconcerting effect upon our theory of the -nervous system. The sympathetic cord of the neck and the vagus nerve -lie very close together, alongside the carotid artery. The vagus is -both afferent and efferent. The sympathetic is wholly efferent—_i.e._, -it conducts impulses, which enter the sympathetic chain within the -thorax, in the direction of the head. If both nerves are cut, and -the end of the vagus turned round, so that it is in apposition with -the upper end of the sympathetic, its regenerating fibres make their -way along the sympathetic cord, headwards, to the superior cervical -ganglion. They arborize about the bodies of its ganglion-cells, just -as the sympathetic fibres used to do. The vagus is a nerve of many -functions. Amongst others, it inhibits the contraction of the heart, -constricts the bronchi of the lungs, dilates the bloodvessels of the -intestines, and helps in regulating the movements of these viscera. -After it has taken the place of the upper segment of the sympathetic -it dilates the pupil, constricts the bloodvessels of the ear, erects -the hairs of the head, as if to the manner born. To take another -example, in a monkey the two nerves supplying respectively certain -flexor and certain extensor muscles of the forearm were cut, and -their ends crossed, so that flexor nerve-fibres grew down to extensor -muscles, and extensor fibres to flexor muscles. There was no bungling -of reflex actions or of voluntary actions when the new roads were first -used. The monkey did not jerk its hand open when it tried to scratch or -to grasp a nut. - -When experimental data first began to accumulate, physiologists -drew diagrams and made models of the nervous system in which they -represented it as composed of conducting arcs. The arcs were superposed -to indicate that they were of various grades—spinal for ordinary -reflexes, bulbar for co-ordinated actions, through the grey matter -in the centre of the great brain for “ideo-motor” actions, through -the cortex of the great brain for voluntary acts. They spoke of -authority and responsibility, comparing the nervous system to an -army or a club. It is premature to attempt a theory of the nervous -system compatible with recent discoveries regarding its structure and -mode of working, but it is clear that the diagrams and metaphors to -which we have just referred were misleading. In place of attempting -to disarticulate the machine, we ought to emphasize its structural -unity. The results obtained by uniting heterologous nerves cannot be -explained by reference to a model made of wires and pieces of cork. -They do not fit in with any organization of human units or with any -postal system or telephonic apparatus for transmitting news. Probably -the lines of thought which will prove most fruitful are somewhat as -follows: (1) An efferent discharge occurs as the result of the opening -of a circuit from a muscle back to the muscle. Afferent impulses—call -them sensory, on the understanding that this does not imply that they -appear in consciousness—are ceaselessly flowing from receptors to -effectors in the muscle. A sensation—in the case of skeletal muscles -usually a skin sensation—reinforces them to discharging-point. If -the spinal cord has been severed from the brain, the up-and-down flow -does not reach beyond its grey matter. It is short-circuited. If the -brain is in normal connection with the spinal cord, sensory impulses -travel upwards to its cortex (without, save in exceptional instances, -arousing consciousness, or, as we should prefer to express it in this -connection, without attracting attention) to a degree which varies with -the several classes of receptor and with the animal. A monkey reduced -to the condition of a “spinal animal”—_i.e._, with its spinal cord -severed from its brain—is less competent than a dog, and a man is far -less competent than a monkey. In other words, a man habitually uses -his brain more than does a monkey, and a monkey more than a dog. The -proportion which brain-weight bears to body-weight roughly indicates -the part the brain plays in conducting the traffic of the body. (2) -Communication within the nervous system is almost unrestricted. If, -before the median nerve was divided at the wrist, receptor A usually -initiated a current which passed through the circuit to effector X, and -receptor B to effector Y, and if the new fibres which grew downwards -lost their way so that the one which used to receive messages from A -attached itself to B, and the one which used to transmit commands to -X attached itself to Y, A is not thereby cut off from X, or B from -Y. Such a mechanical association is restricted to our diagrams. It -does not enter into Nature’s plan. The spinal cord is not scored with -unchangeable paths. A messenger from A could always reach either X or -Y. It was not the path, but the struggle with competing messengers, -which directed him to X. - -When we endeavour to picture the mechanism of the nervous system, we -find ourselves faced by phenomena which appear irreconcilable. One set -of observations leads to the conception of closed paths; another set -points to an open conductor. The experimental crossing of nerves to -which we have just alluded shows that the nervous system is adaptable, -to a degree which seems extraordinary to anyone who attempts to compare -it with any of Man’s devices for establishing communication. Paths -appear to make themselves. On the other hand, the more important, and -therefore dominant, reflex actions, such as swallowing, breathing, -the maintenance of position, are due to the union of receptors and -effectors by lines which are either reserved for their sole use, or, if -shared by other currents, it is on the understanding that they have a -first and altogether prepotent claim. No competing impulses can divert -them or block their way. All reflexes which in the history of the race -have established their right to dominance not only seize and hold a -route through the nervous system, to the exclusion of all competitors, -but, as we have already shown in the case of the swallowing impulse, -the traffic in neighbouring routes is suspended for their benefit. -At the other end of the scale we find reflexes which may be termed -“occasional,” in that, although of frequent occurrence, they exhibit -illimitable variability in form. Occasional reflexes require, as a -preliminary to their transmission, that the afferent impulses which -give rise to them should secure for a time the exclusive use of the -motor neurones by which they are carried out. The receptors bring the -motor neurones into tune with themselves, and while in tune they will -respond to impulses from no others. But the tuning lasts for a short -time only. Either receptor or neurone, or both, soon tire. There is no -danger of a particular reflex being prolonged to the detriment of the -organism as a whole. As an illustration of an occasional reflex, we may -cite the scratching movement of a dog. Its skin is punctured by a flea. -It scratches the place. A second flea bites it somewhere in the same -neighbourhood. The dog does not shift its hind-foot so as to scratch -midway between the two bites. It finishes out one scratch before paying -attention to its second tormentor. The exact position to which the -hind-foot is raised depends upon the position of the irritant; and -since this may be shifted over a very considerable surface, the form of -the reflex varies equally widely. Each of the very numerous receptors -in the skin tunes a slightly different group of motor neurones; and -since a second irritant may reinforce the first, instead of making an -alteration in the group of neurones which the reflex is discharging, it -is clear that there is no fixed path uniting receptor A with neurones -X, Y, Z and receptor B with neurones W, X, Y. If, however, the second -irritation occurs at a spot lying at a considerable distance from -A, in place of reinforcing the scratching movement which A has set -going, it weakens and shortens it. The receptor C, which is calling -for the discharge of a markedly different set of motor neurones, tends -to inhibit those which are already active. These results are tested -with precision upon a “spinal dog” and with the aid of an electric -needle, the other pole from the battery being a large flat plate -placed in contact with the animal’s body. The conception of definite -paths, to which the contemplation of permanent reflexes gives rise, is -inappropriate to occasional reflexes. The latter show so wide a range -of variability and adaptability as to prove that a given receptor -may bring any of a great variety of groups of motor neurones into -connection with itself; just as a given group of neurones may be played -upon by impulses from a great number of different receptors. We have -called it a tuning of the motor neurones. One metaphor is as good as -another. The physical process which in the brainless frog underlies the -preparation for discharging motor neurones in the spinal cord, on the -same side as the leg on which vinegar is placed, so long as that leg -is free, and on the opposite side, when that leg is fixed, is unknown. -We seem to catch a glimpse of a doubleness of action, receptors in -the muscles combining with receptors in the skin in determining the -paths along which impulses shall be reflected—the efficient muscles -sensitizing their own neurones to the tuning influence of impulses from -sounding cutaneous nerve-endings. But it is impossible to formulate a -working scheme in the present state of knowledge. - -=Sense-Organs and Nerve-Centres.=—A vast amount of labour has been -devoted to the study of the external form of the central nervous -system and to unravelling its internal structure; to plotting out its -various groups of nerve-cells, to disentangling its innumerable tracts -of fibres. The surface of the brain and spinal cord has been mapped -and measured. Every millimetre of its substance has been cut into -sections on the micro-tome. Organs which, fifty years ago, appeared -too complicated for investigation have been described in the minutest -detail. An immense accumulation of data is available for purposes of -reference; yet anyone who submits the theory of the nervous system as -it is held at the present day to a general review must allow that the -results of anatomical research enter but little into its construction. -The reason for this is not far to seek. As knowledge has advanced, the -apparent, or rather the expected, complication of the system has given -place to ideas of unity and simplicity. Its external configuration and -the varied arrangement in “nuclei” of its nerve-cells may, without -impropriety, be described as accidental. The form of the body and the -consequent location of the clients of the nervous system determine -the disposition and degree of concentration of its various business -centres. It shows, when followed throughout the whole animal kingdom, -extreme variability of its constituent organs, with absolute uniformity -of plan. Indeed, from the physiological point of view the term “organ” -is scarce admissible. It implies diversity of function in too high a -degree. The several parts into which the central nervous system is -obviously divisible co-operate so intimately as to preclude us from -thinking of them as separate organs. - -If the citadel of the central nervous system is to be captured, all -lines of approach must be tried. Its outward form must be studied, -its minute structure examined with the microscope, its modifications -in various animals compared, its development followed, its reactions -to artificial stimuli tested, its pathological deficiencies and -vagaries watched. Yet, of all the means which have been made use of in -attempting to penetrate its secrets, the study of its history, by the -methods of comparative anatomy and embryology, has probably contributed -most to the development of sound ideas regarding the manner of its -working. The first differentiation visible in the blastoderm—the -globe of cells into which the ovum divides and out of which the embryo -is built—has relation to the formation of the nervous system. If -the earliest stages of its growth are followed, and the different -phases through which it passes are compared with the forms which it -assumes permanently in lower animals, the plan or type upon which -it is constructed shows up distinctly. Looking down the line to the -earliest vertebrata, we can discern clearly the form of nervous system -possessed by their prototype. Not that this “ideal ancestor” ever -existed. Experience teaches that it is unlikely that any animal that -ever lived was absolutely regular and symmetrical in all its parts; -nevertheless, the type can be presented in a perfectly regular scheme. -The ideal ancestor of the vertebrata was segmented, like a caterpillar -or a worm. Its mouth was not at the anterior extremity of the body, but -two (or more) segments behind it. Every segment bore a sense-organ (at -one period two sense-organs) on either side. Beneath each sense-organ -there was a clump of “grey matter.” Each segment also contained -(although not at the earliest epoch) two clumps of nerve-cells and -neuropil in a more central situation. These “ganglia” were united by -longitudinal and transverse commissures. They received the axons of the -cells which lay in the clumps beneath the sense-organs. They gave axons -to various muscles. Such is the type out of which the modern nervous -system has developed: two separate sense-organs and a complete nervous -system for each segment, the sense-organs connected with the ganglion -of the same side, the ganglia of the two sides bound together across -the middle line, and each row of sense-organs and each row of ganglia -united by longitudinal commissures into a chain. From the nervous -system as we see it now the majority of these segmental sense-organs -have disappeared; but the mode of formation of the cerebro-spinal -ganglia shows that they are the clumps of nerve-cells which lay beneath -the vanished organs. In the nose and the eye the grey matter retains -its original situation in the immediate vicinity of the receiving -epithelial cells—as the olfactory bulb and the deeper (anterior) -layers of the retina. The ganglia of the auditory nerve lie within the -bones of the ear. Spinal ganglia are close to the spinal cord. Auditory -and spinal ganglia contain only the cell-bodies of the first collecting -neurones (sensory nerves) together with certain curious bracketing -cells already referred to (p. 324), all the other constituents of the -peripheral clumps of grey matter which are found in the olfactory bulb -and retina having been withdrawn from the spinal ganglia into the axis -of the brain and spinal cord. - -The sense-organs in front of the mouth have had from the beginning -an immense advantage over the others as observing-stations. Whereas -the body-organs collected information regarding the things with which -the animal came in contact, and consequently specialized in touch, -pressure, temperature, and, in the case of fishes, sensitiveness to -the chemical constitution of the medium in which the animal lived, -the head-organs specialized in responsiveness to forces acting from a -distance—particles suspended in air, vibrations of light, pulsations -of sound. Sensitiveness to touch, if it is to be useful, must be -widely distributed. The body-organs therefore broke into scattered -groups of sense-cells. Touch-spots are scattered all over the surface, -although they are set much closer together in the areas of skin which -are usually the first to come into contact with external objects -than they are elsewhere. The efficiency of the sense-organs of the -head—nose, eye, and ear—depended upon their remaining compact. -Progress in animal life, as we understand it—the rise from lower to -higher forms—has depended upon increasing integration of the body and -co-ordination of its functions. The nervous system is the agent which -has accomplished this unification. Each step in advance has depended -upon the provision of more nerve-tissue for the lacing together of -the various parts. We have seen already (p. 329) how intimate is -the union of receptors and effectors of every kind via the spinal -cord and brain. The overwhelming predominance in the direction of -action of the nose, the eye, and the ear has led to the accumulation -in their vicinity of the ever-increasing grey matter. The cerebral -hemispheres, or “great brain,” are pouched outgrowths from the first -pair of ganglia directed towards the olfactory pits. The original eyes -bore a similar relation to the second pair of ganglia—the epithet -“original” implying that the eyes which we now use are not the organs -with which our prevertebrate ancestors saw. First one of the original -eyes disappeared, and then the other. The vestige of the second is -still to be seen in the “pineal body” which is found on the dorsal -side of the brain of every vertebrate animal—in a mammal deeply -hidden in the cleft between the cerebrum and cerebellum. In place of -the pineal eyes two other sense-organs have specialized as eyes. They -are constructed on a different plan, being, to put it shortly, pineal -eyes turned inside out; for whereas in the pineal eyes, as in most of -the eyes of invertebrate animals, the rods and cones, which are the -cells of the retina sensitive to light, are directed forwards towards -the lens, the rods and cones of our permanent eyes are directed away -from the source of light. This change has made it possible to provide -more abundantly for their nutrition, and hence a greater power of -discriminating separate points in space and of distinguishing colours -is conferred upon them. The substitution of other sense-organs for the -original eyes has complicated the pictures which are presented to us by -a brain in its successive stages of growth; but it does not prevent us -from recognizing the general plan. Probably the secondary eyes, like -their predecessors, belonged to a pre-oral segment. The sense-organs -of a segment behind the mouth developed into ears; and the ear was -in its earliest phases, and still is, something more than an organ of -hearing. Its semicircular canals give information of displacements -in space. Knowledge of the position of its body is, to a fish, of -far more importance than its ability to hear breakers on the rocks. -Three looped tunnels, opening at either end into a common chamber, are -hollowed in the bone which contains the ear (_cf._ Fig. 38). Placed -at right angles one to the other, they occupy all three dimensions of -space. Open a notebook until, one of its covers lying horizontally, -the other is vertical, and place a sheet of paper vertically against -the bottom of the pages. A curved line drawn on each of these three -surfaces will represent the three semicircular canals. Arrange another -notebook in the same way, and let the two rest on the table with the -two vertical covers inclining one to the other, anteriorly, at an -angle of 90 degrees. The six surfaces will be in the planes of the six -semicircular canals. Within each bony canal is a membranous tube, to -which nerves are distributed, filled with fluid. When the position of -the head is changed, the fluid within the membranous tubes slides on -their walls. It is left behind at the moment the movement commences. It -overtakes its receptacle when the movement stops. The stimulus received -by the nerve-endings is recognized as indicating an alteration in the -orientation of the head. If the movement of the fluid is violent, -as when one waltzes, the loss of the sense of position disconcerts -the brain to such an extent that giddiness results. For a time the -quiet assurance upon which so much depends, that one knows how the -body stands in relation to its surroundings, gives way to a chaos of -sensations. From the nature of the case, the information which the -semicircular canals afford relates to change. They give no help in -ascertaining the position of the head when it is at rest. This must be -the reason, although the connection is not very clear, for the waning -of the effect in consciousness when stimulation is prolonged, and also -for the very marked after-sensation. At the commencement of a voyage -attention may be unpleasantly attracted to the rolling of the ship. -After a few days it ceases to be noticeable; yet when the voyager, -the night after landing, wakes in the dark, he finds his bed-room as -unsteady as his cabin. Rising hurriedly, the attempt to adjust his -position to the heaving floor (we speak from personal experience) -may result in a heavy fall. Although this phenomenon must be classed -with other “after-sensations,” it is so prolonged as to suggest that -consciousness, having become accustomed to a world which causes a -backward and forward flow of endolymph, misinterprets the absence of -sensation as indicative of change. - -Taste is, practically, a special kind of smell. A fish’s olfactory -membrane, taste-buds, and chemical organs “of the lateral line” serve -the same sense, although, no doubt, they are applicable to the analysis -of different forms of matter in solution. - -Our ideal prevertebrate has now left its primitive undifferentiated -condition. In front of its mouth it bears organs with which it searches -the world. Close behind the mouth are its auditory and orienting -organs. The rest of the surface of the body is endowed with the -capacity of recognizing “taste,” temperature, and contact. Smell, -sight, and orientation determine the development of the brain. - -The cerebrum which has eventually become, as the seat of consciousness, -and hence the apparatus of mind, the dominant factor in the nervous -system, was in the first instance the part of the brain concerned with -the distribution to the muscles of impulses generated in olfactory -organs. There is scarcely any indication in a fish’s brain of the -representation in the cerebral hemispheres of any other sense, even -that of vision. - -A bird’s brain presents a striking contrast to the brain of a fish. -With the exception of the apteryx and other ground-birds of New -Zealand, all birds are apparently destitute of the sense of smell. -Vision is the sense upon which their activity depends. It has invaded -the cerebrum, converting it into an organ in which sensations of -sight are worked up into “mind-stuff.” The optic lobe connection is -restricted to the production of reflex actions in which vision is -immediately followed by movement. - -All the senses are represented in the great brains of mammals. -The cerebrum, which owes its existence to its connection with the -favourably-situated sense-organ of the nose, and grew in importance -when vision invaded it, has now taken in the senses of hearing, taste, -and touch. Only what may be termed in general visceral sense, and the -sense of orientation, are excluded. - -Looking back to the starting-point, we see a segmented animal; its -segments of equal value; its nervous reactions unisegmental, although -linked in functional sequence. If it starts to walk, owing to -stimulation of one of its sense-organs, the impulse to walk spreads -from segment to segment. Comparing the latest product of evolution -with the earliest, we find that nervous tissue has concentrated -at the anterior end of the body. The double chain of ganglia, now -condensed into the axis of the brain and the spinal cord, still -contain all the effector neurones by which muscles are called into -action. Sensory nerves still arborize in the axis, providing the -mechanism for actuating motor neurones. But the vast majority of -intermediate or intercalated neurones have been attracted to the two -huge brain-masses—the cerebellum and cerebrum. In the former all -sensations (not conscious) connected with tone, position, orientation -and equilibrium are worked into appropriate impulses for the regulation -of the muscular system. In the latter all sensations which convey -information regarding the relation of the environment, including the -body, to the ego—the _not-me_ to the _me_—are transformed into motor -discharges which set a-going the movements (and the thoughts) by means -of which the purposes of life are fulfilled; for in the cortex of the -great brain alone is the passage of nerve-currents accompanied by -consciousness. Concentration of nerve-tissue allows of the combination -of sensations. It also facilitates the no less important effect -of mutual influence, interference. Sensations are suppressed, and -therefore the multitude of reactions to which they would give rise are -inhibited, in the interests of restricted and sustained movement or -thought. - -=The Cerebellum.=—Sharks and other swift-swimming fishes have large, -deeply fissured cerebella, for the cerebellum is the part of the brain -which has gathered into itself most of the grey matter associated -with balancing, attitude, posture. The cerebellum is in birds large -and deeply folded. Developed from the ganglia to which the auditory -nerve distributes impulses from the semicircular canals, it has -established connections with all the other nervous tissues concerned -with sensations of position, strain, or pressure, including the -eyes, which afford information regarding the position of our limbs -relatively to the trunk, and of the whole body relatively to external -objects. Morphologically it is a median growth. The adverb is one of -those qualifying terms, convenient in science, which direct thought -without confining it. As used above, it implies that anyone who passes -before his mind the cerebella of all animals from fishes to Man, and -in all stages of growth, from their earliest appearance in the embryo -to their condition in the adult, sees the organ as a median prominence -surmounting the medulla oblongata. The bulgings of its sides which, -in human anatomy, are termed hemispheres, do not disturb its central, -unpaired plan of structure. It has, it is true, a lateral appendage -on either side (the combined flocculus and paraflocculus of mammalian -anatomy), but this lobe, although of great historic interest, is -so small, as compared with the median growth, as not to affect our -general conception of the form of the organ. By transverse fissures the -cerebellum is divided into a series of lobes. - -In appearance the cerebellum varies greatly in the different classes -and orders of Vertebrata. Yet underlying this variety there is -marked unity of plan. A sagittal section of the organ of a shark, of -a bird, of a kangaroo, of a dog, of a whale, of Man, shows that it -is divided, from before backwards, into the same number of lobes in -animals occupying every position from the bottom to the top of the -vertebrate scale. A very little effort to grasp the significance of -this mystic number, nine, convinces one of the hopelessness of any -attempt to correlate the form of the cerebellum with the muscular -development or sensory endowments of vertebrates as a sub-kingdom. -It is the same for animals with limbs and animals without; animals -with well-developed noses or eyes, and animals destitute of one or -other of these sense-organs. This uniformity is extremely significant, -when contrasted with the wide differences exhibited by the cerebral -hemispheres. It shows that, unlike the great brain which mediates -between the several senses and the muscular system, the little brain is -concerned in bringing about adjustments to the environment which are -equally important to all animals, no matter how far they may depart -from the common type. The cerebellum is crossed by deep fissures, -dividing it into narrow convolutions or folia. The folia are grouped -in nine lobes. If the reader has secured as an illustration the brain -of a sheep, he will notice that the lateral regions of the cerebellum -present a complicated appearance owing to the contortion of the folia, -which results from the unequal development on its sides of the several -lobes. In its total size the cerebellum keeps step with the cerebrum, -the right side of one organ being associated with the left side of the -other. - -[Illustration: FIG. 23.—VERTICAL SECTION OF THE CORTEX OF THE -CEREBELLUM, CUT PARALLEL WITH THE LONG AXIS OF A FOLIUM. - - A shows three cells of Purkinje, their espalier - systems of dendrites being seen in profile. A - “mossy fibre” enters the granular layer from the - white matter. About a dozen of the granules are - shown, each with four or five dendrites and a - single axon. The axon bifurcates in the molecular - layer, its two branches running for a considerable - distance to left and right along the folium. B - shows the other nervous elements which are found in - the cortex: a cell of Golgi with a ramified axon, - a climbing fibre, a basket-cell, of which the axon - divides into four branches, and a small stellate - cell.] - -The grey matter which covers the surface of the cerebellum, its -cortex, is singularly regular in microscopic pattern (Fig. 23). It -is divided into three sheets: superficially, the molecular layer in -which the dendrites of the cells of Purkinje branch; beneath this, the -thin layer in which are situate the cell-bodies of these neurones; -thirdly, the layer of small cells, or granules. Cells of Purkinje and -granules have been already described (p. 303). To these must be added -the stellate, bracketing cells of the molecular layer, the axons of -which divide to form baskets about a number of Purkinje-cells, and the -cells of Golgi of the granular layer. These last are comparatively -large cells, which have thornless dendrites, and axons which branch -repeatedly in the granular layer, without passing into the white matter -which underlies the cortex. Two kinds of nerve-fibre bring impulses -to the cortex: (1) “Mossy” fibres, which bear rosettes of filaments -which distribute impulses to the granules; and (2) “climbing” fibres -or “tendril” fibres, which, passing through the granular layer, cling -like ivy to the trunk and principal boughs of the dendritic processes -of Purkinje-cells. The axons of the cells of Purkinje undoubtedly carry -impulses away from the cortex, but their destination is not certainly -known. - -The uniformity of structure of the cerebellum suggests that it “acts -as a whole.” Anatomy gives no warrant for the expectation that work of -different kinds is done by its several lobes. Its simplicity leads one -to hope that its mechanism may some day be understood; but at present -there are so many gaps in our knowledge that it is difficult, perhaps -hardly profitable, to attempt to string together the few anatomical -facts of which we are sure. - -By means of tracts of afferent fibres the cerebellum has a very -extensive connection with the grey matter of the cerebro-spinal axis -(including the optic thalamus) into which sensory impulses of all kinds -are poured. Experimental results indicate that the organ distributes -impulses to the whole length of the cerebro-spinal axis, from the level -of the neurones which govern the muscles which move the eyes to its far -hinder end. No nerve-roots enter it. Its afferent fibres are the axons -of cell-bodies which lie in the posterior horns of the grey matter of -the spinal cord and in the corresponding grey matter of the axis of the -brain, especially that part related to the nerve from the semicircular -canals. Another set of afferent fibres lies at the periphery of the -spinal cord, forming one of the best defined of the spinal tracts. It -is also one of the oldest, being found in the same situation in all -vertebrate animals. Its fibres, which are exceptionally large, are -the axons of cells which form a very definite column—the “vesicular -column of Clarke”—on the median side of the posterior horn. Further -than this we cannot go. We are ignorant of the nature of the sensory -impressions collected by the cells of Clarke. The cerebellum also -receives through its middle peduncle the axons of cells which lie in -the pons Varolii on the opposite side; which cells are discharged by -impulses descending from the cortex of the great brain. It is not -improbable that it gives to the great brain as many fibres as it -receives from it. - -If we had no experimental evidence as to the part which the cerebellum -plays in the harmonious working of the whole nervous system, we -should infer from its structure and connections that it is somewhat -mechanical, a co-ordinator of the activities of other parts rather -than in itself a functionally independent organ. Pathological and -physiological observations very definitely justify this conclusion. -They show that the cerebellum is not essential to life. It may be -completely destroyed by disease or removed by operation without robbing -the individual of any single function or capacity. Disease of the -cerebellum does not diminish the patient’s sensitiveness to every kind -of stimulus, nor does it deprive him of the use of any single muscle; -but it reduces him to the condition of a person who in gait, but not -in mind, is habitually drunk. When he walks he staggers from side to -side; when he stretches out his hand it trembles. His movements are -jerky; his head shakes, his eyes oscillate; he suffers from a feeling -of giddiness; his speech comes haltingly. Cerebellar ataxia, which is a -rare disease, resembles in many respects the much commoner “locomotor -ataxia” produced by disease of the spinal ganglia and the parts of -the cord connected with posterior roots; but careful analysis of the -symptoms shows that they are due, not to want of the sensations which -guide movements, but to inability to regulate the force of muscular -contractions. A man suffering from locomotor ataxia falls when he -closes his eyes, because, not being able to feel with his feet, he is -dependent upon vision for information as to his attitude. When the -cerebellum is diseased, the patient is no less unsteady with his eyes -open than he is with them closed. - -The results of cerebellar disease or injury bring home to us the -fact that a nice adjustment of movements is needed to maintain -equilibrium. A dog from which the cerebellum has been removed retains -all its natural enterprise, all its instincts, all its emotions; but -every action which requires it to maintain its centre of gravity in -an unstable position gives it trouble. Placed in water so that its -body is supported, it swims almost as well as a normal dog. It is, -however, easy to lay too much stress upon the balancing function of the -cerebellum. The disturbance of this function attracts our attention; -yet it is probably but the indirect result of the suppression of -activities of a more widespread character. No animal ventures such -liberties with its centre of gravity as the biped Man accomplishes, -without thinking, every time that he descends a flight of stairs. Yet -the cerebellum of the limbless whale, that lives in a medium which -decentralizes its gravity, so to speak, bears the same proportional -relation to the rest of the nervous system as that of Man. Strangely -enough, it is the only cerebellum in the animal kingdom which so -closely resembles Man’s that it might be passed off as belonging to -a human giant; another reminder of the difficulty of deducing the -functions of the several parts of the organ from a study of their -relative development. What have a man and a whale in common which -determines the identity in form of their cerebella? How has it come -about that two cerebra as widely unlike as a man’s and a whale’s should -be associated with a common form of cerebellum? - -If we apply to grey matter the distinction between sensory and motor -nerve-tissue—having no exact terminology, it is difficult to avoid -these metaphorical expressions—the cerebellum is essentially a sensory -development. It grows from the very margin of the infolding groove, -which, when closed, becomes the central canal of the brain and spinal -cord, its elements being marshalled in intimate association with -sensory root-fibres. Its millions of loops formed by the axons of -granules and the collecting processes of Purkinje-cells, are by-paths -which tap the conductors of sensory impulses. From some—those, -for example, which originate in the muscles and tendons, and in -the semicircular canals—more of the impulse is diverted to the -cerebellum, from others less. The organ has no motor functions. It -does not discharge neurones which control skeletal muscles, or plain -muscle, or glands. Yet it influences the passage of impulses through -sensori-motor chains, and apparently its influence is universal. It -regulates tone, reflex action, voluntary action. There is no part of -the nervous system over which its control is not felt. By its action -on the apparatus which binds the infinity of receptors which the body -contains to its muscle-fibres and other effectors, it unifies the -body. The cerebrum, as we shall see, is the organ which unifies the -personality. In the progress of evolution two functions which were -originally combined have, for convenience of concentration, been -divorced. The great brain has been set free from the more mechanical -part of the work. That it can perform the functions of the cerebellum -as well as its own is proved in cases of congenital deficiency of -that organ. In several instances malformation, amounting to a very -considerable reduction in the size of the cerebellum, was not detected -until after death, there being no symptoms of a sufficiently pronounced -character to call attention to it during life. - -=The Cerebrum.=—All observations made on the great brain prior to -1870 showed it as absolutely inexcitable. Surgeons and physiologists -agreed that cutting, burning, passing electric currents through its -substance, neither yielded evidence of sensation nor movement of any -part of the body. Concerning its structure little was known beyond the -fact that whereas the grey matter, or cortex, which covers its surface -contains nerve-cells, only fibres are to be found in the white matter -which constitutes the greater part of its bulk. It seemed a hopeless -task to attempt to make anything out of a mass of tissue so uniform -in constitution and so irresponsive to experiment. Removing portions -of it appeared to cause a general dulling of the intellect without -loss of any particular mental quality. Physiologists, therefore, spoke -of the cerebrum as “functioning as a whole.” Phrenologists, having -classified the various phases of mental activity as “faculties,” -discovered “bumps” on the surface of the skull which they correlated -with the possession of the several faculties in a marked degree. They -parcelled out the brain in organs concerned with different kinds -of thought; but their localization of function was anatomically as -baseless as their classification of the various aspects of mind, -viewed as a system of philosophy, was absurd. In 1870 it was announced -that electrical stimulation of certain areas of the cortex of the -cerebrum of an animal under the influence of an anæsthetic, and -therefore incapable of voluntary action, induces definite movements. -Although the surgical applications of this discovery have proved -immensely important, its physiological value, as affording a method of -investigating the functions of the brain, is extremely small. Yet the -discovery gave an impetus to the further study of the cortex, which -has been rewarded with many exact results. By the discovery of its -excitability to electric currents it was proved that the whole cortex -has not exactly the same work to do, or—perhaps this is the safer form -of statement—does not do its work in exactly the same way. As soon as -it was known that it is divisible into areas differing in function, -many methods by which the delimitation of the areas might be attempted -were devised. The converging efforts made during the past forty years -by comparative anatomists, histologists, physiologists, pathologists, -and physicians, have resulted in the acquisition of an accurate, if -very restricted, understanding of the construction and mode of working -of the apparatus of thought. Of some of the new data the psychologist -is able to make use; but so far as the physiologist is concerned, it is -the vehicle of mind which is the subject of study, not its contents. - -A new subject has been created since 1870. There is therefore nothing -to be gained, so far as our present purpose is concerned, from the -consideration of views which were current before that date; and since, -as must always occur when a science is rapidly advancing, observations -which logically should have been the first to be made were not thought -of until it became necessary to devise methods of checking results -obtained in other ways, we will consider the various sources of our -information without regard to the chronological order in which they -were opened up. - -The cerebral hemisphere contains two large central masses of grey -matter, the nucleus caudatus and the nucleus lenticularis, often -described as a single structure under the name “corpus striatum.” Their -functions are unknown. The nerve-fibres which connect the cerebral -hemispheres with the rest of the central nervous system form two thick -limbs or crura on the under side of the brain. Each crus turns upwards -into its hemisphere, between the nucleus caudatus and optic thalamus -(the latter belongs to the “between-brain”) on the inner side, and -the nucleus lenticularis on the outer. In this passage the compact -crus, which is somewhat flattened, is termed the “internal capsule.” -Immediately above the three grey masses the internal capsule disperses -as a fountain of fibres which go to all parts of the cortex. Mingled -with these radiating fibres are vast numbers of others, proper to the -hemispheres, which run tangentially. Some, crossing the median plane, -as the corpus callosum, bind the two hemispheres together. Others form -tracts which can be followed from one end or pole of the hemisphere to -the other. Groups of fibres, dipping but little below the cortex, unite -nearly adjacent spots or neighbouring convolutions. - -The folding of the cortex beneath fissures is due to the necessity -of disposing of a certain bulk of grey matter without increasing its -thickness beyond the proper limit. Since the superficial area of a -sphere varies as the square of its radius, whereas its capacity varies -as the cube, it is possible for a fixed relation to be maintained -between the amount of cortex and the amount of white matter in the -brain, only by the folds increasing in depth as the size of the brain -increases. Fissuring is a response to a mechanical need. This does not -imply, however, that the lines along which it takes place are devoid of -morphological meaning. The similarity in pattern of the convolutions -and fissures in various animals, and the regular progress of their -development in each individual, prove the contrary. If they are not -absolutely trustworthy as boundaries of areas of separate function—and -further evidence will be needed before a decision can be pronounced -upon this disputed question—they are in the main satisfactory as -landmarks. - -As the nervous system grows, the axons of its neurones acquire -their fatty (myelin) sheaths in the order in which they come into -functional activity. The passage through them of impulses is the -stimulus which leads to the deposition of fat. The study of the -progress of myelination enabled the anatomist Flechsig to ascertain -the situation within the brain of the tracts of fibres related to -the several senses, and hence the traffic of the areas of the cortex -to which they go. Glistening white streaks appear successively in -the pulpy yellowish-pink substance of the interior of the brain. At -the time of birth all the fibres which enter or leave the cerebral -hemispheres have acquired their myelin sheaths. In the baby’s brain the -sense-organs have established all their connections with the cortex. No -new fibres will appear in the nerves of the eye, the ear, or the other -sense-organs, nor will their end-stations in the cortex be further -multiplied. (The use of the expression “end-stations” is legitimate -so far as sensations are concerned; notwithstanding that all sensory -impulses are retransmitted by neurones in the cerebro-spinal axis.) But -the cortex is very far from having finished its growth. It contains -a large amount of embryonic tissue, which gradually spreads outwards -from the developed areas into the surrounding unoccupied zones. The -taking up of new territory, and the consequent increase in the size -of the brain, is continued into adult life. The study of progressive -myelination enabled Flechsig to divide the cortex into “sensory -centres,” and intervening “association-zones”; although, doubtless, the -difference in function between the portions which receive sensations -direct and the portions in which the products of sensation are worked -up is one of degree, and not of kind. - -[Illustration: =Fig. 24.=—VERTICAL SECTIONS OF THE CORTEX OF THE -CEREBRUM—A, OF THE VISUAL SENSORY; B, OF THE VISUAL ASSOCIATION AREA. - - Between the two sections are shown the principal - types of cell, at the levels at which they - are severally found: _a_, small pyramid; _b_, - medium-sized pyramid; _c_, large pyramid. The size - of a pyramid is an indication of the distance to - which its axon extends before branching; the longer - its traject, the more widespread, it would seem, is - its terminal arborization. The axon of _c_, one of - the very large pyramids found in this association - area, passes to the front of the cerebrum, where - it breaks up in an association area of the tactual - sense of the hand, or of sensations concerned - with the regulation of gait, or in a centre for - movements of the eyeball. _d_, a tangential cell - of the surface; _e_, a Golgi cell with ramified - axon; _f_, a polymorph cell, with its axon directed - towards the surface. In sensory areas, tangential - fibres and granules are more numerous; in - association areas, small and medium-sized pyramids.] - -The structure of the cortex is not quite the same in sensory and -association areas; but it is everywhere so far from showing the -diagrammatic simplicity which characterizes the cortex of the -cerebellum as to make it difficult to summarize the modifications which -distinguish its various regions. To a considerable extent its elements -shade one into the other, differing in size and in orientation rather -than in form. Commonly it is described as divisible into five layers: -(1) A thin superficial layer, containing cells of various forms and -fibres derived from the cells of the deeper strata. Some of the cells -are pluripolar, possessing several axons which run parallel with the -surface. Their destination is unknown. They do not appear to form -baskets like the cells of the molecular layer of the cerebellum. The -dendrites of pyramidal cells extend into this layer. (2) The layer -of small pyramids; cells with a branching apical process, root-like -dendrites from the basal angles of the pyramid, and an axon which sinks -into the white matter. (3) Granules. Carmine or other nuclear stains -show that small cells are present in very large numbers, especially -in the sensory areas; but since they are not, like the granules of -the cerebellum, coloured by the chrome-silver method, their form -and the disposition of their axons are unknown. (4) Large pyramids -exactly similar in form to the small ones. Their apical processes -are very thorny. Their axons give off several collaterals. Pyramids -are the most conspicuous elements in the cortex. Properly speaking, -they do not occur in layers, but are scattered throughout its whole -thickness, although their cell-bodies are not seen in either its most -superficial or its deepest strata. The largest are those of which the -axons either descend into the spinal cord or pass to a very distant -region of the cortex. They are found singly or in small clusters in the -deeper levels. (5) Polymorphous cells, some of them pyramids lying on -their sides, or even directing their axons towards the surface; some -fusiform or irregular cells; some Golgi-cells (p. 340). The axons of -pyramids enter the white matter, and many fibres from the white matter -radiate towards the surface between the pyramids; but the way in which -afferent, sensory fibres are connected with the collecting processes, -dendrites, of the pyramids is not known. We have already referred to -thorns, and to the possible nerve-net (p. 301). Sheets of tangential -fibres also occur in the cortex. A particularly distinct sheet divides -the granules in the visual cortex into two strata. In sections of this -region the sheet of fibres appears as a white line, distinctly visible -without a lens. - -The limits of the several areas can be determined by examining the -structure of the cortex; but the individual peculiarities of the -various regions are not so marked as to indicate that they have -different kinds of work to do; if by kinds of work we wish to imply -that one part is “sensory,” another “motor,” a third concerned with -“intellectual processes.” On the contrary, its relative uniformity -shows unmistakably that all parts are engaged in the same work. -Nevertheless, certain broad conclusions can be drawn with regard to -the form of the neurones more immediately concerned with sensation, -with motion—that is to say, with the discharge to the grey matter -of the cerebro-spinal axis of the impulses which call its neurones -into activity—and with the secondary processes, called collectively -“association,” which occur within the cortex. Granules, as everywhere -throughout the nervous system, are receivers and distributors of -sensory impulses; although a study of the cerebral cortex does -not justify the conclusion that they are necessary links in its -sensori-motor arcs. Large pyramids are occupied with the nutrition -of fibres which have a long traject through the system. Hence they -are “motor.” They constitute a marked feature of the area which -is susceptible to stimulation. They occur also in the visual area -and elsewhere. Small pyramids are associational; that is to say, -their axons do not leave the cerebral hemispheres. They distribute -impulses from sensory areas to association-zones, and from one part -of an association-zone to another. The layer of polymorphous cells -is relatively thicker in animals in which the cortex of the brain -exercises less control over action than in animals in which the cortex -is supreme—in a rabbit thicker than in a monkey; in a monkey thicker -than in Man. This layer is therefore said to be concerned with the -lower functions of the cortex, whatever this expression may mean. Since -the relative abundance of small pyramids is a test of the supremacy -of the cortex, we may speak of them vaguely as concerned with its -higher functions. But a surer test of the capacity of the cortex for -the elaboration of the raw materials of thought which sensory nerves -deliver to it is the relative abundance of the tissue which intervenes -between its cells. The number of cell-bodies to be counted in a square -millimetre of a section of a given thickness is smaller in Man than in -a monkey, in a monkey than in a dog, and in a dog than in a rabbit. - -A comparison of the brains of various mammals in which particular -sense-organs are either deficient or exceptionally well developed -affords the clearest proof of the localization of sensory areas. This, -if it were possible to make satisfactory measurements, would be by -far the best class of evidence as to the part played by the several -senses in an animal’s mental life. Unfortunately, measurement appears -to be out of the question; but a glance at a rabbit’s brain, placed by -the side of a mole’s, shows that vision is localized in the occipital -region. All marine mammals are destitute of the sense of smell; the -brain of a dog, compared with that of a porpoise or a whale, shows -that the sphenoidal region (_cf._ Fig. 25) is associated with this -sense. The brain of an otter exhibits very clearly the area into which -impulses arising in the nerve-endings of the sensory bristles of the -cheek are poured. - -“Nihil est in intellectu quod non prius in sensu fuerit.” The organ -of the intellect is the cortex of the great brain, a sheet of grey -matter which has developed in connection with the various sense-organs. -The cerebral hemisphere of an infant is merely an extension of the -nerve-tissue associated with its sense-organs. Such it remains in a -microcephalous idiot. In the lower animals its capacity of growth after -birth is very small. But in a normal child the inflow of impressions -through sense-organs, the experience acquired regarding itself and its -surroundings, education, whether accidental or directed, causes the -extension of nerve-tissue from the sensory areas into the expansible -intervening zones. - -There is still some uncertainty as to the nature of the sensations -received in the excitable area. They may be termed “kinæsthetic” -(sensations connected with movement) without more exact definition. -Some physiologists consider that tactile sensations, as well as the -obscure sensations, originated in the nerve-endings in muscles, around -tendons, or on joint-surfaces, are distributed to the areas, which, -when stimulated, are shown to represent fingers, hand, arm, and other -parts of the body. Others have sought, though with doubtful success, -for a tactile area, independent of the kinæsthetic centres. When first -discovered, these centres were termed “motor,” and still this term -may be retained, on the understanding that it does not imply that the -exchanges which occur in the kinæsthetic centres are of a different -nature to those which take place elsewhere. The region which they -occupy has become the motor area of the cortex because voluntary -movement is possible only under the guidance of sensations of movement. -A sound or a retinal image may prompt the movement; but the part of the -temporal region, or of the occipital region in which the sound-movement -exchange or sight-movement exchange occurs must act through the motor -area by opening kinæsthetic-movement arcs. Destruction of a part of -the kinæsthetic cortex causes in Man and the higher apes permanent -paralysis for the movements directed by the spot destroyed. In lower -animals the definition of the movement centres is vague, and their -removal produces only temporary results. Their mastery over the muscles -is less complete than in the higher apes and Man. - -Practically nothing is known with regard to localization of function in -the association-zones, with the exception of the localization of the -centres for words; but this exception is so remarkable as to suggest -that if there were any other faculties, interference with which caused -defects as distinct as those which characterize disorders of speech, -it would be found that the association-zones are made up of definite -centres. As the evidence stands with regard to the broadest continental -divisions, we can merely state that it points, although not very -clearly, to the connection of the frontal zone, the region in front of -the kinæsthetic area, with ideas of personality, of other zones with -ideas of environment. Injury to the frontal region has in certain cases -resulted in the victim’s losing his knowledge of himself, his name, and -his relation to his family. On the other hand, gunshot wounds and other -definite injuries have in a large number of cases destroyed portions -of the cortex behind the forehead without causing any recognizable -intellectual change. It is quite certain that this part of the brain -performs no functions which are of a different, or, as it is often -called, higher order than those of other association-zones. It has been -stated that disease of the zone which intervenes between the visual -and auditory areas is more likely to cause hallucinations, disease of -the frontal zone delusions. A patient fancies in the one case that -he sees things that are not there, or hears voices when no one is -speaking; in the other case he imagines himself a king; but evidence -connecting localized disease with mental derangement is very scanty. -The functional disturbance which causes lunacy is usually of a general -character; or, if local to begin with, it becomes general before the -death of the patient makes possible the examination of his brain. - -[Illustration: FIG. 25.—THE SURFACE OF THE LEFT CEREBRAL HEMISPHERE, -CEREBELLUM, AND MEDULLA OBLONGATA. - - Sensory areas are enclosed by broken lines; certain - centres in the association-zones are marked by - dots. The sensory area of smell is on the inner - aspect of the brain; so also is the area of vision - which borders the calcarine and retrocalcarine - fissures, and only rarely extends on to the - external surface, as shown in the diagram. The - sensory area of hearing is largely hidden within - the fossa of Sylvius, the opening into which - is indicated by the dark line above it. The - kinæsthetic-sensory areas for the various muscles - of the body occupy the territory between the - dotted line in front and the bottom of the fissure - of Rolando behind. They do not extend on to the - posterior wall of this fissure. It is impossible - at present to define the boundaries of any of the - centres in the association-zones.] - -Derangements of =speech= throw a flood of light upon the organization -and manner of working of the association-zones; and, owing to the -accident of the continuation of the line of the carotid artery by -the middle cerebral artery, which supplies the speech centre, there -is no other spot in the cortex so likely to be thrown out of gear. A -little plasma coagulates on one of the cardiac valves, or about an -atheromatous spot in the aorta. Detached by the blood-stream, it is -shot into one of the branches of the middle cerebral artery, which -it plugs, causing apoplexy. A larger or smaller number of muscles on -the opposite side of the body are paralysed. If the plugging occurs -on the left side of the brain, it is accompanied by aphasia; but only -if it occurs on the left side, owing to the fact—perhaps the most -remarkable in connection with the localization of speech—that only on -the left side is the cortex trained to utter words. In course of time -the patient may recover the power of speaking, but not until he has, -with almost as much labour as in childhood, educated the right side to -do the work. There are four speech-centres, quite distinct one from -the other. Near the visual area is the centre for seeing words, or -rather the centre for seeing the meaning of words. If this centre be -diseased, a written word is merely a crooked line. Behind the auditory -area is the centre for recognizing the meaning of words heard. If it -is interfered with, the most endearing or commanding phrases produce -no more impression on the hearer than a bird’s song. In front of the -hand-area—its localization is less certain than that of the other -three—is the centre for writing. In it are associated words heard -or seen, with the movements necessary for the making of letters. In -the centre first referred to, as being the one most often thrown out -of gear, which lies in front of the area for the mouth and throat, -words heard or seen are translated into movements of the parts which -give them sound. No other actions illustrate so clearly the “law of -neural habit.” In the infant’s brain sounds of words are distinguished -from other sounds. They are associated with the objects which they -name. Movements of the mouth and throat, made at first ineffectively, -blunderingly, succeed after a time in securing the thing of which -they sound the name to the child’s satisfaction. Thus, two centres -are gradually established in his mind. Sounds and ideas of things are -associated in the one; words and ideas of the movements necessary to -their pronunciation in the other. Either of the four speech-centres -may be placed out of action without the others suffering. A man may -be able to write without being able to read what he has written. He -may read aloud, although apparently deaf to speech. He may be unable -to write or unable to speak, although understanding what he reads or -hears. Aphasia, when partial, illustrates still further the law of -neural habit. The ability to remember nouns, especially proper names, -is most easily lost. Few are the people who, as age advances, do not -suffer from this failing. Even the names which are most familiar elude -the memory. From one point of view this is strange. Nouns-substantive -are the words first learned. Of all words they have the most definite -objective association. But it is just their definiteness which makes -them difficult of approach when the apparatus of mind is working badly. -There are so few paths by which they can be reached. Their mental -associations are limited. A patient who is recovering from the effects -of a lesion which has rendered him partially aphasic may be able to -recall adjectives when he cannot recall nouns. He may say, “Give me -the black,” when he wants ink, and “Give me the white,” when he needs -paper. Or he may retain control of verbs. “Where is the—— what I put -on—what I think with?” may be the circumlocution for hat. - -Psychologists explain the voluntary production of a movement as the -setting flowing of a sensori-motor current. Everyone agrees that it -is impossible to think of the impulses which produce movement as -originating without sensory antecedents. Hence psychologists picture -the nerve-current as originating on the sensory side. Kinæsthetic -images of the sensations which will result from the movement are -described as being called up in the mind by the agitation of the -part of the brain which, by association, is linked with the neurones -which discharge impulses to the appropriate spots in the grey -matter of the spinal cord. The idea of movement flows over to the -muscles. But this conception of the relation of mind to body assumes -too much. It postulates an existent mind in which the images of -movement-sensations—the memories, that is to say, of the sensations -which previously accompanied movement—are stored. The study of the -apparatus of mind does not warrant this assumption of an existent -mind. It finds nothing in the nervous system but apparatus. There is -no mind existent in the brain during sleep. It would appear to be -sufficient to describe the origination of a voluntary movement as -the opening of the channels which convey the afferent impulses which -are ceaselessly pouring into grey matter from nerve-endings in and -about muscles into efferent channels. Our conception of the number -of sensations which reach the realm of consciousness is ludicrously -restricted by our inability to pay attention to more than one sensation -at a time—a restriction, it is needless to remark, which is imperative -in the interests of consistency of behaviour. Two personalities paying -attention to different sequences of sensations would give incompatible -orders. One would command the muscles to cause the body to recline; -the other would direct them to make it stand up. From myriads of -sense-organs impulses are continuously rippling through the cortex -of the brain. The term “impulse” is too heavily weighted by its -association with the idea of currents which are strong enough to prove -effective without the intervention of consciousness; but no other is -available. They ring the bell of consciousness, however little may be -the attention which their summons secures. Attention cannot be directed -to two things simultaneously. It moves, as it were, on a succession of -points. On some it rests longer than on others. They make an impression -which can be recalled; the rest being passed by so rapidly that -they are not remembered, it is as if they had never been perceived. -They blend, as a succession of moving lights blend, in producing a -background to consciousness. Not recognizing their separateness, we -interpret them as fused. A good deal of misleading metaphor has been -used, as it seems to the writer, in accounting for the effect upon -the mind of impressions which make but a weak demand upon attention. -They are spoken of as “marginal” perceptions, from the analogy of -the ineffectiveness of impulses generated at the periphery of the -retina, as compared with those which give rise to direct vision. A -“subconscious,” or even “unconscious,” self is evoked. The self cannot -be less than conscious. Self is the passage of attention from sensation -to sensation. Its relation to the not-self is temporal, not spatial. - -Every sensation which is called up into consciousness, though it -occupy attention for the shortest possible time, tends to give rise to -movement—is, indeed, in its very nature an impulse flowing through -a sensori-motor arc. The circuit for the voluntary execution of a -movement is represented as flowing through kinæsthetic-movement arcs. -This may be necessary for volitional actions, but it is not essential -for reflex actions. A spinal frog will remove an irritant from its back -with its hind-leg, after the roots of all the afferent nerves of the -hind-leg have been cut. In this case the reflex is direct, from injured -skin to muscles of the leg. It is not double—muscular sensations from -the leg, liberated into efferent leg-muscle-nerves by skin-sensations -originated simultaneously in a part anterior to the segments in which -the roots have been cut. - -The unit of sensation to which attention can be directed has yet to -be defined. Like sensations—sensations which are correlated in -experience, that is to say—seem to fuse in consciousness. A sequence -of similar sensations appeals to attention. Unlike sensations interfere -one with another. The apparent fusion is not a composite neural effect -which consciousness views as a single unit. Not even identical images -simultaneously formed on the two retinæ produce a superimposed effect -upon a particular spot in the brain. Different brain-spots receive the -two separate images which the mind views as one. This raises a doubt -as to whether perceptions are, properly speaking, fused. It suggests -that they are separate points upon which attention rests in rapid -succession; but such a hypothesis does not preclude the conception -of the production of a composite sensation by impulses coming -simultaneously from the same sense-organ—_e.g._, a unified neural -effect as the result of several musical tones. - -Every neural agitation which attracts attention has an effect upon -the growth of the nerve-strands in which it occurs. =Memory= is not -an existent. It is the repassage of the same strands. There is no -such _thing_ as memory. It is the neural apparatus which responds in -a similar way to a similar agitation. It is difficult to speak of -association and neural habit, the phenomena upon which not only all -mental life, but all co-ordinated activities, are based, without using -such expressions as “the broadening of the path” or “the thickening -of the conductor” by the impulses which pass through it. Apparently -these analogies may with safety be pressed curiously far. Chaotic -response to stimulation is unknown. Thanks to the nervous system, -action exhibits an ordered relation to stimulation. This relation is -determined by education, giving the term a connotation wide enough -to cover all experience. Nerve-tissue adjusts itself to experience; -and since the nerve-matter which takes the pattern is not labile, -the process of organization is consecutive and the result permanent. -One pattern is not destroyed as another is impressed. Hence temporal -associations are formed. What has been thought once will be thought -again, if the circumstances in which it was thought recur. What has -been done once will be done again under the influence of a similar -sequence of stimuli. The conductors are widened every time that they -are used. But, so far as concerns the mind, a reversed influence comes -into play. The wider the conductor, the less appeal to attention is -made by the impulses which pass through it. It is as if currents which -have to overcome resistance in a narrow path acquire a higher potential -than those which find an open road. And since the making of the road -depends upon attention, the limit of broadening is reached when a -volitional act becomes a habit. The first time that a piece of music is -played consciousness is alert. Marks on the page and movements of the -fingers are felt intensely. With each repetition the need for attention -subsides. - -A skilled movement is impossible in the absence of guiding sensations. -I decide to button my coat. Sensation-paths from the muscles of the -forearm are opened into motor paths extending from the large pyramids -in the arm-centres of the kinæsthetic cortex. But it is not sufficient -that the action be started: it must be guided by the sensations which -movement produces. If my fingers are numb with cold, I cannot button -the coat. The muscles which move the fingers are warm enough beneath -the sleeve, but my attempts to will them to move are as futile as they -would be if the muscles belonged to some other person. The will has -no power over the muscles. It is essential that the sensations which -accompany the act of buttoning the coat flow through the same paths as -hitherto in the cortex of the brain. Flowing through the same paths, -they produce the same effect in consciousness, the same perceptions. In -ordinary parlance, one cannot perform any act unless one can remember -what it felt like to perform it on a previous occasion. It is almost -as sound physiology to describe the voluntary action of fastening a -button as commencing in the skin of the fingers as to describe is as -commencing in the brain. The act is due to the direction of attention -to impulses which flow from muscle to muscle, and from skin to muscle. - -All skill in the use of muscles is acquired by the method of trial and -error. Familiar movements are tried, combined, modified with a view to -the production of a new result. A man accustomed to striking with the -right hand forwards endeavours to swing a golf-club with the left hand -backwards. For a long time the result is anything but a success. At -length the head of the club takes the right curve. It not only hits the -ball with its centre, but it carries it through in the right line. The -ball travels 120 yards or so towards the green. In golfing terminology, -a successful drive is always “an awful fluke”; but the fluke once -accomplished, nothing is easier for the golfer than to drive equally -well on all succeeding occasions. He need merely remember exactly what -it felt like to give the club a perfect swing, and exclude all other -sensations while he is passing these memories through his sensori-motor -arcs! - -The fact that we can deliberately improve an action, fitting it to -the attainment of the object of desire, by suppressing wrong and -emphasizing right sensations, shows how large a part consciousness -plays in the affairs of the nervous system. This brings us to -the frontier of physiology. At this boundary the authority of -the physiologist ends. He cannot define consciousness; he cannot -investigate it. Yet he naturally asks whether the machine which he is -investigating is a machine and nothing more. When the possibilities of -reflex action were first recognized, thought tended to dethrone feeling -and Will in favour of automatism. If the actions of a spinal frog -exhibit so distinct a purposive character, why, it was asked, should -we assume that the frog with a brain is anything more than a reflex -machine? Light, heat, sound are playing upon its sense-organs; surely -these stimuli suffice to set going all the sensori-motor currents which -lead to the various movements which in their totality constitute the -frog’s behaviour! And why assign to a mammal a self-directing authority -which we deny to a frog? The increased complexity of its behaviour is -more than accounted for by the greater variety of its nervous arcs. -All animals, it was argued, including Man, are reflex machines. Their -thoughts and actions are the effects of the play upon their nervous -systems of forces from the outer world. Each inherits a nervous system -of a certain pattern. Its individual development is conditioned by -the sensations which pass through it. The sensations are impressed by -the environment. Therefore the individual is a puppet, his activities -the dance of circumstance. Consciousness is an “epiphenomenon.” Few -physiologists or students of animal behaviour take this material view -of life at the present day. The fact that it leads inevitably to the -conclusion that consciousness is an “epiphenomenon” (Huxley’s term) is -its _reductio ad absurdum_. It is not in harmony with the economy of -Nature that an animal should be endowed with the capacity of feeling -pain and pleasure, if such endowment is useless to it. It can be useful -only by directing activity towards the attainment of pleasure and -the avoidance of pain. This admitted, the mechanical theory falls to -the ground. There is an “It” which feels, selects feelings, chooses -those which have a pleasant tone, wills to perform the acts by which -they are attained. It follows that the value of consciousness lies in -the prerogative which it confers of adapting action, within certain -limits, to circumstance. An animal succeeds in life in proportion as -the nervous system which it inherits reacts satisfactorily to its -environment. A chick which, after being hatched in an incubator, has -been isolated for twelve hours without food, seizes a grain of corn the -instant that it sees it. Its brain contains ready-made sensori-motor -arcs connecting the spot in its cortex in which the visual impression -of the grain is perceived and the motor neurones which control the -pecking muscles. A sheep-dog is quickly broken to sheep, because its -ancestors have been selected by mankind from amongst dogs that readily -adapted themselves to this work. The breeder has selected a pattern of -brain with the same success with which, when appearance is the only -desideratum, he selects a pattern of coat. Beavers set to work at -constructing a dam at the only spot in a valley at which it is possible -to create an artificial lake, because for countless ages Nature has -ruled out the animals which constructed their dams in unsuitable -places. Man also inherits a brain-pattern; but, not being required to -shift for himself soon after birth, he goes through a long period of -infancy and tutelage, during which, by force of circumstance and his -own Will, the pattern is elaborated. His supreme success is due to his -capacity for adapting means to ends. He inherits very few instincts. -Except as regards organic functions, his spinal cord is subservient in -almost all respects to his brain. Most of the actions of an animal are -instinctive—a word which has been sadly misapplied. Its connotation -is negative rather than positive. Owing to the marked pattern of its -brain, an animal finds it difficult to avoid acting in a particular -way. As the nights grow longer and its hours for feeding are curtailed, -a swallow is impelled by its instinct to go South. It makes the same -use of its sensations during its migration, and is as completely -dependent upon them for its guidance as a man would be. The lower we -descend the scale, the more inevitable do an animal’s movements become; -but there can be no doubt but that consciousness is of value to an -animal, as to Man, in that it gives to its individuality the capacity, -within such limits as Nature has selected, of resisting or modifying -its ancestral instincts when they are not absolutely appropriate to the -occasion. - -Sentience implies personality. “No system of philosophy can extrude -the ego.” The difference between the performance of the animal machine -as a physiologist studies it, and its behaviour when under the control -of its own driver, is the difference between reflex action and choice. -The ego interacts with physical forces. It does not come within the -province of the physiologist to explain the source of the force which -interferes with force. He finds no trace of it on either credit or -debit side when making up the body’s accounts. He is unable to enter, -“Item, to the development of consciousness ... so much.” He can form -no conception of this immaterial manifestent which hovers over the -infinitely numerous sensori-motor exchanges which are always occurring -in the cortex of the brain, giving to a particular group of agitations, -now here, now there, a special quality; but the manifestent is needed -to account for the potency of the reinforced agitations which enables -them to take possession of the nerve-paths by which muscles are reached. - -It is for the psychologist to define the application of the terms -“consciousness,” “attention,” “will.” He cannot define the attributes -of the ego which these terms connote. The moralist must show the way -in which they determine, or should determine, conduct. Yet within the -plain limits of physiology, attention, using the word in its every-day -sense, modifies the responses of the nervous system in a degree which -cannot escape observation. It is astonishing to anyone accustomed to -hospital surgery (although even in this field singular exceptions are -met with) to see the grave operations which a veterinary surgeon may -perform, without the animal showing any evidence of pain, provided its -apprehension has not been aroused and its attention directed to what -is being done. A horse standing in front of a crib of oats, untied, -will hardly whisk its tail while the surgeon is making a great wound -in its flesh, and sawing off a bony excrescence. The knife does not -come within the experience of a horse. It has no anticipations, and -its skin, intensely sensitive to the tickling of a fly or the smart -of a whip, is relatively insensitive to a cut. An eminent surgeon of -the last generation (the writer, as a student, “dressed” for him in -his old age) was in the habit, having arranged that his patient could -not see what he was doing, of performing operations of a very painful -nature whilst assuring his patient, “I am merely making a thorough -examination, in order that I may be perfectly certain of the cuts that -I shall have to make to-morrow in the operating-theatre when you are -under chloroform.” We are not concerned with the ethics of his method; -but the assurance, “Now that’s all over; you will never need to have -that operation performed again,” saved many a sufferer from a night of -apprehension and a miserable “coming round.” - -It was stated, during the South African War, that at Ladysmith the -bearer of a critical despatch, who was struck in the palm of the hand -by a bullet which traversed the whole length of his forearm, did -not discover that he was wounded until he saw the dripping blood, -after his errand was successfully accomplished. To deliberately cut -oneself with a razor is most painful, yet shaving in the morning, with -thoughts concentrated on the doings of the day, it is often the sight -of blood which directs attention to the fact that the skin is severed. -Of all evidences of self, the power of paying attention is the most -noteworthy. We can direct attention to certain sensations, which then -become perceptions, and we can deliberately ignore others, within -certain restricted limits. - -The control of the nervous apparatus by the self is a truth which no -student of the physiology of human beings can ignore. Isolated from its -relation to all other scientific truths, it has been made the basis of -a nescience which, although positively merely foolish, is, negatively, -harmful—yet a form of folly which answers well to the needs of persons -of a certain category. - -It may be objected that the picture of the relation of mind to brain -which is here presented—the one, activity, motion, the other a -labyrinth of conducting paths—makes all mental phenomena entirely -dependent upon current sensations. No results could happen if the -sensations were not there. It affords no ground for the explanation of -=mental images=, =hallucinations=, =dreams=. A few lines may be spared -to show that this objection does not hold. We cannot attempt to explain -the conscious control of thought. It is a part of the impenetrable -mystery to which we have just referred. But, granted that it obtains, -the direction by the ego of afferent nerve-currents through the same -strands which formerly vibrated to sensations which drew a picture, and -hence the revival of its image, is no more incomprehensible than the -liberation of afferent impulses from muscles into efferent channels. -Brain-chains are composed of many links. Their interconnection is -illimitable. When I recall the appearance of the house in which I lived -as a child, I throw into the chain impulses (from somewhere) which -traverse the final links, where passage implies consciousness. At the -edge of the lace-work of linked threads the impulses light up a pattern -which childhood’s experience worked into the apparatus of thought. - -If we were to admire the perfection of any special aspect of the -brain’s functioning, the rarity of hallucinations might give us cause -for wonder. That impulses so seldom leave their own paths is more -astonishing than that occasionally, when the brain is excited and its -nutritive conditions deranged, the impulses which the ego can direct -into channels where they revive an image should sometimes, and with -far greater force, make their own way down well-worn paths, lighting -up a picture which deceives the ego. Dreams, by contrast, throw up -in a strong light the part played by attention in intelligent life. -The capacity for alertness is due to the favouring of one set of -impulses by suppressing others. The favoured impulses hold the road. -Concentration of attention is keeping thought to one line by resisting -all temptation to wander into by-paths. The waking condition is the -state in which all nerve-ways are closed, with the exception of those -which consciousness is using. The more severe the closure, the more -vivid is consciousness. In sleep all paths are open. In none is the -potential acquired by impulses in the process of overcoming resistance -high enough to evoke consciousness. A burst of impulses ascends from -the stomach, set a-flowing by undigested fragments of salmon and -cucumber, or mounts from the arm on which the sleeper has been lying -until its circulation has been arrested. They reverberate through the -open corridors of the brain. If they are sufficiently noisy to awaken -the sleeper, he, detecting them in this path and in that, supposes -them to be on the same errands as the impulses which commonly pass -thus. If dreams are analysed, it will be found that, although the -combinations of impressions may be uncommon and extremely bizarre, the -impressions are selected from the most familiar. The images of which -the dream is compounded, which may have lost all normal relations and -may have assumed impossible proportions, are those which the mind -most frequently conjures up. In the large majority of instances some -happening of the day preceding can be recognized as the prompting -cause. A remembered dream is the photograph taken by consciousness -of the sensations which have bombarded it into activity. Especially -if due to impulses originated by visceral discomfort, the dream may -have an unpleasant tone. This may take various forms, but the emotion -most commonly aroused is fear. The objects visualized may have -preposterous dimensions, or they may be not sufficiently distinct for -recognition—elusive imps; but most commonly distress is caused by -the want of harmony of sensations, due to the absence of kinæsthetic -elements. A man is lying on the railway-line; a train is approaching -with increasing speed; he cannot get up. He is in the pulpit, but -cannot speak. Dreams thus confirm the view set forth above as to the -cause of volitional action. Ability to perform an act depends upon the -flow through the kinæsthetic centres of the brain of impulses generated -in the muscles by which the act is to be, or is being, performed. -Kinæsthetic sensations do not under any circumstances play the same -part in mental life as sensations from the skin, the eye, or the ear; -when the body is passive in bed they are not flowing into the cortex. -The dream-photograph shows elements demanding movement, but affords no -evidence that movement is in progress. - - - - -CHAPTER XII - -SMELL AND TASTE - - -In Man the chief function of these senses is to guard the entrances -to the respiratory and digestive tracts. In this they are not -conspicuously efficient, since various poisonous gases, salts, and -powders, escape their vigilance. Merely a selection of the substances -which occur in air and in food are recognized as having odour or -flavour. Smell and taste are only partially distinguished in ordinary -parlance. No odorous substance is spoken of as tasteless when taken -into the mouth. Its volatile constituents, escaping to the chambers -of the nose, are said to afford a certain flavour. On the other -hand, it is recognized that substances which stimulate the tongue -alone—bitters, acids, sweets, and salts, unmixed with volatile -bodies—have no odour. - -Biassed as we necessarily are by the paltry rôle assigned to smell in -our mental life, it seems a little unworthy of the present functions of -the great brain that it should have developed in association with the -nose. Yet smell and taste are the oldest of the senses. Their origin -goes back to the days of chemiotaxis, when the organism, having no -specialized sense-organs, was attracted to its mate or to its food, and -repelled from conditions unsuitable for its well-being, by particles in -solution acting as chemical stimulants. An amœba is chemiotactically -drawn towards its food, one spore of an alga is attracted to another, -by the particles of matter which drift across the interval between them. - -In the life of many animals smell plays as important a part as that of -either of the other senses. One has but to watch a dog “looking” for -its master, already full in view, with its nose, to realize that smell -is the sense on which a dog chiefly relies. We describe it as looking, -because in ourselves the eye has so far outdistanced the other senses -as a channel of information that we speak of “looking” when we mean -seeking, and say that “we see” when we wish to imply that we understand. - -The difference between smell and taste is, in fishes, a difference in -the quality of the sensation, and not in its “modality” or kind; but -in terrestrial animals the olfactory membrane of the nose has become -specialized for the recognition of particles suspended in air, the -tongue for substances dissolved in water. The olfactory membrane, which -lines the upper two of the three chambers of the nose, is covered with -elongated cells of two kinds: (_a_) Columnar cells, fairly thick; and -(_b_) fusiform cells, each carrying at its free extremity a bunch of -exceedingly minute hairs. The fusiform cells are neuro-epithelial cells -of the most primitive type. Before nerve-cells, properly so called, -appeared, certain favourably-placed epithelial cells were connected -by protoplasmic bridges with muscle-fibres, to which they delivered -the impulses which were generated in them by external forces. Later -some of the neuro-epithelial cells sank beneath the surface, where, -as ganglion-cells, they served as intermediaries between groups of -sensory cells on the surface and the nerve-net which lay more deeply -in the tissues. The olfactory membrane perpetuates the earlier stage; -in so far as it consists of elements which are combinations of -sense-cells and nerve-fibres. Each of its fusiform cells sends inwards -a nerve-filament, which, traversing the submucous tissue of the nose -and the bone (cribriform plate) on the base of the skull, between the -orbits, enters the olfactory bulb. The olfactory bulb is a part of -the local nervous mechanism of smell. It is the ganglion of the nerve -of smell _plus_ nerve-elements which in all segments behind the eye -have been withdrawn from the neighbourhood of the sense-organ into the -central nervous system (_cf._ p. 333). - -The way in which odorous particles in air stimulate the fusiform -cells is unknown. The quantity which suffices as a stimulant is so -small as to put chemical stimulation out of the question. A few -grains of musk will scent a room for years. 0·00000004 milligramme of -mercaptan (sulphur-alcohol) is recognizable in a litre of air. This -is a dilution to 1 in 50,000,000,000. Probably even such figures as -these would be thrown into the shade if we could estimate the minimum -amount of human effluvium which will enable a dog to follow his -master’s trail. Explanations have been sought in alterations in the -vibrations of molecules of air caused by the presence amongst them of -relatively heavy molecules of volatile substances; but the difficulty -of accounting for the generation of nerve-impulses in the sensory cells -remains as great as ever. The hairs borne by olfactory cells are so -short that it is impossible that they should project beyond the film -of moisture on the surface of the membrane. This seems to preclude an -answering vibration. Yet an increase in the thickness of this layer -and in its density, due to the presence in it of mucus secreted during -a catarrh, renders the sense-cells incapable of responding to odorous -particles. - -Smell in an animal is not a test of the quality of the air it is -breathing, but a source of information as to the direction in which it -may seek its prey; or, although far more rarely, as to the direction -from which the advance of a foe is to be feared. Hunting animals depend -for the most part on the nose. Hunted animals rely chiefly on the eye. - -If we attempt to analyse our smell-sensations, we find that we can pick -out a number of varieties which appear so unlike as to have nothing -in common: Putrid meat, burning indiarubber, sulphuretted hydrogen, -ammonia, roses, onions, lemon verbena, methylated spirit. Everyone can -make for himself a list of typical odours which seem to have specific -qualities—odours so distinct that he never confuses one with another. -He can also class together scents about which he is often uncertain. -The type-odours he can distinguish when present in a mixture; whereas -odours which are less distinct reinforce or modify one another. It has -been found, by careful experiment, that certain type-odours even tend -to neutralize each other. Musk and bitter almonds, for example, if -present in small quantities and properly proportioned, produce a very -dim sensation, whether supplied as a mixture to both nostrils, or the -one assertive odour to one nostril and the other to the other. This -last observation is of great importance. It proves that their mutual -destruction does not occur on the olfactory membrane. It is not due to -physical interference. The sensation of musk is delivered to one side -of the brain, the sensation of bitter almonds to the other; but when -attention is directed to these two sensations there is found a quality -in the one which is irreconcilable with the quality of the other. - -In certain persons and under certain pathological conditions, -sensitiveness to particular odours, or groups of odours, is absent, -while for the rest the sense is normal. Methylated spirit, prussic acid -and mignonette, constitute a group which not infrequently drops out. -Instances have also been reported of persons unable to smell vanilla -(to which some are hyper-sensitive), and of others insensitive to -violets, although normally sensitive to the scents of other flowers. -The notes sounded in consciousness extend over a long gamut; but -there are reasons for thinking that the number of keys on the clavier -which odoriferous substances strike is limited. Eleven is the number -provisionally adopted. The effect in consciousness varies according as -one key or another is struck, or several at the same time with varying -degrees of force. - -Many attempts have been made to associate the sensation-qualities of -the various odours with the chemical or physical properties of their -odorants, with but little success as yet. To excite the sense of -smell, a gas must be at least a little heavier than air. No volatile -body, it is stated, is so heavy as to be odourless; on the contrary, -speaking generally, heavy molecules are more stimulating than light. -The quality of a smell-sensation would therefore appear to depend upon -the period of vibration of the molecules of the substance which evokes -it; but, as already stated, a consideration of the apparatus which -responds to stimulation by odoriferous particles does not help us to an -understanding of the way in which the particles act upon it. - -[Illustration: FIG. 26.—HIGHLY MAGNIFIED SECTION THROUGH THE WALL OF A -CIRCUMVALLATE PAPILLA OF THE TONGUE, SHOWING TWO TASTE-BULBS. - - These sense-organs are groups of elongated epithelial - cells, set vertically to the surface. Their cells - are of two kinds—the one fusiform, slender, - bearing each a bristle-like process which projects - through a minute pore left between the superficial - cells of the general epithelium; the other thicker - and wedge-shaped. Nerve-fibres are connected with - the fusiform cells.] - -Taste is far more limited in its range of sensations than smell. The -back of the tongue is sensitive to bitters, the tip to sweets and -salts, the sides to acids. Mixtures of these qualities are distinctly -analysable by the sense of taste. Our sensations of taste do not fuse. -Slight differences in the way in which the organs on the different -parts of the tongue react to stimulation enable us to recognize that a -sapid substance is a mixture. When, with a great flourish of trumpets, -saccharin was introduced as a safe sweetener for gouty people, an -attempt was made to provide them with saccharin-sweetened jam. The -effect of the jam upon the person who consumed it was truly humorous. -First a suspicion of tartness, then its adequate suppression, followed -by nauseating sweetness. The sense-organs which subserve the sense -of taste are clusters of fusiform epithelial cells, collected in -“taste-bulbs” (Fig. 26). Each gustatory cell bears a minute bristle, -which projects through the pore left by the cells of the surrounding -epithelium which constitute a globular case for the bulb. As in the -nose, eye, and ear, a second thicker variety of epithelial cell is -also present. The nerve-fibres of the taste-bulbs are not, as in the -olfactory membrane, processes of their cells, but branches of the fifth -nerve which ramify amongst them. On the back of the tongue taste-bulbs -are much more numerous than elsewhere. They are not as sensitive as the -cells of the olfactory membrane; nevertheless, they enable us to detect -1 part of quinine in 2,000,000 parts of water. - -Sensations of taste and smell endure for a long time after stimulation, -because the odorous or sapid substance remains in contact with the -sense-organs. This accounts for the confusion into which a man is -thrown if he sip alternately port and sherry. After a short time he -cannot tell the one from the other. The organs are quickly fatigued, -using the term loosely. How intolerable patchouli would be to the -ladies who use it were it otherwise! If for some time one sniffs the -odour of mignonette, it ceases to be recognizable; whereas, turning to -a rose, the olfactory membrane is found to be as sensitive as usual. -When the sense is fatigued for a particular smell, it is dull for -others of the same group, thus affording an opportunity of classifying -smell-sensations according to their qualities; but the method is -difficult to apply. Taste-organs are greatly affected by temperature. -Quinine is not tasted just after drinking ice-cold water. Alcohol, -ether, or chloroform paralyses the organs much in the same way. -Castor-oil slips down the throat unnoticed if the mouth, just before -swallowing it, has been rinsed with brandy or with a strong solution of -tincture of chloroform. - -Englishmen make but little use of their sense of smell. It might -teach them much regarding the various emanations from putrid matter -which are produced by bacterial action; but, dreading drains, they -decline to cultivate proficiency in the exercise of this sense. The -nose is valued for the warning it gives of “nasty smells,” but is not -allowed to analyse them. Burnt milk, soap-boilings, rancid oils, are -taboo, because they are associated with bungling in the kitchen. With -moderated ardour, we allow our sense of smell to distinguish foods and -beverages, but we are not a race of epicures. The perfumes of flowers -are classed as “nice smells.” The idea of greediness is not associated -with their enjoyment; besides, they remind us of gardens, sunshine, -pretty forms and colours. When bottled, musk, orange-blossom, violets, -lavender, are valued not so much for their own sweetness, as for their -singular efficiency in obscuring nasty smells. Few persons practise -the recognition and distinction of even pleasant odours. Very few, on -first coming across a scented herb or shrub, pay sufficient attention -to its perfume to impress it on their memories. They note the shape -of its leaves and the colour of its flowers, but they are unable to -identify it by its odour when they meet with it again. It is not much -to be wondered at, therefore, that this slighted sense tends to leave -us after middle life. It has been asserted—and probably the statement -is justified—that rarely is the olfactory bulb of a man over forty -free from signs of atrophy. We have no statistics concerning the brains -of Japanese, who regard the sense of smell as one of the chief avenues -of pleasure; but it may be that in this respect their brains present -a contrast to our own. Yet the deadening of the sense is scarcely -noticed, since its results are of little consequence as compared with -those which follow loss of sight or loss of hearing. Many a man, as he -grows older, declares that the cook of his club has lost his cunning, -or frankly asserts that he “no longer cares for kickshaws. Cold beef, -beer, and pickles, are good enough for him.” He little suspects that -his palate has lost its power of distinguishing the flavours of -dainty meats and wines. Others continue to be exacting, because their -imaginations still endow food with the qualities which they remember, -just as people eat preserved asparagus or tinned peas because they -look—however little they taste—like the gifts of Spring. - -Taste accompanies the reception of food in the mouth. We have no -knowledge of the situation of our own olfactory membranes, and -therefore we suppose that a flavour, whether it be due to stimulation -of taste-bulbs or olfactory membrane, is in the mouth. The odour of -a flower we mentally project to a distance, because we associate the -sight of a flower with its perfume. A dog, able to judge the freshness -or staleness of a scent, must project its sensations of smell in the -same way in which we project our sensations of sight. It forms an -estimate, of a sort, of the time that it will take in reaching the -source of the scent. Its excitement increases as the trail grows -fresher. - -Taste and smell are heavily laden with affective tone. When -disagreeable, the feeling which they evoke is near akin to pain. -It may gather head until, like hunger, it causes the discharge of -motor neurones; but under its influence food is ejected, instead of -preparation being made for its reception. - -Taste and smell are senses which afford us no information with -regard to time or space. They give rise to massive sensations. Such -sensations, devoid of detail, produce a frame of mind rather than -thought. The smell of tobacco does not distract attention. On the -contrary, the steady flow of impulses to which it gives rise helps -to inhibit, to subdue, the yapping of more exigent sensations. And -since sensations of smell have no features of their own, they form -a background to sensations of other kinds, entering with them into -memory. No two scenes are exactly alike. One cannot recall another. But -the scent of syringa is always the same. Wherever smelled, it opens the -pathways in the brain in which were first associated a June evening and -syringa, with a scene and a situation upon which memory loves to dwell. - - - - -CHAPTER XIII - -VISION - - -The eye is enclosed in a globe of fibrous tissue, of which the front -part, or cornea, being transparent, admits light. The epithelial -layer which covers the cornea, conjunctiva, is also transparent. No -bloodvessels enter these colourless tissues, unless as the result of -inflammation due to infection or to exposure to sunshine or dust. For -nutrition they are dependent upon the plasma which, exuding from, and -returning to, the vessels which surround them, circulates in their -tissue-spaces. In advancing years, when the circulation is less brisk, -a ring of opaque tissue, arcus senilis, encroaches on the cornea. In -the interior of the globe, just behind the cornea, is a projecting -shelf, formed of a ring of tissue supported by buttresses, ciliary -processes. It is continued inwards as the iris, a muscular curtain. The -“hyaloid membrane” lines the back portion of the globe. Continued on -the inner side of the ciliary processes, it splits into several layers, -which pass, one in front of the lens, others to its edge, to which -they are attached, and still another, very thin, behind it. Since it -holds the lens in place, the anterior portion of the hyaloid membrane -is known as its “suspensory ligament.” Thus the eyeball is divided -into three chambers. The anterior is filled with watery lymph, aqueous -humour. In it, resting on the anterior surface of the suspensory -ligament of the lens, is the iris. The middle chamber contains the -lens. The posterior chamber is filled with a liquid jelly, vitreous -humour. - -By the contraction of the circular fibres of the iris, the aperture of -the pupil is diminished, limiting the light which enters the globe. -This adjustment occurs when the illumination is bright. It is also -brought into action for the purpose of cutting out divergent rays, -which would not be clearly focussed when objects near at hand are -looked at. The posterior surface of the iris and the inner surfaces of -the ciliary processes are covered with dense black pigment. It is this -pigment, showing through the uncoloured connective tissue and plain -muscle-fibres of which the iris is composed, that gives their colour to -grey and blue eyes. In many eyes the iris contains a brown pigment in -its substance. - -[Illustration: FIG. 27.—HORIZONTAL SECTION THROUGH THE RIGHT EYE. - - The slight depression in the retina in the axis of - the globe is the fovea centralis, or yellow spot; - the optic nerve pierces the ball to its inner or - nasal side. The lens, with its suspensory ligament, - separates the aqueous from the vitreous humour. On - the front of the lens rests the iris, covered on its - posterior surface with black pigment. On either side - of the lens is seen a ciliary process, with the - circular fibres of the ciliary muscle cut transversely, - and its radiating fibres disposed as a fan.] - -The back portion of the globe of the eye is covered with a curtain, the -retina, formed by the spreading out of the fibres of the optic nerve -in front of various layers of nerve-cells and the sensory cells of the -organ of vision, rods and cones. The retina lies between the hyaloid -membrane, which encloses the vitreous humour, and a layer of pigment -which “backs” it, as a photographer backs a plate when he proposes to -use it towards a source of light—to take a photograph of a window from -within a room. The serrated margin of the retina is somewhat anterior -to the equator of the eyeball. The pigment which backs the retina is -contained in a sheet of cells which belongs to the pouch of brain that -extended outwards towards the eye-pit (p. 334). Properly speaking, -therefore, it is a layer of the retina. - -[Illustration: FIG. 28.—DIAGRAMS SHOWING THE MODE OF FORMATION OF THE -CRYSTALLINE LENS. - - A, A pit in the epithelium on the surface of - the head has closed into a hollow sphere. - B, The cells of the posterior wall of this - sphere are growing forward, as the fibres of - the lens which traverse its whole thickness, - with the exception of the cubical epithelium - on its front.] - -Three sets of tissues take part in the development of the eyeball. (1) -The epithelium covering the surface of the head is depressed as a pit, -which gradually closes into a hollow sphere. This sphere, when its -cavity is filled up, owing to the great elongation of the cells of its -posterior half, becomes the lens. It breaks away from the rest of the -epithelium of the surface, which clears to transparency as that part -of the conjunctiva termed the “corneal epithelium.” (2) The retina, -as already stated, is a hollow outgrowth from the interbrain. As this -pouch approaches the lens, its anterior half is pushed back into the -posterior half, forming a cup with a double wall. The anterior, or -inner, sheet of the bowl of the cup develops into the nervous layers -of the retina, the posterior sheet into its pigmented epithelium. (3) -Connective tissues are transformed into the other constituents of the -globe—cornea, iris, vitreous humour, etc. The globe is complete, -except at a spot on the nasal side of its posterior pole where the -optic nerve pierces it. - -The bloodvessels of the retina, entering with the optic nerve, ramify -on its anterior surface. Under ordinary circumstances we ignore the -shadows which they cast, as we ignore the blind spot which coincides -with the disc of insensitive tissue presented by the end of the optic -nerve, and many other imperfections; but it was shown by Purkinje many -years ago that by a very simple manœuvre they may be forced upon our -notice. - -[Illustration: FIG. 29.—PURKINJE’S SHADOWS. - - A beam of light traversing the eyeball in the - direction A throws a shadow of the vessel _v_, - lying on the front of the retina, upon the - sensitive layer at its back. When the light is - moved from A to B the shadow moves from _a_ to _b_. - The mind, supposing the shadow to be a dark mark on - the nearest wall or screen, infers that this mark - moves from A′ to B′.] - -By making use of _Purkinje’s figures_, it can be proved that the level -in the retina at which undulations of light give rise to the impulses -which evoke visual sensations coincides with the back of its anterior -sheet—_i.e._, with the layer of rods and cones. A person stares -fixedly at a white sheet in a dimly lighted room while an assistant, by -the help of a lens, focuses a strong light on the front of his eyeball, -to the outer side of the cornea. The rays, traversing the white of -the eye, throw shadows of the retinal vessels on the layers behind -them; but this not being the way in which light normally enters the -eyeball, the person experimented upon supposes that he sees the shadows -in front of him. He mentally projects them on to the white sheet. The -pattern of his retinal vessels appears on the sheet in grey streaks. -When the spot of light is moved, the shadow-pattern shifts, and in the -same direction; since, as the retinal image is reversed, a movement -from right to left is interpreted by consciousness as a movement from -left to right. Given the angle through which the light is moved, and -the apparent displacement of the shadows, it is a simple matter to -calculate the distance behind the bloodvessels of the sensitive layer -of the eye. So definite are Purkinje’s figures that the shadows of -individual blood-corpuscles can be followed, and the rate at which they -are moving in the capillaries of the retina calculated. - -The retina is the organ of vision. Cornea, iris, lens, vitreous humour, -are parts of the camera in which this sensitive screen is exposed; and -of the retina, the sensitive layer is the layer of rods and cones. -Interest therefore centres in these structures. They are disposed with -the utmost regularity on the posterior surface of a thin, reticulated -membrane—the outer limiting membrane. But rods and cones are only the -outer halves of sensory cells, the inner portions of which, reduced -to a minimum in thickness, except where they contain their nuclei, -lie in the outer nuclear layer. Rods are the larger elements. Each -consists of an outer segment, or limb, of relatively firm substance -transversely striated, and liable to break into discs; and an inner -limb of much softer substance, again divisible into two parts, the -outer longitudinally striated, the inner granular. Cones are almost -identical in structure with rods, save that their outer limbs are much -smaller, their inner limbs rather fuller. In frogs and various other -animals, but not in Man, each cone contains at the junction of its two -limbs a highly refracting globule of oil, often brightly coloured, red, -yellow, or green. - -[Illustration: FIG. 30.—THE RETINA IN VERTICAL SECTION—A, AFTER -EXPOSURE TO BRIGHT LIGHT; B, AFTER RESTING IN THE DARK. - - The arrow shows the direction in which light - traverses the retina. C, Retinal epithelium, with - its pigmented fringe. 1, Layer of rods and cones, - separated by the external limiting membrane from - 2, the layer of the nuclei of the rods and cones. - 3, The ganglion-cells of the retina, which are - homologous with the cells of the afferent root of a - spinal nerve. Their peripheral axons ramify beneath - the sensory epithelium (rods and cones and their - nucleus-bearing segments), their central axons - in 4, the inner molecular layer. D, Collecting - cells on the front of the retina; _a a a_, their - axons which conduct impulses to the brain; _b_, an - efferent fibre from the brain.] - -The layers in front of the rods and cones contain nervous elements -accessory to them. In the “inner nuclear layer” are the ganglion-cells -of the retina, homologous with the cells of the ganglia on the -posterior roots of spinal nerves; but, in the retina, bipolar and -extremely minute. On either side of the rather thick layer occupied by -the nuclei of these ganglion-cells (and of cells of other types which, -for the sake of clearness, we omit) is a felt-work of nerve-filaments -in which their two extremities arborize. The most internal, or -anterior, layer consists of a single sheet of rather large collecting -cells and of their axons, which stream towards the optic nerve. Each -cone has its proper ganglion-cell, collecting cell, and efferent -fibre. Rods are served in groups by ganglion-cells and collecting -cells. From this it may be inferred that a cone is a sensory unit, an -inference confirmed, as we shall show presently, by direct evidence. -The connections of the rods show that they also are sensory elements, -although it may be doubted whether they are sensory units. The optic -nerve contains a very large number of fibres—about a million—all -small, but some distinctly larger than the rest. The largest very -probably belong to the collecting cells of rods. But the retina -certainly does not contain a million collecting cells. A considerable -residue of fibres is therefore unaccounted for. It is supposed that -they are afferent to the retina, but we have no knowledge regarding the -nature of the impulses which descend from the brain. - -The retinal pigment is not merely a backing for the sensitive screen. -It undoubtedly plays an important part in vision. That it is not -essential is evident from the fact that albinos, whose eyes appear pink -owing to the absence of pigment, and the consequent showing through -of the blood in the exceedingly vascular membrane which lies behind -the retina, can see; although their visual sense cannot be described -as normal. They are exceptionally sensitive to an excess of light. We -shall return to this subject after describing the differences in manner -of functioning which distinguish rods from cones, differences so marked -as to justify us in speaking of two kinds of vision. - -During twilight warm tones gradually fade out of the landscape; cold -blues and greys predominate. A time arrives when scarlet poppies -look black, although yellow and blue flowers and green leaves can -still be dimly distinguished. In full daylight colours are seen at -their brightest in the high lights; where the light is dim they -tend to appear in different shades of grey. At night, if the sky is -star-lit, all colours give place to a slightly bluish grey in the -high lights, black in the shade. But a not very uncommon abnormality -is night-blindness—inability to see at all when the light is not -bright enough for the recognition of colours. In persons so affected -the rods do not function; for it is with the rods that we see in -weak light. They record differences in intensity between the lower -limit of their sensitiveness and the higher degree of brightness, at -which they are superseded by cones; but they afford no information -regarding colour. Their monochrome is interpreted by the mind as a -bluish grey, apparently because, since they are insensitive to red -rays, the sensations of which they are the source are associated with -the blue end of the spectrum. When the cones are stimulated very -slightly, the reinforcing grey of the rods enables us to distinguish -all other colours, save red, which appears black. In bright light the -rods are in a permanent state of exhaustion; they do not contribute to -vision. Rods respond to stimulation more slowly than cones. This fact -enables us, by a very pretty experiment, to distinguish the two kinds -of vision. A disc of green paper about the size of a threepenny-bit is -pasted on a red surface. Held at arm’s length in a room lighted by a -single candle, the disc looks dull green when the gaze is directed at -it; but if the gaze be directed 2 or 3 inches to one side of it, it -appears brighter than before, but less distinct and almost grey. The -explanation of this is to be found in the fact that at the posterior -pole of the eye there is a shallow cup—fovea centralis—which carries -cones only, without rods. This small depression is the area of direct -vision, the only spot at which we see things quite distinctly. At the -fovea the nuclei and nerve-cells of the retina are withdrawn from in -front of the cones to the margin of the cup, in order that they may -not interfere with the passage of light. The pit and the ring round it -contain some yellow pigment. Hence it is usually termed the “yellow -spot.” When we are looking straight at the green disc, it is focussed -on the yellow spot. It then excites a sensation of greenness; but since -this is not reinforced by any rod-sensations, the green is dull. When -it is focussed outside the yellow spot, it stimulates rods and the -sparse cones which lie amongst them; and the rods being more sensitive -than cones to light of low intensity, the disc looks brighter. If, -while the observer is still gazing fixedly at a spot to the side of the -disc, the red paper be waved rapidly, but gently, to right and left, a -brightish grey cover seems at each movement to slip off the dark green -disc, and to regain its position a moment later, with a jump. The grey -rod-sensation, developing more slowly than the green cone-sensation, -is, as it were, left behind. The two are separated at the moment when -the paper starts to right or to left. - -Astronomers have long recognized that one of the smaller stars which -catches the attention when they are not looking directly at it may -be invisible when the gaze is directed to the spot where it ought -to be. It was visible when focussed on rods, but it is not visible -when focussed on cones. In most birds the retina shows cones alone. -To anyone who for the first time enters a dovecote at night the -experience is very curious. A candle is for him a sufficiently strong -illuminant, but it does not give light enough to enable the pigeons -to see. Although evidently alarmed by the noise made by the intruder, -they allow themselves to be taken down from their perches without -making any attempt to escape. If, startled by the touch of a hand, they -take to flight, they fly against the wall. Pigeons are night-blind. -The retina of an owl bears chiefly rods, the outer limbs of which are -exceptionally long. - -The outer limbs of the rods are coloured reddish-purple. This colour -is quickly bleached by light. If a frog which has been kept for a -short time in the dark be decapitated, its head fixed for ten minutes -in a situation in which a window is in front of it, then carried to a -photographic dark-room, where an eye is taken out by red light, opened, -and the retina removed, a print of the window will be seen upon it. -Such an optogram may be fixed by dipping the retina in alum. - -The retina is easily detached from its pigment-layer. If it has been -bleached by exposure to light, it regains its “visual purple” when -again placed in contact with its pigment. Evidently the visual purple -is renewed from the pigment which lies behind (and around) the rods. - -From the cells of the pigment-layer a fringe of streaming processes -depends amongst the outer limbs of the rods and cones (Fig. 30). In a -dull light the processes hang but a short way down; in a bright light -they react almost to the outer limiting membrane. They supply pigment -to the rods, but their relation to cones is not understood. It is -clear, however, that the cones, although they are not coloured, are -dependent upon the pigment-fringe, since they always remain in contact -with it. Their inner limbs elongate in the dark, lifting them to the -pigment, and shorten in bright light. These movements may merely -indicate that the cones require a backing of pigment, but it would seem -more probable that, like the rods, they absorb a substance which is -sensitive to light, although we cannot recognize it by its colour. - -The responsiveness of the rods to light is due to visual purple. -As every lady is aware, colours, especially mauves and lilacs, are -bleached by light. The chemical change affected by light in the colour -of the outer limbs of the rods is the stimulant which originates -impulses in the nerve-fibres connected with them, and it is generally -believed that cones—the more highly specialized sensory cells—are -stimulated in the same way. Visual purple is particularly abundant in -all animals that range at night, with the exception of the bat. But -its absence in the bat does not militate against the theory that it -is the cause of night-vision, for it has been shown that a blind bat -flies with almost as much freedom, and avoids obstacles—even threads -stretched across the room—with as much skill as one that can see. It -is guided by the bristles of its cheek. So, too, is the cat, which has -the reputation of being able to see in the dark. Undoubtedly a cat’s -eye is an exceptionally efficient organ in dim light, just as it is -exceptionally sensitive to sunshine—it is provided with an iris which -contracts the pupil almost to a pinhole—but the cat trusts to the -bristles of its cheek for information regarding the things which block -its path. - -Most of the peculiarities which distinguish the reactions of the eye -from those of other sense-organs can be explained by its mode of -stimulation—the initiation of a nerve-current by a chemical change. No -stimulus, if sufficiently strong, can be too brief. The retina reacts -to an electric spark in the same way as a photographic plate; but, -unlike the plate, the retina is restored to its previous condition of -sensitiveness in about one-tenth of a second. A visual sensation lasts -about one-tenth of a second. This prolongation of the sensation is, -however, a mental, not a retinal, effect. The mind continues to see an -object which has been illuminated by a flash until the retina is again -in a condition to send brainwards a second impulse. Were our sensations -coincident in duration with the stimulation of our sense-organs, we -should live in a flickering cinematograph. When one is watching a -moving point of light—the glowing end of a match, for example—the -prolongation of sensation has its disadvantages; the moving point -is interpreted as a streak of light. If the illumination be very -brilliant, the object seen may give rise to a prolonged after-image. A -glance at the sun leaves in the mind for seconds, or even for minutes, -the image of a glowing disc. Sensations due to stimulation of the -yellow spot last longer than those which originate in the peripheral -retina. If, in a train, one is being carried at a certain pace, past a -fence composed of upright palings, one sees the separate slats until -the eyes are directed towards them, when they fuse into a continuous -screen. - -The phenomena of negative or complementary images are of retinal -origin. The bright image of the sun, if the stimulus has not been too -violent, gives place to a black disc. If one closes the eyes after -staring at a window, a black surface crossed by bright lines is seen -in place of a white surface with dark frames to the panes. If, after -staring at a red surface, one looks at the ceiling, a green patch is -seen; after yellow, blue. Every colour has its complement, which may be -determined in this way. There is much uncertainty as to the exact terms -in which this phenomenon is to be accounted for, but little doubt as to -its being due to the peculiar mode of reaction of the retina to light. -Chemical substances which have been used up have to be restored, and -during the period in which they are coming back to what may be termed -a neutral condition the retina delivers to the brain impulses of the -opposite sign. - -Contrasts which are experienced simultaneously are more difficult to -understand than those which appear successively. In Fig. 31 the half -of the grey cross which is surrounded by black appears brighter than -the half which lies on white paper. A grey cross on a red background -looks green; on a green background, red; on yellow, blue; on blue, -yellow. If green is on red, it looks greener than if it is on white or -black. These simultaneous contrasts are seen best when the strength of -the colours is reduced by covering them with tissue-paper. It is as if -activity of any one part of the retina is accompanied by activity of -the opposite sign in the remainder. But it is unsafe, in explaining our -various sensations, to lay too much stress on the mode of stimulation. -The mind judges sensations in the light of previous experience. In -anatomical language, the effect of sensations upon the personality -depends upon the paths which impulses follow in the brain, and the -associations which have been established by previous impulses which -have followed the same paths. The retina enables us to distinguish -tone and colour. By the variations in tone, the juxtapositions of -light and shade, we recognize form. All streams of impulses which do -not present tone-variations—do not, that is to say, reproduce the -details of a scene—are interpreted in terms of colour. Every child -discovers that the tedium of the intervals during which it is proper -that his eyes should be closed may be relieved by pressing his knuckles -against the lids. Although the world is shut out, a phosphene offers -itself for his consideration—a yellow or white disc of irregular form -with a red margin, changing into lilac bordered with green, and then -into yellowish-green with a blue edge. Such, if my recollection can -be trusted, were the pictures which I used to see as a boy; but no -adjustment of pressure calls them forth with anything like the same -vividness now. - -[Illustration: FIG. 31.—SIMULTANEOUS CONTRAST. - - The shading of the two V’s is exactly similar; but - the figure in half-tone on black appears brighter - than the figure in half-tone on a white ground.] - -All the senses show a tendency to rebound after activity, exhibiting -contrast-phenomena; but the contrasts of vision are more marked and -varied than those of the other senses, as everyone who is curious in -the observation of his own sensations is aware. Negative after-images -are generally referred to the retina; but various other kinds of -after-image and contrast-phenomena must be attributed to the judgments -passed by the mind upon the sensations which it receives; and not to -physical changes in sense-organs. Positive after-images are well-marked -appearances, although less common, perhaps, than the phenomena of -reversal of sensation of which we have just written. On waking in the -morning, one looks at the window; shifting the gaze to the ceiling, -an after-image of the window appears, just as one saw it, with bright -panes and dark frame. The “dark adapted eye,” being exceptionally -sensitive, yields the same persistent positive after-image as the eye -in its usual condition yields, after being directed towards the sun -at mid-day. Movement-after-images can be explained only by referring -them to misdirection of judgment. If the gaze is fixed on a rock close -beside a waterfall, then shifted to a bank covered with grass or -bushes, the part of the bank which occupies the lateral part of the -field of vision appears to rush upwards, reversing the movement of the -water. When the gaze has been fixed upon falling water—a narrow stream -sparkling in sunlight—a central strip of the field moves upwards, the -margins remaining stationary. If one stares at the spot on the surface -of a basin of water on which drops are falling from a tap, and then -looks at the floor, it is seen to contract towards the spot looked at, -reversing the movement of the ripples in the basin. These observations -reveal a fact of great importance in the physiology of vision. It is, -probably, impossible truly to fix the gaze. The muscles of the eyeball -keep the retinal field in constant movement—larger movements with -minute oscillations superposed. When, as in watching a waterfall, -movement has for a time taken a definite direction, its cessation is -judged to mean reversal. - -The anatomical unit of sensation is a cone. The fovea centralis, the -only part of the retina capable of receiving sensations sufficiently -discrete for reading, contains cones alone. If the gaze be directed -but a very few millimetres on to the white margin of the page, letters -lose their form. In the fovea the centre of one cone is 3·6 µ distant -from the centre of the next. Two stars are visible as separate stars -if they subtend an angle of at least 60 seconds with the eye. Their -images on the retina are then 4 µ apart. Parallel white lines ruled on -black paper, held at such a distance as causes them to subtend angles -of 60 seconds with the eye, appear not straight but wavy, showing that -their images are taken up, not by a continuous substance, but by the -mosaic of cones. So far the explanation of the visual unit is strictly -anatomical; but it must be added that trained observers can recognize -the separateness of objects which subtend angles of much less than 60 -seconds—not more than 5 or 6 seconds. This can be accounted for only -on the hypothesis that images far closer together than the width of a -cone produce a specific effect in passing across the anatomical unit. - -In 1807 Thomas Young, the physicist, formulated a theory to account -for =colour-vision=. He supposed that the retina contains three kinds -of apparatus—_a_, _b_, and _c_—each especially responsive to a -particular kind of light, all three slightly stimulated by rays of all -colours. (Young imagined three kinds of nerve, but modern supporters -of his theory suppose three different substances chemically changed -by light.) A prism spreads out the rays which are combined in white -light into a band in the order of their wave-lengths—those which have -the longest wave-length (0·8 µ) and the slowest rate of vibration (381 -billions to the second) at one end, those which have the shortest -wave-length (0·4 µ) and the most rapid vibration (764 billions to the -second) at the other: between these two extremes every intermediate -grade of length and rapidity. These are a mere fraction—a small -group—of the waves which the æther transmits, but they are all that -we can see. The long, slow vibrations give rise to sensations which we -describe as red; the short, rapid vibrations we describe as violet. -Our names for the tints which intervene are singularly old-fashioned -and unsatisfactory, but all persons agree that they recognize in the -spectrum a certain number of definite colours. Some normal-sighted -persons say twelve, others eighteen. It is largely a question of -terminology. - -Many considerations show that it is quite unnecessary to imagine -that the retina is affected in a different kind of way by every kind -of light, or by each of several groups of waves. If the red of the -spectrum is mixed with yellow, we receive an impression of orange, -which is identical with the impression produced by waves of the mean -length of red and yellow; orange and green give yellow; yellow and -blue, green. Any two complementary colours yield white. By taking -three colours—say, red, green, and violet—we obtain, when they -are duly mixed, not white light only, but light of any other tint, -although not of spectral purity, since it is mixed with white. Young -considered that all the conditions of colour-vision would be satisfied, -all our various sensations provided for, if the retina contain three -kinds of apparatus which light, according to its quality, affects in -varying degrees; and with this theory of three kinds of apparatus—_a_, -_b_, and _c_—the theory of three elementary or fundamental -colour-sensations is indissolubly linked. The colour _x_ produces -its intensest effect when _a_ is stimulated, with the least possible -stimulation of _b_ and _c_; _y_ is the reaction of _b_, _z_ of _c_. -Recent studies of the curves of intensity give us the tints of _x_, -_y_, and _z_ as carmine-red, apple-green, and ultramarine blue. - -The blending of sensations is illustrated with the well-known -colour-top. But perhaps the most striking proof that three elementary -colour-sensations are adequate to produce our visual world is afforded -by photographs taken with the three-colour method. Three plates are -exposed—(_a_) behind a red screen, (_b_) behind a greenish-yellow -screen, (_c_) behind a blue screen. They are fixed in such a way that -the portions acted upon by light are rendered insoluble, whereas the -rest of the film can be dissolved away; _a_ is then stained red, _b_ -greenish yellow, _c_ blue. The three are superposed, and the result -appears to the eye as an exact reproduction of the subject of the -photograph in all its hues. It shows every shade of orange and green -and violet. It is as bright—that is to say, as full of white light—as -the original. - -Various objections may, however, be brought against Young’s theory. -Of these, the most weighty are: (1) The retina does not contain three -kinds of apparatus, as Young supposed; nor can we find three kinds of -photochemical substances, as required by the theory in its modern form. -If we could find them, a fresh difficulty would arise; for we have no -reasons for supposing that one and the same nerve-ending can receive -stimuli of three different kinds. (2) The theory offers no explanation -of negative after-images—the complementary colours experienced -when the eye is closed after staring at a brightly coloured object. -(3) It does not adequately account for the various deficiencies of -colour-blindness. - -It is well recognized that there are various degrees of -colour-blindness, and that the colour-vision of persons considered -normal presents different grades of refinement. Nevertheless, the -abnormalities of colour-blind persons are so marked that cases fall -into definite classes. Those whose cones do not function—which means -that their yellow spots are either undeveloped or diseased—see all -things grey. They are totally colour-blind. Excluding these, the -colour-blind may be grouped in one or other of two divisions—(_a_) -those who confuse red and green, (_b_) those who confuse yellow and -blue. One person out of every thirty-five is red-green blind. The -proportion is even higher if males only are considered, showing -how very unfortunate is our choice of warning signals. A man who -is red-green blind cannot tell the port from the starboard light. -Blue-yellow blindness is, on the other hand, extremely rare. According -to Young’s theory, colour-blindness is due to the absence of one of the -three sets of visual apparatus. But cases do not altogether conform to -this hypothesis. We knew an amateur water-colourist, since deceased, -who derived intense pleasure from the beauties of Nature, and showed -no mean skill in reproducing them with his brush, notwithstanding the -fact that he was red-green blind. Each night his sister arranged his -paint-box for him, and only rarely did he use vermilion to fill in a -foreground of lush green grass. But this mistake, when he made it, did -not destroy his own satisfaction in the picture. It was clear that -red had a value for him, although he confused it with green. It is -impossible for a normal person to see through the eye of one who is -colour-blind, and there is no other means of comparing his sensations -with our own. The mistakes which the colour-blind make in sorting -coloured objects and in naming mixtures of light selected from various -parts of the spectrum show the range of their deficiency, but give us -no information regarding the qualities of the sensations which they -retain. - -The test of colour-sensitiveness usually employed is the grading of a -large number of wools of different tint. The order in which the colours -should be arranged is not a matter of opinion. They must be placed -in the order in which they occur in the spectrum—_i.e._, arranged -according to their wave-lengths. In the cases of colour-blindness -which are most frequently met with the defect may be described as due -to an absence of the sense of redness, or as an absence of the sense of -greenness. The two conditions can be distinguished. But since the eye -is not dark for red (although in certain cases vision is very weak for -the red end of the spectrum) or dark for green, the abnormality cannot -be adequately accounted for on structural grounds. It is not explicable -on the hypothesis that one of three sets of responsive sense-organs -(or nerve-fibres) or photochemical substances is absent from the eye. -Again, it is generally agreed that the sensations of white, yellow, -and blue of the red-green colour-blind are similar to those of normal -persons. This is not in harmony with the theory of the omission from -their eyes of one of three pieces of colour-apparatus. - -Professor Hering, of Leipsic, adopting the generally accepted view that -light effects chemical changes in substances contained in the retina, -to which changes stimulation of nerve-endings is due, formulated a -theory of colour-vision which many physiologists prefer to Young’s. He -imagines that the retina contains three kinds of pigment, each of which -is, as he believes all living substance to be, in a constant state of -change. It is at the same time being built up and destroyed. Using -the terms which connote the opposite directions of metabolism, the -pigment is simultaneously undergoing anabolism and katabolism; the two -processes, when the retina is at rest, maintaining equilibrium. When -light acts upon either of the substances, it hastens, according to its -quality, either the one process or the other; and the chemical change, -whether it be constructive or destructive, stimulates the endings of -optic nerves. Hering assumes, therefore, that there are six elementary -qualities of visual sensation—red, green, yellow, blue, white, black. -Red, yellow, white are due to anabolism of the visual substances; -green, blue, black are due to their katabolism. The installation of -yellow amongst the unanalysable colours is a relief to many minds. It -is almost impossible to think of yellow as a compounded colour. White -also, we feel, is not a compounded colour, despite our knowledge that a -prism scatters from it all the hues of the rainbow. Black, many persons -assert, gives them a definite sensation, and not merely a sense of -rest. (Parenthetically, it may be observed that the _feeling_ that a -colour is pure or mixed is not to be trusted. It may be based upon -the chromatic aberration of the eye, or it may be reminiscent of the -paint-box. We know that we cannot make yellow by mixing red and green -pigments, hence we feel that it is pure. Of green we are not by any -means sure; gamboge and Prussian blue come into our minds.) Except when -the light which falls upon the retina is giving rise to one of the -four pure colour-sensations, all three substances are simultaneously -affected, although one may be undergoing katabolism while the other two -are being built up, or _vice versa_. Hering accounts for simultaneous -contrast by assuming that the activity of any one part of the retina -induces an opposite kind of change in the remainder, and especially -in the vicinity of the primarily active part. When a certain patch -is developing a sensation of red, the rest of the retina develops a -sensation of green. - -The great merit of the theory is, however, to be found in its offering -an explanation of complementary after-images. The green patch seen -with closed eyes after one has stared at a red object is due to -the rebound of metabolism. In returning to a condition of chemical -equilibrium the retinal substance acts as a stimulant which evokes -the antagonistic colour. But it is a theory which makes very large -assumptions. It assumes, for example, the possibility of the existence -of a substance which is built up by light from one end of the spectrum, -and decomposed by light from its centre. Not that Hering regards the -existence of three retinal substances as essential to his theory. He -is prepared to transfer to the brain the seat of the substances, or -the substance, which, by their, or its, anabolism and katabolism, -produces antagonistic colour-perceptions; but in this he is abandoning -physiology for metaphysics. We have no warrant for imagining that there -exists in the brain any substance which, by undergoing physical changes -of various kinds, produces various psychical effects. The problem to -be solved is physiological. Rays of light of different wave-lengths -excite the retina to discharge impulses which are variously distributed -in the brain. The effects which they produce in consciousness depend -upon their distribution. The impulses to which the longest rays give -rise evoke sensations of red, those due to the shortest, sensations of -violet. And what is true of the retina as a whole is true, apparently, -of each individual cone. In what way does light act upon a cone? It -is one of the most fascinating problems in physiology. Round it our -thoughts revolve whenever we are trying to form conceptions of the -nature of stimulation, sensation, and perception. Each of the two -theories which we have expounded above helps to group together certain -of the more striking phenomena of colour-vision, but neither gives a -satisfying explanation of their causation. - -The sensitiveness of the retina is in a remarkable degree adjusted to -the intensity of the light. When a dark room is entered, the pupil -dilates; but one’s power of distinguishing objects continues to -increase after the pupil has reached its maximum size. At the end of -ten minutes the eye may be twenty-five times as sensitive as it was -when the room was entered. This _adaptation to darkness_ is due in -large degree to the substitution of rods for cones as the organs on -which vision chiefly depends. But it cannot be wholly due to this, -since it occurs when one is working with a red light. Probably the red -used in a “dark-room” is not sufficiently near the end of the spectrum -to be completely without influence upon visual purple, but it is a -colour to which rods are comparatively insensitive. Other evidence also -points to an adaptation of cones as well as of rods. - -[Illustration: FIG. 32.—THE FORMATION OF AN IMAGE BY THE REFRACTING -MEDIA OF THE EYE. - - _x_, The common centre of curvature (nodal point of - the several media). Rays which pass through this - point are not deflected. _y_, The principal focus - of the system. All rays which are parallel to the - optic axis converge to this point. The image of - the point A is formed at _a_, the spot at which a - ray parallel with the optic axis meets an unbent - ray—the image of B at _b_.] - -_Accommodation of the eye for distance_ is brought about by a mechanism -which allows the lens to change in shape. It becomes more convex when -a near object is looked at than it was when adjusted for an unlimited -distance, which is its condition when the eye is at rest. Adjustment -for near objects involves muscular action, and is accompanied by a -sense of effort, however slight. Whilst the eye is at rest the lens is -mechanically compressed against the anterior layer of its suspensory -ligament. Accommodation for near vision is effected by the ciliary -muscle, which is placed in the shelf of tissue which projects into -the interior of the eyeball. This muscle is made up of a ring of -circular fibres, and to the outer side of this, of fibres which radiate -backwards and outwards. The longitudinal, or radiating, fibres obtain -their purchase by attachment to the firm wall of the globe just beyond -the cornea. They spread into the front of the loose chorioid membrane -which lines the eye behind the retina. By the joint action of these two -sets of plain muscle-fibres the suspensory ligament is slackened, and -the extremely elastic lens, previously compressed, bulges forwards. -The radius of curvature of its anterior surface changes from 10·3 -millimetres for distance to 6 millimetres for vision at the “near -point.” It was stated, in connection with the development of the lens -(p. 374), that the cells of the posterior half of the hollow sphere -out of which it is formed grow forwards into extremely long fibres, -which traverse its whole thickness. These fibres are bent like the -segments of a carriage-spring. Their anterior ends rest against the -flattened ligament of the lens; the vitreous humour, which is always -under tension, compresses their posterior ends. When removed from -the eye, the lens becomes rounder than it is _in situ_, even when -accommodated for near objects. But in later life it grows stiff. It -ceases to bulge forwards when its ligament is slackened. Hence it -becomes necessary to aid the presbyopic eye with convex glasses when -it is used for near objects, although for distant vision it remains as -effective as ever. If the ciliary muscle is constantly and completely -relieved of the labour of accommodation, it grows lazy, or rather -wastes from want of use. A person who relies on spectacles loses his -power of accommodation; but ophthalmologists agree that self-focussing, -if it give rise to a sensation of strain, is bad for the eyes. In -myopic persons the eyeball is too deep; objects are focussed in front -of the retina. In hypermetropia (“long sight”) the eyeball is too -shallow; objects are focussed behind the retina. Concave glasses -correct the one condition, convex glasses correct the other. Glasses -are also very commonly called for to neutralize another defect—regular -astigmatism—which may be present by itself, or may accompany -insufficient length or too great length of the optic axis. It is due -to unequal curvature of the cornea. Usually the curvature is sharper -in the vertical than in the horizontal meridian (_cf._ p. 269); as a -consequence, points in a vertical line are focussed in front of points -in a horizontal line. Cylindrical glasses, not lenses, are required to -correct this defect. And here it may be well to call attention to the -fact that rays of light are more sharply refracted by the surface of -the cornea than they are by the crystalline lens. The lens has a high -index of refraction (1·45), but it does not lie in air (the index of -refraction of which is 1), but between two humours which have about the -same index as water—namely, 1·336. The bending by the combined action -of the cornea and the lens of rays of light which come from a source so -distant that they may be considered as parallel brings them to a focus -on the retina, when the lens is at its flattest. When the lens is at -its roundest, rays which diverge from a point only 5 inches in front of -the eye are focussed on the retina. The lens is therefore essential for -accommodation, but, after its removal for cataract, vision, even for -near objects, is rendered possible by the use of convex glasses. - -[Illustration: FIG. 33. - - A, The normal eyeball, in which, when the ciliary - muscle is relaxed, parallel rays are brought to a - focus on the retina. B, A hypermetropic eyeball. - Its depth being less than normal, parallel rays - are not brought to a focus on the retina when the - eye is adjusted for distant vision without the aid - of a convex glass. C, A myopic eyeball. Its depth - being more than normal, a concave lens is needed to - diminish the convergence of parallel rays.] - -A star or a distant gas-lamp is seen as a point of light with rays. -Usually this figure, which has given origin to the expression -“star-shaped,” shows three greater rays alternating with three lesser -rays. Such an image is not produced by a point of light near to the -eye, since it is due to the puckering of the lens when flattened -against its ligament. It brings into evidence the three axes on the -front of the lens and the three axes which alternate with them on the -back, with regard to which the lens-fibres are disposed. - -As an adaptation of living tissues to optical purposes the eye is above -admiration, yet it presents many =defects=, which an optician corrects -in the instruments which he manufactures. A remarkable fact in the -physiology of vision is our unconsciousness of the imperfections of its -organ. An unusual experiment is needed to bring them to our notice. If -we look through a common glass lens uncorrected for unequal refraction -of rays of different wave-lengths, we recognize that a bright object -is shown with a colour-fringe, yet we take no cognizance of the -colour-fringes which surround the images of all bright objects focussed -upon our retinæ. If we think about the matter, we recognize a feeling -that blue in a window of stained glass appears farther away than red; -but this might well be due to association. Blue glass is chiefly used -for the sky. If we look at a bright object through purple glass, we -her red with a blue fringe or blue with a red fringe, according as the -eye is focussed for red or for blue. The purple glass having absorbed -all intermediate rays, we become aware that we cannot focus the two -extreme ends of the spectrum at the same place. Since a greater effort -of accommodation is needed to focus red, we judge that the bright -object is nearer to us when it appears red than when it appears blue. - -Spherical aberration is another fault of the lens. The rays which -enter its margin are brought to a focus sooner than those which pass -through its centre. This is due to the fact that its surfaces are -regularly curved, whereas a glass lens is corrected by grinding it -flatter towards the margin. This defect is partly corrected by the -cornea, which has an ellipsoidal surface, and partly by the greater -density of the centre of the lens. Yet it is still necessary for the -eye to be “stopped down” by the iris when a near object is looked at, -although less light is entering the eye than when it is directed to -the horizon—a condition which would lead a photographer to open his -iris-diaphragm. - -[Illustration] - -Of all the imperfections of the eye which the mind ignores, the most -remarkable is the gap in the field of vision, due to the gap in the -sensitive layers of the retina, which occurs where the optic nerve -enters it—the blind spot. Hold this page of the book 10 inches from -the face, keeping the lines of print horizontal. Close the left eye and -look at X with the right eye. The black disc disappears, because its -image is focussed on the blind spot. Since the picture on the retina -is reversed, it is clear that the optic nerve enters the globe to its -inner side, and slightly above its horizontal meridian. But, unless we -employ an unusual test, we are quite unconscious of the fact that a -definite hole is punched in the picture. The mind fills it in, and the -way in which it does so is extremely suggestive. It lies about it—in a -downright ingenuous fashion if it is confident of credence, in a more -subtle way if a simple falsehood is likely to be challenged. In place -of the black disc make nine conspicuous crosses: - -[Illustration] - -Hold the paper in such a position that _X_ falls upon the blind spot. -It ought to disappear, but the mind assures you that there is a cross -at that spot. The mind completes the field. In place of the crosses use -noughts and crosses, thus: - -[Illustration] - -Now let _X_ fall on the blind spot, and allow the eye to go just a -little out of focus. The four marginal crosses draw inwards: - -[Illustration] - -The mind contracts the field. Still denying the gap, but not having -sufficient data from which to invent an object, the fraudulent -nature of which would not be found out the instant that the gaze is -shifted, the mind lies regarding the position on the paper occupied by -surrounding objects. - -Is it quite fair to the mind to say that it lies about the blind -spot? The mind judges sensations in the light of experience. An -association of previous sensations teaches me that the wall of the -room is not pierced by a round hole a foot in diameter opening into -outer darkness. Many sensations to me the fact that the designs on a -wall-paper succeed one another with unbroken regularity. Fixing my gaze -on one of them, I cannot by any effort of attention efface the pattern -which happens to be focussed on the blind spot. I know that I shall see -it the instant that I move the eye. If I let my eye roam until the face -of my wife falls on the blind spot, its image disappears. I know its -lineaments far better than I know the pattern on the wall-paper, but I -cannot fill it into the picture. Her hands are visible, and the work -which is resting in her lap, but in a mysterious way the background -draws together where the face should be. My mind refuses to pass a -false judgment; but it also refuses to see that there is a gap. - -This exceedingly instructive observation teaches the relativity of -sensations. It shows that a sensation has no objective value until -judgment has been passed upon it by the mind. The meaning of this -we express in figurative language, none other being available. We -speak of a new sensation as being compared with sensations previously -received—taken into the picture-gallery of the mind, and placed in its -due position amongst the infinitely numerous records which are stored -there. If we try to make a nearer approach to correlating physical with -psychical activity, we say that sensation has no value save that which -it acquires from its temporal relation in the sequence of sensations -to which attention is directed, and that this value depends upon the -relation which similar sensations have possessed in former sequences. -There is no gap in binocular vision. An object focussed on the inner -(nasal) side of the right eye, where the blind spot is situate, is -focussed on the outer (temporal) side of the left eye. The left eye -sees the object to which the right eye is blind. Since we have almost -invariably used two eyes in the past, experience teaches that there is -no gap in the field of vision. Hence the new group of sensations which -alleges that there is a gap must be corrected. The field must be filled -up in the way which experience shows to be most likely. The retina is -a sheet of rods and cones, each of which has a nervous connection with -the brain proper to itself. The retinal field is associated with the -brain-field. But this does not imply that we may think of the mind as -having a spatial distribution on A or button B in the retina causes -bell A′ or bell B′ to ring in the brain, but it does not follow that -perception A′′ or perception B′′ will be heard in the mind. It will be -heard if this is the association established by custom, since mind is -the product of experience. But the new sensation is creating precedent -as well as being judged by it. - -[Illustration] - -Point A in the right retina is associated by experience with point _a_ -in the left, and point B with _b_. These are termed _corresponding -points_, because they are similarly stimulated in binocular vision. -The mind, therefore, judges that it receives the same information from -each pair of corresponding points. The position of corresponding points -will be understood if the right retina is imagined as put inside the -left, precautions being taken to make the yellow spots coincide, and -to avoid twisting the retinal cups in taking them out of the eyeballs. -Great care is taken to maintain the points in correspondence during the -various movements of the two eyeballs. In addition to the four recti -muscles which move the eyeball upwards, downwards, to right and left, -two oblique muscles give it the requisite amount of rotation. We have -learned to give the same value to the impulses from two corresponding -points. But under changed conditions the correspondence changes. When -a squint develops in childhood, it follows one of two courses; either -the obliquity of one of the eyeballs increases until it looks towards -the nose, and its images cease to interfere with the images in the -dominant eye—they are ignored by the mind—or a fresh correspondence -is established between points in the oblique eye and points in the eye -which looks straight forward. If we are severely critical, we find, -from a study of the form of the eyeball, that it is impossible that the -same rods and cones should occupy corresponding points in different -positions of focus and with different degrees of convergence of the -eyeballs. To permit of this the retinal cups would need to change in -shape. But again mechanical correspondence is of little consequence. In -the light of experience the mind judges that points correspond. When -we are gazing at a flat surface, the mind judges that corresponding -points are giving it similar information. It does not see a flower on -a wall-paper twice as bright or twice as red with two eyes as with -one. If the eyes are normal, the impression received through the two is -precisely the same as the impression received through either singly. -But when we are looking at solid objects, the image on one retina is -not the same as the image on the other. One eye sees farther round -the object on the one side, the other on the other; and it is just -this disparity in the pictures, aided by the feeling that the eyes -are converging, that gives the impression of solidity. Correspondence -of points, on the other hand, is not necessarily sufficient by itself -to convince the mind that the pictures presented by the two eyes are -identical. When a flat triangle such as this is regarded with the two -eyes, its black lines fall on corresponding points; but the figure is -associated in the mind with other sensations—sensations of movement -and touch. Notwithstanding the identity of the retinal images, the mind -tries to see them as disparate. The figure troubles the eyes. At one -moment the meeting-point of the three central lines projects forwards, -at the next it recedes. That similarity of retinal images counts for -something is shown by closing one eye. The uncertainty of shape of the -figure is rendered more troublesome. It changes still more rapidly from -convex to concave. When the point seems to be in front of the page, the -accommodation of the eyes is adjusted for nearness; when behind the -page, for greater distance. But the illusion that the object occupies -three dimensions is not dependent upon the sense of contraction of the -ciliary muscle. When the paper is moved towards the eye, its centre -recedes; it is left behind until the ciliary muscle has had time to -contract. When it is moved away from the eye, it projects until the -ciliary muscle has had time to relax. Accommodation follows judgment, -not judgment accommodation. The mind is extremely suspicious of the -veracity of its newsagents. Disparateness of images, convergence of the -eyeballs, shifting of accommodation for the various levels of an object -in space, should be indisputable evidence of solidity or of hollowness. -Conversely, the absence of either factor should be conclusive proof of -flatness. But the mind does not trust to isolated sensations; it looks -for associations of sensations. When the finger hints, “I could touch -that sharp point,” it is useless for the eye to aver that there is no -point to be touched. - -If two exactly similar photographs are placed in a stereoscope, the -fact that the eyes are not converged gives to the common picture an -appearance of depth, notwithstanding the fact that corresponding points -on the two retinæ are stimulated. If the two photographs have been -taken, as they should be taken for this purpose, with a double camera, -the disparity of the retinal images immensely enhances the impression -of solidity. - -It is impossible to exaggerate the dependence of sensation on -=judgment=. At birth a child commences the long process of education -which enables it to associate the sensations derived from its retinal -images with the movements which place it in contact with things. -It discovers that, when it is necessary to make the eyes converge, -the object is near at hand. It also associates the voluntary action -of contracting its ciliary muscle with nearness. Unconverged and -unaccommodated eyes come to mean distance. So, too, do indistinctness -due to absorption by the atmosphere, blueness due to the same cause, a -small image on the retina. But there are obvious limits to its power -of ascertaining the distance of an object, and therefore, conversely, -of its power of estimating size. We have no idea of the size of the -retinal image of the sun. Very few people would be prepared to believe -that the angle which the sun subtends with the eye barely exceeds -half a degree. (The first finger, viewed in profile, at arm’s length, -covers one degree of arc.) A disc of paper of the right size, placed -at the right distance, looks far too small to represent the sun. The -most brilliant of orbs bulks larger than this in our minds. Everyone -who for the first time looks at the sun through well-smoked glass, -or, better, through a flat-sided vessel filled with ink and water, -is astonished that it looks so small. Nor are we prepared to accept -the evidence of a camera that the sun at the zenith does not produce a -smaller image on the retina than the sun when rising above the horizon. -Yet if a photographic plate is exposed to the rising sun, and again, -without changing its focus, to the sun at the zenith, the two images -are practically equal. There is a slight difference due to the greater -refraction of rays passing tangentially through the atmosphere, but -it is so slight as to bear no relation to the difference between our -two judgments of size. When the sun is rising behind trees and houses, -we compare it with objects which we know to be large and distant; -yet it looks almost as large when rising out of the sea. One of the -causes of the illusion is our conviction that the sky is flattened; -and this, again, is due partly to its paler tint—its less substantial -blueness—near the horizon, and partly to our impression that it is -spread out over a flat earth. When the sun is in what we deem to be the -more distant part of the vault of heaven, we judge it to be farther -from us, and therefore larger than when it is above us. Yet the last -word has not been said in explanation of a phenomenon which has been -studied by mankind since the dawn of science. Helmholtz attributed the -apparent greater distance, and consequent greater size, of the sun -and moon when near the horizon to the indistinctness of their discs. -When its image is so reflected from the zenith as to cause the moon -to appear to rest upon the horizon, it does not, he said, increase in -size. In answer to Helmholtz’s explanation, it may be objected that, -when at midnight he brought the full moon down from the zenith, he did -not bring with her the conditions of light and colour by which she is -customarily surrounded when floating on the horizon. If, when watching -the moon which has just risen, vast in diameter, out of the sea, one -interposes between it and the eye a sheet of paper in which a small -hole has been made, and looks at the moon with one eye through the -hole, it instantly shrinks to the size which it appears to have at the -zenith. It is not even necessary to blot out the whole of its trail of -light on the sea. At the same time, it appears to retreat to a great -distance. This shows how complicated are the associations upon which -judgments of size and distance are based, and to how small an extent -they are determined by the size of the image on the retina. This -observation is most surprising if made one or two nights after full -moon, when twilight is already dim at moon-rise. - -Our estimate of the distance away from us of an object on the horizon -is based upon the time and effort which experience tells us we should -need to spend in reaching it. The untried appears shorter than the -tried. Anyone who compares his feeling of the number of yards he would -have to climb up a pole reaching to the zenith with his feeling of -the number of steps he would need to take to reach the horizon will -recognize that the horizon appears to him to be the farther away. - -[Illustration: FIG. 36.—A SYMMETRICAL ARCH, DIVIDED BY A VERTICAL -LINE, A, WHICH PASSES THROUGH ITS APEX.] - -In representing a solid object an artist conveys theidea that light -is falling obliquely upon it. One side of the object, therefore, is -more strongly illuminated than the other. By depth and gradation of -shade he indicates the extent to which the thing projects forwards, if -solid, or falls back, if hollow. He makes the margin of a ball hazy, -in the expectation that the spectator will look at the spot nearest -to him—an artifice which he may easily press too far, since the eyes -wander restlessly over a flat surface. In representing distance he -is dependent upon giving to the various objects in his picture sizes -equivalent to the sizes of their images on the retina, making them -brighter or paler and more or less distinct. Yet he cannot hope to -simulate the convincing evidence of distance which is afforded by our -sense of the degree of convergence of our eyes. Hence, as Francis Bacon -pointed out, a picture appears more real when one eye is closed than -when both are open. Its middle distance at once falls back. - -[Illustration: FIG. 37.—TWO HORIZONTAL LINES OF EQUAL LENGTH—THE ONE -WITH DIVERGING, THE OTHER WITH CONVERGING, TERMINAL LINES.] - -Innumerable are the illustrations which may be given of errors -of sensory judgment, but none are more striking than the various -figures which may be drawn with converging or diverging lines. The -mind under-estimates acute and over-estimates obtuse angles. It is -impossible to convince oneself that in Fig. 36 the line A bisects a -symmetrical arch. Equally difficult is it to believe that in Fig. 37 -the line with diverging terminal segments and the line with converging -terminal segments are of exactly equal length. In the Ruskin Museum at -Sheffield there is a sketch by the master of the façade of a church -which shows a vertical tower to one side of a triangular pediment, or, -rather, this is what the sketch was meant to show, and does show, when -measured on an architect’s table. In effect the tower appears to be -leaning towards the pediment. Errors of judgment of this type have been -attributed to the curvature of the lines of a rectilinear image on the -retina, the mind judging the distance between two points by the length -of the chord, and not the length of the arc which joins them. This is -very simply illustrated by the example of the apparently greater length -of a filled space than of a vacant one. - -[Illustration] - -A B looks longer than B C. If A B C be represented as a curved line, -the arc A B will, of course, be longer than the chord B C. But it -is not safe to suppose that the mind compares the length of an arc -with the length of a chord. Judgment is based upon experience, and -probably the illusion is due to more subtle causes than the curvature -of the retina. The mind does not look at the retina. If it did, it -would find the reversal of the picture the least of the inaccuracies -which it had to correct. It would find it very difficult, for example, -to superpose in its stereoscope the photographs of a vertical tower -taken simultaneously by the right eye and the left. The curved images -on the retina of the vertical lines which define the angles of the -tower, as seen with one eye, could not be made to correspond with -the images focussed by the other eye. The Greeks felt this when they -settled the form of a column. The canon of the swelling entasis and -increasing taper above it did not destroy the appearance of uniform -thickness which the shaft presented. It gave to the eye just the slight -help which it needs to enable it to picture the shaft as of the same -thickness from base to capital. - - - - -CHAPTER XIV - -HEARING - - -The ear, like the eye, records amplitude of vibration; loudness. It -also records rapidity of vibration, musical pitch, which corresponds -with colour. But it seems to have a more difficult task than the eye, -since it has to analyse, or at any rate has to transmit information -regarding the form of compound vibrations. The meanings of these -distinctions may be illustrated by reference to a tracing on the -cylinder of a phonograph. A needle attached to the posterior surface of -the thin metal plate against which one speaks scratches the surface of -a rotating cylinder of hardened wax. Examined with a lens, the record -is seen to be an irregularly changing line. The depth of the marks is a -measure of loudness. Their varying number in a given time indicates the -changing pitch of the voice which produced them. Their form is a record -of the quality of its tone. The work of the ear, so far as it consists -in the estimation of the amplitude and rapidity of pulsations of sound, -is easy to describe, but the acoustics of form are complicated. - -Light is transmitted as vibrations of æther. They are transverse to -the direction in which the light is travelling. Sound cannot travel -through a vacuum, since it is dependent upon displacements of material -particles. The particles move forwards and backwards in the direction -in which sound is progressing. Sound is a sequence of pulsations, -alternate condensations and rarefactions of the media which conduct -it. Their particles are first pressed together, and then rebound to -positions farther apart. A sequence of to-and-fro movements, each -smoothly continuous throughout the whole duration of a pulsation, -would produce a pure musical tone. Tuning-forks carefully bowed settle -down after a few seconds into unbroken oscillations, which convey to -the air the to-and-fro movements of pure tones. Such tones vary in -nothing but loudness and pitch. If their pulsations are slow, we speak -of the pitch as “low”; if they are rapid, we say that their pitch is -high. But if the sound produced by tuning-forks (and low-toned stopped -organ-pipes) be omitted from the list, no pure tones reach our ears. -The notes of flutes, fiddles, trumpets, pianos, have each a certain -“quality” characteristic of the instrument. Even in a violin the G -string has not the same timbre as the D string. Owing to the elasticity -of the substances which originate and of the substances which -transmit sound, its pulsations are not simple to-and-fro movements, -uninterrupted from beginning to end. Each pulsation is partially -broken at intervals; and the quality of the sound depends upon the -number and relative accentuation of these partial interruptions. Sound -travels through air at the rate of 1,100 feet per second. This figure, -divided by the number of vibrations per second of a tone, gives the -wave-length in air of a tone of that particular pitch. For example, the -middle C has a vibratory rate of 256. Its wave-length is, therefore, -somewhat over 4 feet. The lowest tone of an organ has a wave-length of -37 feet; its highest of 3½ inches. These figures give no information, -however, regarding the movement of the particles which pass on the -sound. When air is transmitting a note—say the middle C—its separate -molecules do not move through a distance of 4 feet. Each molecule -moves but a short distance, varying with the loudness of the tone; but -the “wave” of crowding runs straight forward from the piano-string to -the ear, the molecules at the end of each stage of 4 feet taking on a -backward movement, so that the crowding, so far as the molecules of -that particular section are concerned, returns to its starting-point. -Between the piano-string and the ear there is a crowding and forward -movement at 0, 4, 8, 12 ... feet; a spreading and backward movement at -2, 6, 10, 14 ... feet. Most illustrations which are intended to aid -the mind in forming a definite picture of the transmission of sound -are liable to be misinterpreted, because they translate rectilinear -movements into waves. They represent the movements of the string, and -not the movements of the molecules of air between the string and the -ear; but with the aid of the imagination one may picture the positions -of the particles in this path. The pulse, we will suppose, has just -reached the limit of 12 feet. Half-way from its 8-foot halting place -the molecules are again crowded, although not so densely. One-third of -the distance from the same point there again appears a tendency to -crowd. This latter point marks an interval of one-third of this wave -_plus_ the wave which led up to it. At the end of the ninth foot there -is a crowding, though less marked—this wave _plus_ the two preceding -waves, divided into fourths. Within these intervals are other points -at which the molecules have closed together, the distances from a -nodal point depending upon the number of waves involved, and, speaking -generally, growing less marked as the number increases. Such are the -very complex pulsatile movements which reach the ear. - -Every musical sound produced by a piano, a violin, or other instrument, -is compounded of a fundamental or prime tone, and overtones, partial -tones, or harmonics. The following table shows the more important -partial tones which accompany the prime tone when the middle C on a -pianoforte is struck: - - Number of Number of - Note. Vibrations. Interval. Ratio. Overtone. - - C‴ 2,048 } 7th - } Super-Second 8/7 - } - B″♭ 1,792 } 6th - } Sub-minor third 7/6 - } - G″ 1,536 } 5th - } Minor third 6/5 - } - E″ 1,280 } 4th - } Major third 5/4 - } - C″ 1,024 } 3rd - } Fourth 4/3 - } - G′ 768 } 2nd - } Fifth 3/2 - } - C′ 512 } 1st - } Octave 2/1 - C 256 } =Fundamental= - -The quality of a musical note depends upon the number and relative -loudness of its overtones. When several notes are sounded -simultaneously, they blend into a chord or harmony, provided the -intervals which separate them are equal to the intervals which separate -the simpler overtones. Each of the notes yields overtones. The tones -blend into a concord. Their partials are in unison. The variations in -air-pressure of the compound tone are strictly periodic. If the ratios -of the frequencies of its constituent notes are simple the product is a -rich, full sound, such as a common chord. - -At least one other character of the pulsations of sound must be taken -into consideration if we wish to picture the nature of the force to -which the ear responds. Tones which reach it from several instruments -simultaneously are not necessarily in unison, or even in harmony. The -overtones of a single note sounded on a piano or violin—the statement -does not hold good for bells, nor is it strictly true of flutes or -horns—must necessarily bear a simple proportional relation to their -prime tone. They divide the grand pulsation into fractions “without a -remainder.” But the vibrations of two tuning-forks which are slightly -out of unison interfere one with the other at regular intervals. They -produce “beats.” Everyone is familiar with the curious effect which -is produced upon the eye when one row of railings is seen through -another, or one expanse of wire-netting behind another. Sets of lines -which occupy nearly the same positions in the line of sight combine -to make a large pattern, which overlies the smaller pattern of the -rails or netting. The same thing happens with sounds which coincide at -considerable intervals, although in the case of sounds interference -is as marked as reinforcement. If whilst a tuning-fork yielding 101 -vibrations per second is singing another of 100 vibrations is brought -into play, the vibrations of the second fork are superposed on those -of the first. At a certain moment the forward movement of molecules of -air induced by the first fork is reinforced by a forward push from the -second. But half a second after this coincidence of phase an opposite -result is produced—50½ vibrations of No. 1 have passed, but only 50 -of No. 2. No. 2 is going backwards (inwards), whilst No. 1 is moving -forwards (outwards). The same molecules are impelled backwards by No. -2 and forwards by No. 1. The result is a pause. The compound sound -produced by the two forks reaches the ear in throbs. If the forks -were vibrating at the rates of 101 and 99, there would be two pauses -and two beats in every second; if at the rate of 202 and 198, four. -The number of beats per second equals the difference in frequency of -vibration of the tones. A pianoforte tuner does his work best if he has -a musical ear, yet he may discharge his duties with competence without -one. Having struck a note, he sounds its octave, holding both keys -down, and listens for the beat. If the first note gave no beat with -his tuning-fork, the second is in time when it likewise gives no beat -with the first. We have met a tuner who did his work in this way; but -it must be admitted that his tempering of the intervals of the octave -with which he commenced, and consequently of the other octaves above -and below it, left something to be desired. The result might have been -satisfactory had he been provided with twelve tuning-forks. - -The question as to whether beats, when sufficiently rapid, blend into a -tone has been much discussed, without a decision. Probably they do not. -The complementary question as to the cause of dissonance is also not -completely closed. Two notes harmonize, as we have seen, when the ratio -of their frequencies is a simple fraction. Musicians are not quite -agreed as to the level of numerical complexity at which a compound -tone first produces a feeling of discomfort. A good deal depends upon -its position in the scale and the instruments which are combining -to produce it. A minor third (⁶/₅) is on the safe side. This is the -first chord in our list of intervals in which a beat can be detected. -Slow beats, however, do not distress us. It is the rapid beats of -conflicting overtones which give a harsh, rough character to a compound -note. The level at which a line is drawn between harmony and dissonance -seems to depend to a considerable extent upon musical education, using -the term in its widest sense. In primitive music—Hungarian, Scotch, -Welsh—intricate minor chords predominate. The minute subdivision of -the octave in Indian music is quite incomprehensible to a European -ear. Musical cultivation tends to eliminate complex fractions. It is, -however, to be noted that the history of Western music also shows the -influence of an opposite tendency. Later generations have admitted as -harmonies combinations which earlier generations could not tolerate. - -Pitch, quality, harmony, and dissonance are distinguished by the -human ear. These are the attributes of musical or periodic sounds. -In a separate class must be included noises of all kinds, termed in -acoustics “aperiodic,” because the vibrations which cause them are -not rhythmic. The teeth of a policeman’s rattle may click a hundred -times a second, but it does not make music. Even with a rapidity of -interruption greater than this (at least 500 times per second) a -succession of noises fails to blend into a smooth, continuous sound. -The ear recognizes the loudness, duration, and even to a very high -frequency the repetition of unmusical sounds. - -The ear as a sense-organ can be followed down the zoological scale to -jelly-fish. In its primitive form it is a chamber lined with epithelial -cells bearing hairs, containing an otolith, or ear-stone. Otoliths are -rounded calcareous masses which play an important part in the ears -of all animals up to fishes. Even in man they are found in the more -subdivided form of otoconia. Contact of the otoliths with the sensory -hairs originates impulses in the nerves with which primitive ears -are abundantly provided. Advisedly we use the word “ear” in place of -“auditory organ.” In all animals this organ affords information of -a double nature-movement of the external medium in which the animal -lives, and movements of the animal in the medium. When the animal -moves, its sensory hairs are displaced with regard to the otolith; when -the water in which it is swimming pulsates, its otoliths are shaken -against the sensory hairs. Displacements of the animal and agitations -of the water produce similar effects. The ear in this stage is an -organ of touch. It might well be questioned whether an animal fitted -with a piece of sensory apparatus of this kind is endowed with a sense -which we may properly, after reflecting upon our own sensations, term -“hearing.” It is, however, stated that certain transparent crustaceans, -in which the functioning of the ear-organs may be watched through a -lens, show in these organs hairs of varying length which vibrate to -tones of different frequency. This observation apart, it might be -doubted whether fishes hear, if we mean by the word “hearing” the -recognition and discrimination of tones of high frequency—musical -tones. Their ears serve equally to inform them of the changes in -position of their heads and of the tremblings of the sea. The shocks -transmitted through the sea are near akin to the slower vibrations of -sound, if the fishermen of the Mediterranean are justified in their -practice of beating a wooden clapper which rests upon the seat of the -boat as they row backwards and forwards in front of a curved net. They -believe that the fish are frightened by the noise; but it matters -little whether we describe the fish as hearing a noise, or as feeling -the percussions of the clapper conducted through the water. To the more -rapid vibrations of the clapper, the fish are probably insensitive. -The cochlea, which we have every reason for regarding as the organ by -which sound is analysed, is not possessed by fishes. It makes its first -appearance in reptiles. Birds, it is evident, are able to distinguish -musical tones. Their cochleæ are very short, and are destitute of -“rods of Corti.” For a moment this appears surprising, but it must be -remembered that the range of tones which any bird discriminates is -very short, however nicely it may value the notes within its range. In -mammals the ear is clearly divided into three parts, to which the three -functions which have grown out of the specialization of the sense of -touch are allocated. (1) The semicircular canals are concerned with -the sense of orientation. (2) The utricle and saccule reverberate to -noise—the rumbling of trains, the boom of guns, the beats of dissonant -musical tones. We do not know how to classify the agitations of the -atmosphere which surrounds us and of the earth on which we stand, nor -can we point with any certainty to the groups of stimuli which for -us have taken the place of the grinding of stones on the beach and -slapping of rocks by waves. (3) The organ of Corti in the cochlea -discriminates and analyses musical sounds. To these three sense-organs, -which are situate in the inner ear, certain structures are accessory. - -The concha, which enables a horse or a cat to collect sound and to -localize its source, is in ourselves merely an ornament to the side of -the head. - -[Illustration: FIG. 38.—THE EXTERNAL, MIDDLE, AND INTERNAL EAR OF THE -LEFT SIDE. - - From right to left, the figure shows the concha and - lobule of the ear in profile; the external meatus - (abbreviated); the drum, divided vertically, its - posterior half visible; the hammer-bone, with - the tip of its long arm attached to the drum, an - arrow indicating the point of attachment and line - of action of the tensor tympani muscle; the anvil - attached by a ligament to the bony wall of the - middle ear; the stirrup, with its foot-plate almost - filling the oval window; the labyrinth, with the - three semicircular canals above, and the scala - vestibuli below. The curled black line shows the - situation of the scala media, or ductus cochleæ - (which contains the organ of Corti). Pulsations - of sound which move the membrana tympani are - transmitted by the three bones to the oval window. - They shake the perilymph, producing waves which - travel along the scala vestibuli to the apex of the - cochlea, whence they return by the scala tympani - to the round window (if they do not take a shorter - course through the ductus cochleæ). The Eustachian - tube opens out of the lower part of the middle ear.] - -The external meatus is a curved tube, about an inch long. Frequently -a tuft of hairs guards its entrance. The wax secreted by its wall -serves to attach particles of dust, and to deter insects from entering -the tube. The air at the end of it is at a uniform temperature. It -is closed by the membrana tympani, or drum. This membrane receives -the vibrations of sound; and, in order that it may collect them with -absolute impartiality, it is in every respect the opposite in shape and -structure to the top of a drum. The stretched parchment which covers -a drum is flat. Its tension is uniform in all its parts. Movements -have the greatest amplitude at the centre. Every precaution is taken to -insure its emitting, with as little confusion as may be, the particular -note to which it is tuned. The drum of the ear is shaped like the -mouth of a trumpet, depressed to a point, but convex from this point -outwards. Its elastic fibres, which are partly radial, partly circular, -are at many different tensions. Its deepest part, to which the long arm -of the hammer-bone is attached, is not its centre. - -The “middle ear” is an irregular cavity communicating with the -pharynx by the Eustachian tube. It is filled with air at the same -pressure as the atmosphere. Except during the act of swallowing, when -it is at first shut tightly and then opened, the pharyngeal end of -the Eustachian tube is gently closed. When one is dropped in a lift -rapidly down the shaft of a mine, the difference in pressure between -the external air and the air in the middle ear stretches the drum to -such an extent that deafness to low tones is produced. Conversation -becomes inaudible. The deafness is remedied by swallowing saliva, -and thus opening the end of the Eustachian tube. The commonest cause -of permanent deafness is inflammation followed by thickening of the -mucous membrane of the lower end of the Eustachian tube, with its -consequent closure, due to frequent sore throats. The air in the middle -ear is slowly absorbed. It needs to be constantly renewed through the -Eustachian tube. - -On the inner wall of the middle ear are two small apertures—the -oval window and the round window. Both are closed with membrane. -Into the oval window is fitted the sole-plate of the stirrup-bone. -Three bones—hammer, anvil, and stirrup—combine in transferring the -movements of the membrana tympani to the oval window. They constitute a -jointed lever, which swings about an axis passing through the ligament -of the anvil (Fig. 38), the excursions of the long arm of the hammer -being reduced in amplitude by one-third at the stirrup-plate. As -the oval window has only one-twentieth of the area of the drum, the -movements of the latter are transmitted with concentrated force. Two -points in the mechanism of these bones may be specially noticed: (1) -The head of the hammer is free to rotate in the cavity of the anvil, -checked by a cog. Every inward movement of the drum is faithfully -transmitted to the oval window; but when the drum moves outwards, -the hammer does not necessarily carry the anvil with it. (2) A -muscle—tensor tympani—is inserted near the elbow of the long arm of -the hammer. When high notes are listened to its contraction tightens -the drum, rendering it more responsive to rapid vibrations. It has a -tonic action, but it does not make any special contraction for low -notes. - -Behind the two windows, within the solid bone, is the inner ear, -which our ancestors very aptly termed a “labyrinth.” It is filled -with fluid—perilymph—which is shaken by every movement of the -stirrup-plate. Since water is incompressible, no waves could be raised -in the perilymph were there no second aperture. Every vibration -conveyed by the stirrup-plate after passing through the labyrinth ends -as a vibration of the membrane which closes the round window. - -Nowhere does perilymph come in contact with auditory cells. All the -endings of the nerve of hearing are contained within a membranous -labyrinth which lies within the bony cavities. The way in which the -waves of the perilymph are dispersed over the surface of this closed -sac can be inferred from the diagram (Fig. 38). They sweep round the -utricle and saccule, are lost in the narrow spaces which surround the -semicircular canals, run up the scala vestibuli of the cochlea. The -course of the waves which traverse the cochlea is of especial interest -in connection with the physiology of hearing. - -The cochlea—snail-shell—is a spiral tunnel of three turns, -in hard bone, about an inch in length. A shelf of bone—lamina -spiralis—projects into the tunnel on its convex side. From the free -margin of this spiral lamina two membranes extend to the outer wall -of the tunnel—one firm, containing straight, stiff, and probably -elastic fibres which radiate outwards (the basilar membrane); the -other an extremely delicate film of connective tissue. The tunnel is -thus divided into three compartments, known as the scala vestibuli, -scala media, scala tympani. The scala media belongs to the membranous -labyrinth. Waves transmitted through perilymph pass, as we have already -explained, up the scala vestibuli. At the apex of the cochlea the -two scalæ are in communication; but the aperture is small, and it is -unlikely that waves reach the lower passage from the upper through -this opening. They pass through the thin membrane which roofs the -scala media, shake its endolymph, and reach the lower passage through -the basilar membrane. It is noteworthy that, since the round window at -the lower end of the scala tympani is, with the exception of the oval -window, the only opening of the bony labyrinth, all waves transmitted -through the oval window must travel part of the way or all the way up -and down the cochlea. - -[Illustration: FIG. 39.—A SECTION THROUGH THE AXIS OF THE COLUMN OF -THE COCHLEA. - - The spiral sheet of nerve-fibres which supplies the - organ of Corti is cut in eight places. If the - bundle to the lowest coil of the shell (on the left - side of the diagram) is followed, it will be seen - to bear ganglion-cells where it enters the bony - spiral lamina. This lamina divides the tube into - two canals—scala vestibuli above, scala tympani - below. From the edge of the lamina the membrane of - Corti stretches to the outer wall. Above the organ - of Corti is the membrana tectoria, and above this - a very thin membrane which cuts off the ductus - cochleæ from the scala vestibuli.] - -The organ of Corti is spread out on the basilar membrane. It is an -epithelial structure of extreme regularity and uniformity. Near to -the edge by which the basilar membrane is attached to the spiral -lamina rests a double row of rods of Corti, stiff pillars which lean -one towards the other, over the tunnel of Corti, the convex head of -the outer rod fitting into a concavity in the head of the inner one; -in some places one outer rod fits against two inner rods, as the -latter are rather the more numerous. On the inner side of the inner -rod is seen, in transverse sections a single plump cell filled with -cloudy protoplasm, and bearing on its free surface a tuft of very -short hairs. On the outer side of the outer rod are three or four -hair-cells, each with a cloudy outer segment containing the nucleus, -a granular middle segment, and a stiffish stalk, which attaches it to -the basilar membrane. Between the hair-cells are supporting cells, -thicker below, tapering above, containing in their substance a firm -fibre. Still farther to the outer side are epithelial cells, of no -special interest. The purpose of the rods of Corti and the supporting -cells is to give attachment and support to a reticulated membrane of -exquisite delicacy, through the oblong apertures of which the hairs -of the hair-cells project into the endolymph. The spiral lamina is -traversed by a vast number of fibres of the auditory nerve, which, -losing their medullary sheaths, pass across the tunnel of Corti as -naked axons, to end amongst the hair-cells. Above the organ of Corti, -attached by its edge to the spiral lamina, is a thick, gelatinous, -fibrillated structure—membrana tectoria—which rests as a coverlet on -the surface of the organ. It has been supposed that it serves to damp -the vibrations of the hairs after they have been set in motion by the -waves passing across the scala media; but it not impossibly plays a -more active part in hearing than this. - -[Illustration: FIG. 40.—ORGAN OF CORTI. - - The spiral lamina, on the left of the drawing, - gives attachment to the membrane of Corti, which - stretches to the opposite wall. Below the membrane - is a bloodvessel which runs its whole length - beneath the tunnel of Corti. The tunnel is formed - by pillars—the inner on the left, the outer on - the right—which meet above it. On the left of the - inner pillar is a hair-cell; to the left of this - a nerve-cell with two nuclei. To the right of the - outer pillar is a space; to the right of this four - hair-cells alternating with four supporting cells, - which hold up the reticulated membrane through - apertures in which the tufts of hairs project. - Three nerve-fibres are seen in the spiral lamina; - they cross the tunnel to ramify between the rows of - outer hair-cells. The lamina tectoria rests upon - the tufts of hairs.] - -The organ of Corti is, beyond doubt, the apparatus which analyses -sounds; but the problem of the way in which it responds to tones -of different pitch, or analyses compound tones, is not as yet even -approximately solved. To escape the acoustic difficulties which have -to be faced by anyone who endeavours to expound the theory of the -cochlea as a piece of analytical apparatus, various suggestions as -to the possibility of an action _en masse_ have been advanced. For -example, the basilar membrane has been compared to a telephone-plate -which takes up vibrations and transmits them through the auditory nerve -to the brain. But if the organ of Corti be the transmitter, there is -no ear in the brain to analyse the vibrations given out by a receiving -telephone-plate; and without a receiving plate and a listening ear a -telephone is purposeless. According to this hypothesis, the basilar -membrane vibrates as a whole, moving the hair-cells in various -“patterns”; the pressure of the hairs against the tectorial membrane -causing irritation of the cells which bear them, and hence producing -stimulation of various groups of nerves. Other pattern theories -are somewhat similar. But it is obvious that all hypotheses of the -vibration of the whole of the basilar membrane, or of large parts of -it, simultaneously, leave to the mind the responsibility of reading -the pattern which the impulses generated in the organ of Corti make in -the brain. It is conceivable that every fraction of a semitone which a -musician can discriminate, and every combination of tones which he can -analyse, is transmitted to the brain by a large number of co-operating -nerve-impulses; but such a theory involves a complexity of mental -associations difficult to contemplate. - -According to the general principles enunciated in this book, analysis -of stimuli is the function of sense-organs. It cannot in all cases -be compared with the analysis effected in a physical laboratory; nor -is this necessary; but it must be carried so far that nerve-impulses -which have no specific qualities apart from their source shall give -rise to effects in consciousness which have no basis other than the -topographical distribution of the said impulses in the brain. There -may be sensory impulses of different orders; there may be in the brain -psycho-physical substances which react to impulses of various orders in -various ways; but until we have some hint of the existence of specific -impulses and specific psycho-physical substances, we are not justified -in postulating their existence simply in order that we may escape from -physiological embarrassments. - -The organ of Corti has in the highest degree the appearance of a piece -of apparatus for the analysis of sound. If the basilar membrane, with -the cells which rest upon it, be cut out and laid flat, the suggestion -of some kind of instrument is very strong. It is a long narrow ribbon, -narrowest at the bottom of the spiral, increasing to about twice the -width at the apex. It is crossed by radiating fibres, presumably -elastic. The cells which rest upon it carry vibrating hairs, and -are supplied with nerves. The rods of Corti hold up the reticulated -membrane, which keeps the hair-cells in place. It is not to be wondered -at that when its structure was first discovered it was thought that the -problem of the analysis of musical tones was solved. If two pianos in -perfect tune are in the same room, when one is played the corresponding -wires of the other twang. Anyone who sings into a piano, whilst the -loud pedal raises the dampers, feels an increased fulness in his voice. -This is the familiar phenomenon of resonance. Why should not the fibres -of the basilar membrane resonate to the tones conveyed to the ear—the -shorter ones at the base of the cochlea to high tones, the longer ones -at the apex to low tones? This is the order in which we should expect -the pulsations of sound which ascend the scala vestibuli to be taken -up—the more rapid, near its commencement, the less rapid farther up -it. But an explanation of the physics of the selection of vibrations -of different frequencies by different sets of the elements which make -up the organ of Corti, if such selection occurs, is still to seek. In -the first place, the fibres of the basilar membrane are so exceedingly -short. What could a fibre less than 0·5 millimetre in length make of -the vibrations of a 36-foot organ-pipe? Even if this objection be -waived, as certain eminent physicists hold that it may be, there is not -a sufficient difference in length between the longest and the shortest -fibres to account for the great range of tones which we are able to -discriminate; nor is there any evidence that some fibres are more -tightly stretched than others. - -A further consideration which tempts physiologists to look upon the -organ of Corti (including the basilar membrane) as a series of -resonators is the somewhat remarkable agreement between the number of -separate pieces of apparatus of which it appears to be composed and -the number of different musical sounds which, if it were a series of -resonators, it might be called upon to discriminate. - -The squeak given by a bat at each turn in its flight has a pitch of -about 11,000 vibrations to the second—the sixth E above the middle -C (Tyndall). In a group of persons listening for the squeak there -are usually some who cannot hear it. Above this the range of hearing -is very variable. The suddenness of transition from perfect hearing -to total want of perception makes experiments with small pipes or -with a siren somewhat amusing, when a number of persons are tested -at the same time. One complains that the note is intolerably loud -and shrill, whilst others assert that there is perfect silence. -Thirty-three thousand vibrations is usually regarded as the upper limit -for the human ear, but certain physiologists place it at 40,000, or -even higher. The upper limit is of little consequence, since there -is very little power of discriminating rapidities above the highest -note used in music—the piccolo stop of the organ, with a pitch of -4,096. It is possible that a sound with a lower frequency than 27 -(the contra-bassoon) may be heard as a tone—16 according to certain -writers; but again our power of discriminating very low notes is small. -Over a certain range a skilled musician can tell that a note is out of -tune when it is one sixty-fourth of a semitone higher or lower than -it ought to be. If we assume that by allowing equal sensitiveness for -a range of seven octaves, the excess of the allowance over the actual -sensitiveness towards either end of this stretch would compensate for -the comparatively few distinctions which the ear can make either below -or above it—64 × 12 × 7 = 5,376. A much higher estimate, based upon -observations which seem to show that the ear can distinguish sounds -less than one sixty-fourth of a semitone apart, places the total number -at 11,000. - -On the assumption that one piece of apparatus is tuned to resonate -for every distinguishable sound, between 5,000 and 11,000 pieces of -apparatus would be required. Taking one of Corti’s arches as the -centre-piece of the resonator, although the rods are certainly not -vibratile structures, we find the number to be 3,848 (the number of -the outer rods); if either rod with a hair-cell, or hair-cells, is the -analytical element, 9,438. Counting gives 3,487 inner, 11,700 outer, -hair-cells. The fibres of the basilar membrane are estimated at 24,000; -the fibres of the cochlear nerve at 14,000. It will be understood that -the counting of structures as minute as these yields results which -cannot be more than approximately accurate. Helmholtz, assuming that -each arc of Corti indicates an analytical element, accounted for the -apparent deficiency in their number by assuming that a tone of which -the pitch fell between two arches set both in sympathetic vibration, -the arch which was nearest in pitch to the tone vibrating the more -strongly. In this way he anticipated an objection which has often been -brought against his theory of a long series of resonators. - -In opposition to Helmholtz’s theory it is pointed out that when a -violinist runs his finger up a bowed string, the pitch rises with -perfect smoothness; it does not bump along from resonator to resonator. -Especially in the case of very high tones given out by a siren, it is -urged that at the rare intervals at which a resonator in the ear is -tuned for the tone which the siren is emitting it should sound much -louder than when the tone falls midway between two resonators. But the -whole question of the nature of the response of the analytical elements -is too obscure at present for the discussion of points so nice as this. - -Many who think that Helmholtz’s theory of resonators is based upon -principles of physics and of physiology which must be regarded as -the starting-points of any explanation of the analysis of sounds by -the ear and the mind, hold that it goes too far in searching for a -separate resonator for every distinguishable tone. The cochlea, as we -have already said, does not offer anything like so extensive a choice -as this, if regard be had to the tension or length of its elements, -and not to their numbers. Those who accept it as an axiom that the -cochlea contains a series of responding instruments—but a series far -more limited in range than the gamut of our sound-perceptions—seek -to discover in musical tones qualities which unite them in groups. -Just as in the case of colour-sensations they recognize four (or six) -elementary qualities which excite four (or six) pieces of responding -apparatus, so also in the case of hearing they seek for a limited -number of tone-qualities and a correspondingly limited number of -elementary sensations. The ideal of those who take this view is an -octave of qualities and of elementary sensations sounded in the middle -of the scale when _x_ nerve-endings are stimulated, as the octave -above when 2_x_ nerves respond, the octave below with _x_/2. Such a -conception seems to guide thought round insurmountable barriers. There -is, however, a risk of making too much of the periodic intervals, -because they take so important a place in music. At one side of the gap -which sound bridges between the individual and his environment is an -elastic body shaking at any possible rate within the range of hearing. -At the other side of the gap is the ear. If, having arranged several -thousands of stones along the side of the road in order of size, I -were to state, picking up No. 512, “This is the fundamental of which -No. 1,024 is the octave,” answer would be made to me: “It may be that -the larger could be broken into halves, each as heavy as the smaller -stone; but I recognize no difference between the stones in shape, -colour, or hardness.” A vibrating string divides into equal segments, -each of which vibrates within the vibrations of the whole string, -sounding the octave. We recognize a similarity in quality between -tones and their octaves because we are accustomed to hear the octave, -the most prominent of overtones, in all musical sounds. Hence, from -association, it has become more difficult to distinguish a note from -its octave than it is to distinguish it from its fifth; but it does -not follow that the effect of 1,024 vibrations upon the sensory cells -more nearly resembles the effect of 512 than does that of 768. But at -this point we are compelled to construct some hypothesis as to the way -in which the vibrations affect the sensory cells. The protoplasm of -the cells is not directly sensitive to them. We can account for the -generation of impulses in the nerve connected with a particular cell, -or group of cells, only on the supposition that a resonating mechanism -which responds to vibrations of a certain frequency shakes the cell. -Even then it seems necessary to suppose that there is an accessory -mechanism which disturbs the cell-protoplasm sufficiently to render -the shake effective, probably the hairs rubbing against the tectorial -membrane. Anatomical study gives us no confidence in the theory of the -existence of several thousands of resonators tuned to as many notes -of different pitch. It remains for the physicists to say whether or -not we may picture one of these minute resonators as responding to a -given note in 10 separate octaves, another in 9 ... another in only 1. -The physicists, on their part, may very properly ask the anatomists -to point out the resonators, and even to reproduce them in models of -dimensions which allow of experimental investigation. - -It is generally agreed that the sensation of a chord is compounded of -the sensations to which each of its constituent tones gives rise, and -that our power of analysing the compound is a question of attention. A -musician can direct his attention to either sensation at will. It is -not equally certain that a person who has no knowledge of music can -do the same. Familiarity with musical instruments gives us so exact -a knowledge of the way in which compound tones are produced that it -becomes a difficult matter to decide whether, when we say that we can -pick out the E or the G of the common chord, it means that we can hear -it as distinct from =C= and C′, or whether it means that, knowing the -constitution of the chord, we think about the E or the G when we hear -the compound tone, to the exclusion of its other constituents. Then, -again, the several strings which we try to strike simultaneously do not -actually “toe the line.” Their vibrations are not in the same phase, -even though the strings be in absolute tune. Discrepancy of phase may -favour the singling out of the several constituents of the chord. There -we touch upon a problem which we passed over in silence when attempting -to give an idea of the nature of the pulsations which reach the ear. We -then (p. 405) described the partial pulsations which are superimposed -upon the main pulsation as if they necessarily started simultaneously -with it. We assumed that the phase difference of the partials was -zero. But it is clear that differences of phase of its constituent -tones may produce an almost infinite number of variations in the -form of a compound “wave” of sound. Is the ear variously affected by -different forms of wave? Does difference of phase result in difference -of sensation? In broad terms, the answer to this question must be in -the negative; although it can be shown that in certain cases a change -in phase of the several constituents of a compound tone, without any -alteration in their number or their loudness, makes a change in its -acoustic quality. Any attempt to correlate physical changes—the -movements of air in the outer ear—with the effects which they may be -supposed to have upon the organ of Corti must take into account this -wide range of variation of wave-form. We have called attention to the -difficulties which it introduces; but have no hope of indicating the -way in which they may be overcome. - -Nothing connected with the physiology of the sense of hearing is -more remarkable than its capacity for education. The cochlea of one -human being is as extensive and as elaborate in structure as that of -another, yet some men can make an infinitely more refined use of it -as an analytical apparatus than can others. A native of the Torres -Straits cannot distinguish as two separate notes sounds which are less -than a semitone apart. Sir Michael Costa could distinguish sounds into -the sixty-fourth parts of semitones. The cochlea of a cat is not less -elaborate than that of a man, yet Man’s mental life is based upon the -analysis of auditory sensations. His supreme advance in the animal -scale has depended upon the invention of language, by means of which -he communicates and receives information, thus rendering experience -eternal, notwithstanding the transience of the individuals who acquire -and transmit it. An animal is born, finds out, dies. A man starts with -the wisdom of the race beneath his feet. - -Hearing has a nebulous origin in sensations of movement or -displacement. The connection between the two special senses—the sense -of orientation and the sense of hearing, properly so-called—remains -always intimate. David danced before the Ark of the Lord. All people, -savage and civilized, associate music with movement. High in the -animal scale appears the sense-organ which enables its possessor to -discriminate musical tones. By its use Man has developed with great -rapidity—as secular time is reckoned—an intelligence which removes -him from all other animals a planet’s space. The sounding of his organ -of Corti by pure tones and combinations of pure tones gives him extreme -pleasure, although it in no way ministers to his intelligence. Yet -there is in the enjoyment of music a quality of pleasure which makes -it near akin to the satisfaction which we experience in exercising the -intellect. - - - - -CHAPTER XV - -SKIN-SENSATIONS - - -The senses, according to a time-honoured classification, are five -in number—smell, sight, taste, hearing, and common sensation, or -touch; but such a classification of our sensations and of the organs -which originate them is too crude for modern needs. Already we have -shown that, whereas the nose and the tongue afford the same kind of -information, the ear affords information of two, perhaps of three, -different kinds. Within the realm of common sensation we pick out three -special senses served by specialized sense-organs—touch, cold and -heat—and, possibly, a fourth, served by non-specialized nerves, to -which alone the epithet “common” properly applies. - -The skin is supplied with nerves—naked fibrils—in the richest -abundance. They are most easily demonstrated in the layer which covers -the cornea, thanks to its transparency; in this, as shown in Fig. 41, -having branched on the front of the fibrous tissue of which the cornea -is composed, the nerves pass towards the surface, forming connections -with every one of its cells, or, at any rate, with every cell of the -more superficial of the three or four layers of which the epithelium -is made up. Ramified nerve-twigs of this type do not, under ordinary -conditions, convey any sensations to consciousness. So long as the -skin-cells with which they are connected are healthy, the nerve-twigs -establish for them connections with the central nervous system by which -their nutrition is regulated; but they carry no impulses to which we -can direct attention. The movement of blinking is accompanied by no -sensation until the edges of the eyelids come in contact. A pencil -pressed against the lid evokes touch-sensations from the skin, but -none from the cornea which underlies it. When a tiny beetle injures -the surface of the cornea by scratching the epithelial cells with -its horny wings and legs, the ruptured nerve-filaments convey to -consciousness impulses, or, as we prefer to express it, an influence -which is felt as pain. But even the pain caused by injury to the cornea -is trifling as compared with that which originates in the under-sides -of the lids, where not only is the epithelium supplied with branching -nerve-twigs, but specialized organs of touch are present to localize -the seat of injury. Everywhere the epithelium covering the surface -of the body is so abundantly supplied that a successful staining of -nerve-filaments induces one to think that every epithelial cell has -its nervous affiliation. These are the nerves of common sensation, if -we retain the term; but sensation so common, so obscure, so little -differentiated that we know no more about it than we know about the air -which envelops our hands and faces on a warm, windless day. Yet the -air, when it moves, gives rise to a dim, broad, generalized sensation, -which may be focussed into definiteness by a sensitive nerve. - -[Illustration: FIG. 41.—VERTICAL SECTION OF THE EPITHELIUM WHICH -COVERS THE SURFACE OF THE CORNEA, AND OF A SMALL PORTION OF THE CORNEAL -SUBSTANCE, HIGHLY MAGNIFIED. - - The black lines are naked nerve-fibres (stained with - chloride of gold), which are distributed amongst - the cells of the more superficial strata of the - epithelium in very great abundance. The corneal - substance is composed of sheets of transparent - fibres with intervening cells. As the fibres of the - several sheets cross one another at various angles, - they are cut, some transversely, others in the - direction of their length.] - -An observer who has devoted himself for many years to the investigation -of skin-sensations, and especially of the “referred pains” which are -due to diseases of the viscera, recently caused the large cutaneous -nerve which supplies the thumb side of the forearm and hand to be cut -in his own arm, in order that he might study carefully the revival -of sensations. He found that he never lost his ability to recognize -displacements of the tissues beneath the skin. Pacinian bodies and -other end-organs of deep-lying nerves recorded pressure and tension -caused by pushing or rubbing with a blunt instrument. Seven weeks after -the injury he began to recognize stimuli that do harm—hot things, -cold things, pricking with a pin—although his power of localizing -the spot injured was extremely vague. In seven weeks, that is to say, -the protopathic nerves, which do not follow the same definite lines -as the nerves of the special senses, but form open networks with many -alternative paths, had re-established their skin connections. Only -gradually and very slowly did critical sensations return—the ability -to distinguish degrees of warmth, to recognize as separate two points -of a pair of compasses, to feel a touch with cotton-wool. - -According to a theory set forth in this book (p. 312), pain is not a -set of sensations, but a condition of the central nervous system which -renders it unduly excitable, or excitable in a particular manner, to -impulses which have the same local origin as the nerve-current which -sets up the condition of pain. When a nerve of the skin has been cut, -the epithelial ramifications are renewed before any specialized tactile -or other sense-organs have regained their nervous connections. When -the area which has regained its surface ramifications, but has not -regained its sense-organs, is injured, no localization of pain results. -Indeed, the obscure sensations which are then experienced if the skin -be injured can hardly be described as painful. The ramified nerves -pour their agitation into the grey matter of the spinal cord; but it -is not the agitation _per se_ which causes pain. It is the passage of -impulses through the agitated area that gives to them, when they reach -consciousness, not only a topographical meaning, but also a distressful -feeling. Until the specialized organs of the skin have been restored to -working order, there are no impulses to pass through the agitated grey -matter, and therefore no feelings of pain. According to this view there -are two systems of afferent nerves, the protopathic and the specialized -or critical. The former is very widely and very abundantly distributed -to the surface of the body, the lungs, the alimentary canal, and other -viscera. It has no end-organs, no defined tracts in the central nervous -system, no definite connections with the cortex of the great brain. -The currents which it conducts, if they originate in the visceral part -of this system, have no direct effect in consciousness; but if they -originate on the surface of the body, or in the alimentary canal at -the lower end of the œsophagus, or in certain other situations, they -co-operate with stimuli of heat, cold, or traction. The critical system -works in a more definite way. Its impulses originate in sense-organs. -Starting with a certain potential, they are transmitted by the -discharge of a succession of linked neurones. When they reach the -cortex their potential is sufficiently high to evoke consciousness. -Their distribution in the cortex is as definite as their origin. - -Specialized sense-organs are necessary for the origin of all -sensations. Within the epithelium are certain cells which look as -if they were specialized for sensory purposes. The deeper sheet, or -derma, of the skin is abundantly provided with structures in which -nerves end in the most elaborate and complicated ways (Fig. 42). They -are found especially in the papillæ of connective tissue, which, set -in rows, form the ridges that one can see at the finger-tips and in -various other situations. All of these organs are made up of groups -of epithelial cells which, displaced from the epidermis, have sunk -into the derma, with the nerves connected with them. In their further -development the nervous part of the apparatus is complicated by -branching, the branches being thickened and usually flattened into -ribbons, which lie on the external surfaces of the cells or between -them. A more or less marked capsule is provided for the organ by -condensation of connective tissue. - -Anyone can convince himself that the skin is not uniformly sensitive. -He may test it first for the minimal stimulus which excites a sensation -of touch. With a hair of the head—it must not be a very fine one—cut -across with scissors, and held between finger and thumb at the right -distance from the cut end, the skin of the palm of the hand is prodded. -Every here and there a spot is found which is insensitive to so slight -a pressure. These spots are neither large nor very close together. -If the hairless skin of the arm between the elbow and the armpit -be investigated in the same way, much larger blank areas are met -with—oval patches more than ¼ inch in diameter. When a hairy surface -is tested, it is found that contact with a hair can always be felt; and -when the hairs are shaved, the touch-spots are found to extend around -or from the points at which hairs pierce the epidermis. Touchless areas -lie between them. Hair-follicles receive tufts of nerve-filaments, and -it appears that they are the chief organs of touch. “Touch-corpuscles,” -which are found in great numbers in the papillæ of the skin of the -fingers and elsewhere, may probably be regarded as, genetically, -hair-follicles which have not developed hairs. - -[Illustration: FIG. 42.—SENSE-ORGANS SUSCEPTIBLE TO PRESSURE. - - All are formed on essentially the same plan; a - fibrous capsule invests a group of epithelial - cells amongst which a nerve ramifies. The simplest - form is known as a Grandry’s corpuscle-a nerve - ending in one or two plates between two or three - epithelial cells. These organs are found in great - numbers in the bills of aquatic birds. If a duck - is watched whilst it is gobbling mud at the margin - of a pond, it will be seen to have a remarkable - capacity for discriminating between the shells of - small snails, which it can crush, and stones, which - it needs to drop from its bill. Its bill is also - provided with small Pacinian corpuscles (Fig. 43). - Touch-corpuscles, more elaborate in form than the - one figured, are found in the papillæ of the skin - of the fingers and elsewhere. They appear to be - modified hair-follicles. End-bulbs occur in the - conjunctiva and elsewhere, and especially in the - peritoneum. Together with Pacinian corpuscles, they - are accountable for sensations connected with the - distension of the stomach and intestines.] - -If sensitiveness to pain is investigated by tapping very gently with a -needle—or, better, by using a stiff horsehair fixed in a cleft stick, -from which it projects about ¼ inch—it will be found that every here -and there are spots which are exceedingly sensitive, whilst adjoining -them are areas which are moderately sensitive, and between these -areas small spots or stretches of skin which do not give the smarting -sensation even though the horsehair be pushed until it doubles up. - -[Illustration: FIG. 43.—PACINIAN CORPUSCLE. - - These organs are especially numerous in the - neighbourhood of tendons and ligaments. They - are also present beneath the skin of the hands - and feet. Their capsules are formed of a great - number of concentric lamellæ of connective tissue, - enclosing lymph-spaces. Within the capsule is a - core of finely granular substance, which also shows - a tendency to a lamellar disposition. The structure - of these relatively large sense-organs is highly - suggestive of sensitiveness to pressure, traction, - or rubbing.] - -Testing now for sensitiveness to cold with a cold blunt metal point, -“cold-spots” can be mapped on the skin. If the metal is warmed to about -50° C., “heat-spots” are found. The different kinds of spot are very -irregularly distributed. They may coincide, or overlap, or leave blank -spaces. Their relative abundance varies. In some regions touch-spots, -in others cold-spots, in others heat-spots, are more closely grouped. -The tongue and the hand, and especially the tips of the fingers, are -most sensitive to touch; but whereas the tongue is also exceedingly -sensitive to warmth, the hands are relatively insensitive. Yet, -speaking generally, parts especially sensitive to touch are little -sensitive to temperature, and _vice versa_. Sensitiveness to cold is -much more widespread than sensitiveness to heat. It is concentrated in -the skin covering the abdominal viscera. A cold douche directed between -the shoulders is doubtfully felt as cold. There is no doubt whatever -about it when it strikes the skin over the stomach. - -From these observations it appears that the skin contains three -sets of organs sensitive respectively to touch, cold, and heat. -Certain investigators hold that it also contains specific organs, or -nerve-endings, sensitive to painful stimulants; but in this case there -is the obvious difficulty of distinguishing between pain and touch. At -no spot can pure pain be evoked free from any consciousness of touch. - -To a certain extent the combinations of epithelial cells and -nerve-endings in the skin fulfil the negative requirement of -sense-organs; each kind, whilst specially sensitive to its own specific -stimulant, is insensitive to stimulants of other kinds. But mutual -exclusion is not absolute in the case of cold and warmth. If a warmed -metal point be applied to a cold spot, it produces a sensation of cold. -Our feelings of warmth and cold are to a large degree comparative. -Luke-warm water feels cold to hands just taken out of hot water; -moderately cold water appears luke-warm to hands that have been in -contact with ice. The sensory apparatus for cold and heat soon adapts -itself, or, in physiological language, it is soon fatigued. If after a -prolonged bath at the body temperature a foot be plunged into very hot -water and withdrawn quickly, the feeling which first ensues is one of -cold. It is indistinguishable from the feeling provoked by dipping the -foot into cold water. The sensation of cold subsequently gives place -to one of painful warmth. This does not indicate that the heat-spots -have been waked out of their lethargy by excessive stimulation. On the -contrary, it is the cold-spots which, when they were first stimulated -by the very hot water, answered “Cold,” that now cry out “Hot”; for -both cold-spots and heat-spots, when strongly stimulated, yield the -same sensation. Indeed, it appears that the mind relies upon the -simultaneous stimulation of adjacent heat-spots and cold-spots for the -assurance that the thing with which the skin is in contact is really -hot. If two metal points, one kept warm and the other cold, are applied -simultaneously to two closely adjacent spots of skin, the resulting -sensation is “hot.” When the cold point is withdrawn, or replaced by a -second warm point, the sensation sinks to “warm.” - - - - -CHAPTER XVI - -VOICE AND SPEECH - - -A cut carried horizontally backwards across the cartilage which -projects forwards as Adam’s apple, a quarter of an inch below its -notch, would show that it is =V=-shaped, the point of the =V= in front. -Each limb of the =V= is a broad plate. In the mid-line is a gap, the -rima glottidis, through which the windpipe communicates with the -pharynx (Fig. 45). It is overhung by the stiff leaf-shaped epiglottis, -the edge of which can be felt with the finger behind the tongue. -(γλωττίς, the mouthpiece of a reed-pipe, is the term commonly used, for -short, for the rima glottidis.) When air is being drawn into the lungs, -the glottis is widely open. In speaking or singing it is almost closed. -It is tightly shut whilst food is passing down the gullet. - -The glottis is bounded, as to its anterior two-thirds, by two -membranous folds, the vocal cords. In its posterior third it has a -triangular cartilage, the arytenoid, on either side. A distinction is -sometimes drawn between the anterior part, bounded by the vocal cords, -and the whole glottis, the former being termed “rima vocalis”; but it -is scarcely justified, for, although it is true that the anterior part -is essentially the organ of voice, and its margins alone vibrate when -high notes are sung, the anterior ends of the arytenoid cartilages -also vibrate during the production of low notes. (The substance of -these processes is not, properly speaking, cartilage; it resembles -the epiglottis in containing a great abundance of elastic fibres.) -And here we must warn the reader not to picture to himself a vocal -“cord” as a kind of fiddle-string. It bears no resemblance to a cord, -as we ordinarily understand the word; it is but a fold of mucous -membrane, such as one might pinch up between finger and thumb from the -inner side of the cheek. Its capacity for vibration depends upon the -tenseness which is given to it by the pressure of the lymph with which -it is distended, and vast numbers of exceedingly slender elastic fibres -which traverse it. - -[Illustration: FIG. 44.—THE ANTERIOR HALF OF THE LARYNX SEEN FROM -BEHIND. - - The drawing shows the folds of mucous membrane, - the vocal cords, which stretch from the tips of - the arytenoid cartilages to the recess behind - the median portion of the thyroid cartilage. To - the outer side of each vocal cord is seen the - thyro-arytenoid muscle (cut across), consisting - of a broad outer portion, chiefly concerned in - closing the glottis during the act of swallowing, - and a smaller internal portion, which regulates the - length and the thickness of the segment of the cord - allowed to vibrate.] - -[Illustration: FIG. 45.—THE APERTURE OF THE GLOTTIS SEEN FROM ABOVE. - - The leaf-like structure in front of it is the - epiglottis; the two triangular structures at the - back, the arytenoid cartilages; the white bands - on either side, the vocal cords. A, The glottis - is widely open during inspiration. Arrows show - the lines of action of the muscles which rotate, - and approximate, the cartilages. Attached to - their outer angles, and pulling these angles - forwards, the lateral crico-arytenoid muscles; - pulling them backwards and inwards, the posterior - crico-arytenoid muscles. Drawing the cartilages - together, the arytenoid muscles. B, The glottis - during speaking in a deep chest-voice, or when a - low note of the lower register is being sung. C, - During the production of a high note of the lower - register. D, During the production of a note of the - head-register. E, During the act of swallowing; - the arytenoid cartilages are drawn towards the - epiglottis the aperture is folded into a =T=; - the pharynx (the tube behind the glottis) is - distended.] - -The first cartilage below the thyroid—it may be felt with the -finger—is termed “cricoid” (κρίκος, a ring), from its resemblance to -a signet-ring. Narrow in front, its large signet projects upwards, -within the =V= of the thyroid, behind, and on the top of the signet -rest the two arytenoids. Each arytenoid is a triangular pyramid, -its anterior, external, and upper angles prolonged into processes. -It is united with the cricoid by a swivel joint, which allows its -anterior process to swing inwards or outwards under the influence of -two antagonistic muscles attached to its outer angle—the lateral -and posterior crico-arytenoids. Another muscle attached only to the -arytenoids draws them together. Still another muscle—or two muscles, -for it is in two separate bands—unites the anterior process of the -arytenoid with the back surface of the thyroid just on the outer side -of the attachment into that cartilage of the vocal cord. The internal -thyro-arytenoid muscle is a comparatively narrow band; the external -thyro-arytenoid muscle is thick and broad.[3] By the simultaneous -contraction of the encircling muscles the larynx is closely squeezed -together, the anterior portion of the slit forming a =T=, with the -transverse limb in front. This occurs only in swallowing. Under the -co-operating contractions of the several muscles, the glottis assumes a -variety of shapes. The external crico-arytenoids rotate the anterior -angles of the arytenoid cartilages inwards (Fig. 45, A). If at the same -time the arytenoid muscle draws the cartilages together, the glottis is -reduced to a slit (Fig. 45, C). The posterior crico-arytenoid muscles -rotate the cartilages outwards. If the arytenoid muscle is at the same -time relaxed, the glottis gapes to its fullest extent (Fig. 45, A). The -freer the opening, the less is the resistance to the blast of air, the -gentler the vibrations of the cords, the lower the voice. The closer -the slit, the greater is the resistance which the air in the windpipe -has to overcome in passing through it, and consequently the more ample -the vibrations into which it throws the vocal cords. - -The vocal cords are the tongues of a reed-pipe, which, commencing -in the chest at the point where the great bronchi join to form the -windpipe, comprises the larynx, and, above the larynx, the complicated -chambers of the throat, mouth, and nasal cavities, including the spaces -within the bones of the head which open out of them. The pitch of the -voice depends upon (1) the length of the vocal cords, and (2) their -tension. The first factor is fixed for every individual. The voice is -base, baritone, tenor, in a man; contralto, mezzo-soprano, soprano, -in a woman—in proportion as the cords are long, of medium length, or -short. A man’s vocal cords measure, on the average, 15 millimetres, a -woman’s 11 millimetres. When a boy is from twelve to fifteen years of -age his vocal cords double in length, and the “breaking” of the voice -occurs as he gives up trying to get high notes out of his longer cords, -and allows them to produce manly tones of an octave lower. - -The lower posterior angles of the thyroid cartilages articulate with -the cricoid. If the four cartilages are freed from all soft tissues -without disturbing the thyro-cricoid, or crico-arytenoid joints, and -if, while the thyroid is held in one hand, a finger of the other is -placed on the front of the cricoid, it will be found that as this is -depressed the arytenoid cartilages which rest upon its signet are -tilted upwards and forwards within the thyroid; as it is raised, -they are tilted away from it. In life this movement is effected by -a muscle—the crico-thyroid (Fig. 46)—attached to the front of the -cricoid cartilage and to the under border of the lateral plate of the -thyroid. This is the muscle of supreme importance in the production of -the voice. The thyroid cartilage is slung in a fixed position by the -hyoid bone (to be felt in the neck above it). The crico-thyroid muscle, -being unable to depress the thyroid, raises the front of the cricoid -cartilage, tilts back the arytenoids, tightens the vocal cords. As the -voice ascends the scale, the tension of the cords is progressively -increased, and their vibrations rendered proportionately more rapid. -The range of the human voice is about three and a half octaves; of -individual voices about two octaves; if the shrill cry of a baby, -which may reach the third G above the middle C, or even higher (E⁗ or -F⁗), be excluded. Exceptional voices have a range far greater than two -octaves. Falsetto voice is produced by throwing half of the vocal cord -out of vibration (the way in which this is accomplished is not clear), -and at the same time raising the back of the tongue to the wall of the -throat in such a manner as to cut off all the lower part of the upper -resonating chamber, leaving it only the mouth and the cavities of the -nose. - -[Illustration: FIG. 46.—THE LARYNX FROM THE RIGHT SIDE. - - From above downwards: the hyoid bone, thyro-hyoid - membrane, thyroid cartilage, cricoid cartilage, - trachea. The upper and posterior angle of the - wing of the thyroid cartilage is suspended from - the hyoid bone; its lower and posterior angle - articulated with the cricoid cartilage. On the - summit of the cricoid cartilage it articulates the - arytenoid. Dotted lines indicate the position of - the vocal cord. The crico-thyroid muscle, which - raises the front of the cricoid, tilting the - arytenoid cartilage backwards and tightening the - vocal cord, extends, fan-like, from the front of - the cricoid to the lower border of the wing of the - thyroid.] - -So far the mechanism of voice is easily understood. As the scale -is ascended, the vocal cords are progressively tightened by the -contraction of the crico-thyroid muscles. But an analysis of the -feelings experienced during singing (and of the quality of the sounds -produced) shows that by themselves these muscles are not able to make -changes in the tension of the cords sufficient to account for the full -range of the voice. Or, put in another way, the tension of the vocal -cords is not altered to the extent which would be necessary if upon it -alone depended a range of from two to three octaves. It is obvious that -by some means the length or thickness, or both, of the portions of the -cords vibrating is changed as the scale is ascended. If commencement be -made on a low note, a point is reached, after a certain number of notes -have been sung, at which a sudden change occurs. There is an alteration -in the quality of sound, the more marked, the less well trained the -singer. The singer experiences a feeling of relief. If a finger be -placed on his crico-thyroid muscle, a relaxation of its anterior fibres -can be detected. As he proceeds up the scale, these fibres again -tighten. At a certain point there is again a change in the quality of -voice, and in the feelings which accompany its production. The two -points at which change occurs are said to divide the voice into three -“registers”—the lower, or chest-register, the middle, and the upper, -or head-register. A great effort is needed to hold either register -above its natural range. - -The physiology of the registers is a subject far too thorny for -handling in this book. The larynx can be watched with the laryngoscope -during the production of notes of different pitch, but observers are -not in accord regarding the appearances which it presents, or their -interpretation. The possibilities of changing the reed which vibrates, -the vocal cord, otherwise than by increasing the direct pull upon it -exerted by the crico-thyroid muscle, appear to be as follows: (1) -During the production of the lowest notes the elastic portion of the -arytenoid cartilage may be included with the cord. It may be thrown out -of vibration by its rotation inwards (under the action of the lateral -crico-arytenoid muscle) until it is pressed against its fellow. (2) -Certain portions of the cord may be damped by partial contractions of -the internal thyro-arytenoid muscle. It has been frequently stated, -although the statement is not accepted by all anatomists, that some of -the fibres which take origin from the arytenoid cartilage end in the -cord, instead of passing right through to the thyroid. It is supposed -that by their contraction they throw the posterior portion of the -cord—even, it is asserted, as much as its posterior two-thirds in the -higher head-notes—out of vibration. (3) It appears that the width -(thickness) of the cord vibrating is also regulated by the contraction -of the thyro-arytenoid muscle. Those who regard the diminution in -the thickness and width of the vibrating fold of mucous membrane and -underlying elastic tissue as the chief factor in the adaptation of -the larynx for the middle register lay great stress upon the sense of -relief from muscular effort which accompanies the transition. Less -force is needed to tighten the thinner cord. They also call attention -to the loss in volume of the voice when the lower register is left, and -to its greater softness. The lower is spoken of as the thick register, -the middle as thin, and the upper (on the hypothesis that part only of -the cord vibrates) as the small register. - -Singing reveals the possibilities of the larynx as a musical -instrument. In =speech= the larynx plays a part, but the form of the -syllabic sounds and the relative prominence of overtones in the vowels -is of more importance than pitch. Flexibility of voice is dependent -upon ability to increase or diminish at will the size of the resonating -chambers of the throat, mouth, and nose, or the freedom of access -to them. Conversation is carried on in the lower or chest-register. -When a practised speaker mounts a platform, he spends the first few -minutes in ascertaining the pitch of the hall—that is to say, the -pitch of his voice to which the room resonates most freely. Having -found the proper tone, he endeavours to maintain a uniform tension -of his vocal cords, and therefore a uniform pitch. He relieves the -monotony of speech by suitable variations of its overtones. Nothing is -more uncomfortable to listen to than an oration delivered in cadences. -The speaking voice should be full, round, and musical, and free from -affectation—as guiltless of the intoning or preaching quality as it -is of harshness or of vulgar flatness. A flexible voice is capable of -producing, as occasion calls for them, tones of any and every quality. -With the throat and mouth set for the syllable “haw,” it is impossible -to do justice to such words as “king” and “queen.” The voice-tones -of a superior person are as distasteful to the hearer as those of a -vulgarian. Unpleasant also is a nasal twang, illogically so called, -since it is due, not to the opening of the resonating chambers of the -nose, but to the restriction of the entry of air into them. In this it -is somewhat similar to the effect produced by a severe cold. Resonance -in the nasal chambers produces a clear, ringing voice. - -A little consideration of the varying qualities of different -voices suffices to show how largely they depend on resonance. When -vowel-sounds are analysed, it is found that the distinctive character -of each of them is dependent upon the overtones which it contains. For -every vowel the overtones are fixed, or very nearly so, no matter what -may be the pitch of the note to which the vowel is sounded. - -It is much to be regretted that the alphabet was settled before the -physiology of speech was understood. Were it based upon reasonable -principles, children would be spared the bewilderment which overtakes -them when they endeavour to establish in their minds some kind of -relation between the names of consonants and their effects upon the -blast of air as it passes through throat and mouth, and between tongue -and palate, teeth and lips. The vowels, had physiologists defined them, -would have been real pure vowel-tones—_ōō_, _o_, _ah_, _ēē_—sounds -which can be sustained for an indefinite time, and allowed to die -away without deterioration in their quality. _A_ (_é_ as pronounced -in France) is doubtfully pure—it has a tendency to tail off in -_ēē_; _ī_ is frankly a diphthong, _ai_ (_ah-ēē_). Try to hold a long -final note on the syllable “nigh”! An international standard of -vowel-sounds would have been fixed, by giving the vibrating periods of -the tuning-forks for which in each several case the resonating chambers -are shaped, and defining the relative accentuation of each overtone. -Greatest boon of all, the irruption of the Essex dialect would have -been dammed. It would not have been allowed to inundate London, or to -submerge Australia, debasing our English tongue. In Cockney speech -vowels degenerate down the line of greatest indolence. _Aw_ becomes -_or_, or _ar_; _a_ becomes _i_. It requires a greater effort to -pronounce a full _a_ than a flat _a_, a definite flat _a_ than _i_. -And worse than a Cockney’s unwillingness to take the trouble necessary -for the production of dignifiedvowel-tones is his reluctance to make -the effort required for the holding of any tone. In his mouth virile, -self-reliant vowels are replaced by emasculated diphthongs, which -collapse as they present themselves to the ear. It costs trouble to fix -the mouth-chamber before a vowel is sounded and to hold it steady until -it is finished. _Ah_ slides down through _ai_ to _ēē_; _i_ slips into -_ēē_. “Cow” becomes _kyow_; “you,” _ye-u-ow_; “cart,” _kyart_. And just -as the effort needed for the filling of the vowels is shirked, so also -is grudged the expenditure of an accessory blast for their aspiration. - -When a vowel is whispered, although the vocal cords do not vibrate, the -blast passing through the resonating chambers produces the overtones -characteristic of the vowel. Anyone who feels his own larynx while he -sings, to the same note, the various vowels between _ōō_ and _ēē_—he -may please himself as to the number of _ai_, _eu_, and _ŭ_ vowels he -interposes between these two extremes—will recognize that it is pulled -farther and farther upwards by the muscles which surround it. The -cavity of the mouth is at the same time made shorter and broader for -each succeeding vowel. Singing the several vowels before a piano, and -at the same time striking various keys, it is felt in the mouth that -the resonance of that chamber is reinforced by certain selected notes. -Certain tuning-forks, when sounded in front of the mouth shaped for -a vowel, ring out more loudly, because the mouth-cavity resonates to -their prime tones. The overtones of the vowels can be analysed in this -way. Conversely, by sounding simultaneously an appropriate selection -of tuning-forks, each with the right degree of force, the overtones of -a vowel can be synthesised. Thus if whilst one tuning-fork is sounding -B₁♭ (B♭ above middle C), two others be added giving B₂♭ (loud) and -F₃ (soft), the composite sound resembles the vowel _o_. If to these -same three forks, with F₃ sounding more strongly, B₃♭ and a loud D₄ be -added, the sound changes to _ah_. - -The organ of voice is a combination of a reed-pipe with resonating -chambers, the shape of which can be changed at will. The quality -characteristic of a vowel is given to it by adding to the note produced -in the larynx sounds due to the resonance of the throat and mouth. -On the assumption (not allowed by all authorities) that, since the -resonating chambers are not sound-producers, they can only add to the -larynx-tone, as “formants” of a vowel, its own harmonics—sounds which -they have picked out of it—it follows that, if, when the prime is -changed, the resonators were not adapted to the new note, they would -be dumb. If this attitude in regard to the question be justified, -there must be a certain amount of variation in the quality of a vowel -as the scale is ascended. But a vowel is not a musical tone; it is -a conventional sound. Its whole value depends upon its retaining, -as nearly as may be, the same quality, whatever be the pitch of its -prime tone. By adjusting the form of the throat and mouth, we can not -only prevent one vowel from passing into another, but we can keep it -so nearly true to itself as to convince the ear that its quality is -unchanged: _ōō_ remains _ōō_, and _ah_ _ah_, although the form of the -sound as produced on C♯ is different to its form when sung to C. - -Apart from the general distinction that low notes are taken more -easily with vowels requiring a large mouth-cavity, and high notes with -those providing a small one, there are certain very distinct relations -between vowel-sounds and musical tones which need to be borne in mind -in setting words to music. A singer changes a word when he feels that -its vowel-tone does not allow him to give to the note to which it is -set the fullest expression of which he is capable. - -An account of the physiology of the production of consonants is to be -found in most text-books of grammar. - -FOOTNOTE: - -[3] A bullock’s larynx is an admirable object of study. In almost all -points of form and structure it is practically identical with the human -larynx, and its large size makes it easy to dissect. - - - - -INDEX - - - Absorption from alimentary canal, 129 - Accelerator nerves of heart, 237 - Accommodation of the eye for distance, 391 - for light, 390 - Acromegaly, 93 - Addison’s disease, 91 - Adrenalin, action on the kidney, 209 - formed in suprarenal capsule, 92 - Air, quantity inspired, 173 - quantity needed by individual, 191 - Air-cells of lungs, 168 - Albumin made by plants, 12 - Alcohol, effect on nerve conduction, 301 - Alimentary canal, morphology of, 98 - nerves of, 104 - Altitude, highest, attained by climbers, 187 - Alveoli of lungs, their number, 169 - Amides produced from proteins, 119 - Amœba, irritability of its protoplasm, 10 - Amyl nitrite, effect on vascular system, 237 - Anæmia, treatment with iron, 67 - Anæsthetics, influence on protoplasm, 11 - Analysis by animals, 12 - Angina pectoris, 237 - Angler fish, its nerve-cells, 31 - Animal machine and its driver, 354, 358 - Animals, hunting _versus_ hunted, 366 - not reflex machines, 358 - relative insensibility to the knife, 361 - Antitoxins, formation by protoplasm, 20 - Aorta, diameter of, 232 - Aphasia, 352 - Apnœa, condition of arrested respiration, 181 - Appendicitis, increased frequency of, 101 - Appetite, a safe guide, 114 - Arteries, blood-pressure in, 234, 239 - structure of wall of, 233 - Artificial respiration, 183 - Asphyxia, 182 - Association-zones in the cortex of the great brain, 348 - Asthma, due to reflex contraction of small bronchi, 167 - Astigmatism, correction by glasses, 393 - due to modern print, 269 - Attention, effect of, in heightening pain, 361 - - Bacteria, diminution of number in intestine on milk diet, 138 - of alimentary canal, 135 - of Bulgarian sour milk, 138 - of the River Ganges, 141 - in an infant’s intestine, 136 - their rôle in nature, 20 - Balance-sheet of body, how drawn up, 149 - Balloon, highest altitude attained in, 187 - Basket-cells in nervous system, 324, 340 - Bat’s squeak, number of vibrations, 418 - Bats, flight not dependent on vision, 381 - Beats in music, explanation of, 407 - Beetle, muscle of, 261 - Belladonna, physiological action, 109 - Bile, composition, 117 - function in regard to absorption of fat, 133 - relation to digestion, 117 - Bile-pigment, origin from hæmoglobin, 69, 82, 118 - Bioplasm, the essential substance of a living cell, 148 - Birds, sense of hearing of, 410 - Blind spot, how filled in, 395 - Blisters, 41 - Blood, amount ejected by heart, 219 - circulation-time, 219 - composition of, 59 - gases of, amount, 190 - tension, 61 - lodged in abdominal veins, 234, 236 - Blood-corpuscles, cellular nature, 28 - life-story, 62 - number, 61 - origin, 63, 64 - structure, 60 - Blood-platelets, 74 - Blood-poisoning, 57 - Blushing, 243 - Bowman’s description of kidney, 200 - discs in muscle, 259 - Brain. _Cf._ Cerebellum, Cortex of cerebrum - blood-supply of, 352 - Bread, digestion of, 120 - Breathing, mechanism of, 171 - Bruises, explanation of play of colours, 69 - Bulgarian milk-germ, 138 - - Capillary vessels, circulation of blood in, 232 - migration of leucocytes from, 232 - structure of their walls, 38 - Carbohydrate foods, chemical composition, 147 - Carbonic acid, carried by blood, 60 - liberation in lungs, 61, 189 - Carbonic oxide, compound with hæmoglobin, 187 - Carnivora, absorption of fat from alimentary canal of, 133 - Cartilage, growth, 28 - Catalysis, 17 - Cell theory, 26 - Cells, constituent parts, 26, 28 - size, 30 - specialization of function in, 35 - Cells of Purkinje in the cerebellum, 303, 340 - Cellulose, digestion of, 137 - Cerebellum, cases of deficiency of, 341 - connections with cerebro-spinal axis, 340 - development of granules of, 299, 303 - lobes, 338 - minute anatomy, 339 - phylogeny, 338 - relation to tone of muscles, 342 - Cerebral hemisphere, an outgrowth towards olfactory pit, 334 - in animals with various sensory endowments, 349 - Cerebro-spinal fluid, 50 - Chemical activity of protoplasm, 12 - messengers, 89, 123 - processes in plants, 15 - Chemiotaxis of leucocytes, 56, 364 - Children, brain in, 346 - development of astigmatism in eyes of, 269 - Chill, catching a, 242 - Chloroform. _Cf._ Anæsthetics - Cholesterin, 118 - Chromatolysis in nerve-cells, 320 - Chrome-silver method of colouring nerve-tissue, 293 - Chyme, food converted into, 126 - Circulation of the blood, 218 - Circulation-time, 219 - Cirrhosis of liver, 42 - Coagulation of blood, 69 - Cochlea, anatomy, 413 - Cockney dialect, the degradation of vowel-sounds, 439 - Coke-fire, poisonous fumes from, 186 - Cold-spots in skin, 429 - Collaterals of nerves, 297 - Colon, length and disposition of, 101 - Colour-blindness, 385 - Colour-vision, 385 - Colours, reason for apparent fading in twilight, 378 - Conductivity of protoplasm, 248 - Consciousness, does not come within physiological investigation, 360 - its part in animal life, 359 - Control experiments, their value, 72 - Convolutions of brain, 345 - Cooking, effect upon digestibility of meat, 120 - Corneal epithelium, sensitiveness of, 424 - Corpus striatum of brain, 344 - Cortex of cerebrum, discovery of excitability of, 344 - fissures and convolutions, 345 - functional areas, 352 - myelination of its fibres, 345 - sensory and association areas, 346 - structure of, 347 - variations in different animals, 349 - Corti, organ of, its structure, 414 - theories of function of, 416 - Coughing, mechanism of, 180 - Crayfish, tone of claw-muscle of, 273 - Cretinism, 85, 90 - Cricket, chirp of, 261 - Crypts of Lieberkühn, 103 - Curdling of milk, 75 - - Dancing, association of sound with movement, 422 - Day’s work, food required for, 151 - Deafness due to sore throat, 412 - Degeneration of nerves after section, 326 - Depressor nerve of the heart, 237 - Diabetes, excretion of more carbohydrate than contained in food, 143 - Dialysis, explanation of the process, 40, 128 - Diaphragm, function in respiration, 171 - Diastases, destructive ferments, 18 - Diet, limits of possible variations in, 153 - of labouring classes, 152 - Digestibility of bread, meat, fish, etc., 120, 125 - Digestion, mechanism of, 96 - vascular changes during, 235 - waits on appetite, 114 - Digitalis, action on heart and kidney, 209 - Diphtheria, antitoxin of, 20 - Diuretics, 209 - Dog’s sense of smell, 370 - Dreams, theory of, 362 - Dropsy, 42 - Drowning, resuscitation from, 183 - Drugs, physiology of, 95 - Ductless glands, 94 - Dyspnœa, difficult respiration, 181 - - Ear, anatomy, 411 - bones of, 412 - differentiation into separate sense-organs, 410 - in fishes, 410 - phylogeny, 409 - Eel’s blood injected into mammal, 20 - Effector, an organ which exhibits change in response - to stimulation, 253 - Egg-albumin destroyed by blood, 19 - Electric organs, 288 - phenomena of muscles, 279 - Emotions, their relation to vaso-motor changes, 242 - Energy, expended by body, 151 - source of the body’s, 152 - of stimulus compared with energy of muscular response, 254 - Engines, body compared with, 152, 256 - Epiglottis during swallowing, 433 - Equilibrium, maintenance of, in walking, 342 - Erepsin, ferment of intestinal juice, 119 - Errors of sensory judgment, 402 - Excretion, 195 - Eye, accommodation for distance, 391 - adaptation for darkness, 390 - blind spot, 394 - optical defects of, 393 - phylogeny, 334 - refractive media, formation of image by, 391 - Eyeball, abnormalities in shape of, 392 - anatomy, 373 - development, 374 - muscles of, indefatigable, 269 - - Fat, absorption of, 131, 132 - accumulation of, relation to foods consumed, 144 - chemistry, 132 - digestion, 133 - laid down in connective tissues, 145 - stored in liver, 145 - Fatigue, causes of, 45, 268 - Fermentation, 16 - Ferments, chemical nature, 18 - classification, 16, 18 - physiological importance, 18 - Fibrin of blood, its antecedents, 75 - Fireflies, source of their light, 291 - Fish, sense of smell of, 365 - supposed to be frightened by noise, 410 - Flatulence, cause of, 114, 125, 136 - Foods, classification, 142 - history of, after absorption, 142 - relative value, 147, 151, 153, 157 - residue after digestion and absorption, 194 - Foramen ovale of heart, sometimes perforate, 218 - Frigate-bird, turbinate bones of, 166 - Frog, supposed to be found entombed in rock, 164 - Functional interdependence of organs, 94 - Functions transferred to other organs, 87 - - Gall-stones, cause of formation of, 118 - Galvani’s observation of contraction of a frog’s muscles, 277 - Ganges, purifying water of, 138 - Ganglia of sympathetic chain, function, 325 - Ganglion-cells of retina, 376 - spinal, 299, 333 - Gaseous tension, meaning of expression, 188 - Gases of blood, their exchange in the lungs, 184 - Gastric glands, structure, 123 - juice, amount secreted, 114 - composition, 114 - digestive action, 115 - Gelatin as article of diet, 158 - Giant cells, 65 - Glands, vaso-motor nerves of, 109, 241 - Glycogen, formula, 147 - as muscle food, 148 - stored in liver, 147 - Goitre, cause of, 84 - Granules, appearance of, in glands, 110 - of cerebellum, development of, 299, 303 - Grey matter, formation of paths in, 356 - Growth, a function of protoplasm, 24 - a reaction to work, 47 - - Hæmatin, 68 - Hæmatoidin, 68 - Hæmochromogen, 68 - Hæmoglobin, crystalline form, 66, 186 - formula, 66 - as oxygen carrier, 66, 186 - spectrum, 68, 185 - Hæmophilia, non-coagulability of blood, 76 - Hallucinations, 362 - Headache, a pain in the scalp, 106, 319 - the brain’s warning of fatigue, 269 - from strain of eye-muscles, 268 - Hearing, analysis of compound vibrations, 405 - capacity dependent upon education, 422 - Hearing, Helmholtz’s theory of analysis of sounds, 419 - range of sensations, 418 - sense of, 404 - upper limit, 418 - Heart, anatomy, 217 - automatism of, 238 - development, 218 - murmurs, 229 - muscular tissue, minute structure, 261 - nerves regulating beat, 237, 239 - sounds of, 228 - valves, their mechanism, 226 - work done by, 219, 223 - Heat, production of, by muscles, 254, 256 - Heat-spots in skin, 429 - Helmholtz’s theory of organ of Corti, 419 - Hering’s theory of colour-vision, 388 - Hormones, meaning of term, 89, 124 - of pancreas and liver, 127 - of stomach, 123 - Humours in ancient medical theory, 79 - Hunter, experiment of grafting cock’s spur in its comb, 47 - Hydrochloric acid, part taken in digestion, 114 - Hydrophobia, protective inoculation, 78 - Hyperpnœa, excessive respiratory efforts, 182 - Hypoblast, a layer of the embryo, 97 - - Illusions of movement, 335, 384 - of size and distance, 400 - Immunity, acquisition of, 20 - Impulse of the heart, 225 - rate of passage in muscle, 280 - in nerve, 278, 280 - theory of nerve conduction, 282 - Inhibition, explanation of term, 311 - of reflex actions, 311 - Insects, efficiency of their muscles, 261 - Instinct, due to brain-pattern, 359 - Intelligence of animals, 359 - Internal secretions, 83 - Intestinal juice, digestive action, 119 - Intestine, large, sacculation of its walls, 101 - small, folds and glands of mucous membrane, 102 - Intestines, movements of, 103 - nerves of, 105 - size and situation, 100 - Iodine, importance of, to economy, 89 - Iodothyrin, goitre due to deficiency of, 90 - Iris, its function in regulating admission of light to eye, 394 - Iron in food, 67 - in hæmoglobin, 67 - use of, in treatment of anæmia, 67 - Irritability, a function of protoplasm, 10 - - Japanese, cultivation of sense of smell by, 370 - Judgment of angles, 402 - of distance and size, 401 - of meaning of sensations, 396, 399 - - Kidney, ancestral history, 195 - elimination of indigo by, 207 - of birds and reptiles, 200, 207 - hydrostatic mechanism, 189 - minute anatomy, 196 - Kinæsthetic sensations, absence from dreams, 363 - part played by, in voluntary actions, 354 - representation in cortex of brain, 350, 352 - Knee-jerk, 274 - - Labyrinth of ear, 413 - Lactate of ammonia, relation to urea, 13 - Lacteals, lymphatic vessels of alimentary canal, 43, 131 - Lactic acid produced in muscle, 46, 146 - Larynx, closure during swallowing, 433 - structure of, 430 - Latent period of muscle after nervous impulse reaches it, 278 - Laughter, respiratory mechanism of, 180 - Lecithin produced by metabolism of nerve-tissue, 118 - Leech, ganglion-cells of, 298 - Leucocytes as protective agents, 52 - death of, 54, 57 - migration of, 49 - number in lymph and in blood, 49, 61 - origin of, 33, 51 - source of fibrin-ferment, 74 - Leucocythæmia, excess of leucocytes in the blood, 215 - Levers to which muscles are attached, 286 - Light, emission of, by animals, 291 - Lime, influence upon coagulation of blood, 75 - curdling of milk, 75 - Lithates, or urates, constituents of calculi, 213 - Liver, destruction of red blood-corpuscles in, 83 - form and structure of, 160 - former theories of its functions, 129, 163 - manufactures urea and uric acid, 146, 162 - of well fed sheep, 147 - origin of, in vertebrate phylogeny, 34 - Liver stores food, especially glycogen, 46, 145, 147, 161 - Locomotor ataxy, 341 - Ludwig’s view of mechanism of kidney, 200 - Luminous glands, 291 - Lung, exchange of gases in, 173, 184, 190 - nerve-supply, 178 - structure, 168 - Lymph, amount of, in body, 37 - composition, 49 - relation to blood, 51 - Lymph-spaces, 37, 43, 49 - Lymphatic glands, structure of, 54 - Lymphatic vessels, 43 - - Malapterurus, electric organs, 288 - immense neurones of, 295 - Manometer for measuring blood-pressure, description of, 238 - Man’s ancestry, 153 - Massage of abdominal viscera, 101 - of muscles, 48 - Meal, the story of a, 120 - Meat, diet consisting solely of, 157 - digestion of, 121 - extracts of, as articles of diet, 159 - Megacaryocytes, 65 - Memory, physiological explanation, 356 - Metabolism, chemical change in living tissue, 12, 273 - Methæmoglobin, 69 - Microscope, its discovery, 26 - Migration of birds, 359 - Milk, call for secretion of, by a hormone, 94 - chemical and physical constitution, 132 - digestion of, 127 - Milk diet, reduction of bacteria in alimentary canal on, 138 - Mind, physiology of, 354 - Mosquitoes, production of sound by, 261 - Motile cells, 32 - Mountain sickness, 187 - Mountains, highest climbed, 187 - Mucous membrane, use of term, 97 - Murmurs, in chest, in diseases of lungs, 169 - of heart, 229 - Muscle, change in appearance under microscope during - contraction, 263 - chemistry of contraction, 266 - contraction a phenomenon of osmosis, 258 - electric phenomena of, 278 - means of promoting growth of, 271 - measurement of its power, 285 - nature of impulse which leads to contraction of, 282 - Muscle of heart, its minute structure, 224 - of insects, its efficiency, 261 - plain, its minute structure, 258 - plasma, its coagulation, 266 - rhythm of voluntary contraction, 279 - theory of its structure as a mechanism liberating energy, 234, 255 - tone of, 272 - tracings taken of contracting, 278 - voluntary, its minute structure, 259 - wastes when its nerve is severed, 274 - work done by, proportional to load, 286 - Muscles, arrangement in regard to the bones which they move, 286 - co-operation in lifting a weight, 287 - Muscular energy, source of, 235 - Muscularis mucosæ of alimentary canal, 103 - Musculi papillares of heart, 227 - Music, chords admissible in, 408 - Indian, division of octave, 408 - primitive, prevalence of minor chords, 408 - Musical tones and overtones, 406 - Myelination of nerves, order of, 345 - Myxœdema, dependent on disease of thyroid gland, 85 - Myxomycetes, fusion of cell-bodies of, 27 - - Nasal chambers, air warmed in, 166 - Negroes, their long heels, 285 - Nerve, conduction in, theory of, 282 - degeneration, 326 - electrical phenomena, 279 - indefatigable, 282 - regeneration, 326 - structure, 296 - Nerves, depressor, 237 - experiment of crossing, 327 - fifth, 316 - of heart, 239 - of intestines, 426 - protopathic and critical systems of, 425 - secretory, of the salivary glands, 109 - splanchnic, 236 - superior laryngeal, 178 - vagus, 104 - vaso-motor, 239 - Nerve-cells last throughout life, 148 - limitations of their functions, 321 - store of energy in, 320 - transfer of impulses from cell to cell, 177, 300 - their relation to muscle-fibres, 274 - varying size of, 295, 322 - Nerve-centres, 176 - Nerve-force, improper use of expression, 281 - Nerve-impulses, distribution in grey matter, 305 - reinforcement of, 320 - resistance to, at synapses, 306 - Nerve-nets, pericellular, 301, 319 - Nervous system, neuronic and extra-neuronic conduction, 310 - phylogeny of, 332 - Neuro-fibrillæ, 298 - Neurone, origin of term, 293 - transmission of current by, 328 - various types of, 296, 323 - Night-blindness, 378 - Nissl’s bodies, source of nervous energy, 320 - Nitric oxide, combination with hæmoglobin, 186 - Nitrogenous equilibrium, 150 - food, stimulating effect of, 157 - waste, 210 - _Nœud vital_ of Flourens, 176 - Normal diet, 151 - Normal salt-solution, 82 - Nucleo-proteins, source of uric acid, 215 - - Odours, classification of, 366 - Œdema, or dropsy, 42 - Olfactory membrane, structure, 366 - Optic nerve, number of fibres, 378 - Organ of Corti, structure, 415 - theory of function, 417 - Organs that have lost their prime functions, 87 - Orientation, sense of, 335 - Osmosis, 40, 128, 201 - cause of muscular contraction, 235 - Osteoblasts, bone-forming cells, 32 - Osteoclasts, bone-eating cells, 65 - Oxygen, amount required per diem, 166 - carried by red blood-corpuscles, 66 - - Pain, influence of, upon action, 359 - referred from viscera to surface of body, 316 - relation to sensation, 313, 425 - theory of, 312, 425 - Pancreas, structure, 116 - Pancreatic juice, constitution, 116 - fat-splitting ferment of, 133 - Papillæ of the tongue, various forms of, 97 - Parathyroids, 86 - Pepsin, digestive action, 115 - Peptone prevents coagulation of blood, 77 - Pericellular nerve-nets, 301 - Perspiration, cools the surface of the body, 236 - repressed during fever, 257 - Peyer’s patches of lymph-follicles in intestine, 53 - Phagocytes, germ-eating leucocytes, 60 - consumption of red blood-corpuscles by, 82 - Phosphenes, developed by pressure on eyeball, 383 - Phosphorescence, cause of, 291 - Phrenology, 343 - Pictures, suggestion of solidity in, 401 - Pineal body, phylogeny, 334 - Pituitary body, 93 - Plants, anæsthetized by ether, 12, 24 - their metabolism, 15 - their respiration, 24 - Pleura, lining membrane of chest, 172 - Pleurisy, pain of, 313 - Pleuritic fluid, absorption of, 223 - Pneumonia, changes in lung during, 169 - Portal system of bloodvessels, 80 - regulator of vascular tone, 236 - Power of muscles, 285 - Precipitins formed in blood, 19 - Proteins, absorption by alimentary canal, 145 - chemical constitution, 6 - dietetic value, 157 - fate after absorption, 212 - Protopathic nerves, 425 - Protoplasm, arrangement in cells, 30 - constitution, 7 - Huxley’s definition, 6 - Pulse, cause of, 244 - records of, 245 - variations, 247 - Purgatives, theory of action, 128 - Purkinje-cells of cerebellum, 303, 340 - shadows of retinal vessels, 375 - Pus, origin of, from leucocytes, 57 - Pyramids of cortex of great brain, 346 - - Rabbit’s ear, vaso-motor changes in, 235 - Receptor, an organ specially sensitive to stimulation, 253 - Referred pains from viscera, 316 - Reflex action, inhibition of, 311 - of scratching, 330 - vinegar experiment with frog, 307 - Regeneration of nerves, 326 - Renal-portal circulation, 199 - Renewal of tissues, 148 - Rennin, ferment of milk, 16 - Resistance in nervous system, laws of, 177, 307 - Respiration, artificial, 179 - effect on circulation, 221 - a function of protoplasm, 23, 164 - movements of, 171 - nervous mechanism, 175, 179 - in tissues, 165, 193 - Respiratory centre in medulla oblongata, 176, 178, 182 - Respiratory quotient, 174 - Retina, structure, 374 - Retinal pigment, relation to vision, 381 - Rice ordeal, arrest of secretion of saliva, 112 - Rigor mortis, 266 - Rods and cones, respective functions in vision, 378 - Rowing, value of, as exercise, 287 - - Saccharin, taste of, 367 - Saline frog, respiration in, 193 - Saliva, chemical constitution, 107 - function of, 96, 107 - Salivary glands, mechanism of secretion, 108 - nerves of, 109, 236 - Salts, absorption of, in alimentary canal, 128 - Scientific method, definition of, 71 - Scratch reflex, in dog, 330 - Sea-sickness, 106 - Secretin, hormone of pancreas and liver, 127 - Secretion, accumulation of granules in cells, and their discharge, 110 - a response to stimulation, 111 - not a process of filtration, 110 - Semicircular canals, their functions, 410 - their positions in space, 335 - Sensations, their apparent fusion, 356 - many which escape attention, 318, 355 - neutralization of one by another, 356 - Sense-organs, origin in vertebrata, 336 - Sensory areas in cortex of the great brain, 348 - Sensory nerves, their connection with cerebro-spinal axis, 304 - Shell-fish, poisonous extract of, 41 - Shivering due to loss of heat from skin, 257 - Sight. _Cf._ Vision - Skate, electric organs of, 289 - Skilled movements, dependent upon kinæsthetic sensations, 357 - Skin, experiment of cutting nerve, 424 - variety of sensations from, 423 - Sleep, condition of neurones in, 362 - Sleeping sickness, 33 - Smallpox, protection against, 78 - Smell, disappearance of sense of, in later life, 370 - dog’s dependence upon sense of, 366 - reason for mental associations with sensations of, 371 - sensitiveness to mercaptan, 365 - Smells, nice and nasty, 369 - Smoking, mental effect of, 371 - Sneezing on looking at bright light, 317 - Sore throat, cause of deafness, 412 - Soul, Aristotle’s definition, 32 - Sound, mode of conduction, 404 - rapidity of vibrations of, 406, 418 - Sounds of the heart, 228 - periodic and aperiodic, 409 - Spectacles, defects of eyeball which call for, 392 - Speech, derangements of, due to disease of the brain, 353 - mechanism of, 437 - Sphygmographs for recording pulse, 245 - Spinal dog, reflex action in, 330 - frog, reflex action in, 307 - ganglia, development of cells, 299 - Splanchnic nerves, regulation of blood-pressure by, 236 - Spleen, destruction of blood-corpuscles in, 80 - structure, 79 - Squint, correction of double vision in, 397 - Starch, formula, 15 - Star-shapes due to puckering of crystalline lens, 393 - Starvation, statistics of, 156 - Stiffness of muscles, cause of, 45, 271 - Stimuli to muscles and nerves, 248 - Stokes, discovery of spectrum of blood, 68 - Stomach, digestion in, 120 - glands of, 123 - referred pains from, 316 - shape and size, 99 - Stone in the bladder, its cause, 213 - Subconscious self, 355 - Sugars, digestion of, 120, 136 - formulæ, 15 - Sun, apparent size near horizon, 399 - Suprarenal capsules, their structure and function, 91 - Sweetbread as article of diet, 215 - Sympathetic system of nerves, 243, 325 - diameter of fibres, 325 - Synapses of nerve-cells, resistance interposed at, 306 - Synaptases, constructive ferments, 18 - Synthesis by plants, 15 - - Tapeworms, resist digestion in the intestines, 21 - Taste, confusion with sense of smell, 364 - localization on tongue, 367 - sense of, in fishes, 365 - sensitiveness to quinine, 369 - Taste-bulbs, their structure, 368 - Tattooing, removal of pigment by leucocytes, 55 - Tea, its dietetic value, 122 - Teeth, 96 - Tendon, the growth of, from cells, 28 - Tension of gases in the lungs, 190 - Tetanus, the vibratile contraction of muscle, 279 - Thoracic duct, discharges lymph into veins, 43, 131 - Thorax, negative pressure in, 222 - Thorns on dendrites of nerve-cells, 300 - Thyroid body or gland, forms an internal secretion, 86 - relation to goitre, 85 - structure of, 85 - Tight-lacing, deformation of organs which it causes, 220 - Tigroids, in nerve-cells, stores of energy, 320 - Tissues, respiration in, 165, 193 - Tone of muscles, 272 - Tongue, as organ of taste, 367 - Tonsils, function as guardians of the fauces, 53 - structure, 52 - Torpedo, electric organs of, 290 - Touch, sensations of, 426 - Toxins produced by microbes, 20 - - Urea, amount relatively to proteins consumed, 155 - antecedents of, 146, 212 - chemical formula, 211 - secreted during period of starvation, 156 - Uric acid, amount secreted daily, 213 - artificial production of, 13 - chemical formula, 13, 214 - diathesis, its relation to diet, 140 - due to metabolism of leucocytes, 53, 216 - form in which excreted, 207 - made in the liver of birds, 13 - Urticaria due to abnormal composition of lymph, 41 - - Vaccination, protective value of, 22 - Valves of heart, their mechanism, 226 - Vascular system, tone of, 236, 240 - Vaso-constrictor nerves, 236 - Vaso-dilator nerves, 236 - Vegetables, dietetic value of, 139 - digestion of, 125, 137 - Vermiform appendix, 88 - Villi of intestine, absorption of food by, 130 - fat seen in, during active digestion, 134 - Viscera, their insensitiveness to injury, 316, 426 - Vision, colour contrasts, 382 - duration of images, 382 - judgment of distance and size, 411 - solidity, 401 - stereoscopic, doctrine of corresponding points, 397 - Visual purple, 381 - Vital action, definition of expression, 205 - Vivisection, 4 - Vocal cords, structure, 431 - how modified in singing, 435 - Voice, breaking of, in boys, 434 - falsetto, how produced, 435 - range of human, 435 - registers, 436 - Vomiting, 105 - Vowels, synthesis by tuning-forks, 439 - - Wandering cells, 33 - Warmth, appreciation of, by skin, 429 - Waste substances, classification, 194 - how eliminated from body, 59 - Waterfall, negative after-image of, 384 - Water-weed, experiment proving that it respires, 24 - Wear and tear of bioplasm, 145 - Wisdom-tooth, tending to disappear, 96 - - Yawning, beneficial effect on circulation, 222 - nervous mechanism of, 180 - Young’s theory of colour-vision, 385 - - Zymogen, 110 - -THE END - -BILLING AND SONS, LTD., PRINTERS, GUILDFORD - - - - - - -End of the Project Gutenberg EBook of The Body at Work, by Alexander Hill - -*** END OF THIS PROJECT GUTENBERG EBOOK THE BODY AT WORK *** - -***** This file should be named 63089-0.txt or 63089-0.zip ***** -This and all associated files of various formats will be found in: - http://www.gutenberg.org/6/3/0/8/63089/ - -Produced by Paul Marshall, Turgut Dincer and the Online -Distributed Proofreading Team at https://www.pgdp.net (This -file was produced from images generously made available -by The Internet Archive) - -Updated editions will replace the previous one--the old editions will -be renamed. - 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You may copy it, give it away or re-use it under the terms of -the Project Gutenberg License included with this eBook or online at -www.gutenberg.org. If you are not located in the United States, you'll have -to check the laws of the country where you are located before using this ebook. - -Title: The Body at Work - A Treatise on the Principles of Physiology - -Author: Alexander Hill - -Release Date: August 31, 2020 [EBook #63089] - -Language: English - -Character set encoding: UTF-8 - -*** START OF THIS PROJECT GUTENBERG EBOOK THE BODY AT WORK *** - - - - -Produced by Paul Marshall, Turgut Dincer and the Online -Distributed Proofreading Team at https://www.pgdp.net (This -file was produced from images generously made available -by The Internet Archive) - - - - - - -</pre> - - -<div class="transnote bbox hhonly"> -<p class="indent">The cover image was created by the transcriber, and is placed in the public domain.</p> -</div> - -<hr class="chap" /> - <h1>THE BODY AT WORK</h1> -<hr class="chap" /> - -<div class="figcontainer"> - <a id="FRONTIS" name="FRONTIS"> </a> - <div class="figsub"> - <img src="images/frontispiece_01.jpg" alt="" width="275" height="264" /> - <p class="center"><br /><big><b>A.</b></big></p> - </div> - <div class="figsub"> - <img src="images/frontispiece_02.jpg" alt="" width="275" height="358" /> - <p class="center"><big><b>B.</b></big></p> - </div> -</div> -<div class="figcontainer"> - <div class="figsub"> - <img src="images/frontispiece_03.jpg" alt="" width="275" height="345" /> - <p class="center"><br /><big><b>C.</b></big></p> - </div> - <div class="figsub"> - <img src="images/frontispiece_04.jpg" alt="" width="275" height="266" /> - <p class="center"><big><b>D.</b></big></p> - </div> - <p class="center"><span class="smcap">Fig. 1.</span>—<span class="smcap">Photomicrographs - of Cells of the Cortex of the<br /> Cerebellum and Cerebrum.</span></p> - <p class="center"><a href="#NOTE_FRONTIS"><i>For description see p.</i> x.</a></p> - <p class="author"><i>Frontispiece.</i></p> -</div> -<hr class="chap" /> - -<p class="f200"><b>THE BODY AT WORK</b></p> - -<p class="center">A TREATISE ON THE PRINCIPLES<br />OF PHYSIOLOGY</p> - -<p class="center space-above3">BY<br />ALEX HILL, M.A., M.D., F.R.C.S.</p> -<p class="f90">SOMETIME MASTER OF DOWNING COLLEGE, CAMBRIDGE</p> - -<p class="center space-above3">WITH 46 ILLUSTRATIONS</p> - -<p class="center space-above3">LONDON</p> -<p class="f120">EDWARD ARNOLD</p> -<p class="center">1908</p> - -<p class="center">[<i>All rights reserved</i>]</p> - -<hr class="chap" /> -<p><span class="pagenum"><a name="Page_v" id="Page_v">[Pg v]</a></span></p> -<div class="chapter"><h2>PREFACE</h2></div> - -<p>Few subjects are as well provided with text-books as physiology; yet -it may be doubted whether the interests of the amateur of science have -been adequately cared for. From his point of view there are certain -obvious drawbacks to even the most admirable of text-books. Writing for -medical students, their authors assume that their readers have passed -through two years of preliminary training in physics, chemistry, and -biology; they take for granted that they will have the privilege of -supplementing their study of the theory of physiology with practical -work in a laboratory; they treat all parts of the subject with equal -thoroughness. In this book I have endeavoured to describe the phenomena -of life, and the principal conclusions which have been drawn as to -their interdependence and as to their causes, in language which will -be understood by persons unacquainted with the sciences upon which -physiology is based. I have omitted all reference to experimental -methods and to the technique of the science, save when a knowledge of -the means by which information has been obtained is essential to a -comprehension of its bearing. I have passed over such sections of the -subject as are generally considered unsuitable for ordinary discussion. -And since this book neither aims at being an introduction to the -systematic study of physiology, nor poses as an aid in the preparation -for professional examinations, I have treated with some thoroughness -the more recondite and the more suggestive results of recent research, -and have tried to indicate the trend of modern thought regarding -<span class="pagenum"><a name="Page_vi" id="Page_vi">[Pg vi]</a></span> -problems as yet unsolved. I have endeavoured to reflect the intrinsic -interest of the science apart altogether from its medical applications.</p> - -<p>An author who attempts the popular exposition of a science must -stand sufficiently far away from his subject to lose sight of its -details, whilst keeping its outlines clearly in view. The difficulty -of finding such a position is probably greater in the case of -physiology than in that of any other science. Few of its conclusions -are indisputable—even those which seem to be most in accord with -the balance of evidence. If my treatment of any vexed questions is -unjustifiably dogmatic, this will, I trust, be attributed to the -desire to present a definite picture, and not to forgetfulness of -considerations which seem to call for qualified statements. All -physiologists will agree that a book which recorded every piece of -evidence which is difficult to reconcile with the views generally -adopted would not only extend to an inordinate length, but would leave -a very indefinite impression on the mind of the reader.</p> - -<p>In many cases the value of a conclusion depends upon the reputation for -insight and accuracy of the physiologist who recorded the observations -upon which it is based. It is no want of appreciation of the genius of -the workers who have contributed most largely to the advance of the -science which has led me to omit, save in a few classical instances, -the names of all authorities. It is solely due to a desire to lighten -this book of all details not essential to the comprehension of the -propositions which it sets forth.</p> - -<p>The illustrations are reproductions of blackboard drawings. A few of -them have already appeared in my <i>Physiologist’s Notebook</i> and <i>Primer -of Physiology</i>; but the large majority are now printed for the first time.</p> - -<p class="author">ALEX HILL.</p> -<p><i>November, 1908.</i></p> - -<hr class="chap" /> -<p><span class="pagenum"><a name="Page_vii" id="Page_vii">[Pg vii]</a></span></p> - -<div class="chapter"><h2>CONTENTS</h2></div> - -<table border="0" cellspacing="0" summary="TOC" cellpadding="0" > - <tbody><tr> - <td class="tdr"><small>CHAPTER</small></td> - <td class="tdc"> </td> - <td class="tdr"><small>PAGE</small></td> - </tr><tr> - <td class="tdr">I.</td> - <td class="tdl_ws1">PROLEGOMENA</td> - <td class="tdr"><a href="#Page_1"> 1</a></td> - </tr><tr> - <td class="tdr">II.</td> - <td class="tdl_ws1">THE BASIS OF LIFE</td> - <td class="tdr"><a href="#Page_6"> 6</a></td> - </tr><tr> - <td class="tdr">III.</td> - <td class="tdl_ws1">THE UNIT OF STRUCTURE</td> - <td class="tdr"><a href="#Page_26">26</a></td> - </tr><tr> - <td class="tdr">IV.</td> - <td class="tdl_ws1">THE FLUIDS OF THE BODY  </td> - <td class="tdr"><a href="#Page_37">37</a></td> - </tr><tr> - <td class="tdr">V.</td> - <td class="tdl_ws1">INTERNAL SECRETIONS</td> - <td class="tdr"><a href="#Page_84">84</a></td> - </tr><tr> - <td class="tdr">VI.</td> - <td class="tdl_ws1">DIGESTION</td> - <td class="tdr"><a href="#Page_96">96</a></td> - </tr><tr> - <td class="tdr">VII.</td> - <td class="tdl_ws1">RESPIRATION</td> - <td class="tdr"><a href="#Page_164">164</a></td> - </tr><tr> - <td class="tdr">VIII.</td> - <td class="tdl_ws1">EXCRETION</td> - <td class="tdr"><a href="#Page_194">194</a></td> - </tr><tr> - <td class="tdr">IX.</td> - <td class="tdl_ws1">THE CIRCULATION</td> - <td class="tdr"><a href="#Page_217">217</a></td> - </tr><tr> - <td class="tdr">X.</td> - <td class="tdl_ws1">MUSCLE</td> - <td class="tdr"><a href="#Page_248">248</a></td> - </tr><tr> - <td class="tdr">XI.</td> - <td class="tdl_ws1">THE NERVOUS SYSTEM</td> - <td class="tdr"><a href="#Page_293">293</a></td> - </tr><tr> - <td class="tdr">XII.</td> - <td class="tdl_ws1">SMELL AND TASTE</td> - <td class="tdr"><a href="#Page_364">364</a></td> - </tr><tr> - <td class="tdr">XIII.</td> - <td class="tdl_ws1">VISION</td> - <td class="tdr"><a href="#Page_372">372</a></td> - </tr><tr> - <td class="tdr">XIV.</td> - <td class="tdl_ws1">HEARING</td> - <td class="tdr"><a href="#Page_404">404</a></td> - </tr><tr> - <td class="tdr">XV.</td> - <td class="tdl_ws1">SKIN-SENSATIONS</td> - <td class="tdr"><a href="#Page_423">423</a></td> - </tr><tr> - <td class="tdr">XVI.</td> - <td class="tdl_ws1">VOICE AND SPEECH</td> - <td class="tdr"><a href="#Page_431">431</a></td> - </tr><tr> - <td class="tdr"> </td> - <td class="tdl_ws1">INDEX</td> - <td class="tdr"><a href="#Page_441">441</a></td> - </tr> - </tbody> -</table> - -<p><span class="pagenum"><a name="Page_viii" id="Page_viii">[Pg viii]</a></span></p> -<hr class="chap" /> -<p><span class="pagenum"><a name="Page_ix" id="Page_ix">[Pg ix]</a></span></p> - -<div class="chapter"><h2>LIST OF ILLUSTRATIONS</h2></div> -<table border="0" cellspacing="0" summary="TOC" cellpadding="0" > - <tbody><tr> - <td class="tdr"><small>FIG.</small></td> - <td class="tdc"> </td> - <td class="tdr"><small>PAGE</small></td> - </tr><tr> - <td class="tdr">1.</td> - <td class="tdl_ws1"><span class="smcap">Photomicrographs of Brain-Tissue</span></td> - <td class="tdr"><a href="#FRONTIS"><i>Frontispiece</i></a></td> - </tr><tr> - <td class="tdr">2.</td> - <td class="tdl_ws1"><span class="smcap">The Organs of the Chest and Abdomen</span></td> - <td class="tdr"><a href="#I_PXI">xi</a></td> - </tr><tr> - <td class="tdr">3.</td> - <td class="tdl_ws1"><span class="smcap">Mucous Gland, Capillaries, and Connective-Tissue Spaces</span></td> - <td class="tdr"><a href="#I_038">38</a></td> - </tr><tr> - <td class="tdr">4.</td> - <td class="tdl_ws1"><span class="smcap">Blood-Corpuscles</span></td> - <td class="tdr"><a href="#I_060">60</a></td> - </tr><tr> - <td class="tdr">5.</td> - <td class="tdl_ws1"><span class="smcap">Spleen-Pulp, with Phagocytes</span></td> - <td class="tdr"><a href="#I_081">81</a></td> - </tr><tr> - <td class="tdr">6.</td> - <td class="tdl_ws1"><span class="smcap">Duodenum and Neighbouring Organs</span></td> - <td class="tdr"><a href="#I_099">99</a></td> - </tr><tr> - <td class="tdr">7.</td> - <td class="tdl_ws1"><span class="smcap">A Lobule of the Liver and Liver-Cells</span></td> - <td class="tdr"><a href="#I_160">160</a></td> - </tr><tr> - <td class="tdr">8.</td> - <td class="tdl_ws1"><span class="smcap">The Diaphragm during Inspiration and Expiration</span></td> - <td class="tdr"><a href="#I_172">172</a></td> - </tr><tr> - <td class="tdr">9.</td> - <td class="tdl_ws1"><span class="smcap">The Kidney</span></td> - <td class="tdr"><a href="#I_197">197</a></td> - </tr><tr> - <td class="tdr">10.</td> - <td class="tdl_ws1"><span class="smcap">The Heart in Longitudinal Section</span></td> - <td class="tdr"><a href="#I_218">218</a></td> - </tr><tr> - <td class="tdr">11.</td> - <td class="tdl_ws1"><span class="smcap">Horizontal Section of the Heart, showing its Valves</span></td> - <td class="tdr"><a href="#I_225">225</a></td> - </tr><tr> - <td class="tdr">12.</td> - <td class="tdl_ws1"><span class="smcap">Section of the Wall of a Small Artery</span></td> - <td class="tdr"><a href="#I_233">233</a></td> - </tr><tr> - <td class="tdr">13.</td> - <td class="tdl_ws1"><span class="smcap">Kymograph</span></td> - <td class="tdr"><a href="#I_238">238</a></td> - </tr><tr> - <td class="tdr">14.</td> - <td class="tdl_ws1"><span class="smcap">Sphygmograph</span></td> - <td class="tdr"><a href="#I_244">244</a></td> - </tr><tr> - <td class="tdr">15.</td> - <td class="tdl_ws1"><span class="smcap">Blood-Pressure Tracings</span></td> - <td class="tdr"><a href="#I_245">245</a></td> - </tr><tr> - <td class="tdr">16.</td> - <td class="tdl_ws1"><span class="smcap">Minute Structure of Muscle-Fibres</span></td> - <td class="tdr"><a href="#I_262">262</a></td> - </tr><tr> - <td class="tdr">17.</td> - <td class="tdl_ws1"><span class="smcap">The Biceps Muscle in Action</span></td> - <td class="tdr"><a href="#I_286">286</a></td> - </tr><tr> - <td class="tdr">18.</td> - <td class="tdl_ws1"><span class="smcap">Electric Organ of Skate</span></td> - <td class="tdr"><a href="#I_289">289</a></td> - </tr><tr> - <td class="tdr">19.</td> - <td class="tdl_ws1"><span class="smcap">Minute Structure of a Nerve-Fibre</span></td> - <td class="tdr"><a href="#I_296">296</a></td> - </tr><tr> - <td class="tdr">20.</td> - <td class="tdl_ws1"><span class="smcap">Ganglion-Cells with Neuro-Fibrillæ of the Leech</span></td> - <td class="tdr"><a href="#I_298">298</a></td> - </tr><tr> - <td class="tdr">21.</td> - <td class="tdl_ws1"><span class="smcap">The Development of the Granules of the Cerebellum</span></td> - <td class="tdr"><a href="#I_304">304</a></td> - </tr><tr> - <td class="tdr">22.</td> - <td class="tdl_ws1"><span class="smcap">Tigroids and Neuro-Fibrillæ</span></td> - <td class="tdr"><a href="#I_321">321</a></td> - </tr><tr> - <td class="tdr">23.</td> - <td class="tdl_ws1"><span class="smcap">Minute Structure of the Cortex of the Cerebellum</span></td> - <td class="tdr"><a href="#I_339">339</a></td> - </tr><tr> - <td class="tdr">24.</td> - <td class="tdl_ws1"><span class="smcap">Minute Structure of the Cortex of the Cerebrum</span></td> - <td class="tdr"><a href="#I_347">347</a></td> - </tr><tr> - <td class="tdr">25.</td> - <td class="tdl_ws1"><span class="smcap">Functional Areas of the Cortex of the Cerebrum</span></td> - <td class="tdr"><a href="#I_352">352</a></td> - </tr><tr> - <td class="tdr">26.</td> - <td class="tdl_ws1"><span class="smcap">Taste-Bulbs</span></td> - <td class="tdr"><a href="#I_368">368</a></td> - </tr><tr> - <td class="tdr">27.</td> - <td class="tdl_ws1"><span class="smcap">Horizontal Section of the Eye</span></td> - <td class="tdr"><a href="#I_373">373</a></td> - </tr><tr> - <td class="tdr">28.</td> - <td class="tdl_ws1"><span class="smcap">Development of the Crystalline Lens</span></td> - <td class="tdr"><a href="#I_374">374</a></td> - </tr><tr> - <td class="tdr">29.</td> - <td class="tdl_ws1"><span class="smcap">Purkinje’s Shadows of the Vessels of the Retina</span></td> - <td class="tdr"><a href="#I_375">375</a></td> - </tr><tr> - <td class="tdr">30.</td> - <td class="tdl_ws1"><span class="smcap">Retina adapted for Obscurity and for Bright Light</span></td> - <td class="tdr"><a href="#I_377">377</a></td> - </tr><tr> - <td class="tdr">31.</td> - <td class="tdl_ws1"><span class="smcap">Simultaneous Contrast</span></td> - <td class="tdr"><a href="#I_383">383</a></td> - </tr><tr> - <td class="tdr">32.</td> - <td class="tdl_ws1"><span class="smcap">Formation of an Image on the Retina</span></td> - <td class="tdr"><a href="#I_391">391</a></td> - </tr><tr> - <td class="tdr">33.</td> - <td class="tdl_ws1"><span class="smcap">The Form of the Eyeball in Short Sight, Normal Sight,and Long Sight</span></td> - <td class="tdr"><a href="#I_392">392</a> - <span class="pagenum"><a name="Page_x" id="Page_x">[Pg x]</a></span></td> - </tr><tr> - <td class="tdr">34.</td> - <td class="tdl_ws1"><span class="smcap">The Blind Spot</span></td> - <td class="tdr"><a href="#I_394">394</a></td> - </tr><tr> - <td class="tdr">35.</td> - <td class="tdl_ws1"><span class="smcap">An Optical Illusion</span></td> - <td class="tdr"><a href="#I_398">398</a></td> - </tr><tr> - <td class="tdr">36.</td> - <td class="tdl_ws1"><span class="smcap">An Optical Illusion</span></td> - <td class="tdr"><a href="#I_401">401</a></td> - </tr><tr> - <td class="tdr">37.</td> - <td class="tdl_ws1"><span class="smcap">An Optical Illusion</span></td> - <td class="tdr"><a href="#I_402">402</a></td> - </tr><tr> - <td class="tdr">38.</td> - <td class="tdl_ws1"><span class="smcap">External, Middle, and Inner Ear</span></td> - <td class="tdr"><a href="#I_411">411</a></td> - </tr><tr> - <td class="tdr">39.</td> - <td class="tdl_ws1"><span class="smcap">The Cochlea</span></td> - <td class="tdr"><a href="#I_414">414</a></td> - </tr><tr> - <td class="tdr">40.</td> - <td class="tdl_ws1"><span class="smcap">The Organ of Corti</span></td> - <td class="tdr"><a href="#I_415">415</a></td> - </tr><tr> - <td class="tdr">41.</td> - <td class="tdl_ws1"><span class="smcap">Nerves of the Corneal Epithelium</span></td> - <td class="tdr"><a href="#I_424">424</a></td> - </tr><tr> - <td class="tdr">42.</td> - <td class="tdl_ws1"><span class="smcap">Touch-Corpuscles</span></td> - <td class="tdr"><a href="#I_427">427</a></td> - </tr><tr> - <td class="tdr">43.</td> - <td class="tdl_ws1"><span class="smcap">Pacinian Corpuscle</span></td> - <td class="tdr"><a href="#I_428">428</a></td> - </tr><tr> - <td class="tdr">44.</td> - <td class="tdl_ws1"><span class="smcap">The Glottis</span></td> - <td class="tdr"><a href="#I_432">432</a></td> - </tr><tr> - <td class="tdr">45.</td> - <td class="tdl_ws1"><span class="smcap">The Larynx in Longitudinal Section</span></td> - <td class="tdr"><a href="#I_433">433</a></td> - </tr><tr> - <td class="tdr">46.</td> - <td class="tdl_ws1"><span class="smcap">The Larynx from the Right Side</span></td> - <td class="tdr"><a href="#I_435">435</a></td> - </tr> - </tbody> -</table> - -<hr class="chap" /> -<div class="chapter"> -<p class="f150" id="NOTE_FRONTIS"><b>NOTE ON THE FRONTISPIECE</b></p> -</div> - -<p>Four photomicrographs of cells or parts of cells of brain-tissue, -coloured by the chrome-silver method (<a href="#Page_293"><i>cf.</i> p. 293</a>).</p> - -<p><b>A.</b> Cell of Purkinje from the cerebellum of a man aged 45. At -the bottom of the photograph is seen the rounded cell-body, with -the commencement of its axon. The summit of the cell-body bears an -elaborately branched system of dendrites, spread out in the plane of -the section.</p> - -<p><b>B.</b> A single basket-cell of the cortex of the cerebellum (very -highly magnified). The oval cell-body gives origin to four dendritic -processes which branch. Thorns are to be seen on the larger process -which ascends on the right. From the same process, near its origin, -springs a delicate axon which thickens as it proceeds to form a basket -at the right hand lower corner of the photograph. Two other branches -of the same axon, which form baskets around other Purkinje-cells, are -faintly visible, although out of focus.</p> - -<p><b>C.</b> Seven or eight pyramids from the cortex of the cerebrum of a -hedgehog. A little below the centre of the photograph is seen a large -pyramid with a single thorny apical process which bifurcates, several -basal dendrites and an axon. In the upper part of the photograph are -seen the apical processes of a number of pyramids of which the bodies -were not included in the section.</p> - -<p><b>D.</b> The margin of the cortex (subiculum cornu Ammonis) from the -same specimen. A single row of pyramids extends across the photograph. -They are remarkable for the richness of branching of their basal -processes, which has earned for the cells which comprise this sheet the -name of “double pyramids.”</p> - -<p>All four sections were cut vertically to the surface.</p> - -<hr class="chap" /> -<p><span class="pagenum"><a name="Page_xi" id="Page_xi">[Pg xi]</a></span></p> - -<div class="figcenter"> - <a id="I_PXI" name="I_PXI"> </a> - <img src="images/i_pxi.jpg" alt="" width="500" height="583" /> - <p class="center"><span class="smcap">Fig. 2.</span>—<span class="smcap">Diagram - showing the Relative Positions of the Organs<br /> of the Chest and Abdomen.</span></p> - <div class="blockquot2"> - <p class="neg-indent space-below2">The ribs from the first to the tenth have been cut - across in the lateral line. The eleventh and twelfth ribs do not reach sufficiently - far forwards to be cut. With the exception of a short segment near - its junction with the ascending colon, the small intestine has been - removed. The trachea is seen to divide into bronchi beneath the arch of - the aorta. The right lung has three, the left two lobes. The kidneys - are situate behind all the other viscera. On their upper ends rest the - two suprarenal capsules. The lower edge of the right lobe of the liver - follows closely the line of the ribs and costal cartilages. Below the - left lobe of the liver the stomach comes to the anterior abdominal - wall. The transverse colon (large intestine) comes to the anterior wall - below the stomach. Below the latter the wall is in contact chiefly with - coils of small intestine. The vermiform appendix rests on the posterior - wall. Spleen and pancreas are not shown in the diagram.</p> -</div></div> - -<p><span class="pagenum"><a name="Page_xii" id="Page_xii">[Pg xii]</a></span></p> -<hr class="chap" /> -<p><span class="pagenum"><a name="Page_1" id="Page_1">[Pg 1]</a></span></p> -<p class="f200"><b>THE BODY AT WORK</b></p> -<hr class="chap" /> - -<div class="chapter"> - <h2>CHAPTER I<br /><span class="h_subtitle">PROLEGOMENA</span></h2> -</div> - -<p>Physiology is the science of the body at work. It is the study of life. -Anatomy records how plants and animals are constructed. It maps and -measures. Physiology ascertains what they do, endeavours to explain how -they do it, and conjectures why.</p> - -<p>A knowledge of structure is essential to the right understanding of -function; but the physiologist does not contemplate structure with -a view to divining possibilities of action. He has no interest in -structure as such. To him it is a matter of perfect indifference -whether the tendon of a muscle is at its origin or its insertion. He -would rather not know which end of the muscle terminates in a tendon. -It is waste of his time to notice such a fact, save for the negative, -the protective value of the information. If he did not know how the -muscle and tendon are related, he might possibly imagine the muscle as -doing something of which it is incapable. Observers of living things -are often credited with studying structure with a view to determining -function. The reverse is the true order of thought and observation. -Living things perform certain acts. Having no inherent knowledge of -our own microcosm which enables us to say how it works, we cannot, -by reflecting upon our own internal operations, explain its various -activities. Nor can we make use of the results of introspection when -endeavouring to account for the acts of other beings. Our knowledge -of how things are done is altogether extrapersonal, objective. It is -the result of trial, failure, success in the use of apparatus, our own -essays, or those of others. The body is a combination of organs—a term -<span class="pagenum"><a name="Page_2" id="Page_2">[Pg 2]</a></span> -used somewhat loosely to designate any piece of the animal mechanism -which has a distinct function to perform. The physiologist studies -the results of the activity of an organ. He watches it in action, and -endeavours to explain the process by which it produces its effects. -Then follows the anatomist, who, taking it to pieces, examines it -with the utmost thoroughness which scalpel and forceps or microscope -allows, with a view to ascertaining whether its structure will support -the physiologist’s hypothesis as to its mode of action. This in the -vast majority of cases has been the history of scientific progress. -The physiologist has preceded the anatomist in drawing inferences as -to the manner in which things are done. The anatomist, after a further -examination of structure, has either admitted the plausibility of -his explanation, or has interposed the objection that the part was -incapable of working in the way supposed.</p> - -<p>This comparison of anatomy and physiology must not be pushed too far. -Enough has been said to emphasize the distinction between them. The -one treats of form, the other of function. The one looks at structure, -the other at action. Anatomy in its limited and logical sense has -nothing to do with the uses of a part; its business is to measure it. -Physiology has nothing to do with the measurements of parts; its duty -is to watch for movement. Every living thing may be contemplated either -in its statical or in its dynamical aspect. Physiology looks at it from -the latter point of view.</p> - -<p>Surveying his province, the physiologist asks himself: “Who are my -subjects? What am I to find out about them? What methods, in addition -to direct observation, may I use to obtain this information?” His -oversight embraces all living things. It is no longer reasonable to -make a distinction between human and animal physiology, or between the -physiology of animals and the physiology of plants. No human being can -take all science for his field. If he contents himself with scratching -its surface, he will assuredly raise but a meagre crop, and that mostly -weeds. But he is far behind the spirit of his age if he declines to -sow in his own little patch seeds of thought which have blossomed in -other localities, however remote. The man whose purpose in studying -physiology is to obtain a knowledge of the working of the healthy human -<span class="pagenum"><a name="Page_3" id="Page_3">[Pg 3]</a></span> -body, in order that he may know how to set right the accidents, -perversions, and premature decay to which human flesh is prone, would -remain an empiric of the most rigid type did he not apply to the -elucidation of his problems all conclusions reached from the study -of other organisms which are likely to prove pertinent. There would -be no science of human physiology had observation and experiment -been limited to Man. There would be no science of medicine, it -may be added, had not the mode of working of the human body, and -the influence of drugs upon it, been inferred from the results of -experiments upon animals—experiments which could never have been made -upon men. Blisters, blood-letting, mercury-poisoning, would still be -the physician’s remedies for all human ills. “Give the watch a good -shaking. It sometimes does good. If that fails, I cannot advise you -what to do, as I know nothing about the working of a watch.” Even -though we open the living human body, as must be done for the purpose -of making good such defects as are amenable to surgical treatment, -and for a little while observe its wheels go round, we are unable, -from fear of damaging the wheels, to introduce the mechanical tests -which would tell us how and why they revolve. The man must be allowed -to recover with uninjured organs. But, thanks to anæsthetics, there -is no test which may not be applied to a live animal with as much -propriety as to a dead one. Anæsthetics abolish the distinction, in its -ethical applications, between life and death, because we are under no -obligation, as in the case of the human being, to allow an animal to -recover. Many experiments upon animals will be recorded in this book, -and since the book is intended for the general public, who have been -singularly misled regarding the nature and methods of vivisection, an -opportunity is taken thus early of insisting that anæsthetics have made -all things, not only possible, but legitimate. It is unnecessary to -commence the description of each experiment with the statement that the -animal was first placed in a condition of complete anæsthesia, or to -end it with the statement that it was destroyed before it had recovered -from the effects of the anæsthetic. The reader may take these facts for -granted. In discussing the propriety of operating upon a living but -unconscious animal, we are playing a word game as old as Plato’s day. -<span class="pagenum"><a name="Page_4" id="Page_4">[Pg 4]</a></span> -What is life? What is the relation of the personality to the animal -machine which it occupies and operates? For a few minutes a heart -removed from the body continues to beat. In a physiological sense it -is alive, although the body from which it was removed is dead. Yet -the personality does not reside in the heart, as many generations of -philosophers believed. It is merely an accident that the body dies when -the co-ordinating mechanism, the heart, ceases to pump blood through -its vessels. Nor is the personality limited to the brain. Without the -sense-organs which place the brain in relation with the body, and owing -to the movements of the body—by which the sources of sensations of -smell, sight, hearing are ascertained—with the world of which it forms -a part, there would be no personality, no Ego. Is it, then, coextensive -with the body which exhibits it? A soldier returning crippled from the -wars does not finish out his days with his personality curtailed. We -are no nearer than was Plato to a definition of life. Such a discussion -soon takes us out of the realm of science. Science is limited to -the sphere in which the whole is greater than the part. Take away -consciousness, and personality ceases. Guarantee that consciousness -shall never return. The animal is dead. When considering the propriety -of vivisection we must regard life and consciousness as inseparable. -There can be no question of right or wrong in regard to experiments on -a dead animal, even though a sensitive mind, from association, shrinks -from contemplating them. A person who dislikes the idea of dissecting -a dead animal is influenced by purely subjective and personal -considerations; nor is he prompted by sympathy with an unconscious -animal when he recoils from the spectacle of its still moving organs. -The term “vivisection” conveys too large a meaning. A negative term is -needed, some word which will hold the emotion of pity in check. Pity -is misplaced when devoted to the unconscious subjects of physiological -experiment; and, happily for animals, as for Man, anæsthetics suspend -conscious life. Only a person who has undergone a surgical operation -can understand how resolutely the intellect declines to adopt as part -of itself things which have not come within its own experience. The -nurse’s testimony, that a long interval separated the placing of the -mask upon the face and the commencement of that dull half-consciousness -<span class="pagenum"><a name="Page_5" id="Page_5">[Pg 5]</a></span> -which gradually reawakened into interest in one’s surroundings cannot -be set aside. The nurse says that during that interval knife, saw, -and cautery were busy at their work. Her story is accepted, but it -is not believed. All physiological operations are conducted under -anæsthetics. In by far the larger number the experiment is continued -until life terminates, under anæsthetics. The only ground upon which an -objection to vivisection can be based is the ground that it involves -the infliction of pain, and it is with regard to this that the greatest -misapprehension exists in the public mind. Only in experiments which -have for their object the study of the effects of the removal of a -certain part, the diversion of a duct, the elimination of the control -of a particular nerve, is there any possibility, under existing -conditions, that an animal will suffer. In such experiments as these, -observations cannot commence until after the animal has recovered. -The operation is conducted under anæsthetics, and with the utmost -precautions, to prevent any disturbance of the animal’s general health. -The injury is in almost all cases of a comparatively limited nature, -and it is certain that it involves very little pain to the animal -when it has recovered from its anæsthesia, since, thanks this time to -aseptic surgery, there is no inflammation or other secondary trouble.</p> - -<p>The field of physiology embraces the phenomena exhibited by all living -things, whether plants or animals. The vegetable physiologist works -in one part, the comparative physiologist in another. The work of the -human physiologist is more limited in scope. Yet there are few problems -relating to Man’s mechanism concerning which the physiologist can have -direct knowledge. His theories are based upon the results obtained by -experimenting upon animals.</p> - -<hr class="chap" /> -<p><span class="pagenum"><a name="Page_6" id="Page_6">[Pg 6]</a></span></p> -<div class="chapter"> - <h2>CHAPTER II<br /><span class="h_subtitle">THE BASIS OF LIFE</span></h2> -</div> - -<p>Protoplasm was defined by Huxley as “the physical basis of life.” It -is the material substance which lives. There is no life in anything -which does not consist of, or is not supported upon, or permeated by -a system of filaments of protoplasm. Huxley’s definition indissolubly -links in thought protoplasm and life. But it is doubtful whether the -definition is in any sense axiomatic. The adjective “physical” has too -narrow a range. If the biologist could say to the chemist, “Here is a -substance which was alive. If I could restore to it the energy which -it has lost, if I could impart to it the movement which I recognize -as life, it would again be alive,” he would offer the chemist a -substance susceptible to the methods of his science, something which -he could analyse. If, approaching the physicist with a group of -chemical products, he could say, “Into these protoplasm broke up on -dying. I cannot assure you that while it was alive they were combined -into molecules within your meaning of the term. There may be no such -‘substance’ as protoplasm in the sense in which you understand the -word, but so long as this mass lived these various familiar compounds -were bound together in a supermolecular form. Death was their falling -apart. If I could cause them to recombine, they would be alive,” he -would give the physicist a problem within the range of his methods. The -physicist could devise a method for measuring these units. The science -<span class="pagenum"><a name="Page_7" id="Page_7">[Pg 7]</a></span> -which can weigh an electron, the thousandth part of an atom, need not -fear failure in its attempt to gauge the size of units of structure -composed of groups of heavy molecules, albumins, globulins, and other -proteins,<a name="FNanchor_1_1" id="FNanchor_1_1"></a><a href="#Footnote_1_1" class="fnanchor">[1]</a> -with the inclusion, perhaps, of fats, sugars, inorganic salts. But -herein lies the biologist’s dilemma. He cannot assert that there exists -a homogeneous substance, protoplasm. He cannot assert that there exists -a definite tectonic grouping of heterogeneous substances which, so -long as it is maintained, constitutes a physical basis capable, and -alone capable, of exhibiting the phenomena of life. Protoplasm is -still a hypothetical substance—a name. Truly, in the absence of -nitrogen-containing compounds of very complicated chemical constitution -there is no life. All living things yield on chemical analysis -approximately the same nitrogenous substances. No one can say whether -the capacity for living is dependent upon the molecular—that is -to say, the chemical—constitution of the basis, or whether it -is dependent upon the arrangement of its molecules, its form. It is -even open to question whether instability, the capacity for incessant -change, both in chemical composition and in form, be not the condition -which differentiates living matter from dead. “Physical basis” is too -hard a term for this elusive concept of the matter which exhibits life.</p> - -<p>If it were possible by a process of elimination to ascertain the -substances which must be present in protoplasm, the physiologist might -formulate a reasonable hypothesis as to the nature of this “basis.” -But there is no part of any living thing, or, at any rate, no part -which is not microscopic in its dimensions, which can be pointed -out as protoplasm and nothing besides. It is impossible to isolate -anything which can be described as pure protoplasm. Nor is it possible, -by comparing various tissues which are acknowledged to be rich in -protoplasm, to ascertain what chemical substances are common to them all.</p> - -<p>If it were feasible, by analysing a number of specimens of protoplasm, -to make sure that, although <i>x</i> is absent from one, <i>y</i> from another, -and <i>z</i> from a third, some one thing, <i>P</i>, is always present, then -<i>P</i> might be regarded as the physical basis, even though it were evident -that <i>P</i> alone was not protoplasm. Protoplasm would be <i>P</i> combined -with either <i>x</i>, <i>y</i>, or <i>z</i>. Globulins and albumins and other -proteins are always present, but in varying proportions; but it is impossible -to make certain that either of these chemical substances is more important -than the rest. Nor is it possible to assert of either that it is essential. -<span class="pagenum"><a name="Page_8" id="Page_8">[Pg 8]</a></span></p> - -<p>Chemically, protoplasm is a mixture of substances, chiefly proteid, -in a condition in which it is capable of manifesting the phenomena -of life. But whether it be more complex and of heavier molecule than -either globulin, nucleo-protein, albumin, fibrin, or any other of the -nitrogenous compounds which take its place when it is dead; or whether -it be as simple as either of these, but differ from them all in its -instability, in the constant flux of its atoms, which causes it at one -time to incline towards one of them, at another time to another, are -questions which cannot at present be answered.</p> - -<p>The uncertainty as to the chemical nature of protoplasm is responsible -for an unfortunate irregularity in the use of the term. It is <i>ex -hypothesi</i> the most active, the most living part of an animal cell. -If the cell has a nucleus and an envelope, the protoplasm must lie in -the space between the two. This part of the cell is therefore often -termed, without qualification, the “cell-protoplasm.” Frequently the -abuse of the word is carried still further. Young cells, leucocytes, -nerve-cells, etc., which have no envelope, consist of a nucleus -embedded in soft cell-substance. The latter is termed its protoplasm. -The cell is described as consisting of nucleus and protoplasm, the term -assuming an anatomical signification. Not only is such a use of the -term bad, because it indicates a confusion of thought, but it brings -with it a train of ambiguities. What are the limits of the protoplasm? -If the cell-body be firmer towards its exterior than it is within, is -the denser substance protoplasm, or is it not? It has not the qualities -which are attributed to protoplasm in so marked a degree as has the -substance which it surrounds. Hence a distinction is made. The one -is “ectoplasm,” the other “endoplasm.” Within the cell-body are many -collections, often in the form of granules, of substances which have -not the protoplasmic attributes. They constitute the “deuteroplasm” -of certain cytologists. But these enclosed substances may be as far -removed from protoplasm as starch grains. It is absurd to use the -termination “plasm” for such well-defined products of cell activity -as these. The subject is, unfortunately, obscured by conflicting -terms. Nomenclatures which were invented with the object of giving -definiteness to our ideas have served but to perplex them. The term -<span class="pagenum"><a name="Page_9" id="Page_9">[Pg 9]</a></span> -“protoplasm” should be reserved as a synonym for the substance which -is most alive, the substance in which chemical change is most active, -the substance which has in the highest degree a potentiality of growth. -Anatomical distinctions are better expressed in anatomical terms. We -shall treat of such distinctions when considering the organization of -the cell.</p> - -<p>In the meantime it may be well to consider the attributes which appear -to belong to this most living substance. Its chemical composition can -be inferred only from the compounds found on analysis to be present -in a mass of organized substance which there is reason for thinking -was rich in protoplasm while it was alive. The compounds found vary -within certain limits. The quantity of water associated with these -compounds is still more variable. Water is essential to the existence -of protoplasm. Its power of combining with water in variable quantities -is one of its characteristics. Tissue rich in protoplasm yields on an -average about 75 per cent. of water. Part of the protoplasm within a -cell holds more water associated with it, part less.</p> - -<p>Closely associated with its power of holding water is its tendency to -assume an architectural form. In large vegetable cells, such as those -of the hairs within the flowers of Tradescantia, the protoplasm may be -seen, under the microscope, arranged in threads containing granules -which are incessantly streaming up and down them. The spaces between -the threads are filled with water. Such mobile protoplasm cannot be -said to have a structural form. But in the greater number of cells, and -especially in animal cells, the protoplasm is disposed in a network, -with usually a tendency for the strands of the network to set in lines. -In attempting to define these very variable networks, the microscopist -is obliged to speak with caution. He finds it very difficult to -distinguish between appearances which he is justified in regarding as -inherent in the cell-substance, whether alive or dead, and appearances -which he may have induced by the action of reagents whilst preparing -the tissue for examination. Rarely can he assert that he sees a network -in a living cell. When examining a dead cell, he is bound to recognize -that the preservatives and hardening reagents which he used may have -<span class="pagenum"><a name="Page_10" id="Page_10">[Pg 10]</a></span> -caused the proteins to coagulate in a particular pattern. If he obtains -the same pattern with several different methods, he infers that the -appearance which he sees is that of a structure existing in the living -cell; but he is never quite sure that it is not an arrangement produced -by reagents after death.</p> - -<p>The tendency of protoplasm to dispose itself in the form of a network -or sponge-work is of the greatest interest in its bearing upon the -theory of its activity in effecting chemical change. The body itself, -as we shall find later, is a network of tissues enclosing lymph. -The lymph in the tissue-spaces contains foods and waste products in -solution. The tissues are constantly taking from it the former, and -discharging into it the latter. Every cell is, microscopically, a -tissue. The strands of its protoplasm are perpetually sorting foods -from its cell-juice, adding to its cell-juice waste products. By -diffusion, foods, including oxygen, pass from lymph to cell; waste -products, including carbonic acid, pass from cell to lymph. If water -be added to gum, the gum swells. The mixture is homogeneous. Diffusion -takes place slowly through the mucilage. When water is taken up by -protoplasm, the protoplasm swells; but the mixture is not homogeneous. -The protoplasm expands as a wet sponge expands, although the relation -of the enclosing reticulum to the water which it encloses is far more -complicated. It is, as it were, a sponge made of gum. Some water is -combined with the protoplasm; the remainder fills its spaces. There is -an active surface relation between the free water and the protoplasmic -threads. As water rises in a capillary tube, as it passes from the inside -to the outside of a flannel shirt, so it circulates within the cell.</p> - -<p><b>Irritability</b> is a property commonly attributed to protoplasm, -but it is a little doubtful whether there be not again some danger -of an illogical use of terms. An amœba, one of the unicellular -organisms found in ponds, has the power of moving. If a piece of a -water-plant—the stalk of duck-weed is a suitable object—be examined -with the microscope, these little animals are usually to be found upon -its surface. They feed upon algæ more minute than themselves. When they -come in contact with something suitable for food, their body-substance -flows around it. The food is coagulated. So much of it as is digestible -is digested; the remainder is extruded. Constantly parts of the -<span class="pagenum"><a name="Page_11" id="Page_11">[Pg 11]</a></span> -body-substance are protruded, other parts retracted, in the search -for food. Such movement is a response to stimulus. Stimuli received -at one part of the body-substance are transmitted to another. The -body-substance is irritable. It acknowledges stimuli; it conducts -them. But if the amœbæ are watched until, owing to lack of oxygen or -other cause, they die, their irritability comes to an end. It is a -phenomenon of life. Again the physiologist is in a dilemma. Either -protoplasm is not protoplasm when death has supervened, or protoplasm -is not irritable as such. It is somewhat paradoxical to ascribe to the -physical basis of life a property which depends upon its being alive.</p> - -<p>Yet the influence on protoplasm of anæsthetics makes it difficult to -understand how it can be either physically or chemically a substance -which loses its form or changes its constitution whenever it ceases -to display the usual evidences of its existence. Chloroform and -similar agents suspend irritability. Yet irritability returns as their -influence passes off. They appear to hold it in check without—at any -rate visibly—changing the nature of the irritable substance.</p> - -<p>All parts of the minute body-substance of an amœba are equally -irritable. In higher animals irritability is concentrated in the -nervous system. The form of irritability to which consciousness is -adjunct is restricted to the cortex of the great brain.</p> - -<p>Chloroform and similar agents are termed “anæsthetics” because they -abolish the irritability of the cortex of the great brain, before -their effects upon other parts of the nervous system are sufficiently -pronounced to endanger the working of the animal machine. Pain ceases -to be felt before the dose of anæsthetic is sufficient to suspend the -irritability of the centres of reflex action. All protoplasm, whether -animal or vegetable, is susceptible to the influence of these agents. -They cause it to enter into a state which resembles death in all -respects save the impossibility of revival. There is a great demand -in the Paris flower-market for white lilac in the winter. The plant -cannot be forced until after a period of rest. By withholding water and -placing the bushes in a cool, shady place, horticulturists endeavour -to send them prematurely into their winter sleep. Recently it has been -found that from three to four weeks can be gained by placing the bushes -<span class="pagenum"><a name="Page_12" id="Page_12">[Pg 12]</a></span> -for a couple of days in an atmosphere charged with the vapour of -ether. Some change of state is evidently produced in protoplasm by -anæsthetics. It ceases to be capable of receiving or transmitting -stimuli. But we cannot picture the change as being sufficiently -pronounced to justify the hypothesis that so long as it is irritable -protoplasm is a complex substance which is resolved, as it loses its -irritability, into simpler compounds familiar to the chemist. Perhaps -it would be more correct to say, we cannot picture these chemical -substances as reuniting into protoplasm when the effect of the -anæsthetic passes off. Rather are we driven to think of living matter -as a mixture of many substances in a state of molecular interchange, -and to suppose that the activity of this interchange is diminished by -anæsthetics.</p> - -<p><b>Chemical activity</b> is a property of protoplasm. In its network -combinations and decompositions are effected more extensive in range -than any which a chemist can cause to occur in his laboratory. From -ammonia, carbonic acid, and water, a plant makes albumin. A chemist -cannot make albumin, no matter how complex may be the nitrogenous -substances which he endeavours to cause to combine. Albumin is resolved -by animals into water, carbonic acid, and urea. Cells of the gastric -glands set a problem which puzzles the chemist by making hydrochloric -acid from sodic chloride without the intervention of a “stronger” acid. -Many other illustrations of the same kind might be cited. Although the -tissues of animals act chiefly as destroying agents, their protoplasm -is not without constructive power. There is apparently no limit to the -capacity for synthesis of plants. The chemistry of living things may -be divided into two provinces, absolutely antagonistic in the series -of reactions which they comprise. The one series is constructive, -synthetic; the other destructive, analytical. Construction involves -the locking up of energy. It is endothermal. Destruction results in -the setting free of energy. It is exothermal. To accomplish synthesis -energy must be added. Plants obtain it from the sun’s rays. Animals -disperse energy, set free by the analysis of substances formed in -plants, in maintaining their bodies’ warmth and movement. -<span class="pagenum"><a name="Page_13" id="Page_13">[Pg 13]</a></span></p> - -<p class="space-below1">The chemistry of the laboratory and the -chemistry of protoplasm present certain contrasting features. A -chemist reaches the compound which he wishes to form by effecting -a series of interchanges. For example, he wishes to form uric acid -by uniting a nucleus contained in lactic acid with urea. First he -introduces chlorine and ammonia into the molecule of lactic acid. He -makes trichlorlactamide. Then he heats (supplies energy to) a mixture -of trichlorlactamide and urea. Two of the chlorine atoms carry off -hydrogen atoms from the urea. A third leaves the trichlorlactamide with -its ammonia. Water also breaks away. Uric acid remains.</p> - -<table border="0" cellspacing="0" summary=" " cellpadding="0" > - <tbody><tr> - <td class="tdc">Trichlorlactamide</td> - <td class="tdc">Urea</td> - <td class="tdc">Uric Acid</td> - <td class="tdc" colspan="3"> </td> - </tr><tr> - <td class="tdl"><b>CCl₃CH.OH.CO.NH₂ +</b></td> - <td class="tdl_ws"><b>2(NH₂)₂CO =</b></td> - <td class="tdl_ws"><b>C₅H₄N₄O₃</b></td> - <td class="tdl_ws"><b>+ NH₄Cl</b></td> - <td class="tdl_ws"><b>+ 2HCl</b></td> - <td class="tdl_ws"><b>+ H₂O.</b></td> - </tr> - </tbody> -</table> - -<p class="space-above1">In this example the trichlorlactamide may be -said to exchange its chlorine and ammonia for urea. When he planned -the reaction, the chemist foresaw what would happen. He knew that -if he weakened the grip of the lact radicle upon them, chlorine and -hydrogen, chlorine and ammonia, oxygen and hydrogen, would take the -opportunity of getting away together. The lact radicle and urea would -be left with dangling arms, which must “satisfy their affinities” by -linking up. It would be rash to assert that any reaction is impossible -to Nature’s chemistry; but it may safely be said that the reactions -which protoplasm effects are, so far as we know them, of a different -type from this laboratory example. Uric acid is the chief excrement -of birds. It is made in the liver. If the liver is shut off from the -circulation, lactate of ammonia is excreted in the place of uric acid. -It is therefore, in all probability, lactate of ammonia which the liver -transforms into uric acid. We cannot pretend to say how this is done, -although an empirical formula for the change might be drafted easily -enough.</p> - -<p>Lactate of ammonia has the formula <b>NH₄C₃H₅O₃</b>. Uric acid, <b>C₅H₄N₄O₃</b>, -contains a much higher percentage of nitrogen. It could be produced -from lactate of ammonia by the condensation of the nitrogen-containing -nucleus and the addition of a sufficient amount of oxygen to complete -the oxidation of the superfluous carbon and hydrogen into carbonic acid -<span class="pagenum"><a name="Page_14" id="Page_14">[Pg 14]</a></span> -and water. It is of little interest to count the number of atoms -concerned in this process. If a bird be fed upon urea, or even upon -various salts of ammonia, its liver will change them into uric acid. -Lactate of ammonia is the nitrogen-containing compound with which the -liver has normally to deal. It can handle almost any other combination -of nitrogen with equal ease. In the protoplasm of the liver the atoms -in the molecule of lactate of ammonia are rearranged. The molecules are -condensed; water is set free; oxidation occurs. It seems almost as if -molecules, when in contact with protoplasm, lose their individuality. -Their atoms fall into new groups. Chains which the chemist finds -so difficult to break—chains from which he can remove a link only -by insinuating another and a stronger—are, when in contact with -protoplasm, groups of isolated links. The links rearrange themselves. -They join into new circlets, larger, smaller, more open, closer. As -grains of sand on a metal plate group themselves in harmony with the -vibrations caused in the plate by drawing a violin bow across it, so -the atoms answer to the forces which set protoplasm vibrating. There -is no waste of force. The chemist may need to enclose sawdust and lime -in a crucible heated in an electric furnace if he wishes to compel -them to combine as carbide. He supplies energy enormously in excess of -the amount which the new compound will lock up. Under the influence of -protoplasm the reactions which occur are exactly proportional to the -amount of energy supplied. Or, if it be a reaction by means of which -energy is set free, it occurs spontaneously. No energy is absorbed in -setting it going. All the energy liberated is effective. The chemist -very frequently needs to heat a substance in order to cause it to -decompose, even though it be falling from a less stable to a more -stable state.</p> - -<p>Vital chemistry and mineral chemistry are so widely different in their -methods that one is tempted to think of them as different in kind. We -find it very difficult to look at both from the same point of view. -Men’s minds are preoccupied with the things that they have to do for -themselves. The chemistry of the laboratory is seen as a science -circumscribed by the laboratory walls. If it were possible to stand -outside, it would be evident that it is only a part of the science -of molecular change. Matter changes its state under the influence of -<span class="pagenum"><a name="Page_15" id="Page_15">[Pg 15]</a></span> -force. Many rearrangements are effected by the chemist which do not -occur in nature. He has an almost infinite range of action. Yet many -of the rearrangements of matter and force which are occurring in the -dandelion on his window-sill (if the fumes of sulphuretted hydrogen -have not killed it) he is unable to reproduce. It is largely a question -of waste. Nature works with greater precision than the chemist; but the -chemist could do all that Nature does if he had but the same control of -force.</p> - -<p>We have spoken of the reactions which occur in protoplasm as divisible -into two great series—the one ascending, constructive, endothermal; -the other descending, destructive, exothermal. In the one series -energy is locked up; in the other series it is set free. Synthesis and -analysis are names applied to the two series respectively. Synthesis -is characteristic of plants, although analysis is also perpetually -occurring. Plants fix carbon from the air and liberate oxygen. They -also respire, setting free carbonic acid. Analysis is characteristic of -animals, although synthesis is not excluded.</p> - -<p>Of the chemical processes which occur in plants very little is known. -Few halting-places between raw materials and finished products can be -marked. The final products are sugars and starches, oils, proteins, -and a vast number of other substances—alkaloids, glucosides, etc. -Condensation, dehydration, and deoxidation are the methods by which the -synthesis of these compounds is accomplished. These methods are adopted -simultaneously in varying degree. The large group of bodies known as -sugars and starches are, with few exceptions, built on the <b>C₆H₆</b> model; -in fruit-sugar, <b>C₆H₁₂O₆</b>, six atoms of carbon are linked to one another -and to six molecules of water. The formula of starch is <b>(C₆H₁₀O₅)ₙ</b>. Not -only has water been removed from the molecule, but an unknown number of -molecules have been linked together. This condensation and dehydration -is effected whenever sugar carried in cell-sap is deposited as starch -in seeds or tubers. These compounds are hexatomic. The chemist -pictures them as made by the union in the first place of six atoms. As -small drops unite to form larger ones, so small molecules, under the -direction of the protoplasm of plants, close together. -<span class="pagenum"><a name="Page_16" id="Page_16">[Pg 16]</a></span></p> - -<p>The reactions which characterize animal protoplasm are of a different -kind. They belong to the descending series. Close molecules are -unfolded. Water is incorporated with them. Hydrogen and carbon are -oxidized into water and carbonic acid. The conversion into sugar of -glycogen or of starch may be taken as an illustration of expansion. -Starch, <b>(C₆H₁₀O₅)ₙ</b>, becomes maltose, <b>C₁₂H₂₂O₁₁</b>, and then dextrose, -<b>C₆H₁₂O₆</b>. The grouped molecule of starch opens out. The breaking of the -double molecule of maltose into two molecules of dextrose is a further -illustration of progress towards simplicity. Hydration, union with <b>H₂O</b>, -accompanies this expansion. Hydrolysis is the secret of almost all -digestive acts. Starch is hydrolysed into sugar, fat hydrolysed into -glycerin and fatty acid, proteins hydrolysed into peptones.</p> - -<p>All the chemical transformations which protoplasm is able to accomplish -are of the nature of fermentations. The term <b>fermentation</b> -was first applied to the effervescence which occurs in grape-juice -when its sugar is being converted into alcohol, carbonic acid gas, -and certain substances which appear in relatively small quantities. -It was discovered later that the yeast which effects this change -is a unicellular plant. The term “fermentation” was extended to -the production of vinegar from alcohol, and eventually to all such -reactions as are carried out by living organisms, or by the secretions -or products of living organisms, without the destruction of the agent -which is effective in the process. A ferment is an organic body which -brings about changes in other bodies without itself undergoing change. -At the end of the process, however prolonged, there is as much ferment -as there was at the beginning, and its chemical nature is the same. -Rennin has been made to curdle nearly a million times its weight of -milk, pepsin to digest half a million times its weight of fibrin. As -the ferment is not consumed, there is no relation, except one of speed, -between the ferment and the quantity of fermentable substance which it -is able to transform. We said that a ferment is an organic body. It is -necessary to introduce the qualification “organic,” because certain -reactions termed “catalyses” which occur in mineral chemistry resemble -fermentations in respect of the non-destruction of the agent which -serves as intermediary. If a solution of cane-sugar containing a very -<span class="pagenum"><a name="Page_17" id="Page_17">[Pg 17]</a></span> -small quantity of sulphuric acid is boiled, the cane-sugar is -“inverted.” It is changed into a mixture of fruit-sugar and levulose. -The ferment invertin of the gastric juice and of intestinal juice -produces a similar effect; and just as invertin remains unchanged, so -also the sulphuric acid is found in the mixture unchanged in nature and -in amount after an unlimited inversion of cane-sugar. Great stress was -formerly laid upon the similarity between fermentation and catalysis. -It has now been shown that catalytic actions are not necessarily of the -same nature as fermentation, although the results and, as far as is -visible, the means are similar. For example, finely divided platinum -(or, better, palladium) causes an indefinite quantity of oxygen and -hydrogen to unite. The reaction comes within the category of catalyses. -But it is widely different from a fermentation. The metal causes -hydrogen to condense, and actually absorbs it into its surface layer. -In the liquid form hydrogen cannot resist combination with oxygen. This -may be termed a “physical phenomenon,” adopting the common distinction -between chemistry and physics. There is no reason for thinking that -fermentations can be explained in so simple a way. They may, however, -be grouped under the designation “catalyses.” As the initial conditions -and final results are similar, it is inevitable that fermentations and -catalyses should obey the same “laws” as to mass action, speed, effect -of accumulation of products of action, and the like; but it does not -follow that invertin and sulphuric acid produce their effects in the -same way. Fermentations are instances of catalysis, but all catalytic -actions are not fermentations.</p> - -<p>So far from dwelling upon the resemblance between fermentation and -the catalysis of mineral chemistry, chemists nowadays incline to -regard fermentation as essentially a reaction of life. It is very -difficult, when attempting to present ideas which are new to thought, -to adapt, without ambiguity, existing words. It would be absurd to -talk of a substance removed from yeast or bacteria or blood-corpuscles -by a process which involves cooling with liquid air, grinding with -powdered glass, solution in water, precipitation with absolute alcohol, -and resolution in water, as alive. Yet, unlike any known mineral -product, it is easily killed. Ferments are not destroyed by cold, but -their activity is arrested. They are most active at about the body -<span class="pagenum"><a name="Page_18" id="Page_18">[Pg 18]</a></span> -temperature. Their activity is annihilated by heating them, in -solution, to the temperature at which albumin coagulates—a little -over 50° C. Although they are not alive, their behaviour very closely -resembles that of living matter. They can be obtained only from living -things. They produce their effects even though they are present in -almost infinitely small quantity. It is impracticable to make a -chemical analysis of a ferment, owing, in the first place, to the very -small amount available for analysis, and, in the second place, because -of the impossibility, with existing methods, of obtaining a ferment -pure. The amount of ferment present in even a great mass of yeast, -or in many pounds of salivary gland or pancreas, is extremely small. -However prepared, it is always accompanied with proteid substances. -It is impossible to say whether ferments, like proteins, have heavy -nitrogen-containing molecules. The fact that they are not diffusible -suggests that they have.</p> - -<p>It would be straining language to term fermentation a phenomenon of -life; worse, to define life as a sequence of fermentations. Yet it -is safe to say that all the chemical changes carried out by living -organisms are fermentations. Fermentation and the chemistry of life are -almost synonymous terms.</p> - -<p>A very large number of ferments are already known. Each has its own -specific work to do: “To every fermentable substance is fitted a -ferment, as a key to a lock.” It will be understood, from what has been -already said regarding our inability to determine the composition of -any ferment, that we cannot say whether or not these various ferments -differ one from another in chemical constitution. They are classified -according to their action, and not according to their nature. Those -which build up are termed “synaptases” (συνάπτω, I unite); those -which decompose, or hydrolyse, “diastases” (διάστασις, separation). -The termination “ase” is added to the name of the substance upon which -the ferment acts, except in cases in which other terms have already -become so general as not to be displaceable: amylase, hydrolysing -starch; sucrase, inverting cane-sugar; protease, hydrolysing proteins. -Unfortunately, there is little uniformity in this nomenclature; -amylopsin, invertin, pepsin, are terms used as often as those -terminating in “ase.” As a distinguishing termination, “in” or “sin” -is less desirable than “ase,” owing to the fact that it has been -<span class="pagenum"><a name="Page_19" id="Page_19">[Pg 19]</a></span> -appropriated already as the termination of the names of -albuminoids—<i>e.g.</i>, gelatin, chondrin, mucin.</p> - -<p>The various ferments are substances which protoplasm sets aside for -specific purposes. Primitively, contact with the substance to be -fermented determined the nature of the ferment assigned to the task. -There are reasons for thinking that protoplasm still retains its power -of making a suitable response; cases may be cited in which the lock -presented to protoplasm shapes the wards of the key. In such cases the -fermentable substance provokes the formation of the ferment. But, for -the most part, in situations where particular ferments are regularly -needed, protoplasm has acquired the habit of making such ferments and -no others. The cells of salivary glands accumulate ptyalin, the cells -of gastric glands accumulate pepsin, during the intervals between meals.</p> - -<p>The capacity of protoplasm for producing a new ferment when it is -needed is shown by such examples as the following: Blood-plasm contains -a variety of proteid substances. If a solution of white of egg be added -to it, the mixture is clear and uniform. Yet egg-albumin is treated by -the blood as a foreign body, a poison. When injected into the veins of -a living animal, some of it is excreted by the kidneys, some destroyed -in the blood-stream. If several successive doses of egg-albumin are -injected into an animal (it is most convenient to inject it into the -peritoneal cavity), the power of the blood to destroy the intruder is -greatly increased. If now a specimen of blood be taken, and the plasma -or serum mixed with egg-albumin, the mixture is no longer clear. The -egg-albumin is precipitated. The blood of the animal thus “prepared” -has developed a ferment, termed a “precipitin,” which throws down -egg-albumin. If instead of egg-albumin, which, although a foreign body, -is comparatively innocent, a substance which is distinctly poisonous, -toxic, be injected into an animal, the first dose, if a large one, will -prove fatal. If, however, the first dose be small, and succeeding doses -progressively larger, the animal acquires the power of tolerating a -quantity of the poison much larger than would have proved fatal in the -first instance. A classical example of this, because it afforded an -opportunity of directly observing under the microscope the difference -<span class="pagenum"><a name="Page_20" id="Page_20">[Pg 20]</a></span> -between “unprepared” blood and blood from an immune animal, is the -acquisition by a mammal of the power of tolerating the injection of the -blood of an eel. Eel’s blood contains a toxin which destroys the red -blood-corpuscles of a mammal. The dissolution of the blood-corpuscles -may be watched with the microscope. If successively increasing doses of -serum of eel’s blood be injected into the body of a rabbit, the rabbit -acquires the power of resisting the toxin. Further than this, the serum -of the immune rabbit injected into a rabbit which has not been prepared -confers immunity upon the latter. If the blood of the prepared animal -be mixed with the blood of an unprepared rabbit and with eel’s serum, -and the mixture examined under the microscope, it will be seen that -red blood-corpuscles are no longer dissolved. The immune serum is able -to save the blood-corpuscles of the unprepared blood from destruction. -During its course of preparation the rabbit developed an antitoxin.</p> - -<p>If germs of diphtheria are injected into the blood of a horse, the -first injections give rise to marked febrile symptoms. After a number -of injections the horse becomes completely tolerant of the virus. Not -only does its blood develop sufficient antitoxin to protect it against -the toxin of diphtheria, however large may be the quantity injected -into its system, but the serum of the prepared horse, when injected -beneath the skin of a child suffering from diphtheria, carries with it -sufficient antitoxin to destroy the toxin which has gained admission to -the child’s blood.</p> - -<p>Many more instances might be cited of this capacity of developing -“antibodies” of protoplasm. The leucocytes of the blood are incessantly -adapting their chemistry to the needs of the economy. All the tissues, -it may be supposed, possess the power of developing resistant ferments; -but the leucocytes (<a href="#I_060">Fig. 4</a>) are the undifferentiated -cells, the maids-of-all-work. They have not specialized as makers of ptyalin -or makers of pepsin. They are not completely given up to lifting weights, -like muscles, or carrying messages, like nerves.</p> - -<p>Bacteria are the world’s scavengers. To them ultimately belongs the -task of reducing organic matter to the salts which plants reorganize. -The cycle of life would be broken if bacteria were suppressed. No -sooner has an animal fallen than these little agents commence their -<span class="pagenum"><a name="Page_21" id="Page_21">[Pg 21]</a></span> -beneficent task of resolving its carcass into air and soil. Birds -and insects may interrupt their work. They may steal portions of the -derelict, use them for fuel, or patch them between their own ribs. But -they, too, will soon lie breathless on the ground; and the bacteria -are always ready to finish their interrupted task. Why should they -wait until the slight change occurs, important to us, but of little -consequence to them, which marks the transition of living protoplasm -into dead proteins? There is nothing in the constitution of protoplasm -which makes it harder to break up than protein. There is no quality -inherent in living matter which makes it resistant of decay. We resent -the officiousness which prompts bacteria to obtain entrance into the -ship while it is still under full sail, with a view to commencing the -work of demolition. Deep in our minds lies the conviction that it is -contrary to the rules of Nature. We are especially annoyed at the many -ruses bacteria adopt to disguise their personalities. The bacteria of -the soil we can keep at a proper distance. But bacteria of the stream, -bacteria of milk, bacteria of the breath that would betray us with -a kiss! It is hard to recognize that they are fairly and squarely -playing their part. Birds and insects we can beat off with our hands. -Our invisible enemies are everywhere. They are constantly insinuating -themselves through scratches in the skin, through abrasions in the -mouth, through surfaces of the intestine left unprotected owing to -the desquamation of its epithelium. But if we are constantly open to -attack, we are policed by myriads of zealous leucocytes, ever ready to -reduce the invaders to impotence. The germs which have found entrance -fire off a toxin. The leucocytes reply with an antitoxin. There is -absolutely no limit to the power of protoplasm to protect itself, if -only it be not taken by surprise. It can resist any organic poison if -it is allowed a sufficient time to produce the antipoison. The ferment -of pancreatic juice, trypsin, is a poison which is unlikely to find its -way into the blood. When injected it produces disastrous results owing -to its immense activity in digesting proteins. An animal “prepared” by -the injection of successive doses of trypsin develops an antitrypsin. -Injection of pancreatic juice no longer does it any harm. Tapeworms -which live in the intestines are bathed in pancreatic juice; they are -constantly exposed to its digestive action. They are not digested, -<span class="pagenum"><a name="Page_22" id="Page_22">[Pg 22]</a></span> -because they secrete an antibody which prevents the development of -the activity of trypsin. It is not in this case, strictly speaking, -antitrypsin. It is antikinase, a substance which, if extracted from the -bodies of tapeworms and added to pancreatic juice, renders it incapable -of digesting albumin. The antikinase does not destroy trypsin, but -destroys kinase, the co-operation of which is essential to its activity.</p> - -<p>Not only has protoplasm the power of meeting with an antiferment any -ferment which might prove prejudicial to its own integrity; but after -it has been once attacked it continues to defend the vulnerable spot. -Its tactics are, it must be confessed, somewhat like those of the dusky -warrior who, during his first lessons in the art of boxing, made a -point of covering with his fist the place where he had just been hit; -but even its power of remembering its last injury is of supreme value -to the human race. Before the age of sanitary science, and even, in -certain backward communities, in these days of its beneficent rule, -conditions producing disease were not necessarily set right as soon -as the epidemic was over. The close-packed inhabitants of a ghetto -were continuously exposed to germs of typhoid fever, small-pox, -whooping-cough. But after their protoplasm had once responded to -the need for the production of an antigerm, it either continued for -many years to keep a stock in hand, or it kept the recipe within -easy reach. The memory of protoplasm is amazing. It is commonly said -that vaccination is an absolute protection for seven years. There is -no doubt but that the immunity from small-pox which it induces, if -gradually lessening, lasts for life. The disease, if it attacks a -person who has been vaccinated in infancy, is relatively harmless.</p> - -<p>Inoculation, vaccination, is the boxing-master’s method of utilizing -the self-protective instinct of the dusky warrior. Knowing that his -pupil will for a long while continue to cover an injured spot, he asks -himself: “Where is he most likely, when it comes to a serious contest, -to be hit?” Then he gives him a gentle tap in that particular place. -Does he need to know how to defend himself against small-pox? Give him -cow-pox. Is he likely to receive a knock-down blow from typhoid fever? -Just show him what it feels like to have a gentle shake. Educate his -<span class="pagenum"><a name="Page_23" id="Page_23">[Pg 23]</a></span> -protoplasm to make antityphoid ferment, by giving him the typhoid germ -in such an attenuated form that it cannot do him any harm.</p> - -<p>The chemistry of protoplasm is a science which is growing rapidly, -or, to speak less arrogantly and more correctly, our knowledge of the -ways of protoplasm, the Chemist, has greatly increased during the last -few years. We can but watch protoplasm at work. Our experiments, so -called, are but windows which we open in the walls of his laboratory. -We cannot take the work out of his hands. The methods of mineral -chemistry are useless in this search for knowledge. And, naturally, -the longer we watch, the more details do we discover in what seemed at -first a generalized procedure. We recognize that several manipulations -are required in the carrying out of a reaction which hitherto we -believed to take place in a single stage. This is not the place in -which to give an account of a subject regarded as belonging, owing to -its applications, to the province of pathology. But Nature is one, -however many be the companies into which we divide the explorers of -her secrets. We have attempted the merest outline of the observations -made up to the present, and have submitted the results for the sake of -the light which they throw upon the way in which ferments are prepared -as they are wanted to meet the needs of normal every-day digestion and -metabolism, rather than for the purpose of showing the methods by which -protoplasm combats disease.</p> - -<p>Amongst the chemical phenomena of life is respiration. -<b>Respiration</b> in this very general sense means oxidation. The -force which is exhibited in living is obtained from the union of -organic materials with oxygen under the direction of protoplasm. -This is true of plants as well as of animals. It is true even of the -subdivision of bacteria, termed anaerobic, because they cannot live in -air. They secrete ferments which enable them to decompose compounds -which contain oxygen, in order that they may use the oxygen for -respiration. It might have been supposed that green plants which are -receiving radiant energy from the sun would convert this energy into -the forces which enable protoplasm to display the phenomena of life. -But this is not so. The energy which green plants obtain from the -sun is used in constructive metabolism, and not in maintaining life. -Life-force, if we may use the expression, is derived from the oxidation -<span class="pagenum"><a name="Page_24" id="Page_24">[Pg 24]</a></span> -of the substances which the sun’s rays enable the plant to make. A -plant, equally with an animal, respires. The distinction between the -constructive metabolism of a plant and its respiration may be brought -out in a striking way by administering to it sufficient anæsthetic -to stop the former without stopping the latter. It may be paralyzed -without being killed. If a water-weed—potamogeton is the most -convenient—enclosed in a bell-glass filled with water and inverted -over a dish of water, is placed in sunshine, bubbles of gas rise from -the plant. They accumulate at the top of the bell-glass. If the gas be -removed and analysed, it is found to be oxygen with a small admixture -of carbonic acid. If a second bell-glass containing water-weed be -exposed under the same conditions in all respects, save that a small -quantity of chloroform is added to the water, the gas that collects -at the top of the bell-jar will be much less in amount. It will be -found to be carbonic acid without admixture of oxygen. The power which -chlorophyll possesses of decomposing carbonic acid with fixation of -carbon and liberation of oxygen is suspended by the anæsthetic; whereas -respiration is not interfered with.</p> - -<p>Lastly, we must attribute to protoplasm a <b>capacity of growing</b>. -The activity of protoplasm depends upon constant molecular interchange. -It incorporates molecules of food. It excorporates molecules of waste. -If food is abundant and “vitality” exuberant, it takes in more than it -gives out. It grows.</p> - -<p>If we attempt to formulate a definition of protoplasm, we find that our -ideas are far from clear, owing to want of knowledge. The questions, -What is protoplasm? What is life? are equally unanswerable. Their -definition is reciprocal. Protoplasm is the substance, the material, -which exhibits life. Life is the complex of phenomena exhibited by -protoplasm. All parts of the body are alive, in their degree. The -nucleus of a cell lives, as well as its cell-body. Its capsule may be -less alive—that is to say, less vibrant—than the soft cell-substance -which it encloses; but it lives. So-called intercellular substance, or -matrix, is alive. In growing cartilage the matrix does not behave as a -dead substance. It does not crack and gape under the pressure of the -<span class="pagenum"><a name="Page_25" id="Page_25">[Pg 25]</a></span> -dividing and multiplying cell-bodies which it contains. If the windows -of a house were endowed with the power of spontaneously enlarging, the -walls would be crushed. They would bulge, break, tumble. The matrix -of cartilage offers as little resistance to the enlargement of the -cells which it encloses as the plasma of blood to the multiplication -of blood-corpuscles. It grows with the cell-bodies, and must be -considered as divisible into areas, each of which is the periphery of -a cell. Muscle is alive. So, too, are bone, teeth, hair, nails. But as -we proceed outwards we find the quality of aliveness growing less and -less apparent, until at last we acknowledge that it is unrecognizable. -Vibrations diminish in amplitude and in rapidity, until the material of -which the body is made appears to be at rest.</p> - -<p>Biologists apply the term “protoplasm” to the <i>most living</i> substance -of which plants and animals are composed. It may be that there is an -entity, protoplasm. It may be that in certain situations this exists in -an unmixed state. It may be that the degree of aliveness of a tissue -or constituent part of a tissue varies as the quantity of protoplasm -which it contains. The tendency of protoplasm to dispose itself in a -reticulum in the meshes of which other substances accumulate favours -such a view. The cells of the deeper layers of the skin are rich in it. -The superficial layers are composed chiefly of keratin. It is possible -that the network opens out, and its strands grow thinner and thinner, -as keratin accumulates. But it cannot be demonstrated that this is the -case. There is no completely satisfactory reason for concluding that -the life of a cell of the skin resides in its protoplasmic network, -while its keratin is inert.</p> - -<p>Many attempts have been made to prove that living cells contain -something which dead cells do not contain; but no evidence which will -bear sifting has, as yet, been adduced in support of this thesis.</p> - -<hr class="chap" /> -<p><span class="pagenum"><a name="Page_26" id="Page_26">[Pg 26]</a></span></p> -<div class="chapter"> - <h2>CHAPTER III<br /><span class="h_subtitle">THE UNIT OF STRUCTURE</span></h2> -</div> - -<p>Immediately after its discovery in the seventeenth century, the -compound microscope was applied to the study of minute plants and -animals, their organs and tissues. In this connection and for this -purpose the microscope has steadily improved, until perfection has -almost been attained. Calculations based upon the physical properties -of refracting media show that the limits of the assistance which it -can give to the eye have been very nearly reached. One of the first -results of the application of the microscope to the study of parts -of plants was the discovery of their cellular structure. Robert -Brown, looking at slices of cork, saw that its tissue is divided into -compartments. It is difficult to ascertain who it was that first used -the word “cell.” The resemblance of a slice of vegetable tissue or the -surface view of a petal of a flower to honeycomb is so striking that -the same comparison probably occurred to the mind of everyone who saw -it. Further study with better instruments showed that the cells are not -empty. Each cell contains cell-juice, or cell-substance, and in the -centre of the cell-substance a miniature cell, the nucleus. Naturalists -therefore extended the connotation of the term. A cell was no longer a -space with enclosing walls; it had contents. A nucleus was invariably -a constituent of the cell. The cell was regarded as an anatomical -unit, consisting of a wall, cell-contents, and nucleus. In 1839 -Theodor Schwann, using his microscope in the study of animal tissues, -recognized the similarity between animals and plants. Animals also, -he discovered, are aggregations of cells. He enunciated the <b>Cell -Theory</b>. Philosophers are always ready to generalize. It is their -business. Seeing that vast numbers of organisms are single cells, that -<span class="pagenum"><a name="Page_27" id="Page_27">[Pg 27]</a></span> -they feed, breathe, divide, and reproduce their kind—in fact, carry -out all the functions of life—as isolated cells, they conceived the -idea that a visible plant or animal is a community of cells, each an -organism in itself. As bees are units of a swarm, as men and women -are units of a state, cells are units which for the sake of mutual -protection remain associated in a multicellular body. The physiological -or sociological aspects of this conception we shall consider shortly; -but the anatomical basis of the cell theory was laid without a -sufficient testing of the facts upon which it rests; or, rather, one -ought to say that, although the axiom, enunciated by Virchow when he -applied the cell theory to tumours and other morbid growths, <i>Omnis -cellula a cellulâ</i>, holds good, the applications of the theory which -certain of its later exponents have made are not necessary sequents.</p> - -<p>Every plant, every animal, commences its existence as a single cell. -An organism which is permanently unicellular divides. Each of the -separate cells into which it divides is a new individual. Higher -plants set aside certain cells as ovules, which in due course, after -conjugation with pollen grains, grow into plants. In the same way the -ova of animals, by repeated cell division, reproduce the species. The -individual commences as a single cell. Its complicated body, composed -of various organs and various tissues, is formed by the multiplication -of cells. Each of the innumerable cells of which it is composed has -the structure, and may therefore be presumed capable of performing -all the various functions, of a unicellular organism. But it does not -follow that the cells retain their individuality. Even unicellular -plants (<i>e.g.</i>, the extraordinary vinegar and tan fungi, myxomycetes) -may for a time merge their individuality in a common mass formed by the -aggregation of many cells.</p> - -<p>The cells of higher plants are not always, or even generally, -anatomically distinct. Their protoplasm, the essential part of every -cell, is united with the protoplasm of neighbouring cells by threads -which traverse the cell-walls. The cells of the connective tissues -of animals are united into a web, or syncytium. This is especially -noticeable during early stages of growth. Nerve-cells are connected -together by conducting filaments (neuro-fibrillæ). It is possible that -nerve-cells and the muscle-fibres which they innervate are from the -<span class="pagenum"><a name="Page_28" id="Page_28">[Pg 28]</a></span> -beginning united by nerve-filaments—that the nerve-cell and -muscle-cell grow apart without severing this thread-like connection. -Certain anatomists regard the nerve strand which connects a cell in -the central nervous system with a number of muscle-fibres, placed, -it may be, at a great distance from the nerve-cell, as the bridge -which has never been broken in the process of cell division and -displacement, which made one primitive cell into a nerve-cell and a -group of muscle-cells. Muscle-fibres are not separate cells, but cell -complexes. Each muscle-fibre contains scores, in some cases hundreds, -of nuclei (<a href="#I_262">Fig. 16</a>). It is a cylinder, perhaps 2 inches long, -in which cell division is incomplete. Tendons are bundles of exceedingly slender -fibres which lie side by side, like silk threads in a skein. The row of -cells which gives rise to a tendon undergoes incomplete cell division. -Their nuclei divide, and a small quantity of soft body-substance is -set apart for each nucleus. The rest of the mass consists of fused -cells. It constitutes a continuous rod, which becomes fibrillated as -it grows. Vegetable cells are separated by cell-walls. Animal cells -tend to develop intermediate partitions; but the partitions are so -thick that they can no longer be described as walls. In cartilage the -cell-bodies are embedded in a great mass of intercellular substance, -or matrix. In this intercellular substance elaborate developments may -take place. Elastic fibres may make their appearance in it to form -elastic cartilage, as in the case of the epiglottis. In these various -instances, although it is perfectly true that tissues are formed by -cell division, the cells are not, strictly speaking, separate units. -They are not completely divided one from another. It is impossible to -recognize their anatomical boundaries.</p> - -<p>But there is a much more serious difficulty in applying the cell -theory—the difficulty of deciding what are the essential parts of -a cell. Long ago it was recognized that many animal cells—white -blood-corpuscles, for example—have no cell-wall. It was therefore -decided that cell-body and nucleus are the only essential parts. But -what is to be said of the red blood-corpuscles of mammals? (<a href="#I_060">Fig. 4</a>). -Are they cells? They have neither cell-walls nor nucleus; nor does -their substance present the structure which is usually associated with -the “body-substance” of cells. They are not produced, if the view held -<span class="pagenum"><a name="Page_29" id="Page_29">[Pg 29]</a></span> -by many histologists be sound, by cell division, in the ordinary sense -of the term, but appear as spots, gradually growing into discs inside -the body of a blood-forming cell. The discs are extruded when they -reach their full dimensions. Yet the tissue, blood, is composed of -these blood-discs and the intermediate substance blood-plasm. Mammalian -blood might be dismissed as a non-cellular fluid secretion containing -formed elements, if it were not for its history. In all animals below -mammals the red corpuscles are cells with nuclei and cell-bodies. The -absence of nuclei in mammals is due to the recognition by Nature of the -fact that, as the blood-cells will never be called upon to divide, it -is a waste of material to provide each of them with a nucleus. Not only -would the nucleus be useless, but it would take up space, diminishing -the capacity of the corpuscle for carrying hæmoglobin. The process of -cell division is in consequence curtailed. There are, it is true, other -ways of looking at this problem. The cells which line the bloodvessels -stand in some sort of nutritive relation with the blood. When the -lining cells of the bloodvessels are injured or inflamed, the blood -clots. But here again it is somewhat straining a point to say that -these lining cells are the cells of the blood, and the blood a kind of -intercellular substance; especially as a distinction would have to be -made between mammals with non-nucleated blood-corpuscles and birds with -complete blood-cells.</p> - -<p>The physiologist, if he is to feel sure of his ground, needs to know -the minute anatomy as well as the naked eye anatomy of the body. But -what is there that he does not need to know? He must be chemist, -physicist, biologist, pathologist, and expert in various other branches -of science. Microscopic anatomy, or histology, as it is commonly -termed, will be called upon in this book only when it has evidence to -give which bears directly on physiological problems. We have dwelt -at some length upon the cell theory because the physiologist needs -starting-points. He needs to have in his mind a conception of the -fundamental structure of the body. Protoplasm is the material which -lives. We begin with protoplasm albeit our conception of protoplasm is -so difficult to formulate that we are obliged to admit that in using -the term we are almost guilty of playing with words. Protoplasm is the -<span class="pagenum"><a name="Page_30" id="Page_30">[Pg 30]</a></span> -most living substance. The substance which is most alive always -presents itself to us as an imperfectly transparent, viscous material, -which proves on analysis to contain a large quantity of certain -proteins mixed with various organic and inorganic compounds. Protoplasm -is organized into, or distributed amongst, cells, which in any given -tissue present a fairly uniform size. What determines the size of -cells? Speaking generally, cells are small—say about 0·01 millimetre -in diameter. In early stages of growth, cell division occurs as soon -as the cell attains to something like this size. It would seem that -when nutriment is abundant cells add to their protoplasm more than they -lose. Having attained certain dimensions at which the conditions most -satisfactory for cell life reach their limit, cell division occurs. The -big drop falls into two smaller drops, each of which grows more rapidly -than the big one was growing at the time when it began to divide. -But if there be an optimum size for nutritive purposes, this limit -is suspended in many cases, and for various reasons. Take the ovum -itself as an example. It is vastly bigger than the cells into which -it divides. The yolk of a hen’s egg is, when first formed, a single -cell. By the time the egg is laid cell division has already set in. In -the embryo there are cells which surpass the average dimensions—the -unexplained “giant cells” which appear in the liver as soon as it can -be recognized as such (<a href="#Page_65"><i>cf.</i> p. 65</a>). These disappear from -the liver, but are for a time evident in the spleen. The large cells found in the -marrow of bone, some with a great single nucleus, others containing a -bunch of separate nuclei, also show that there is no fixed limit of -size. It is generally considered that the giant cells of marrow—or, -at any rate, those which are multinucleated—are leucocytes which are -engaged in scooping out the bone; consuming the hard tissue on the -inner surface of the hollow cylinder in order that, by deposition of -new material on the outside of the cylinder, the size of the whole -bone may be increased—leucocytes battening on bone which, owing to -interference with its blood-supply, is breaking down. They have not -time to divide. Nourishment is superabundant. Although much too large -for a vigorous standard of cell life, they continue to grow, putting -off the duty of cell division until the supply of nutritious food -begins to run short. -<span class="pagenum"><a name="Page_31" id="Page_31">[Pg 31]</a></span></p> - -<p>The most remarkable variations in size are to be found amongst the -cells of the nervous system. It may be given as one of the most -distinctive characters of nervous tissue that its cells have no fixed -or standard dimensions. A nerve-cell enters into connection with -other nerve-cells and with muscle-fibres by means of branches, or -cell-processes, as they are termed. The cells may be globular, as in -the sympathetic system, or star-shaped. Each cell gives off a certain -number of processes, which divide like the branches of a tree, and one -process which may run for a very long distance without dividing. This -latter thread-like process places it in communication either with a -distant part of the central nervous system or with the muscle-fibres -which it controls. By means of such a thread a cell in the spinal cord -may be connected with muscle-fibres of the hand or of the foot. The -thread is really a bundle of filaments (neuro-fibrillæ) which separate -to supply a number of muscle-fibres. It is, in its whole length, a -part of the cell in which it originates. The size of the cell varies -as the number of filaments in this bundle (termed the “axon”), and -possibly also as their length. Hence it comes about that nerve-cells -may be amongst the smallest, or they may be the very largest, in the -body. The so-called “granules” of the cortex of the cerebellum and -of the cerebrum are almost as small as red blood-corpuscles (<a href="#I_339">Fig. 23</a>). -Each of them has five or six minute branched processes and an -exceedingly delicate axon. The large cells of the cerebral cortex, -which send their axons far down the spinal cord, and the large cells of -the spinal cord which supply the muscles of the body, have a diameter -ten or twelve times as great as that of a granule. But larger still -are the nerve-cells which supply the electric organs of the torpedo -and other electric fishes (<a href="#Page_295">p. 295</a>); and largest of all are -the cells which innervate the curious “fishing-rods” of the strange angler fish -(<i>Lophius piscatorius</i>). It is difficult, owing to their irregular -shape, to say how large these cells are; but they are visible to the -naked eye.</p> - -<p>The anatomical unit of structure is the cell. Cells are the bricks of -which the body is built. Some are large, others small, as befits the -part which they take in the construction of the body. If the tissue be -merely a supporting tissue, connective tissue, cartilage, bone, its -<span class="pagenum"><a name="Page_32" id="Page_32">[Pg 32]</a></span> -cells are uniform in size and small. If it have functions to perform -which in some cases are carried out best by small cells, in other cases -by large ones, the cells are adapted in size to the work that they have -to do. Of the various kinds of wandering cells, some—the bone-forming -cells (osteoblasts), for example—are small; others—the bone-eating -cells (osteoclasts)—relatively large. Nerve-cells, like telephone -exchanges, are large or small according to the size of the area which -each supplies.</p> - -<p>All animals of complex organization, from starfishes and sea-urchins -to Man, are inhabited by <b>motile cells</b>. In addition to the bricks -which enter into the construction of its fabric, each fixed in its -place and definitely united to its neighbours, the animal contains -leucocytes which wander through its tissue-spaces or float down the -streams of lymph or blood. We are disposed to speak of these wanderers -as inhabitants of the body, to distinguish them from the elements -which enter into the construction of their habitation. It is difficult -to avoid the temptation of describing the body as a habitation. -Allegorical as Aristotle’s distinction between body and soul—between -the habitation and that which inhabits—may seem, when contrasted with -the exact language of modern science, it would save many a periphrasis -if we might still use the monosyllable “soul.” The fixed tissues -constitute a unity, bound together by nerves. The work done by glands -and muscles is done in response to directions conveyed by nerves. It -is impossible to say where the control of the nerves ceases—to point -out any fixed tissue which is not co-ordinated with other tissues, -nor susceptible to the influence of the environment as impressed upon -the central nervous system, through the medium of sense-organs. The -fixed tissues constitute a habitation for the “soul.” They share in -a common life. The wandering cells are as independent of control as -the parasites which occasionally find entrance into the body. Each -must have a soul of its own in Aristotle’s sense. Like parasites, they -carry on all the business of nutrition, respiration, cell division, -without reference to the needs of the fixed tissues. They take what -they require from the lymph as it leaves the intestines loaded -with the products of digestion; they take it from the lymph in the -tissue-spaces; they take it from the blood. When nutriment or oxygen -runs short, they do not share the privations of the fixed tissues. Only -<span class="pagenum"><a name="Page_33" id="Page_33">[Pg 33]</a></span> -indirectly is their well-being affected by that of the body as a whole; -only accidentally is the death of the body the occasion of their -death. The same might be said of such parasites as the “blood-worms” -of Egypt, or the trypanosomes (the cause of “sleeping sickness”) of -Equatorial Africa. Occasionally, in the rare disease lymphocythæmia -leucocytes multiply exceedingly, not, apparently, in response to a -call for their presence in large numbers, but in defiance of the -needs of the economy, and with baneful results. To the indispensable -services which wandering cells render, frequent reference will be -made. In the present connection, and while we are searching for the -principles of construction of the animal body, it would be desirable, -if we could do so, to define the status of wandering cells. If they -entered the body from without, they would be parasites of commensal -type, intruders who share in the food and shelter of the body in -return for service. But they do not enter from without. They are cells -of the growing body which, detaching themselves from the cells which -are forming tissues, assume a wandering life. They are not to be -recognized in the embryo until development is considerably advanced. -Their origin is far from clear, but histologists believe that, although -they are not recognizable as wandering cells in the earliest stages -of growth, they, or rather their parent cells, are set apart at a -very early date. Probably they are not formed in the embryo proper, -but in the “extra-embryonic area,” from which they emigrate into the -embryo. In this sense they come in from outside. But, after all, the -extra-embryonic area equally with the embryo is a product of the ovum. -Looking at the body as a whole, we recognize a common life, a soul -in Aristotle’s sense, which inhabits the framework of fixed tissues; -and at the same time we see a multitude of independent cells, each an -organism in itself, produced, like amœbæ, from similar independent -cells by cell division, absorbing the body-fluids, consuming invading -germs and fragments of decaying tissues, dying, disintegrating, in -their turn absorbed. Wandering cells are autonomous in the largest sense.</p> - -<p>All multicellular plants and animals are formed by division of a -primitively single cell, the segments remaining in contact. As the -<span class="pagenum"><a name="Page_34" id="Page_34">[Pg 34]</a></span> -scale of life is ascended, the cells which are massed together in -the body, whether of a plant or of an animal—we are still unable to -find any word other than body for the thing as a whole—tend more and -more to differ in appearance. Some are large, others small. Some have -cell-walls; others have none. Some remain “protoplasmic”; others are -largely composed of “metaplasm.” Better terms are wanted to connote -“most living substance” and “less living substance” respectively. It -would be easy to coin suitable words, but, alas! the nomenclature -of physiology is already hopelessly encumbered, and there is little -prospect that a bad word will die when a good one is available in its -stead. Differences in structure indicate differences in function. A -division of labour has set in. The cell starts with capacities for -every function. Its particular situation renders it desirable that -it should cultivate one capacity at the expense of the rest. It -specializes in a particular direction. If it happens to be placed -in the centre of the body on the course of the bloodvessels which -bring to the embryo food and oxygen from its mother, it develops a -great capacity for taking up food. It accumulates in its substance -a vast quantity of nutriment which it cannot consume, holds it, and -passes it on into the blood-stream as it is required. Thus the liver -is formed. In the embryo it attains to a great size, equal to about -one-half the whole body-weight; but whether storing food be its chief -function at this stage, or whether the other special functions for -which it is responsible are equally important, remains a question -for further research. In subsequent life its main work is to store -food. After birth, when the child prepares its own food by processes -of digestion in its stomach and intestines, the blood-supply of the -liver is so modified that the blood from the digestive organs is -passed through it. Now and for the rest of life the liver is the -storehouse of food, the larder of the body. It is a reservoir from -which supplies are distributed as required. A liver-cell retains many -primitive characters. It is soft and destitute of envelope. But under -the microscope it appears, unless it be taken from a starving animal, -unlike any other cell (<a href="#I_160">Fig. 7</a>). It is always loaded with -masses of glycogen. Sometimes it contains fat globules also. This is perhaps -the simplest of all instances of specialization of function. An amœba can -<span class="pagenum"><a name="Page_35" id="Page_35">[Pg 35]</a></span> -take up food. Presumably it always absorbs as much as it can get, the -simple law of growth with cell division making it impossible for it -ever to get too much. The cells which in the liver are so fortunate as -to be placed on the route along which food is carried into the body -retain the appetite of an amœba, but lose its capacity for growth and -cell division. They return to the blood-stream, when it is deficient in -food, the stores which they took up when food was in excess.</p> - -<p>The specialization of a gland-cell is opposite in kind to that of a -liver-cell. It takes up no more food than it requires, but it has -developed a great capacity of producing from the food a substance which -would no doubt be needed for its own purposes were it an isolated cell, -but which the gland-cell places at the service of the body as a whole. -An amœba can digest proteid substances. A cell of the pancreas produces -the ferment necessary for the digestion of proteins, and secretes it -into the alimentary canal.</p> - -<p>To take another instance of specialization. An amœba responds to -stimulation by changing its shape. It contracts in one direction, -expands in another. A muscle-fibre has developed the capacity of -contraction at the expense of all other functions. During the course -of its growth it changes from a round cell into one that is elongated. -The elongation is in the direction in which it acts with greatest -efficiency. Its cell-substance is very highly specialized in order that -it may have the maximum capacity of contraction in this direction.</p> - -<p>Sensory cells develop to a maximum the capacity of responding to -external force; nerve-cells, the capacity of conducting the impulses -generated in sensory cells. The body is a republic in which every -citizen develops to the highest degree the capacity of doing the thing -which his situation makes it desirable for him to do.</p> - -<p>The possibility of isolated cell life, and the necessity within certain -limits of cell division, have led biologists to dwell too much upon -the independence of the separate cells of which the body is composed. -Protoplasm organizes itself into cells, but cells are not necessarily -anatomically distinct. They may be the partially separate elements of a -syncytium, or there may be but the faintest traces of cell separation. -<span class="pagenum"><a name="Page_36" id="Page_36">[Pg 36]</a></span> -The objection to looking upon cells as isolated, self-complete units -does not hold good to the same extent when they are viewed from a -physiological standpoint. A cell is an administrative area. For -purposes of nutrition, respiration, and cell division it is autonomous. -It is responsible for its own local affairs. If a part is cut off from -it, this part loses its vitality; this, at least, is the conclusion -drawn from the atrophy of the axons of nerves when they are cut off -from the cells of which they are outgrowths. Apparently we must -understand by “the cell,” when speaking of the cutting off of a part, -the portion of the cell which retains the nucleus; although we must -be careful not to lay too much stress upon the nucleus as the centre -of cell life. Red blood-corpuscles, as already pointed out, have no -nuclei, and yet they live. Cell growth, estimated by mere increase in -size, does not depend upon the nucleus. Many cells of the skin and its -appendages increase considerably after the nucleus shows changes which -clearly indicate that it is far advanced towards decay. But increase -in protoplasm, cell growth in a legitimate sense, and especially cell -division, are dependent upon the presence of an active nucleus. While, -therefore, histologists no longer formulate the cell theory in the -restricted terms in which it was enunciated some years ago, they still -regard the cell as the unit of structure and unit of function. The body -is built of cells, and whatever is done by the body as a whole is done -by its individual cells.</p> - -<hr class="chap" /> -<p><span class="pagenum"><a name="Page_37" id="Page_37">[Pg 37]</a></span></p> -<div class="chapter"> - <h2>CHAPTER IV<br /><span class="h_subtitle">THE FLUIDS OF THE BODY</span></h2> -</div> - -<p>From one-fourth to one-third of the whole body is fluid. If the skin -be regarded as a water-tight bag, three-fourths or rather less of its -contents are solid, one-fourth liquid; and even its apparently solid -contents, the tissues, contain much water. Water is an essential -constituent of protoplasm. It is also present in cell-juice. The -estimate given above does not include the fluid within the cells, but -only the fluid with which the cells are bathed. In a general sense -this extracellular fluid, excluding blood, is termed <b>lymph</b>. It -occupies the spaces of a gauzy “connective tissue,” which connects, -or separates—the terms are equally appropriate—muscles, nerves, -glands, and other tissues of specialized function. Nowhere, except, in -a fashion, in the spleen, does blood come in contact with a cell. The -lymph which more or less surrounds them is the bath from which cells -receive their food and oxygen, into which they excrete carbonic acid -and tissue-waste. The network of lymph-spaces is traversed by capillary -bloodvessels with walls composed of flattened connective-tissue cells. -Such cells are usually spoken of as elements of an “endothelium.” As -the epithelium covers the surface of the body, so endothelium lines -its cavities. Endothelial cells are thin scales or tiles with sinuous -borders dovetailed one into another. That the tiles which form the -walls of capillary vessels are not cemented together in any proper -sense is shown by the facility with which white blood-corpuscles, -leucocytes, by their amœboid movements, push them asunder when making -their way from the blood-stream into the tissue-spaces, or <i>vice -versa</i>. They offer no more resistance to a leucocyte than a pair of -<span class="pagenum"><a name="Page_38" id="Page_38">[Pg 38]</a></span> -curtains hanging in front of a door offers to a child. Yet so long as -the endothelial cells are alive they keep their edges in such close -apposition as to constitute a continuous membrane which shuts off -blood from lymph. They are always close enough together to prevent red -blood-corpuscles from escaping from the capillary vessels; but their -resistance to the passage of the different constituents of plasma -varies greatly. The membrane which they compose is more complete and -less pervious, or less complete and more pervious, in accordance with -the nature of the tissues which surround it, and their varying needs. -The blood-passages of the liver may be described as filters. The escape -of red blood-corpuscles into lymphatic vessels is prevented, but they -offer practically no resistance to the plasma. Plasma—“lymph,” as it -is termed as soon as it is outside bloodvessels—passes through the -walls of the capillaries of the liver unchanged in constitution. Where -<span class="pagenum"><a name="Page_39" id="Page_39">[Pg 39]</a></span> -they traverse glands (other than the liver), muscles, skin, and various -other structures, the walls of capillary vessels, while offering -practically no resistance to water and diffusible salts which can -pass through membranes, prevent proteid substances from passing from -blood to lymph, except in extremely small quantities. In this way an -exquisite balance is automatically maintained. Water and salts pass -out as they are needed. But they never pass out in excess, because the -protein-containing blood-stream tends to keep them in, in virtue of the -same attractive force which enables it to suck in the oxidized products -thrown into the lymph by the tissues. Whatever a tissue needs it takes -from the lymph. Suppose that bone is being formed. Large quantities of -lime and phosphates are needed for the calcification of the cartilage -in which it is modelled. The cartilage absorbs lime and phosphates -from the lymph which bathes it. Lime salts and phosphates immediately -begin to diffuse from blood into lymph. The hurrying blood-stream -brings up further supplies from the walls of the intestine, products -of digested milk and other foods. Lymph contains (although not in the -same proportions) everything which blood contains. Many an analogy may -be found in the world of economics, although no illustration would be -sufficiently complete. From the lymph tissues take the fuel that they -need, the oxygen with which to burn it, the foods for their own repair, -the raw materials for their arts. Into it they throw their smoke, their -drainage, the slag and refuse of their factories. The blood replaces -the supplies as they disappear. It absorbs all waste. Lymph occupies -streets, market-place, passages, corridors. The blood-stream is a -closed system, rolling down the streets and through the market-place, -on its never-ceasing circuit from port and mine to open air and open -sea. From the alimentary canal it picks up food and fuel; the lungs -give it oxygen, and disperse its carbonic acid; the kidneys purge it of -non-gaseous waste.</p> - -<div class="figcenter"> - <a id="I_038" name="I_038"> </a> - <img src="images/i_p038.jpg" alt="" width="600" height="442" /> - <p class="center"><span class="smcap">Fig. 3.</span>—<span class="smcap">A Ductule - and Two Acini of a Mucous Gland of the Mouth, with a <br />Muscle-fibre cut - Longitudinally; Capillary Bloodvessels and Connective Tissue.</span></p> - <div class="blockquot2"> - <p class="neg-indent space-below2">Stellate connective-tissue cells form a labyrinth of intercommunicating - lymph-spaces which separate the gland-cells and the muscle-fibre from - the walls of the capillary bloodvessels. The capillaries contain - circular red blood-corpuscles and nucleated leucocytes. Some of the - leucocytes are squeezing their way either out of a capillary into a - lymph-space or <i>vice versa</i>. A granular leucocyte is to be seen in a - lymph-space at the bottom of the picture.</p> -</div></div> - -<p>The facility with which the constituents of blood pass out to the -lymph, and the constituents of lymph pass into the blood, depends -upon the condition of the walls of the capillary vessels. Water -and substances dissolved in water might pass through the wall of a -capillary vessel in either of three ways—by filtration, by osmosis, or -by secretion. A filter is a porous barrier, which allows water and all -<span class="pagenum"><a name="Page_40" id="Page_40">[Pg 40]</a></span> -substances dissolved in water to traverse it. The solution passes -through unchanged in composition. Only solid particles are kept back. -The rapidity with which fluid passes through a filter varies as the -difference between the pressure on the one side and the pressure on -the other. A membrane does not allow of filtration. Water and things -dissolved in water pass through it by osmosis. Some things it will -not allow to pass; such, for example, as gum, mucin, white of egg. To -others it offers resistance in varying degrees. Most of the things that -can diffuse through a membrane are capable of crystallization; but the -membrane exercises some control over the passage of even crystallizable -substances when in solution. If a membranous tube containing water in -which proteins, sugar, and various salts are dissolved is hung in a -basin of pure water, the proteins remain in the tube; the sugar and -the salts pass through its wall into the surrounding water. But they -pass at different rates. Those of small molecular weight pass more -quickly than those whose molecule is heavy. After a time a condition of -equilibrium is established. No more salts pass out of the tube. If now -the contents of the tube and the contents of the basin are analysed, -it will be found that the tube contains all the proteins, some of the -sugar, and some of each of the salts, although not in the proportions -in which they were present at the commencement of the experiment. The -water in the basin contains some sugar and some of each of the salts, -but not in the same proportions in which they are found in the tube. -As a matter of fact, the same number of molecules would be present, -per unit volume, on each side of the membrane—in the tube and in the -basin. In this respect the percentage composition of the two solutions -would be the same. But some of the molecules being heavy, others light, -the weight of salts which unit volume of the solution in the tube would -contain would not be the same as the weight of salts in unit volume of -the solution in the basin. A membrane exerts a discriminating action on -the substances which pass through it. Secretion is osmosis in disguise. -It may be even filtration in disguise. A gland-cell (like an amœba) -takes things up and passes them out without regard to their osmotic -equivalent. It seems to exercise a choice. It seems to act in disregard -<span class="pagenum"><a name="Page_41" id="Page_41">[Pg 41]</a></span> -of the laws both of filtration and of osmosis. So, at least, it -appears to us when we are looking at the result in ignorance of what -has happened inside the living cell. The passage from blood to lymph -and <i>vice versa</i> through the wall of a capillary vessel is in certain -situations or at certain times a mere process of filtration; at others -a process of restricted filtration. If the wall is behaving as a -perfect membrane, it is a process of diffusion, or osmosis. It seems -unnecessary to regard it, in any case, as a process of secretion. -The more widely the capillaries are dilated, the less resistance do -they offer to exudation. The narrower their calibre, the greater is -the restraint which they place on the escape or entrance of fluid. -When the skin of the palm of the hand is not sufficiently thick to -protect the soft tissues beneath it from the injurious effects of the -prolonged pressure of an oar or an axe, the capillary vessels of the -under-skin dilate; more lymph transudes; the skin is raised up as a -blister. The same thing happens when the capillaries are dilated and -paralyzed by scalding water. The fluid of a blister has much the same -constitution as blood-plasm, except that it contains less proteid -substance. These results might be regarded as purely mechanical—the -direct effects of pressure or heat upon the membranous capillary wall. -But the “vital” element is more important. The capacity of endothelium -to act as a barrier depends upon its nutritive condition—its vital -integrity, as it might be termed; which no doubt in the last resort -means its chemical relation to the fluids which bathe it. Now and again -blebs, like blisters, are formed on the skin—the herpes which appears -about the mouth; urticaria, which is more generally distributed; and -various other cutaneous disorders. Frequently a connection can be -traced between these eruptions and the consumption of a particular -food. An attack of urticaria results not uncommonly from eating -lobster, mussels, rook-pie, or some few other articles of diet. Various -things—bad fish, for example—may produce the same effect; but -shell-fish have an especially evil reputation. If extract of lobster -or of mussels be injected into the blood of an animal, the amount of -lymph which leaves the blood is markedly increased. The extract acts as -a poison upon the endothelium of the capillary walls. It increases its -<span class="pagenum"><a name="Page_42" id="Page_42">[Pg 42]</a></span> -permeability in all conditions in which lymph escapes in undue quantity -from the blood-stream, or escapes more rapidly than it is absorbed; -the nutritive condition of the endothelium is disturbed. Its unusual -permeability is due in part, no doubt, to the dilatation of the -capillary tube, the stretching of its membranous wall; but it is due -also to the diminished vigour of the endothelial cells. They have lost -to a certain extent their capacity for holding their edges in perfect -apposition.</p> - -<p>When the circulation is sluggish, owing to the inefficiency of the -heart, the tissues become œdematous. In other words, lymph accumulates -in the tissue-spaces. When the skin of a healthy person is pressed, it -returns to its natural position as soon as the pressure is removed. -If there is a tendency to dropsy—for ages the term “hydropsia” has -been thus familiarly clipped—the finger leaves a pit behind it when -pressed upon the skin. It is some little time before the lymph in the -connective-tissue sponge readjusts the surface. Excessive escape of -lymph from the blood, or its insufficient return into the blood, may -also be the result of obstruction to the flow in the great veins. When -the veins of the leg are varicose, the weight of the column of blood -in the distended vessels impedes its circulation. After standing, the -tissues about the ankle become œdematous. The œdema disappears on lying -down. A hardening (cirrhosis) of the liver impedes the circulation of -the blood which comes to it through the portal vein from the walls of -the alimentary canal. The capillaries of the stomach and intestine -are distended. Lymph accumulates in the abdominal cavity, producing -ascites, another form of dropsy.</p> - -<p>It is almost hopeless to attempt to disentangle the various factors -which disturb the balance between blood and lymph—excessive outflow -from blood, deficient inflow from lymph, stretching of the endothelium -of the capillary tubes, imperfect nutrition and consequent imperfect -apposition of the endothelial scales, increased permeability of the -scales. The exudation which accompanies inflammation would seem to be -due to the diminished vitality of the endothelium rather than to a -mechanical factor, such as increased blood-pressure in the capillaries, -and their consequent distention. Ascites is, apparently, a purely -<span class="pagenum"><a name="Page_43" id="Page_43">[Pg 43]</a></span> -mechanical result of the resistance offered to the passage of blood -through the liver; but pleurisy, the accumulation of lymph in the -space between the lungs and the chest-wall, cannot be explained in -the same way. There is no undue pressure on the vessels in which the -blood circulates through the inflamed pleura (the investing membrane -of the lungs and lining membrane of the chest), yet the walls of the -capillaries fail to maintain a proper balance between blood and lymph.</p> - -<p>Hitherto we have spoken of the lymphatic system as a labyrinth of -communicating spaces containing stagnant fluid, which is kept in a -fitting state by egress and ingress out of and into blood. Such a -mental picture is substantially correct. But the system is complicated -by the presence of lymphatic vessels. Cells of the connective-tissue -sponge-work arrange themselves side by side. They flatten into -endothelial scales. The borders of the scales close up. They form -lymphatic channels, wider than blood-capillaries, but strictly -comparable in every other respect. The lymph capillaries unite into -larger vessels. The larger vessels are connected by cross-branches; -they form plexuses. Their walls are strengthened with fibrous tissue. -Like the veins, they are abundantly provided with valves, which -check any tendency to a backward flow on the part of the fluid which -they contain. Lymphatic plexuses surround and accompany the larger -bloodvessels. They are disposed on the surface of muscles and glandular -tissues. They are abundant beneath the skin. Nearly three centuries -ago the lymphatic vessels of the mesentery, which collect products of -digestion, especially fat, from the walls of the alimentary canal, -were recognized owing to the milkiness of their contents after a -meal. They were, on this account, termed “lacteals.” Other lymphatic -vessels, owing to their transparent walls and colourless contents, are -not easily seen; but they are readily injected with mercury or other -fluids which render them conspicuous. In the upper part of the thigh, -in the armpit, or in the neck, they are about large enough to admit a -crow-quill. Those from the lower limbs, from the viscera, and from the -walls of the abdomen converge to a receptacle which lies in front of -the spinal column. The receptaculum chyli is continued upwards as the -thoracic duct, which pours the lymph into the great veins of the left -<span class="pagenum"><a name="Page_44" id="Page_44">[Pg 44]</a></span> -side of the neck and of the left arm just where they join together.</p> - -<p>The thoracic duct provides for the overflow of lymph from the spaces -of the body. There is no circulation of lymph. Lymph from the liver -and from the intestines is constantly draining into the thoracic -duct, and thus returning to the blood-stream by a short direct route, -entering it without the necessity for reabsorption through the walls of -capillary vessels. By no means all of this fluid has exuded from the -blood-stream. Much of it is water which was poured into the stomach -as gastric juice, and into the intestines as the secretions of the -pancreas and other glands, or imbibed through the mouth and absorbed -by the lymphatics of the alimentary canal. The remainder of the water -taken up from the alimentary canal enters its bloodvessels. The diluted -blood flows to the liver, loaded with digested products which the -liver will store. As the blood parts with them the additional water -which has served for their transport exudes from the capillaries of -the liver into lymphatics, which empty it into the thoracic duct. -Large quantities of water are used in washing out digested products. -Secreted into the alimentary canal by the digestive glands, it passes -out through its wall as the vehicle of digested products. Collected -by lymphatic vessels, it is either carried directly into the thoracic -duct, or passed from lymph into blood, carried by blood to the liver, -again transferred from blood to lymph, and borne by the lymphatic -vessels of the liver to the thoracic duct.</p> - -<p>Water exuded from blood into lymph may be reabsorbed into the blood -near the place where it was poured out, or it may reach the blood via -the thoracic duct. It would seem that the former is the natural, the -latter the emergency route; the former the course taken when an organ -is tranquil, the latter a necessity when the organ is active. If the -large lymphatic vessels of a limb are cut, no lymph escapes from them -so long as the limb is at rest. When the muscles contract lymph begins -to flow. If the limb is flexed and extended by hand, lymph flows. If -the muscles are squeezed or massaged, lymph flows. As the flow is set -up both by active contraction of the muscles and by passive movements -in which the muscles do not take part, it clearly must be due to -<span class="pagenum"><a name="Page_45" id="Page_45">[Pg 45]</a></span> -external pressure on the lymphatic vessels. As they are provided with -valves, squeezing them converts them into pumps. The fluid which they -contain is bound to go forwards. Additional fluid is squeezed into -them from the tissue-spaces. To a large extent, therefore, the outflow -of lymph from contracting muscles is to be explained as the result -of the pressure which the swelling muscles exert upon the lymphatic -vessels within their sheaths. But there is another factor which must -not be overlooked, although it cannot readily be estimated. When a -muscle is actively contracting its bloodvessels dilate. There is a -greater exudation of lymph; and reabsorption by blood is not equal to -the exudation. The surplus leaves the limb by the lymphatic vessels. A -gland is never at rest. In the intervals between the ejection of its -secretion its cells are preparing materials for the next outflow. Lymph -is always flowing from a gland; its amount increases as the activity -of the gland increases. More lymph leaves the blood when the gland -is exceptionally active than when it is relatively quiet. Some of it -is not reabsorbed into the blood. A certain proportion of the waste -products of the active gland are hurried away by the overflow system in -the direction of the thoracic duct.</p> - -<p>Lymph is the reservoir of nutriment upon which every cell in the body -draws. It is improbable that in health and under normal conditions -the activity of any organ is ever restricted for want of sufficient -food. As food is removed from lymph, it is instantly replaced by fresh -food from the blood. There is some evidence—not very clear—that the -removal of waste products offers greater difficulty than the renewal -of supplies of food. When the activity of muscles has been excessively -prolonged they ache. It has been supposed that their unwillingness to -do more work is due, not to the exhaustion of the food which they use -up when contracting, but to the inadequacy of the lymph and blood to -carry off all refuse. This, at least, is the explanation of fatigue -which is usually offered, although it is difficult to understand why -the arrangements for removing waste products which have worked to -perfection for eight hours should during the ninth hour become rapidly ineffective. -<span class="pagenum"><a name="Page_46" id="Page_46">[Pg 46]</a></span></p> - -<p>If a frog’s muscle, cut out of the body, has been made to contract -until it refuses to work any longer, it again responds to stimulation -after a solution of salt has been passed through its bloodvessels. -The salt-solution brings no food; the only thing it can do is to -wash away waste products. But this experiment upon a tired, isolated -muscle does not necessarily throw light upon the nature of fatigue in -muscles under normal conditions. The isolated muscle is using up, in -contracting, food which it has stored. Cut off from the circulation, it -has no means of getting rid of the lactic acid and other products into -which food is changed. They may well have accumulated to a poisonous -extent long before all the food has been used up. Hardly more cogent -is the argument based upon the benefit which a tired man experiences -from hot baths, massage, and the like. They take away the feeling -of tiredness, but it does not follow that this result is due to the -removal of waste products. Quickening the circulation of blood brings -about renewal of the lymph. Renewal of lymph means fresh supplies of -food as well as removal of waste products. Even human muscles are not -perfect as machines. They will not work for an unlimited spell. There -comes a time when they must have rest. Something goes wrong in the -admirable adjustment which has hitherto provided exactly the right -amount of food and exactly the necessary freedom from the products of -action. A feeling of fatigue is the signal that the apparatus is not -in a condition to work longer; but whether this feeling is due to a -dislocation of the balance of supply and loss, or to some deterioration -of the apparatus which calls for rest and renovation, it is at present -impossible to say. It is not due to the exhaustion of muscle food. A -more powerful stimulus, the urgency of fright or some other strong -emotion, or an electric current applied directly to the muscle or -its nerve, will still induce vigorous contraction. The muscles of a -hare that has been coursed until it can run no farther still contain -glycogen, muscle food.</p> - -<p>Glycogen is stored in the liver. Fat, if it is assimilated in excess of -the needs of the body, accumulates in the connective tissues. Proteins, -if in excess, are either destroyed by oxidation, or partly destroyed -and partly converted into fat. Increasing the amount and richness of -the food does not, if nutrition is already at its best, improve the -<span class="pagenum"><a name="Page_47" id="Page_47">[Pg 47]</a></span> -quality of the blood. The surplus of food is either stored or burnt. -The composition of lymph is unaffected. Its quality is not improved by -taking more food than enough. A perfect balance is maintained. Every -cell is able, when conditions are normal, to obtain as much nutriment -as it needs. It cannot get more. It cannot lay by food and shirk work. -If it did it would grow. Reaching its optimum size, it would divide. -Additional tissue would be formed. But when it does more work it needs -more food; and it is a matter of common experience that the system is -so adjusted that food is supplied to the tissues, not reluctantly, -but with a slight tendency towards generosity. Working harder than -usual, they find the lymph by which they are bathed somewhat richer in -the materials that they need than the necessities of the case demand. -They are able not merely to obtain all they want, but a little more. -Activity favours growth.</p> - -<p>Many attempts have been made to show that if a part of the body has -more than its share of food it grows to an excessive size. John Hunter -grafted a cock’s spur into its comb. It grew to monstrous dimensions. -Such a result favours the view, but it is not quite conclusive. -Undoubtedly the comb was richly supplied with blood, but it does not -follow that the cells of the spur were able in their new situation -to take advantage of this supply. Besides, the spur when projecting -from the head was not subject to the accidents to which it was -exposed whilst on the leg. Its size was not kept down by friction. -Nor was it as hard and compact as it would have been in its normal -situation. It is scarcely possible to devise any experiment that would -be satisfactory now that the relations between blood and lymph and -lymph and tissues are understood. In certain pathological conditions, -however, hypertrophy is the result of the hyperæmia of chronic -inflammation; and there is little doubt that, if we could arrange for a -certain group of cells to receive lymph richer in food and freer from -waste products than the perfect adjustment of supply to needs normally -allows, the cells would grow.</p> - -<p>Under perfectly healthy normal conditions growth can be induced only -by use. Nature supplies the fuel which is used during activity, and a -balance of food available for the construction of additional machinery. -<span class="pagenum"><a name="Page_48" id="Page_48">[Pg 48]</a></span> -The muscle which is called upon to do work develops a greater capacity -for work.</p> - -<p>When nutrition is not at its best, the growth of muscle may be favoured -by external pressure which squeezes lymph out of its tissue-spaces, -and therefore leads to increased exudation from the blood. It is not -improbable that in badly nourished tissues the circulation of blood is -somewhat torpid and the lymph stagnant. A feeble circulation usually -results in some œdema. The muscles, or rather the connective tissue -which envelops and penetrates them, feels doughy, instead of being, -as it should be, firm and elastic. Under these conditions massage is -undoubtedly of service. Squeezing the muscles displaces lymph, and, -if the pressure is properly directed, drives it along the lymphatic -vessels. Fresh lymph exudes from the capillary bloodvessels, and the -muscle-fibres, surrounded with a more abundant supply of nutriment, -benefit, as, in a vigorous person, they benefit from use.</p> - -<p>Lymph is an exudate from blood. Its composition therefore depends -upon that of blood-plasma, but it tends to differ from it owing to -the influence of two causes. In the first place, the walls of the -capillary bloodvessels restrict exudation. Red blood-corpuscles cannot -pass through them. Proteins which are non-diffusible are, according -to the circumstances of the tissues, held back to a greater or to a -less extent. The pseudo-capillaries of the liver let them pass, as has -already been said. The capillaries of the limbs restrict their passage -to such proportions as, it may be supposed, are absolutely necessary -for the nutrition of the tissues. In the second place, tissues remove -food from lymph and add to it waste products. Hence the lymph issuing -from a limb, after full contact with the tissues, contains less of the -former and more of the latter—less sugar, for example, and rather -more oxidized nitrogenous substances, lecithin and other things termed -collectively “extractives,” because they can be extracted from dried -blood or lymph by ether. The reaction of lymph is alkaline. After a -time it coagulates, but coagulation is slower, and the clot less firm -than in the case of blood.</p> - -<p>As the composition of lymph depends upon the source from which, and -the conditions under which, it has been obtained, it is unnecessary to -<span class="pagenum"><a name="Page_49" id="Page_49">[Pg 49]</a></span> -state the results of a chemical analysis. It suffices to say that lymph -contains all the substances which are present in the plasma of blood, -but not necessarily in the same total amount or in the same relative -proportions. Speaking generally, leucocytes are present in about the -same numbers as in blood—6,000 to 8,000 to the cubic centimetre; -but leucocytes are everywhere present: in blood, in the lymph, in -lymph-vessels, in the tissue-spaces. As they are not passively floating -bodies like red blood-corpuscles, but active migratory organisms, -they tend to accumulate in one situation and withdraw from another, -in accordance with the opportunities which the different localities -afford. They desert effused lymph, blisters, ascitic fluid, and the -like. They are not found in the lymph in the pericardium. There are -fewer in the lymph coming from the intestines after a meal than in the -same lymph during the intervals between meals. Their departure from -effused lymph might easily be explained. It is not so easy to account -for their comparative absence from the lymph in the lacteals when it -is heavily charged with fat and other products of digestion. Such -leucocytes as are present at this time are loaded with fat granules -which they have stolen from the chyle, as the lymph in the lacteals is -usually termed. One would need to be very intimate with a leucocyte -before one ventured to give reasons for all its movements. Lymph -contains the same proteid substances as blood, and in the same relative -proportions, but usually in smaller quantity.</p> - -<p>Incidental reference has been made to the great -lymph-spaces—peritoneal, pleural, and pericardial. The brain and -spinal cord are separated from their outer membranes by a lymph-space. -There are also spaces within the brain—the ventricles—and a central -canal in the spinal cord. The aqueous and vitreous humours of the eye -are also lymph-spaces, although the latter contains some remnants of -tissue. The joint cavities are lymph-spaces. So also are the bursæ -which surround tendons or separate them from bones. It is not, however, -justifiable to include all these cavities in a single category, either -from the point of view of their purpose, their mode of formation, or -the nature of their contents. The peritoneal, pleural, and pericardial -spaces are parts of the great primitive body-cavity, or cœlom. The two -first are potential rather than actual. Normally they contain just -<span class="pagenum"><a name="Page_50" id="Page_50">[Pg 50]</a></span> -sufficient fluid to moisten the apposed surfaces of the endothelium -which lines their walls and covers the organs which they contain. There -is no fluid in them which can be collected and labelled “peritoneal” -or “pleural” fluid. The purpose of the spaces is to allow of movement -without friction—in the one case of the intestines, in the other of -the lungs. It is possible to take a spoonful or so of fluid out of the -space which surrounds the heart. It has the usual composition of lymph. -It contains proteins, but is not spontaneously coagulable. Leucocytes -are absent, a fact which probably accounts for its not clotting. -The fluid inside the cerebro-spinal system is extremely dilute. Its -principal salt—its principal constituent, indeed—is sodic chloride. -It contains hardly a trace of proteins, and these in a modified -condition—proteoses. It also contains pyro-catechin, a benzoic -alcohol. This substance has long been recognized as a constituent of -cerebro-spinal fluid, owing to the fact that, like sugar, it reduces -copper salts when heated with them in an alkaline solution. It appears -to be one of the products of proteid decomposition. Although exuded as -lymph from the bloodvessels of the chorioid plexuses, the composition -of cerebro-spinal fluid has been profoundly changed by the activity—it -might almost be called the digestive activity—of the epithelium which -lines the cerebro-spinal canal. There is a theory that the ancestors -of all vertebrate animals were organized on a very different plan from -that of their distant descendants. Our cerebro-spinal canal was their -stomach and intestine. It would appear that the lining epithelium of -these organs, although disused for millions of years, cannot resist the -temptation to digest the lymph which they contain! The fluid in joints -contains mucin (the essential constituent of mucus), or a substance -resembling mucin. In this case the joint-membrane has added something -to lymph without removing or destroying any of its other constituents.</p> - -<p>Other illustrations might be given showing how the plasma of blood is -altered in composition while it is passing out of, or after it has -passed out of, capillary bloodvessels. Perhaps it would be more logical -to start on the outer side of the walls of the capillaries; since blood -may, very properly, be regarded as a tissue, dependent, like all other -tissues, upon diffusion from lymph for the nutrient materials that it -<span class="pagenum"><a name="Page_51" id="Page_51">[Pg 51]</a></span> -needs. In the wall of the alimentary canal it receives supplies <i>via</i> -the lymph. It drops them in the liver, its <i>garde-manger</i>, to pick them -up again as they are wanted. The torrent of lymph which the thoracic -duct discharges into the veins of the neck conveys the fat which could -not traverse the walls of the capillary bloodvessels, and much of -the reserve of food which the blood had deposited in the liver. Only -about one-quarter of the fluid of the body (one-thirteenth of the -body-weight) is included within the blood-system; but this enclosed -fluid, owing to the fact that it is kept in circulation by the heart, -replenishes and purifies the much larger quantity which does not -circulate. The unenclosed lymph has in particular situations a chemical -composition which varies widely from that of the blood. Imagine a -marsh through which a river flows—the vast plains of water-plants on -the Nile above Fashoda, for example. There is a constant interchange -between the flowing water of the river and the stagnant water of -the marsh. In any given part of the marsh the quality of the water -will depend upon what it has been able to take from, and what it has -given back to, the river; upon what the water-plants have taken from -it, and what they have added to it. Boats which cannot penetrate the -walls of reed keep to the open channel of the Nile. Fish swim, now in -the river, now in the narrow passages and open pools of the marsh. -So it is, in a way, with the fluid in the spaces and cavities of the -lymphatic system and in the bloodvessels which traverse them, and with -its migratory inhabitants. In our extravagant analogy read leucocytes -for fish. Fish have two reasons for wandering from river to marsh. -Amongst the water-weeds they hunt for food; they seek quiet places in -which to breed. In this matter the analogy holds good. A leucocyte may -be overtaken with cell division anywhere—in the blood-stream or in a -lymph-vessel. But cell division very rarely occurs except in certain -favoured spots. The breeding-places chosen by leucocytes are sheltered -situations in connective tissue where the blood-supply is abundant, -and the eligibility of such a spot is much increased by its being near -to a field where their services are likely to be called for. The nests -of connective tissue made by the leucocytes are of three kinds, termed -respectively diffuse adenoid tissue, lymph-follicles, and lymphatic -<span class="pagenum"><a name="Page_52" id="Page_52">[Pg 52]</a></span> -glands. The connective tissue beneath the mucous membrane of the -whole of the respiratory tract—trachea, bronchi, and bronchioles—is -diffuse adenoid tissue. It presents no special structure, but its -spaces are packed with leucocytes in various stages of cell division, -and young leucocytes, or lymphocytes, as they are usually named. Some -of the lymphocytes make their way into the blood or into the lymph. -Others, acquiring their full dimensions, scour the epithelium which -lines the respiratory tract for germs and other foreign bodies which -are drawn into the tract with inspired air. They may be seen pushing -aside the cells of the lower strata of the epithelium, on their way to -the surface, or returning to the subepithelial connective tissue with -germs, or particles of soot, or débris of epithelial cells which they -have taken into their substance (<a href="#I_060">Fig. 4, B</a>).</p> - -<p>The tonsils are examples of follicular lymphoid structures. They lie -one on either side of the entrance to the gullet, between the two -folds (the anterior and posterior pillars of the fauces) by which -the soft palate is continued to the side of the tongue. Normally the -tonsil is not visible, but when inflamed it may project sufficiently -to be seen; and its surface may then be covered with mucus and pus. -It is liable to become enlarged in childhood, owing to chronic -inflammation. A section of the tonsil shows it to consist of clusters -of lymph-follicles lying beneath the mucous membrane. The term -“follicle” is unfortunate. It conveys no idea of the form or structure -of one of these masses of lymph-cells; and it is, besides, applied to -things of an entirely different character—for example, the pits of -mucous membrane which sink down between the masses of lymphoid tissue -in the tonsil. The expression “follicular tonsillitis” does not refer -to the lymph-follicles, but to the epithelial pits. It is a condition -in which a drop of pus is to be seen in the mouth of each of the pits. -A lymph-follicle is a small rounded clump of connective tissue, denser -on its periphery than in its centre. Its bloodvessels are disposed -chiefly on the periphery. Lymphatic streamlets arise in the centre. -Its outer portion is closely packed with dividing lymph-cells and -young leucocytes, which as fast as they are formed migrate towards -the centre, and eventually escape from the follicle by the lymphatic -vessels. The connective tissue which invests and separates the -<span class="pagenum"><a name="Page_53" id="Page_53">[Pg 53]</a></span> -follicles is full of leucocytes. Removal of the tonsils is followed by -no ill effects. They are not essential to our well-being. Nevertheless, -they have important functions to perform. They are barracks crowded -with leucocytes, which guard the pass into the alimentary canal. Their -leucocytes incessantly patrol the mucous membrane, capturing germs, -removing fragments of injured epithelium, striving to make good the -mischief to which this part of the alimentary canal is peculiarly -liable. The enlargement of the tonsil which results from frequent -sore throat is a response to the demand for an increase in the supply -of these little scavengers, in order that they may cope, not only -with objectionable things outside the walls, but with the still -more pernicious germs which during an attack of sore throat succeed -in breaking through the epithelium. It is the invaders which elude -the vigilance of the leucocytes that cause fever and other general -symptoms. Other notable groups of lymph-follicles are found in the -middle portion of the small intestine, where they form oval patches, -about three-quarters of an inch long by half an inch broad—Peyer’s -patches. The leucocytes which are developed in them search the walls of -the intestine for germs. During an attack of enteric fever the patches -become inflamed, and one of the greatest risks which the patient -runs is the risk of ulceration of a patch and the perforation of the -intestinal wall.</p> - -<p>The abundant provision for the multiplication of leucocytes shows that -the destruction of these cells must occur on an equally large scale. -Every day large numbers die. Where this occurs, and how their dead -bodies are removed, is not certainly known. Doubtless they are eaten -by their fellows, their substance oxidized, and the products—carbonic -acid, water, and nitrogenous waste—thrown into the lymph. There is -some reason for thinking that a part of the nitrogenous waste is -excreted in the form of uric acid (<a href="#Page_216"><i>cf.</i> p. 216</a>). -The daily production, and consequent destruction, of leucocytes shows that their -metabolism is a factor which cannot be overlooked when we are making up the body’s -accounts.</p> - -<p>The fixed tissues receive their nutriment in a digested condition. -Leucocytes digest it for themselves. In many cases, although not in -all, the cells of fixed tissues last throughout life, so far as their -<span class="pagenum"><a name="Page_54" id="Page_54">[Pg 54]</a></span> -outer form is concerned, although their molecules are oxidized and -replaced by new material. It is not improbable, therefore, that there -is a difference between the metabolism of the fixed tissues and the -metabolism of leucocytes. The whole of a wandering cell, its nucleus -included, breaks down and has to be removed. We do not know that -this occurs in the case of a fixed cell. On the strength of evidence -which points, apparently, to a chemical relationship between nuclear -substances and uric acid, it has been inferred that the two chief -nitrogenous products which are excreted by the kidney are divisible -into the one which in the main represents the oxidation of fixed cells, -urea, and the other, uric acid, largely derived from the oxidation of -wandering cells.</p> - -<p>The valiant leucocytes do their best to cope with all the rubbish, -whether living or dead, that needs removal. They flock to any situation -in which germs are numerous or tissue has been destroyed. If all goes -well they take the foreign matter into their substance—dead tissue is -matter foreign to the body—and either digest it in the course of their -ordinary progress, or retreat with it, if they cannot digest it, to the -nearest lymphatic gland. But in their efforts to reach objectionable -matter they are apt to wander too far from the healthy lymph from which -they obtain oxygen for their own respiration. Unable to breathe, they -die. They lose the power of extruding pseudopodia. Their extensible, -prehensile processes are drawn in. Assuming a globular form, they float -helplessly in what once was lymph. Their body-proteins are largely -changed to fat. As “pus cells,” they are thrown off in the discharge -from an ulcer, or accumulate in the cavity of an abscess. A pus cell is -a dead and fattily degenerated leucocyte.</p> - -<p>The third kind of breeding-place of leucocytes, a lymphatic gland, has -a more elaborate structure than the tissues with which we have already -dealt. Lymphatic glands are about the size of beans, and of the same -shape. They are found in the course of lymphatic vessels in situations -where they are not exposed to pressure, such as the back of the knee, -the groin, the front of the elbow, the armpit, in the neck above the -collar-bone, and on either side of the sterno-mastoid muscle, behind -<span class="pagenum"><a name="Page_55" id="Page_55">[Pg 55]</a></span> -the angle of the jaw. There are a number in the abdomen and in the -thorax. Each lymphatic gland is invested by a strong fibrous capsule. -Its artery enters, and its vein and efferent lymphatics leave, the -concave side (the hilus) of the gland. The lymphatic vessels which -bring lymph to it pierce the capsule on its convex side. It is -divisible into two parts: (1) The adenoid tissue which surrounds the -artery and its branches; (2) the open network of “lymph-ways” which -invest this adenoid tissue. Leucocytes divide in the adenoid tissue. -The young lymphocytes drop out into the lymph-ways. As a stream of -lymph, brought by the afferent vessels, is always flowing into the -lymph-ways, and out by the efferent vessel or vessels, the lymphocytes -are carried with it towards the thoracic duct. A lymphatic gland is -therefore an organ for adding leucocytes to lymph in the course of the -lymph-stream. It has, however, another and equally important function. -Leucocytes which have picked up germs or other foreign matter pass on -with the lymph to a lymphatic gland. After entering its lymph-ways -they leave the lymph-stream, squeeze into the adenoid tissue of the -gland, and there come to rest with their burden. They remain in the -gland until the foreign matter is digested, or, if it be indigestible, -until they undergo dissolution, when the particles of soot or pigment -are deposited from their débris in a harmless state. When the skin is -tattooed, much of the Indian ink and other pigment remains where it was -inserted with the needle, but some of it is picked up by leucocytes and -carried to the nearest lymphatic gland.</p> - -<p>Lymphatic glands are barriers which stop the spread of infection. -They are the stations to which our police carry captured germs. The -skin of the heel is abraded. Germs from the soil, or elsewhere, which -have accumulated in a dirty stocking—owing to the warm moisture -enclosed by an impervious boot, the woollen covering of the foot is a -peculiarly healthy place for germs—enter the opened lymph-spaces of -the subcutaneous tissues. Leucocytes hasten to the spot. They seize the -invaders with their pseudopodia, engulf them in their body-substance, -enter lymphatic vessels, and are rolled away by the lymph-stream. The -instinct which brings them in ever-increasing numbers to the breach in -the protecting skin can be explained only in terms of force. From our -own conscious action to the causes which determine the movements of a -<span class="pagenum"><a name="Page_56" id="Page_56">[Pg 56]</a></span> -leucocyte, or of an amœba, is so deep a drop that we prefer to -recognize in the latter a merely chemical attractive force. -“Chemiotaxis” we term the influence which draws leucocytes to the place -where food is abundant; although it is also the place, one must admit, -where in the interests of the body as a whole they run great risk of -asphyxiation. It is appetite which draws a schoolboy to a bun-shop; a -sense of duty prompts a fireman to risk his life in a chamber filled -with smoke. We have no desire to humanize a leucocyte; but it is -difficult to emphasize too strongly its independence. It would be -absurd to use terms which imply that a leucocyte has a self-directive -power; yet it is equally misleading to describe its migration to -the seat of injury, its retreat with ingested germs to a lymphatic -gland, its wriggling from the lymph-ways of the gland into the shelter -of its adenoid tissue, in terms which imply that the forces which -direct it are known, and their mode of action understood. The success -which attends the inroads of germs is due to their amazing capacity -for multiplication when they reach lymph or blood. It is useless to -attempt to form an idea of the rapidity with which they divide, since -we have no data upon which to base calculations. If the leucocytes -fail to deal with the first few that enter, germs soon swarm within -the lymph-vessels. This leads to an inflammation of the walls of the -vessels, which may then be seen as red lines beneath the skin. These -red lines lead upwards towards the nearest lymphatic gland. The glands -in the space behind the knee are not usually affected when the focus of -infection is in the foot. The red lines can be traced up the inner side -of the knee and the front and inner side of the thigh to the groin. The -glands in this situation swell until they can be easily felt. If the -mischief is in the hand, the gland at the elbow may be affected, but -most of the lymphatics pass by it on their course to the glands in the -armpit. If a sore throat is the source of infection, the glands beneath -the angle of the jaw enlarge. Thus various glands block the further -progress of infection. In doing this their resources may be strained to -the uttermost; they may enlarge, become tender, grow soft, fill with -pus, break down and discharge the pus without the aid of a surgeon’s -knife, although as soon as pus is recognizable within them it is wise -<span class="pagenum"><a name="Page_57" id="Page_57">[Pg 57]</a></span> -to let it out. If germs pass through these first stations into the -lymph-vessels beyond them, abscesses are formed in other situations. A -condition of “blood-poisoning,” so called, is set up.</p> - -<p>The readiness with which leucocytes sacrifice themselves in their -efforts to remove germs and decaying tissue is a matter of almost -every-day experience. The fatty matter produced in the sebaceous -glands of the skin normally overflows on to the surface. It serves to -render the skin supple and impervious to water. Germs get into one of -the sebaceous glands of the face or of the eyelid. The contents of -the gland begin to decompose. Leucocytes enter it for the purpose of -removing the putrescent substance. They lose their vitality and turn -into pus corpuscles. The pimple or the stye bursts, and pus and fatty -matter are discharged together.</p> - -<p>That the conversion of leucocytes into pus cells is due to want of -oxygen has been shown by the following experiment: A minute piece of -phosphorus is placed beneath the skin. Leucocytes gather round the spot -with a view to removing the tissue which the phosphorus has destroyed. -But phosphorus has so strong an affinity for oxygen that it exhausts -the supply in the area of tissue which surrounds it. The leucocytes -die before reaching the tissue immediately adjacent to the piece of -phosphorus. Their dead bodies form round it a raised ring of pus cells. -We can explain this readiness of leucocytes to sacrifice themselves -in their efforts to reach foreign matter which needs to be removed, -only by saying that the attraction of the food is greater than the -repulsion of lymph destitute of oxygen. An amœba placed in comparable -circumstances gives up the quest of food, however strongly chemiotaxic, -and retreats towards water which contains oxygen sufficient to provide -for its respiratory needs.</p> - -<p><b>Blood.</b>—A portion of the body fluid is enclosed within vessels -and kept in circulation by the heart. The heart pumps blood into the -aorta. This trunk gives off large arteries, which in turn divide until -the finest capillary vessels are reached. The capillary tubes reunite -to form veins, which, with the exception of those which collect food -from the digestive organs, convey the blood right back to the heart. -The veins which drain the stomach and intestines (the organs in which -food is prepared for absorption) and the spleen (the organ in which -<span class="pagenum"><a name="Page_58" id="Page_58">[Pg 58]</a></span> -worn-out red blood-corpuscles are in a sort digested) break up in the -liver into a second set of small vessels. The pseudo-capillary vessels -of the liver reunite to form the hepatic veins, which add the blood -that has passed through that organ to the rest of the blood which is -passing up the inferior vena cava to the heart. A second capillary -circulation is found in the kidney also.</p> - -<p>The heart is four-chambered (<a href="#I_218">Fig. 10</a>). Its left ventricle -drives the blood round the systemic or greater circulation, the blood returning -to the right auricle. The right ventricle drives the blood through the -lesser or pulmonary circulation, from which it returns to the left -auricle. The walls of all bloodvessels, except capillary tubes, are -sufficiently thick to prevent the escape of any of the constituents of -blood. To support the pressure of the blood which they contain, the -arteries and the larger veins need walls of considerable thickness. The -walls of the capillaries allow an interchange between blood and lymph -in the manner already described (<a href="#Page_39"><i>cf.</i> p. 39</a>).</p> - -<p>Blood fresh from the lungs, whether still in the pulmonary veins or in -the systemic arteries, is scarlet in colour. Venous blood is darker and -purple-red, the depth of its tint varying with the extent to which it -has parted with its oxygen. It looks less opaque than arterial blood. -With this exception, the physical properties and chemical composition -of blood are remarkably constant in all parts of the body. Arterial -blood contains more oxygen, venous blood more carbonic acid. Other -chemical differences can be recognized, but they are relatively very -small. The constancy in the constitution of blood is its most notable -character. Bleeding, unless excessive, does not greatly affect it. -The number of corpuscles is of course diminished, but even these are -replaced with great rapidity. The plasma, after bleeding, soon recovers -its proteins and salts. A similar readjustment occurs if normal saline -solution (water containing 0·9 per cent. sodic chloride), or even a -strong solution of salt, is injected into the blood. Within certain -limits it is very difficult to disturb the balance of its constituents. -It gets rid of substances added in excess, or replaces substances -removed, with remarkable facility. If sugar (glucose) be injected into -a vein, it escapes through the capillary walls into the lymph. After a -<span class="pagenum"><a name="Page_59" id="Page_59">[Pg 59]</a></span> -short interval the lymph contains more sugar than the blood. If an -excess of protein, whether of a kind foreign to the blood or its own -serum-albumin, be injected, it is removed by the kidneys. The blood has -various sources from which it can draw out reserves of anything that is -lacking, and various ways of getting rid of anything that is in excess. -It draws upon the lymph in the tissue-spaces for water. It discharges -salts into the lymph. It also takes salts from the lymph. It draws upon -the liver for sugar, and probably for proteins also. In a starving -animal the blood still contains sugar long after fresh supplies have -ceased to reach it from the intestines. The lungs remove its carbonic -acid. The kidneys free it from everything which cannot be otherwise -removed. It is essential to the well-being of the organism as a whole -that a uniform standard of composition should be maintained by the blood.</p> - -<div class="figcenter"> - <a id="I_060" name="I_060"> </a> - <img src="images/i_p060.jpg" alt="" width="600" height="565" /> - <div class="blockquot2"> - <p class="neg-indent"><span class="smcap">Fig. 4.</span>—<span - class="smcap">Red Blood-Corpuscles presenting, some the Surfaces, - others the Edges, of their Discs, together with Single Representatives - of Four Types of Leucocyte.</span></p> - - <p class="neg-indent space-below2">A, the most common type, highly amœboid - and phagocytic. Its protoplasm is finely granular, its nucleus - multipartite. B, a leucocyte closely similar to the last, but larger, - and containing an undivided nucleus. It is shown with a cluster of - particles of soot in its body-substance. C, a young leucocyte, or - “lymphocyte.” D, a coarsely granular leucocyte. Its granules stain - brightly with acid dyes—<i>e.g.</i>, eosin or acid fuchsin.</p> -</div></div> - -<p><i>Composition.</i>—The structural composition of the blood, and the -relation of its several constituents to each other, is best studied -under the microscope. A thin transparent membrane in which blood is -circulating through small vessels—the web between the toes of a -frog’s foot, the mesentery, the membrane of a bat’s ear—affords an -opportunity of observing blood in circulation. In any of the smaller -vessels, whether artery or vein, a column of red corpuscles is seen -moving in the axis of the stream. This column is surrounded by a layer -of clear plasma. Amongst the red corpuscles a few leucocytes may be -detected floating placidly down the current. Others are seen in the -peripheral layer of plasma, tending to creep along the wall of the -vessel rather than submit to be moved forward, as passive objects, by -the current. If an irritant be applied to the membrane, the vessels -dilate; yet, notwithstanding their wider calibre, the current becomes -slower. The red corpuscles mass together. Apparently their constitution -is slightly altered by this commencing inflammation, in such a manner -that they cease to be clean, independent discs which slide past each -other like small boats on a river; they exhibit a tendency to stick one -to another. In the capillary vessels leucocytes may now be observed, -not merely creeping along the inner surface of the endothelium, but -squeezing themselves between its scales; making their way out of the -vessel into the tissue-spaces through which the vessel passes. Such an -<span class="pagenum"><a name="Page_60" id="Page_60">[Pg 60]</a></span> -observation gives the clue to the functions of the several constituents -of the blood. The red corpuscles carry oxygen in chemical combination -with their colouring matter. From them it passes into solution in the -plasma; from the plasma through the walls of the capillary vessels -into lymph; the tissues take it from the lymph as they require it. As -fast as it is removed from lymph it is renewed from plasma. Carbonic -acid excreted by tissue cells is dissolved in lymph. From lymph it is -transferred to plasma. The reception of carbonic acid by these fluids -is not quite so simple as the transference of oxygen from blood to -lymph. It is aided by the presence of alkaline carbonates which are -always ready to form “acid” salts: not acid to litmus-paper—the blood -is always alkaline—but containing more than one unit of acid to one -of base. Sodic carbonate has the formula <b>Na₂CO₃</b>. With an additional -molecule of carbonic acid it becomes <b>Na₂CO₃CO₂(HO)</b>—bicarbonate. When -in solution it can hold still more carbonic acid. If carbonic acid were -<span class="pagenum"><a name="Page_61" id="Page_61">[Pg 61]</a></span> -merely dissolved in lymph and plasma, it would be impossible for the -blood to carry it away with sufficient rapidity; just as it would be -impossible for blood to bring sufficient oxygen were it not for the -colouring matter (hæmoglobin) which forms a temporary, easily divorced -union with it. But from a physical point of view it comes to the same -thing. As the tension of oxygen in plasma falls, it dissolves more from -the hæmoglobin. When the tension of oxygen in lymph is less than its -tension in plasma, the former borrows from the latter. If the tension -of carbonic acid in lymph is higher than in blood, it passes to the -blood. The rapidly circulating blood at frequent intervals traverses -the lungs. The whole blood of the body is exposed to air in the lungs -once every minute. Oxygen tension being higher in pulmonary air than in -venous blood, this gas is taken up. Carbonic acid tension being higher -in venous blood than in pulmonary air, this gas escapes. The plasma in -the capillary vessels which traverse the tissues exchanges gases with -the lymph with very great rapidity.</p> - -<p>The specific gravity of blood varies from 1·056 to 1·059. The -corpuscles are heavier than the plasma. Its reaction to test-paper is -alkaline, owing to the presence of bicarbonate of soda and disodic -phosphate. The alkalinity is greatest when the body is at rest; it is -diminished by severe muscular exercise. Blood contains about 5,000,000 -red corpuscles, and 7,000 or 8,000 leucocytes, to a cubic millimetre. -Red blood-corpuscles are biconcave discs destitute of nucleus, and, so -far as can be seen, devoid of any investing membrane. Seen in profile -they appear biscuit-shaped, because the centre is hollowed out. Their -largest diameter is 7·5 micromillimetres (¹/₃₂₀₀ inch)—a measurement -of great importance to anyone who works with a microscope, because it -serves as a standard by which to estimate the size of other objects. -They are soft, but fairly tough and highly elastic. In circulating -blood a corpuscle may occasionally be seen to catch on the point where -two capillary vessels unite. It bends almost double under the pressure -of the column of corpuscles behind it, and then springs forward.</p> - -<p>A red corpuscle is a vehicle for hæmoglobin. If blood is diluted with -water, or if it is alternately frozen and thawed, the hæmoglobin -<span class="pagenum"><a name="Page_62" id="Page_62">[Pg 62]</a></span> -separates from the corpuscles, which can then be seen as colourless -discs. Hæmoglobin constitutes 40 per cent. of the weight of a -moist corpuscle, or 95 per cent. of its weight after it has been -dried. This is an enormous charge for a corpuscle to carry, and the -question of how it carries it has been much discussed. It is not in -a crystalline state. A corpuscle examined by polarized light is not -doubly refractive. Microscopists know that if there were any crystals -in the corpuscle it would appear bright on a dark ground when the -Nicholl prisms are crossed. It cannot be in solution, since the water -which the corpuscle contains would not suffice to dissolve it. It must -be combined with some constituent of the corpuscle. But whether it -is uniformly distributed throughout the disc, or in a semifluid form -enclosed in spaces in a sponge-work; or whether the corpuscle is a -hollow vesicle enclosing fluid hæmoglobin—a view which was long ago -maintained, and has recently been revived—are questions which still -await further evidence.</p> - -<p>Red blood-corpuscles, properly so called, are found only in vertebrate -animals, although invertebrate animals, from worms upwards, possess -genuine blood, and in some of them it contains hæmoglobin, or a -similar pigment in the form of globules. These might be likened to -the non-nucleated corpuscles of mammals, but it must be remembered -that the non-nucleated cells of mammals have been evolved from the -nucleated blood-corpuscles of birds, reptiles, amphibians, and fishes. -Below fishes red blood-cells are not found. Hæmoglobin is usually -dissolved in the blood of invertebrate animals. It is impossible to -trace any relationship between the coloured globules of invertebrates -and the blood-cells of fishes. The coloured globules must be regarded -as deposits or accretions of hæmoglobin held together by a proteid substance.</p> - -<p>The nucleated red corpuscles of submammalian vertebrates multiply by -cell division while circulating in the blood-stream. A good subject -in which to look for dividing corpuscles is the blood of a newt in -spring-time, when rapidly increasing activity calls for an additional -supply. There is nothing to distinguish the method of division of a -nucleated blood-corpuscle from that of any other cell.</p> - -<p>The life-story of the red blood-corpuscles of mammals is one of the -<span class="pagenum"><a name="Page_63" id="Page_63">[Pg 63]</a></span> -most fascinating that the histologist has to tell. He wishes that -he could tell it with assurance; but, unfortunately, there are many -uncertainties, due to conflicting testimony, in its earlier chapters. -It is unlikely that a blood-corpuscle lives for long. A month or six -weeks is probably the term of its existence. The rapidity with which -the stock is replenished after bleeding shows that there must be ample -provision in the body for making blood-corpuscles. The rate at which -they disappear after they have been added in excess shows that there -is an equally effective mechanism for destroying them. If half as many -again as the animal already possesses be injected into its veins, the -number is reduced to its normal limit in about ten days. It is clear -that they can be made and can be destroyed with great facility, and -it seems a legitimate inference that production and destruction are -constantly taking place. Regarding the way in which they are destroyed -there is no uncertainty. We shall refer to this subject when describing -the functions of the spleen. But how are they made? We can sketch their -history in outline, but the evidence is conflicting with regard to all -matters of detail.</p> - -<p>In early stages of embryonic life all red blood-corpuscles are -nucleated, as they are permanently in birds and the other classes -of vertebrates below mammals. In embryonic mammals they multiply by -division whilst circulating in the blood, just as in the newt. But -it is generally believed that this is not the most important source -of new ones. During the earliest stages of growth they are being -formed in enormous numbers. Such instances of division as can be seen -in circulating blood appear to be all too infrequent to account for -their rapid multiplication, and there can be no doubt but that a more -complicated method of production is more important. Their formation -is described as taking place “endogenously.” Certain cells termed -“vaso-formative,” or “vaso-sanguiformative,” reach a considerable size, -and become stellate in form, or branched. Their nuclei divide without -the cell dividing. Each nucleus accumulates a little hæmoglobin round -it. A space filled with fluid appears inside the cell. The nuclei -project into this space. Then they drop off with their envelopes of -hæmoglobin. The outer shell of the big vaso-formative cell becomes the -wall of a capillary bloodvessel. By its branches it links up with other -<span class="pagenum"><a name="Page_64" id="Page_64">[Pg 64]</a></span> -vaso-formative cells, making a network of vessels. The fluid inside -it is the plasma of the blood. The nuclei and their envelopes are -blood-corpuscles. This, if it be a true story, is a comprehensive way -of making bloodvessels and blood at the same time. Doubts have been -thrown upon its accuracy, but many leading histologists strenuously -maintain that this description is correct.</p> - -<p>At a certain period all nucleated red corpuscles disappear from -mammalian blood. Non-nucleated corpuscles take their place. How are -the latter formed? For a short stage of embryonic life nucleated cells -containing blood-pigment are seen, or are supposed to be seen, in the -liver—there is, unfortunately, great difficulty in distinguishing -them with certainty from young liver-cells; later they are seen in -the spleen; throughout the whole of life they are to be seen in the -marrow of bone. The nucleated cells give origin to the non-nucleated -corpuscles. It is hardly legitimate to call these cells persistent -embryonic corpuscles. Yet the chain which connects the cells which -in the embryo are capable of dividing into pairs of nucleated red -blood-corpuscles, and the cells which, assuming the rôle of parent -cells, do not accumulate hæmoglobin for their own purposes, but for the -benefit of the red corpuscles which split off from them, is probably -unbroken. In this sense they are persistent embryonic corpuscles which -have deserted the blood-stream, and have taken shelter in certain -tissues which are particularly favourable for cell division. The -situations in which they hide themselves are singularly suggestive. In -the liver there is an abundant supply of nutriment, more abundant than -in any other part of the body of the embryo. Later, in the spleen, red -blood-corpuscles are being destroyed. Materials available for making -new ones must therefore be set free. The inside of a hollow bone is -a peculiarly sheltered situation. The fat cells of marrow accumulate -there after a time; but within some bones the marrow develops very -little fat; hence it shows the red colour, which is due to its abundant -bloodvessels. This “red marrow” is the most important seat of the -manufacture of red blood-corpuscles in adult life. Unfortunately, when -we try to answer the question, How are they formed? we are obliged to -speak with caution. Some histologists assert that the nucleated cells -divide, and that one of the two daughter cells accumulates hæmoglobin, -<span class="pagenum"><a name="Page_65" id="Page_65">[Pg 65]</a></span> -and loses—that is to say, extrudes—its nucleus. Others maintain -that the nucleated cells become irregular in form; that hæmoglobin -accumulates in the projecting portion of the cell; that this projecting -portion breaks off as a non-nucleated corpuscle. It would be indiscreet -at the present time to pronounce in favour of either of these reports, -although the decision is of theoretical importance. If the former -account be true, red blood-corpuscles are nucleated blood-cells which -have lost their nuclei. If the latter account be in accordance with -fact, it is hardly justifiable to regard them as cells. They are parts -of cells which finish their existence independently of the cell body -and nucleus to which they belong. As circumstantial evidence, favouring -the theory that cell division is normal and the nucleus subsequently -lost, may be pleaded the existence in marrow, and also in the embryonic -liver and spleen, of certain very peculiar cells. These cells have long -been known as giant cells, and all attempts at accounting for them -have broken down. They are relatively of immense size: their diameter -may be twenty times as great as that of a red blood-corpuscle. Each -contains a huge irregular, bulging nucleus. Hence the cells are termed -“megacaryocytes” (big-nucleus cells). They must not be confounded -with the polycaryocytes (cells with several nuclei), which eat up -degrading bone, although it must be confessed that megacaryocytes and -polycaryocytes appear to be genetically connected. It is supposed -that megacaryocytes consume the nuclei which red corpuscles extrude -during the process of their conversion from nucleated cells. Traces -of nuclei, or things which often look like nuclei, are found in their -body-substance. Their own overgrown misformed nuclei appear to be -the result of an excess of nuclear food. It is certainly remarkable -that megacaryocytes are not found below mammals. They do not occur -in any animal in which red blood-corpuscles retain their nuclei. -Polycaryocytes are found in numbers in the bones of growing birds. -They are evidently scooping out bone from situations in which it has -to be displaced in order that the shape of the bone as a whole may be -changed. But there are no megacaryocytes in birds. On the other hand, -megacaryocytes are present in the liver, and later in the spleen, of -mammals at the periods when blood-formation is occurring most actively -<span class="pagenum"><a name="Page_66" id="Page_66">[Pg 66]</a></span> -in these organs. From the liver they disappear early. In most -mammals they disappear from the spleen about the time of birth; but -in some—the hedgehog, for example—they are found in the spleen -throughout the whole of life.</p> - -<p>Hæmoglobin is a substance which has the property of uniting with oxygen -to form oxyhæmoglobin—a compound from which the oxygen is, again, -very readily withdrawn. It is extremely soluble, but may be made to -crystallize by adding alcohol to blood, after setting the hæmoglobin -free from the corpuscles by freezing and thawing. From the blood of Man -and most other animals it crystallizes in the form of rhombic prisms, -whether in the oxidized (oxyhæmoglobin) or non-oxidized condition. -The addition of oxygen does not affect its crystalline form; although -crystalline, it is absolutely non-diffusible. This is due to the great -size of its molecule, which is probably larger than that of any other -substance which is capable of crystallizing.</p> - -<p>The percentage composition of hæmoglobin conforms closely with that -of albumin and other proteins, with this most important difference: -it contains a definite proportion of iron—0·336 per cent. That the -percentage of carbon, hydrogen, nitrogen, sulphur, and oxygen should -agree with that commonly found in proteins is inevitable, since it -may be split into a part which contains all the iron, hæmatin, and a -proteid part resembling albumin; and the latter constitutes 96 per -cent. of its weight.</p> - -<p>There is no doubt but that its value as a vehicle of oxygen depends -upon the presence of iron. In the matter of taking up and dropping -oxygen, hæmatin behaves somewhat in the same manner as hæmoglobin; -whereas if iron be removed from hæmatin the “iron-free hæmatin” -loses its respiratory value. It is almost certain that a molecule of -hæmoglobin contains a single atom of iron. On this supposition its -molecular formula may be calculated. It is not quite the same for all -animals, although the variations are slight. For the blood of the horse -it is as follows:</p> - -<p class="center"><big><b>C₇₁₂H₁₁₃₀N₂₁₄S₂FeO₂₄₅.</b></big></p> - -<p>This means a molecular weight of 16708. We give the figures, because -<span class="pagenum"><a name="Page_67" id="Page_67">[Pg 67]</a></span> -the properties of hæmoglobin will be better understood if its -prodigious molecular weight is borne in mind. In a sense, the reason -for the great size of its molecule is not far to seek. The atomic -weight of iron (Fe = 56) is much greater than that of either of the -other elements contained in hæmoglobin. The molecule needs to be -very great to float an atom of iron. As it is, the corpuscles are -heavier than the plasma which surrounds them, in the proportion of -about 13 to 12. Although hæmoglobin is a crystallizable substance, -its immense molecule is absolutely non-diffusible. It cannot pass -through a membrane. This is of no consequence as regards the relation -of hæmoglobin to the walls of the capillary bloodvessels, since it -is contained in corpuscles; but it is of great importance as regards -its relation to the discs which carry it. A very small quantity of -enveloping substance suffices to prevent it from diffusing into the -plasma of the blood. The great molecules are held together and isolated -from the fluid in which they float by a minimal amount of insoluble globin.</p> - -<p>The iron needed for the making of hæmoglobin is obtained both from meat -and vegetables. The constituents of an ordinary diet provide from 2 to -3 centigrammes of iron a day. The whole of the blood contains about -4·5 grammes. When corpuscles are being destroyed in the spleen, the -iron which their pigment contains is largely reabsorbed and rendered -available for further use. The iron in a mixed diet is more than -sufficient to counterbalance any loss. Milk contains extremely little -iron. Before birth the liver and spleen accumulate a store of iron -which lasts until the end of the nursing period, unless this be unduly -prolonged. If it be prolonged, the child is apt to become anæmic. -Iron has been administered in the treatment of anæmia ever since its -presence in the red clot of blood was recognized a hundred and fifty -years ago. Physicians are agreed that in the anæmia of young people it -is of value; but observations made with a view to obtaining definite -data as to the increase in number of blood-corpuscles which results -from the administration of iron, without any other alteration in the -diet or the habits of the patient, have not given accordant results. -Some observers have obtained an increase with organic compounds of -iron, others with inorganic compounds; some are in favour of small -doses, others of very large ones. As in the treatment by drugs of other -<span class="pagenum"><a name="Page_68" id="Page_68">[Pg 68]</a></span> -abnormal conditions, it is difficult to isolate the effect of the drug -from the effects of improvements in the general regimen. Yet physicians -agree that iron accentuates the beneficial effects of fresh air and -improved diet.</p> - -<p>When the surface of the body is struck, the effect of the blow is -marked at first by redness. There is nothing to show that small -bloodvessels have been ruptured and blood effused beneath the skin. -Next day the injured area is reddish-purple. The bruise turns blue, -green, yellow, and eventually disappears. In the process of absorption, -oxyhæmoglobin undergoes decomposition. First its proteid constituent is -removed, leaving a coloured pigment containing iron, termed “hæmatin”; -soon reduced by loss of oxygen to hæmochromogen. When Sir George Stokes -first described the spectrum of blood (<a href="#Page_185"><i>cf.</i> p. 185</a>), -he showed that as hæmoglobin may exist in an oxidized and in a non-oxidized condition, -distinguished by their spectra, so also may the coloured residue -which is left after the proteid constituent has been removed from -hæmoglobin. This coloured residue he termed, when oxidized, “hæmatin”; -when not oxidized, “reduced hæmatin.” Stokes’s reduced hæmatin is -now termed “hæmochromogen.” Hæmochromogen stands for the coloured -nucleus of hæmoglobin. Although it is not present in hæmoglobin as -hæmochromogen—hence we must not speak of hæmoglobin as made of a -protein, <i>x</i>, plus hæmochromogen, <i>y</i>—it is to its coloured -residue that hæmoglobin owes its value as a carrier of oxygen. Later, iron -is removed from hæmochromogen, leaving hæmatoidin, a substance often -found at the seat of old hæmorrhages, where it may remain unchanged -for a very long time. Hæmatoidin is apparently identical with the -yellow pigment of bile, bilirubin. The green colour which shows itself -in the bruise seems to indicate that the more oxidized bile-pigment, -biliverdin, is formed in the first instance. Red corpuscles, when -destroyed in the spleen, pass through transformations similar to those -which blood undergoes when effused beneath the skin. Their protein is -used by the phagocytes which eat them. Their iron is reserved for the -use of the blood-forming cells of the red marrow of bone. The pigment -which remains as the residue of hæmoglobin is carried by the splenic -vein to the liver, which secretes it as bile-pigment. So much of the -<span class="pagenum"><a name="Page_69" id="Page_69">[Pg 69]</a></span> -bile-pigment as is reabsorbed by the wall of the alimentary canal is -eventually excreted as the pigment of urine.</p> - -<p>Such is the history of the changes which blood-pigment undergoes within -the living body. To a certain extent its chemistry can be followed in -the laboratory; but it must be remembered, when we are treating of the -chemistry of a substance as complex as hæmoglobin, that the products -which can be obtained from it in the laboratory are not necessarily -those into which it is transformed in the body. In the laboratory -oxyhæmoglobin is easily changed into methæmoglobin, a substance of the -same percentage composition, but with its oxygen more firmly fixed. -Methæmoglobin can be decomposed into a proteid substance and hæmatin. -Hæmatin, when acted on by reducing agents, becomes hæmochromogen. -Hæmochromogen, when subjected to such a reducing agent as a mixture -of tin and hydrochloric acid, gives rise to coloured bodies closely -resembling bile-pigments—not as they are secreted by the bile, but -as they appear in the urine. It is impossible to prove that the changing -colours of a bruise indicate a sequence of chemical transformations -from hæmoglobin to bile-pigment, but it is not improbable that such -a description is correct. The test commonly used to ascertain the -presence of bile-pigment, <i>i.e.</i>, bilirubin, is the play of colours -which it exhibits when oxidized by fuming nitric acid. From yellow it -turns to green, to blue, and then to purple, more or less reversing -the colours of the bruise. It is fairly certain that effused blood -undergoes changes along lines which, if not identical with those -through which blood passes on its road to bile-pigment, are at any rate -very similar.</p> - -<p><b>Coagulation of Lymph and Blood.</b>—Two or three minutes after -blood has been shed it begins to clot. In ten minutes the vessel into -which it has been received may be inverted without spilling the blood. -After a time the jelly, holding all the corpuscles, shrinks from -the sides of the jar. It squeezes out a transparent, straw-coloured -fluid—serum. The clot continues to contract until, in a few hours, -about one-half of the weight of the blood is clot, the other half -serum. Lymph coagulates like blood, but most specimens clot more -slowly, and the product is less firm. -<span class="pagenum"><a name="Page_70" id="Page_70">[Pg 70]</a></span></p> - -<p>When the process is watched through the microscope—a few drops of the -almost colourless, transparent blood of a lobster afford an excellent -opportunity of studying the formation of the clot—innumerable -filaments of the most delicate description are seen to shoot out from -many centres. They multiply until they constitute a felt-work. In -the case of blood obtained from a vertebrate animal, this felt-work -holds the corpuscles in its meshes. Its filaments exhibit a remarkable -tendency to contract. They shorten as much as the enclosed corpuscles allow.</p> - -<p>The filaments may be prevented from entangling the corpuscles by -whipping the blood, from the instant that it is shed, with a bundle of -twigs or wires. The fibrin collects on the wires, while the corpuscles -remain in the serum. If this fibrin is washed in running water until -all adherent serum and corpuscles are removed, it appears as a soft -white stringy substance which, when dried, resembles isinglass.</p> - -<p>Clotting is a protection against hæmorrhage. As it oozes from a scratch -or tiny wound, blood clots, forming a natural plaster which prevents -continued bleeding. It has little if any influence in resisting a -strongly flowing stream of blood. But a clean cut through a large -vessel is an accident which rarely happens as the result of natural -causes. It is not the kind of injury to which animals are liable. When -an artery is severed by a blunt instrument, the muscle-fibres of its -wall contract. They occlude the vessel. The blood clots at the place -where the vessel is injured, and plugs it. This happens also when a -surgeon ties an artery. He is careful to pull the ligature sufficiently -tight to crush its wall. His sensitive fingers feel it give. He stops -before the thread has cut it through. As will be explained later, the -clotting of blood is promoted by contact with injured tissue. If in -tying an artery its wall be not crushed, the blood in it may remain -liquid. When it is skilfully tied, the blood clots, forming a firm plug -which is practically a part of the artery, by the time that the silk -thread used in tying it is thrown out, owing to the death of the ring -of tissue which it compressed. After a tooth has been extracted, the -cavity is closed and further bleeding stopped by clotted blood.</p> - -<p>When large vessels have been severed, the copious hæmorrhage which -follows induces fainting. For a short time the heart stops, or beats -<span class="pagenum"><a name="Page_71" id="Page_71">[Pg 71]</a></span> -very feebly. The blood-pressure falls. The bloodvessels contract. A -clot has time to form. An emotional tendency to faint at the sight of -blood is a provision for giving the various causes which stop bleeding -an opportunity of coming into play. It is a useful reflex action, -always supposing that the person who is liable to it faints at the -sight of his own blood. Amongst other reasons for the greater fortitude -of women—they are far less subject to this emotional reflex than -men—might be alleged the circumstances of life of primitive people. It -was the part of their women-folk to dress wounds, not to receive them.</p> - -<p>The phenomenon of coagulation has attracted attention from the earliest -times. It was a phenomenon that needed explanation, and culinary -experience suggested analogies close at hand. Hippocrates attributed -the clotting of blood to its coming to rest and growing cold. The blood -which gushed from a warrior’s wound formed a still pool by his side. -It set into a jelly as it cooled. Until the second quarter of the -nineteenth century this theory was deemed sufficient. It then occurred -to two men of inquiring mind to institute control experiments. John -Davy placed a dish of blood upon the hob. William Hunter kept one -shaking. In both experiments the blood clotted more quickly than it did -in vessels of the same size, containing the same amount of the same -blood, left upon the table.</p> - -<p>Even before this date an observation had been made regarding the -circumstances in which clotting occurs, which has thrown much light -upon the causes of the phenomenon. In 1772 Hewson gently tied a vein -in two places. At the end of a couple of hours he opened the vein. The -blood was still liquid, but clotted in a normal manner after it was -shed. Scudamore showed that blood clots more slowly in a closed than in -an open flask. A new theory, as little trustworthy as Hippocrates’, was -based upon these observations. Blood clotted because it was exposed to -air. A record of all observations of the circumstances of coagulation, -and of all the theories to which they have given rise, would make an -exceptionally interesting chapter in the history of human thought. It -would bring into singular prominence stages in the development of what -is now known as the “scientific method.” Not that Science has a method -<span class="pagenum"><a name="Page_72" id="Page_72">[Pg 72]</a></span> -of her own. Philosophers of all classes would follow the same method -if their data allowed of its application. The peculiarity of the data -with which Science deals is that they can be brought to a test of -which the data of historical, or political, or economic theory are -not susceptible. They can be confronted with control experiments. The -control experiment is the alphabet and the syntax of the scientific -method. No hypothesis is admissible into the pyramid of theory until -it has passed this test. A natural phenomenon is observed. Every -measurement which is applicable is taken and recorded—time, weight, -temperature, colour. Scientific observation implies the tabulation of -all particulars which are capable of statistical expression. Reflecting -upon the relation of the phenomenon to other phenomena of a like -nature, the philosopher—it is the philosophy of physiologists which -interests us—formulates an hypothesis as to its cause. At this point -the real difficulty of applying the scientific method begins. It is -easy to formulate hypotheses. It is very difficult to devise control -experiments. An experiment must be arranged which will provide that, -while all other conditions in which the phenomenon has been observed -to occur are reproduced, the condition which was <i>ex hypothesi</i> its -cause shall be omitted. This digression into the philosophy of science -may seem to be somewhat remote from our line of march, but it may -perhaps hasten our progress in the comprehension of the story of -physiology. There is no other science in which the control experiment -plays an equally important part. Unless this is realized, the whole -trend of experimental work will be misunderstood. Scudamore explained -coagulation as due to contact with air. Based on the observations we -have cited, no hypothesis could have seemed more reasonable. With -a view to checking this hypothesis, blood was received into a tube -of mercury. It coagulated in the Torricellian vacuum. Scudamore’s -hypothesis, like many earlier and later, when confronted with a control -experiment, was turned away, ashamed.</p> - -<p>Clotting is a property of plasma. Red corpuscles play no part in the -process. Coagulation does not occur in a living healthy vessel. It -occurs when the vessel, and especially when its inner coat, is injured. -It is hastened by contact with wounded tissues, especially with wounded -skin. Contact with a foreign body also starts coagulation. If a silk -<span class="pagenum"><a name="Page_73" id="Page_73">[Pg 73]</a></span> -thread is drawn through a bloodvessel, from side to side, fibrin -filaments shoot out from the thread, as well as from the wound -inflicted on the vessel by the needle which was used to draw it through.</p> - -<p>Plasma contains a substance which sets into fibrin. It has been -termed “fibrinogen.” It is present in lymph, and in almost all forms -of exuded lymph. If sodium chloride (common salt) is added to plasma -until it is half saturated—until it has dissolved half as much as -the maximum quantity which it can dissolve—fibrinogen is thrown down -as a flocculent precipitate. It can be redissolved and reprecipitated -until it is pure. When fibrinogen was separated from plasma a step -was taken towards the explanation of coagulation. Under certain -conditions fibrinogen sets into fibrin. The question which then -presented itself for solution was as follows: What is the substance -which, by acting upon or combining with fibrinogen, converts it into -fibrin? The clue to the solution of this question was obtained from -the consideration of certain observations made by Andrew Buchanan -in 1830, but long neglected, because their significance was not -understood. Buchanan had observed that some specimens of lymph exuded -into a lymph-space—the peritoneal cavity, for example—will clot; -others will not. He noticed that they clot when, owing to puncture of -a small bloodvessel during the process of drawing them off, they are -tinged with blood. Determined to ascertain which of the constituents -of blood is effective in rendering non-coagulable effusions capable of -clotting, he added to them in turn red blood-corpuscles, serum, and the -washings of blood clot. Either of the two latter was found to contain -the clot-provoking substance. Thirty years later a German physiologist -prepared fibrinogen from effused lymph by precipitating it with salt. -He also treated serum in a similar way, precipitating a protein which -he termed fibrinoplastin. When these two substances were dissolved -and the solutions mixed, he obtained a clot, which he regarded as -a compound of fibrinogen and fibrinoplastin. Subsequently he found -that the mixture did not always clot, but he discovered that if he -coagulated blood with alcohol, and washed this residue, the washings -added to the mixed solution just referred to invariably produced a -clot. Thinking that the substance which he obtained from his -<span class="pagenum"><a name="Page_74" id="Page_74">[Pg 74]</a></span> -alcohol-coagulated blood could not be proteid, he termed it -“fibrin-ferment.” He neglected the control experiment. He failed to -ascertain whether or not all three substances were needed. Had he -tried adding fibrin-ferment to fibrinogen, he would have discovered -that the further addition of fibrinoplastin was unnecessary. He did -not ascertain, as he might have done, that the weight of fibrin -formed is somewhat less, not greater, than the weight of fibrinogen -used. (Fibrinogen gives off a certain quantity of globulin when it -changes into fibrin.) He was also wrong in supposing that the water -which he added to alcohol-coagulated blood dissolved no protein. -His “fibrin-ferment” is always associated with a protein. Since it -may also be obtained from lymphatic glands, thymus gland, and other -tissues which contain lymphocytes, it has been inferred that it is -itself a protein, of the class known as nucleo-proteins. The fact that -it is destroyed at so low a temperature as 55° C. has been supposed -to confirm the theory that it is a protein. But with regard to the -chemical nature of fibrin-ferment, as of all other ferments, we are at -present in the dark. Under ordinary circumstances, when blood clots, -the fibrin-ferment, or plasmase, or thrombin—it has received various -names—is set free by leucocytes. Fluids which contain fibrinogen clot -on the addition of a “ferment” which is either secreted by leucocytes -or set free from leucocytes when they break up—as they are very apt -to do, as soon as the conditions upon which their health depends are -interfered with.</p> - -<p>Freshly shed blood contains minute particles, termed “platelets,” in -diameter measuring about a quarter that of a red blood-corpuscle. When -the inner coat of a vessel is injured, platelets accumulate at the -injured spot. They form a little white heap, from which coagulation -starts. Evidently they supply the ferment, or a precursor of the -ferment. As yet their origin has not been traced. They are too large to -be the unchanged granules of granular leucocytes, but that they are in -some way derived from leucocytes seems probable.</p> - -<p>The further study of coagulation has shown that the conditions under -which it occurs are more complicated than the simple explanation just -given would seem to imply. This explanation holds good, so far as it -goes, but facts connected with the details of the process have recently -<span class="pagenum"><a name="Page_75" id="Page_75">[Pg 75]</a></span> -been brought to light which warn the physiologist that as yet his -theory of coagulation is incomplete.</p> - -<p>The presence of salts of lime has an important relation to coagulation. -If blood is received into a vessel in which has been placed some -powdered oxalate of potash, or soap, or any other chemical which -fixes lime, the blood does not coagulate. All other conditions are as -usual, but lime is withdrawn from the plasma. The non-coagulation of -oxalated plasma was interpreted as indicating that lime, under the -influence of fibrin-ferment, combines with fibrinogen to form fibrin; -that fibrinogen altered by fibrin-ferment combines with lime. This -hypothesis was based upon the analogy of the curdling of milk. Milk -cannot curdle if lime be absent. If rennin (milk-ferment), prepared -from milk from which lime has been removed, be added to a solution -of caseinogen (the coagulable protein of milk), also prepared from -lime-free milk, no curd is produced. The addition of a few drops of a -solution of chloride of lime results in the immediate curdling of the -mixture. Evidently rennin so alters caseinogen as to bring it into a -condition to combine with lime. But the analogy does not hold good for -blood. In the case of plasma, lime acts, not upon fibrinogen, but upon -the fibrin-ferment—or rather upon a precursor of fibrin-ferment—in -such a way as to render it effective. Leucocytes produce a prothrombin, -which in contact with lime salts is converted into thrombin, which -coagulates fibrinogen.</p> - -<p>Fibrinogen is the substance which fibrin-ferment combined with salts -of lime changes into fibrin. Yet even now the story is not complete, -if the theory of coagulation is to be brought up to date. A perfectly -clean cannula is passed into an artery of a bird. If it be thrust -well beyond the place where the vessel has been cut, if the vessel -be tied so gently as to avoid injury to its inner coat, and if the -blood which first passes through the cannula be allowed to escape, the -blood subsequently collected will not clot. It contains fibrinogen, -lime salts, and fibrin-ferment, ordinarily so called; but the ferment -is ineffective. The addition to the blood of a fragment of injured -tissue, or of a watery extract of almost any tissue, immediately sets -up coagulation. This observation brings fibrin-ferment into line with -<span class="pagenum"><a name="Page_76" id="Page_76">[Pg 76]</a></span> -other ferments. Digestive ferments are secreted as zymogens, which -require to be influenced by a kinase before they acquire fermentative -activity. So, too, must thrombogen be changed into thrombin, under the -influence of thrombokinase, before it can act upon fibrinogen. Almost -all tissues yield the kinase which actuates fibrin-ferment. The utility -of this provision is manifest. A bird’s blood contains everything -necessary to form a clot with the exception of thrombokinase. The -injury which brings the blood into contact with a broken surface -supplies this ferment of the ferment. Fibrin-ferment, rendered active, -at once changes fibrinogen into fibrin. The same interaction is -necessary before the blood of a mammal is susceptible of clotting. But -a mammal’s blood is even readier to clot than is the blood of a bird; -for not only will a broken surface provide it with thrombokinase, -but the leucocytes contained within the blood, when injured, also -yield it. And the leucocytes are exceedingly sensitive of any change -of circumstance; on the slightest indication that conditions are not -normal they set free, perhaps owing to their own disintegration, the -kinase which turns thrombogen into thrombin.</p> - -<p>There is a constitutional condition, fortunately rare, in which blood -does not coagulate. A person subject to this abnormality is said to -suffer from hæmophilia. It is alleged that this condition is due to -deficiency of lime in the blood; and the deficiency of lime is said to -be due to excess of phosphates. The subject suffers from phosphaturia. -His kidneys get rid of the superabundance of phosphates by excreting -them in combination with lime. If this explanation be correct, there -is a chronic insufficiency of lime in the blood, because it is being -constantly withdrawn in the process of removing phosphates.</p> - -<p>The difficulty in the way of establishing a complete theory of the -coagulation of blood increases when the phenomena of incoagulability -are considered. Blood may be rendered incapable of clotting in a -variety of ways. Leeches and other animals which suck blood have the -capacity of rendering it incoagulable. If the heads are removed from -a score of leeches, thrown into absolute alcohol, dried, ground in -a pepper mill, extracted with normal saline solution, a dark turbid -<span class="pagenum"><a name="Page_77" id="Page_77">[Pg 77]</a></span> -liquor is obtained. This liquor, after filtration and sterilization at -a temperature of 120° C., injected into the veins of an animal, renders -its blood incoagulable.</p> - -<p>The preparation sold by druggists under the name “peptone,” when -injected into the veins of a dog, renders its blood incoagulable. -Commercial “peptone” is a mixture of many substances. Its -anticoagulation-effect is not due to the peptone which it contains. -It has been supposed to be due to imperfectly digested albumin and -gelatin (proteoses), but products of bacteric fermentation (toxins -and ptomaines) are more probably the active bodies. Not only is -the peptonized blood of a dog incoagulable, but if this blood be -injected into the veins of a rabbit (an animal upon which the direct -injection of peptone has no effect), it diminishes the coagulability -of the rabbit’s blood. If peptonized blood be mixed in a beaker with -non-peptonized blood, it prevents the coagulation of the latter. There -is little doubt but that the poison, whatever it may be, acts upon the -leucocytes; and there are some reasons for thinking that the poison is -not contained in the “peptone,” but is secreted by the liver of the -animal into which the “peptone” has been injected.</p> - -<p>A still more remarkable property in relation to coagulation must be -assigned to leucocytes. The blood of a dog which has been rendered -incoagulable by injection of peptone recovers its coagulability after -a time. If a further injection of “peptone” be made, the animal is -found to be immune. Injection of “peptone” no longer renders its -blood incoagulable. In a similar manner the blood develops a power of -resisting the action of agents which induce its coagulation whilst -circulating in the vascular system. Nucleo-proteins contained in -extracts of lymphatic glands and other organs when injected into the -veins of living animals cause their blood to clot, provided they are -injected in sufficient quantity. If they are injected in quantity less -than sufficient to induce coagulation, they render the animal immune -to their influence. A larger quantity given to an animal thus prepared -fails to take effect. This brings the phenomena of coagulation and -resistance to coagulation to the verge of chemistry. They extend into -the domain in which pathology reigns. Tempting though it be to record -other facts with regard to these phenomena which recent investigation -<span class="pagenum"><a name="Page_78" id="Page_78">[Pg 78]</a></span> -has brought to light, it is probably judicious to leave the problem at -the frontier. Across the frontier lies a fascinating land, rich with -unimaginable possibilities for the human race. Settlement is rapidly -proceeding in this country, which is charted, like other border-lands, -with barbarous names: “antibodies,” “haptors,” “amboceptors,” “toxins,” -“antitoxins,” and the like—finger-posts to hypotheses which show every -sign of hasty and provisional construction. But certain facts stand -out, in whatever way theory may, in the future, link them up. The virus -of hydrophobia, modified by passing through a rabbit, develops in human -beings, even when injected after they have been infected, the power of -resisting hydrophobia. The serum of a horse which has acquired immunity -to diphtheria aids the blood of a child, which has not had time to -become immune, in destroying the germs of this disease. It is a contest -between the blood and offensive bodies of all kinds which find entrance -to it, whether living germs or poisons in solution; with victory -always, in the long-run, on the side of the blood, provided its owner -does not die in the meantime. And not only is the blood victorious -in the struggle with any given invader, but having repulsed him, it -retains for a long while a property which neutralizes all further -attempts at aggression on his part. In the past, physicians have fought -disease with such clumsy weapons as mercury, arsenic, and quinine. Now -they anticipate disease. In mimic warfare with an attenuated virus the -blood is trained to combat. Smallpox which has been passed through -the body of a cow is suppressed by the blood’s native strength. The -exercise develops skill to deal with the most virulent germs of the -same kind. In cases in which physicians cannot anticipate disease in -human beings, they train the blood of animals to meet it; and, keeping -their serum in stock, they can, when the critical moment arrives, -reinforce the fighting strength of the patient with this mercenary aid.</p> - -<p><b>The Spleen.</b>—The spleen is placed on the left side of the body, -and rather towards the back. It rests between the stomach and the inner -surface of the eighth, ninth, tenth, and eleventh ribs. It is quickly -distinguished from other organs by its brown-purple colour, a sombre -hue to which it owed its evil reputation with the humoralists. The -<span class="pagenum"><a name="Page_79" id="Page_79">[Pg 79]</a></span> -liver’s yellow bile tinged man’s mental outlook, preventing him from -seeing objects in their natural brightness; but the spleen made black -bile, which, mounting to the brain, displayed its malign influence upon -the action of that organ, as, or in, the worst of humours.</p> - -<p>The spleen is invested with a capsule of no great toughness. Inside the -capsule is “spleen-pulp.” When the fresh organ is cut across, it is -seen that, although most of the pulp is of the colour of dark venous -blood, it is mottled with light patches. In some animals—the cat, -for example—these whitish patches are small round spots, regularly -arranged at a certain distance from the capsule. The distinction -into “red pulp” and “white pulp” marks a division into two kinds of -tissue with entirely different functions. The white pulp is lymphoid -tissue, lymph-follicles developed in the outer or connective-tissue -coat of the branches of the splenic artery. Its function is to make -lymphocytes, of which, for reasons which will shortly appear, the -spleen needs an abundant supply. The constitution of the red pulp is -entirely different, and peculiar to the spleen. The branches of the -splenic artery divide in the usual way into smaller and still smaller -twigs until the finest arterioles are reached; but these arterioles -do not give rise to capillary vessels. At the point at which in any -other organ their branches would attain the calibre of capillaries, -the connective-tissue cells which make their walls scatter into a -reticulum. They are no longer tiles with closely fitting, sinuous, -dovetailed borders, but stellate cells with long delicate processes -uniting to constitute a network. The blood which the arterioles bring -to the pulp is not conducted by closed capillary vessels across the -pulp to the commencing splenic veins. It falls into the general -sponge-work. The venules commence exactly in the same way as the -arterioles end. Stellate connective-tissue cells become flat tiles -placed edge to edge. The endothelium of an arteriole might be likened -to a column of men marching shoulder to shoulder, three or four -abreast; the connective tissue of the pulp, to a crowd in an open -place. The column breaks up into a crowd. On the other side the crowd -falls into rank as the endothelium of veins. The capsule and the red -pulp are largely composed of muscle-fibres. These relax and contract -<span class="pagenum"><a name="Page_80" id="Page_80">[Pg 80]</a></span> -about once a minute. By their contraction the blood is squeezed out of -the sponge.</p> - -<p>If the spleen be enclosed in an air-tight box (an oncometer), from -which a tube leads to a pressure-gauge—a drum covered with thin -membrane on which the end of a lever rests, or a bent column of -mercury on which it floats—the pressure-gauge shows the changes in -volume of the spleen. The long end of the lever, which records the -variations of pressure in the gauge, may be made to scratch a line -on a soot-blackened surface of travelling paper. A record of the -variations in volume of the organ, which can be studied at leisure, -is thus obtained. It shows that the spleen is sensitive to every -change of pressure in the splenic artery. Small notches on the tracing -correspond to the beats of the heart. Larger curves record the changes -of blood-pressure due to respiration. A long slow rise and fall marks -the rhythmic dilation and contraction of the spleen itself.</p> - -<p>One of the three large arteries into which the cœliac axis divides -delivers blood to the spleen direct from the aorta. The splenic vein -joins the portal vein shortly before it enters the liver. Thus the -spleen is placed on a big vascular loop which directs blood, not long -after it has left the heart, from the aorta, through the spleen, to the -liver.</p> - -<p>The peculiar construction of the splenic pulp which brings the blood -more or less to rest within its sponge-work, and the transmission to -the liver of the blood which leaves the spleen, indicate that it is an -organ in which blood itself receives some kind of treatment. It is not -passed through it, as it is through all other parts of the body, in -closed pipes. The spleen is a reservoir, or a filter-bed, into which -blood is received. -<span class="pagenum"><a name="Page_81" id="Page_81">[Pg 81]</a></span></p> - -<div class="figcenter"> - <a id="I_081" name="I_081"> </a> - <img src="images/i_p081.jpg" alt="" width="500" height="578" /> - <p class="center"><span class="smcap">Fig. 5.</span>—<span class="smcap">A Minute - Portion of the Pulp of the Spleen,<br /> very highly magnified.</span></p> - <div class="blockquot2"> - <p class="neg-indent space-below2">Stellate connective-tissue cells form spaces - containing red blood-corpuscles and leucocytes. In the centre of the - diagram is shown the mode of origin of a venule. It contains two - phagocytes—the upper with a nucleus, two blood-corpuscles just - ingested, and one partially digested in its body-substance; the lower - with two blood-corpuscles.</p> -</div></div> - -<p>The red blood-corpuscles of mammals are cells without nuclei, and with -little, if any, body-protoplasm. They are merely vehicles for carrying -hæmoglobin. We should deny to them the status of cell, if it were -possible to prescribe the limit at which a structural unit ceases to be -entitled to rank as a cell. They are helpless creatures, incapable of -renewing their substance or of making good any of the damage to which -the vicissitudes of their ceaseless circulation render them peculiarly -liable. It is impossible to say with any approach to accuracy how long -they last, but probably their average duration is comparatively short. -The spleen is a labyrinth of tissue-spaces through which at frequent -intervals all red corpuscles float. If they are clean, firm, resilient, -they pass through without interference. If obsolete they are broken up. -In the recesses of the spleen-pulp, leucocytes overtake the laggards of -the blood-fleet, attach their pseudopodia to them, draw them into their -body-substance, digest them. The albuminous constituent of hæmoglobin -they use, presumably, for their own nutrition. The iron-containing -colouring matter they decompose, and excrete in two parts; the iron -(perhaps combined with protein); the colouring matter, without iron, -as the pigment, or an antecedent of the pigment, which the liver will -<span class="pagenum"><a name="Page_82" id="Page_82">[Pg 82]</a></span> -excrete in bile. Hæmoglobin is undoubtedly the source of bilirubin, and -general considerations lead to the conclusion that it is split into -protein, iron, and iron-free pigment in the spleen; but the details -of this process have never been checked by chemical analysis. Neither -bile-pigment nor an iron compound can be detected in the blood of the -splenic vein. The only evidence of the setting free of iron in the -spleen is to be found in the fact that the spleen yields on analysis -an exceptionally large quantity of this metal (the liver also yields -iron), and that the quantity is greatest when red corpuscles are being -rapidly destroyed.</p> - -<p>As a rule, it is very difficult to detect leucocytes in the act of -eating red corpuscles; but under various circumstances their activity -in this respect may be stimulated to such a degree as to show them, -in a microscopic preparation, busily engaged in this operation. The -writer had the good fortune to prepare a spleen which proved to be -peculiarly suitable for this observation (<a href="#I_081">Fig. 5</a>). -His method was an example of the way in which a physiological -experiment ought not to be conducted. Having placed a cannula in the -aorta of a rabbit, just killed with chloroform, he was proceeding to -wash the blood out of its bloodvessels with a stream of warm normal -saline solution, when the bottle from which the salt-solution was -flowing overturned. Fearing lest an air-bubble should enter the -cannula, he hastily poured warm water into the pressure-bottle, and -threw in some salt, in the hope that it would make a solution of -about 0·9 per cent. The salt-solution was allowed to run through -the bloodvessel for rather more than an hour. When sections of the -spleen were cut, after suitable hardening, every section was found -to be packed with leucocytes gorged with red corpuscles. Some of -the corpuscles had just been ingested; from others the hæmoglobin -had already been removed. It may be that, for some unknown reason, -the destruction of red corpuscles was occurring in this particular -rabbit with unusual rapidity at the time when it was killed; but it -seems more probable that the animal’s leucocytes were provoked to -excessive activity by changes in the red corpuscles brought about by -salt-solution which was either more or less than “toxic.” As a score -of attempts to reproduce the experiment, with solutions of different -strengths, have failed, it is impossible to be sure that this is a -valid explanation. - -<span class="pagenum"><a name="Page_83" id="Page_83">[Pg 83]</a></span></p> - -<p>There must be something in the condition of worn-out red corpuscles -which either makes them peculiarly attractive to predatory leucocytes -or renders them an exceptionally easy prey. It does not require much -imagination to picture the drama which is enacted in the spleen. -Slow-moving leucocytes are feeling for their food. The majority of red -corpuscles pass by them; a few are held back. The leucocytes, like -children in a cake-shop, cannot consume all the buns. A selection -must be made, and preference is given to the sticky, sugary ones. Red -corpuscles when out of order show a tendency to stick together. When -blood is stagnating in a vein, or lying on a glass slide in a layer -thin enough for microscopic examination, its red discs are seen after -a time to adhere together in rouleaux. The parable of a child in a -cake-shop is not so fanciful as it may appear.</p> - -<p>The differentiation of function of organs is not as sharp as was -formerly supposed. Evidence of their interdependence is rapidly -accumulating. The activity of various organs is known to result in -the formation of by-products termed “internal secretions,” which -influence the activity of other organs, or even of the body as a -whole. The spleen enlarges after meals. This may be merely connected -with the engorgement of the abdominal viscera which occurs during -active digestion, or it may indicate, as some physiologists hold, that -an internal secretion of the spleen aids the pancreas in preparing -its ferments. The spleen enlarges greatly in ague and in some other -diseases of microbial origin. This has been regarded as evidence -that it takes some part in protecting the body against microbes. But -whatever may be the accessory functions which it exercises, they are -not of material importance to the organism as a whole, seeing that -removal of the spleen causes no permanent inconvenience either to men -or animals. Its blood-destroying functions are taken on by accessory -spleens, if there be any, and by lymphatic glands. The marrow of bone -also becomes redder and more active. Under certain circumstances, red -corpuscles, or fragments of red corpuscles, are to be seen within -liver-cells; but it is uncertain whether blood-destruction is a -standing function of the liver.</p> - -<hr class="chap" /> -<p><span class="pagenum"><a name="Page_84" id="Page_84">[Pg 84]</a></span></p> -<div class="chapter"> - <h2>CHAPTER V<br /><span class="h_subtitle">INTERNAL SECRETIONS</span></h2> -</div> - -<p><b>Thyroid Gland.</b>—On either side of the windpipe, rather below -the thyroid cartilage (Adam’s apple), lies a somewhat conical mass of -tissue. The two masses are connected by an isthmus; lobes and isthmus -make up the thyroid gland. The whole weighs about an ounce. In health -it is so soft that only the finger of an anatomist could detect it -through the skin and the thin flat muscles which connect the hyoid -bone and the thyroid cartilage with the breast-bone. It makes no -visible prominence on the front of the neck. The thyroid gland is, -however, liable to enlargement, especially amongst the people who live -in certain districts. In the Valais, “goitre,” as it is termed, is -so frequent that anyone walking up the Rhone Valley is sure to meet -a number of persons—for the most part women—whose swollen necks -overhang their collar-bones, like half-filled sacks. Goitre is even -more common in the Valle d’Aosta, on the Italian side of the Alps. In -England this condition, comparatively rare, is known as “Derbyshire” or -“Huntingdonshire” neck.</p> - -<p>In the majority of cases the tumour in the neck develops slowly, and -does not reach its full dimensions until after middle life. Goitre in -this form, although inconvenient, causes no serious discomfort. But -when it appears in early life, it is associated with an extraordinary -complex of malformations and ill-performed functions. The condition -into which a goitrous child sinks is known as cretinism. With the -exception of the skull-case, its skeleton does not attain to its -proper proportions; and, since the soft parts do not equally submit to -arrest of growth, the dwarf is heavy and ungainly, with large jowl and -<span class="pagenum"><a name="Page_85" id="Page_85">[Pg 85]</a></span> -protuberant abdomen. The appearance of distortion is extraordinarily -heightened by hypertrophy of the skin and the subcutaneous connective -tissue. Ears, eyelids, nose, lips, fingers, are thick and heavy. The -hair and nails are coarse. The skin is folded, wrinkled, rough.</p> - -<p>The bodily ungainliness of a cretin has its counterpart in the -deformity of his mind. He is an idiot whose deficiency is chiefly -marked by apathy.</p> - -<p>Cretinism exhibits itself in varying degrees. The description that we -have just given would not be accurate for all. For the sake of brevity, -we have chosen a case which might be that of a goitrous cretin of a -certain type, or that of a cretin whose thyroid gland, in lieu of -showing what looks like overgrowth, has failed to properly develop. -Nothing is more remarkable with regard to this organ than the fact -that the condition associated with its overgrowth and the effects of -its atrophy, or inadequate growth, are the same. A consideration of -the function of the gland will suggest an explanation of this seeming -paradox.</p> - -<p>The inconvenience caused by goitre induced surgeons, about twenty-five -years ago, to remove the tumour in simple uncomplicated cases. Owing -to the accessibility of the gland, the operation is both safe and -easy; but its removal was found to be followed by symptoms of a very -serious nature, especially overgrowth and œdema of subcutaneous -tissue, muscular twitchings and convulsions, mental dulness. About -the same date, physicians recognized that the disease myxœdema—so -called because the œdema is not watery, as in dropsy, but firm and -jelly-like—is due to deficiency of the thyroid gland.</p> - -<p>No other organ of the body has so weird an influence upon the -well-being of the whole. No other organ has an equally mysterious -ancestral history. Assuredly the thyroid gland was not always such -as we see it now. In prevertebrate animals it must have been quite -different, both in structure and in function. From fishes upwards, -however, its structure is always the same. It is composed of spherical -vesicles or globes. Every globe is lined by a single layer of cubical -epithelial cells. Its cavity is filled with a homogeneous semi-solid -substance known as “colloid.” The globes are associated into groups or -<span class="pagenum"><a name="Page_86" id="Page_86">[Pg 86]</a></span> -lobules. They are in contact with large wide lymphatic vessels. The -organ has a lavish supply of blood. It is also well supplied with -nerves. Colloid is the secretion of the epithelial cells which line -the globes. As these globes have no openings, the secretion must be -passed by osmosis into the lymphatic vessels. There is abundant reason -for believing that by this route the products of the gland reach the -blood, and are distributed by the blood to all the tissues of the body. -And here it is important to notice that associated with the thyroid -gland are certain very small masses of tissue termed “parathyroids.” -There may be four of these—two on the course of the large arteries -which supply the thyroid gland from above, two related with the almost -equally large arteries which supply it from below; but the number -varies. The parathyroids do not contain vesicles. They are solid -masses of epithelial cells, traversed by bloodvessels and lymphatics. -Yet, like the epithelial cells of the vesicles, they secrete colloid. -Granules of this substance are to be seen within their cells. We cannot -pass over the parathyroids without this reference, since, small though -they are, they seem to be quite as important as the thyroid gland -itself, judging from the effects which follow their removal.</p> - -<p>In all vertebrate animals the thyroid gland has the characters which -we have described. What was it like in the ancestors of the vertebrate -races? Its microscopic appearance in vertebrates, the only animals in -which we know it, is so anomalous as to convince an histologist that -it is a makeshift; it looks like an organ which, at a period no longer -visible through the mists of time, had a quite different function to -perform. This function it has lost—some other organ has taken it -on—yet it must do something which is useful to the organism. Otherwise -it would not have been preserved. It has been retained for the sake -of its by-function, for the sake of the internal secretion which it -produces. This is now the only work it has to do.</p> - -<p>What was its prime function? It is an axiom of biology that an animal -in its individual development recapitulates, albeit with many omissions -and abbreviations, the ancestral history of its race. The thyroid -gland appears in the embryo as a diverticulum of the anterior wall -of the pharynx. It is remarkable in being a single, median, unpaired -<span class="pagenum"><a name="Page_87" id="Page_87">[Pg 87]</a></span> -diverticulum, whereas almost all other organs are bilaterally -symmetrical. The parathyroids are formed on the two sides in connection -with certain of the branchial pouches. In its earliest development -the thyroid gland resembles any other gland—a salivary gland, for -example. Until a late stage it retains its connection with the back -of the mouth. Occasionally indications of this primitive connection -persist throughout life. In most cases the place where the duct of -the thyroid gland used to open is clearly marked. At the back of -the tongue—too far back to be seen without the aid of a dentist’s -mirror—there is a <b>V</b>-shaped row of large papillæ (papillæ -circumvallatæ). Just behind the meeting-point of the two limbs of the -<b>V</b> a pit is to be seen—foramen cæcum. This pit is the vestige -of the mouth of the duct of the thyroid gland which opened into the -pharynx in the ancestors of fishes. It is an inconceivably long time -since fishes diverged from other races of animals. We do not know which -of the various orders of invertebrate animals now existent most nearly -resembles our prepiscine ancestor. The organ which has developed into -the thyroid body of mammals may possibly have disappeared from all -the other descendants of the common stock from which vertebrates and -their nearest relatives in the invertebrate sub-kingdom were evolved; -but it is much more likely that it has been preserved, and is still -performing its prime function in the higher invertebrate animals. -Probably it is a functional organ in a cuttle-fish or a scorpion or a -worm, but so unlike the thyroid gland of vertebrates that we fail to -recognize its homology. There are other instances in the body of the -persistence of an organ long after it has fallen into such ruin that -not even archæologically-disposed biologists can guess what it was -like, or what purpose it served in the days when it was at the height -of its efficiency; but perhaps there is none other which so pregnantly -illustrates the physiological doctrine of functional interdependence. -Nature shows herself amazingly conservative in retaining primal -organs—the pituitary body, the thymus gland, the thyroid gland, the -suprarenal capsules—organs which millions of years ago forgot the -very rudiments of their craft; but her conservatism is not mere force -of habit. Although she no longer has any use for the wares which she -created these pieces of apparatus to make, she cannot do without their -<span class="pagenum"><a name="Page_88" id="Page_88">[Pg 88]</a></span> -refuse. Even the vermiform appendix may have its use. Dr. Gaskell’s -theory of the thyroid gland involves a transformation so fantastic that -it would provoke a smile of incredulity were we to set it forth without -a prologue far more lengthy than our space permits. Yet Dr. Gaskell may -be right. We can but guess as to the nature of the prime functions of -the thyroid and parathyroids. For many geological epochs they have not -been exercised. But whatever else they did when they were indispensable -constituents of the organism, their activity was accompanied by the -secretion of colloid. Colloid is not made by other organs; therefore -the otherwise obsolete thyroids are retained. It is of course not -impossible that, in a certain degree, Nature, like a thrifty housewife, -finds a new use for superseded apparatus; but we are probably justified -in believing that the use is never really new. Not wanting the organ -for its original specific purpose, Nature relegates to it alone work -which hitherto it had shared with other of her tools.</p> - -<p>A comparatively short while ago the attention of physiologists was -wholly concentrated upon the obvious or prime functions of organs. -Muscles contracted. The stomach digested. The pancreas secreted -pancreatic juice. The brain made thought. Now they understand, to put -it somewhat metaphorically, that gastric juice is made in the calves -of the legs; the ferment of pancreatic juice in the small intestine; -thought of a certain emotional quality in the large intestine. The -chemistry of the laboratory is far behind the body’s chemistry. -We cannot detect in the blood coming from contracting muscles the -stimulant—possibly a precursor of pepsin—to which the stomach -reacts, although the magical benefit of exercise seems to prove that there -is a chemical connection between the activity of the muscles and the -activity of the glands of the alimentary canal. It has been proved by -experiment that a substance produced in the epithelium of the small -intestine is the messenger upon whose call depends the potency of -pancreatic juice. The clearing of the brain effected by a judicious -pill shows that poisons of some kind are absorbed into the blood from -an overloaded large intestine. None of the organs lives altogether for -itself. The chemical products which it throws off, absorbed by the -<span class="pagenum"><a name="Page_89" id="Page_89">[Pg 89]</a></span> -blood, regulate the activity of other organs. Formerly the several -parts of the body were looked upon as independent. Their activity was -regarded as a direct response to the commands of the nervous system. -If it varied in kind, the variations were supposed to depend upon the -quality of the nervous impulses which reached the organ. Evidence is -rapidly accumulating that many exhibitions of function are evoked by -the calls of “hormones,” or chemical messengers, not by command of the -nerves.</p> - -<p>Internal secretions, using the expression in its general sense, are -necessary for the co-ordination of the work of the various parts of -the animal mechanism. Colloid is the internal secretion of the thyroid -gland and of the parathyroids. Unlike most other internal secretions, -it is a substance easily analysed, and startlingly definite in its -chemical characteristics. It is composed mainly of a protein which -contains iodine. From this protein a substance termed “iodothyrin” -may be obtained, of which no less than 10 per cent. is iodine; but -it is uncertain whether iodothyrin is preformed in the gland. The -exact nature of the active substance of the internal secretion of the -thyroid gland matters little. Whether it be iodothyrin or a protein, -its activity depends upon the fact that it contains iodine in large -quantity. Iodine amounts to from 0·3 per cent. to 0·9 per cent. of the -weight of the whole thyroid gland in Man.</p> - -<p>Iodine is very widely distributed in Nature. It is present in the air, -in rain-water, in herbage. It is also present in all parts of the -body, although in quantities which are infinitely minute. It is found -in sea-water, and is relatively abundant in marine plants. There is -no reason for supposing that it is deficient in districts in which -<i>goitre</i> is common. It would appear more likely that the soil has -properties which result in the fixing of iodine in a form in which it -is not available for plant-food, and that in consequence animals are -unable to obtain a sufficient supply. Careful analyses have shown that -the thyroid glands of sheep bred in mountainous districts where goitre -is common contain but one-thirtieth part as much iodine as the thyroid -glands of sheep bred in places where goitre is rare. In ancient times -burnt sponge and seaweed were esteemed useful in the treatment of -<span class="pagenum"><a name="Page_90" id="Page_90">[Pg 90]</a></span> -goitre. Later, iodide of potassium given internally, and tincture of -iodine as an outward application, were the approved remedies. It is -now known that myxœdema and certain forms of goitre may be checked, -and even cured, by administering uncooked thyroid gland or even -tabloids of dried extract. Fortunately, it is not necessary to inject -it subcutaneously; the iodine-containing compound is so stable as to -resist the action of gastric juice.</p> - -<p>Iodine stored in the thyroid and parathyroid glands is distributed -to all the tissues. The remarkable symptoms which indicate that the -tissues are not receiving an adequate supply may occur under either of -two conditions. Iodine may be deficient in the food, or the thyroid -gland may be incompetent; the former is the commoner cause. And here -we see the explanation of the formation of a goitre. By increasing the -size of the organ which selects iodine, Nature attempts to obtain and -store an adequate supply for distribution to the tissues.</p> - -<p>Cretinism has been observed in animals. If attention were directed to -this inquiry, it might be found that it is not so exceedingly rare as -would be judged from the few observations that have been recorded. A -cretin, if a wild animal, falls an easy prey. If a domesticated animal, -little trouble is taken to insure its survival. A myxœdematous pig -is a dwarf with coarse, sparse hair, thick, warty hoofs, large jowl, -heavy ears. It is apathetic. A piglet presenting these characteristics -is not altogether uncommon in a litter. Among chickens and pigeons, -also, individuals appear which might, judging from their uncouth -appearance and mental dulness, be suffering from cretinism. The only -way of proving that this is the case is to feed them on thyroid glands; -it does not matter from what animal the gland is obtained. Operative -cretins, produced by removing the thyroid gland soon after birth, -recover their natural characters on a diet containing a daily allowance -of thyroid gland. The coarse hairs, or wiry towsled feathers, fall off, -and are replaced by a smooth, supple growth. The thickened skin becomes -soft and pliant. Mental apathy gives place to alertness. They make up -for lost time by growing more rapidly than other animals of the same -age, which have not been operated upon, although they never surpass the -normal stature.</p> - -<p><span class="pagenum"><a name="Page_91" id="Page_91">[Pg 91]</a></span> -<b>Suprarenal Capsules.</b>—Each of the kidneys is capped by a -pyramidal body weighing about ⅛ ounce. Small though it be, this organ -is essential to life. As Dr. Addison was the first to discover, in -1855, its disease results in a cycle of symptoms which invariably has -a fatal termination. A college friend of the writer suffered from -“slackness.” Before he had finished a set of tennis, he abandoned -the game, and spent the rest of the afternoon lying on the grass, -wrapped in a rug. After hall, although he earnestly desired to conquer -the subtleties of the Greek grammar, he fell asleep over his books. -As his countenance was not ruddy merely, but bronzed like that of a -man who has just returned from a yachting cruise, he was the butt of -many a joke. Although already a qualified medical man, who had been -in practice—he had come to the University with a view to adding the -degree of M.D. to his M.R.C.S.—he had no suspicion that he was ill. -Thought he wanted “freshening up.” Took a trip across the Atlantic. -Stumbled over a rope on landing; broke his thigh. Spent two months in -a New York Hospital, but the bone did not mend. At last, the surgeons, -growing anxious, sent him back to London. He was seen by a leading -physician, who told him that he was suffering from Addison’s disease. -Two months later he died of failure of the heart. Disease of the -suprarenal capsules is usually of tuberculous origin. Its symptoms: -muscular weakness and excessive liability to fatigue; abnormal -pigmentation of the skin; lowered blood-pressure, and consequent -sensitiveness to cold; cardiac weakness. As the pigmentation of the -skin and mucous membrane is not invariable, and since it may occur -without disease of the capsules, it is not improbable that it is due -to disease of the abdominal sympathetic ganglia, which are usually -affected at the same time as the capsules.</p> - -<p>The suprarenal capsules are composed of columns of epithelial cells, -which radiate from a large vein in their centre. They are abundantly -supplied with blood and with nerves. The cells near the vein are much -larger than those in the peripheral portions of the columns. Amongst -them are nerve-cells resembling those of the sympathetic system.</p> - -<p>The history of the suprarenal capsules is almost as obscure as that of -<span class="pagenum"><a name="Page_92" id="Page_92">[Pg 92]</a></span> -the thyroid gland. In the embryo they are relatively very large—larger -at one period than the kidney. At this period bloodvessels are formed -in them with great rapidity by a curious process of boring through and -channelling out of their cells. There are other facts connected with -their development in the individual and their varying form in different -classes of vertebrate animals which point to a “previous existence,” -but there is nothing to indicate that they were ever open glands. In -all vertebrates they are closed masses of cells, the only function of -which, so far as we know, is to produce an internal secretion; but the -importance of this chemical messenger in bringing about the proper -working of other organs is almost startlingly evidenced by the collapse -which follows disease, or removal of the organ which produces it.</p> - -<p>The suprarenal capsules yield a substance which has been termed -“adrenalin.” It contains nitrogen, is crystallizable and dialysable; -but its chemical relationships have not been made out as yet. It is not -destroyed by boiling, nor by digestion with gastric juice. Injected -into a vein, it causes, amongst other effects, an immense rise in -blood-pressure, even though the amount injected be extraordinarily -small. Applied locally as a wash or spray, a solution of 1 part in -10,000 produces marked blanching of the surface; and it is useful, -in consequence, as a means of checking bleeding in small operations, -especially those on the eye or the nose. It is a most energetic poison. -Even ¼ milligramme is sufficient to kill a rabbit. In short, adrenalin -acts like the most powerful drugs known to physicians; and this drug, -manufactured by the suprarenal capsules, is constantly added to the -blood. Disastrous consequences follow a failure in the regular supply.</p> - -<p>The tone of the vascular system is maintained by adrenalin. The nature -of its influence upon muscles is not known, but probably the complete -loss of muscular strength, which is one of the most noticeable symptoms -of disease of the suprarenal capsules, is an indirect result of the -lowering of blood-pressure. The muscles, it must be remembered, make up -about one-third of the weight of the body of a muscular man. For the -exchange of their waste products for food, they are dependent upon an -efficient circulation. They are unable to display their normal vigour -when the vascular system is not up to its work. -<span class="pagenum"><a name="Page_93" id="Page_93">[Pg 93]</a></span></p> - -<p><b>The Pituitary Body</b> is another ductless gland of dubious history. -It is a round body, the size of a small marble, which occupies a deep -recess in the floor of the skull, beneath the centre of the brain. -It is composed of epithelial cells collected into irregular groups. -No homologue of the pituitary body can be found in the invertebrate -sub-kingdom. Its strange mode of development in vertebrate animals—it -is present in them all, from fishes to mammals—and the mystery in -which its prevertebral existence is hidden, provoke to speculation. -We must be content to state that it is undoubtedly masquerading under -an assumed name. “Pituitary body” is reminiscent of a long-abandoned -theory that it secretes fluid into the upper chamber of the nose.</p> - -<p>Disease of the pituitary body is associated with a perversion of -growth even stranger than that due to disease of the thyroid gland. -The condition has been termed “acromegaly,” to indicate that all -extremities—toes, fingers, nose, lips, tongue—undergo enlargement.</p> - -<p>With these three organs—the thyroid gland, the suprarenal capsules, -and the pituitary body—we must leave the subject of internal -secretions. Each of these organs is a ductless gland. Each has a -history which the zoologist is unable to transcribe. The document is -a palimpsest, the earlier script so faint as to be illegible beneath -the dark letters which a new era has written over it. Even the modern -script is smudged and blotted. The laws which it sets forth seem, as -a rule, to be destitute of sense, but a sinister meaning is evident -at times. We are tempted to regard these codes as obsolete, until the -mischief which follows their suppression calls our startled attention -to the fact that they are, in the most lively sense, extant. Myxœdema, -Addison’s disease, acromegaly, are ominous warnings that the three -ductless glands are no mere monuments of a past epoch, which owe -their survival to Nature’s indolence. They teach us that we must not -attribute the persistence of such organs to a conservatism which -resists innovation, or suppose that they would long ago have been -wiped off the statute-book if her inertia could have been overcome. -Undoubtedly Nature gives us many excuses for adopting this attitude of -mind. The “chestnuts” on a horse’s legs, the “dew-claws” of a dog’s -foot, are vestiges which would have disappeared if every part of the -<span class="pagenum"><a name="Page_94" id="Page_94">[Pg 94]</a></span> -body had to establish its claim to be regarded as useful before it -became entitled to share in the common supply of food; so, at least, we -are disposed to think. But, tempting though it be to attribute to sheer -conservatism the retention of an organ which has been superseded in its -original functions, and for which we cannot recognize any new use, it -is a temptation which must be severely checked. It is safer to suppose -that the fact that it has been retained is <i>prima-facie</i> evidence that -the body has need of it.</p> - -<p>There can be no doubt as to the importance of the internal secretions -of the three chief ductless glands. What about other organs—the glands -which make external secretions, for example? Does each of them make -also an internal secretion which influences the activity of other -organs? It is very difficult to prove the production of internal -secretions by such organs as the salivary glands, the pancreas, the -kidneys, because all the effects which result from their removal may -be due to the suppression of their external secretions. It is almost -impossible to distinguish the consequences which might be due to -the abolition of an internal secretion from those which ought to be -attributed to the loss to the body of the chief functions of the organ. -Certain physiologists are inclined to think that all organs—not only -the glands, but the liver, spleen, muscles, etc.—produce chemical -messengers which are discharged into the blood; and recent discoveries -tend to justify this view. As the time approaches when milk will be -wanted for the nourishment of offspring, it begins to appear in the -breast. Hitherto this has been attributed to nervous control. It is now -known that the secretion is provoked by a chemical messenger. If this -messenger, extracted from the organ in which it is formed, be injected -into the veins of an animal which has no call to secrete milk, it sets -up a condition of activity in its mammary glands. Such an illustration -of the possibilities of chemical, as distinguished from nervous, -control inclines us to attribute the harmonious working of the body in -large measure to the mutual influence of its several parts, instead of -invoking in every case, as used to be the custom, the directing power -of a somewhat bureaucratic nervous system. -<span class="pagenum"><a name="Page_95" id="Page_95">[Pg 95]</a></span></p> - -<p>It is curious to note that an internal secretion is essentially a drug. -Faith in drugs has suffered eclipse in latter days, and with good -reason. The medicines of fifty years ago so little resembled Nature’s -pharmacy that there is cause enough for astonishment at the credulity -of a generation that believed them to be charms by the exhibition of -which they could direct the working of the body. To be quite just, -our forebears did not exactly adopt this view. They still believed in -remedies. Docks grew in the same hedgerow as nettles. Therefore the -juice of the dock was an antidote to nettle-stings. Washerwomen found -wasps vexatious, but, fortunately, “blue-ball” cured the pain of their -stings, and prevented the swelling which otherwise would have occurred.</p> - -<p>A new pharmacology is rapidly developing. The physiological action of -every substance likely to be of service as a drug is put to the proof. -Having ascertained what is wrong, and knowing exactly what effects his -drugs are capable of producing, the physician devises the adjustment -which he may attempt without risk of making matters worse. He then -seeks, if possible, a chemical messenger near akin to the messenger -whom Nature herself would send; at least, this is the ambition of the -modern pharmacologist.</p> - -<hr class="chap" /> - -<p><span class="pagenum"><a name="Page_96" id="Page_96">[Pg 96]</a></span></p> -<div class="chapter"> - <h2>CHAPTER VI<br /><span class="h_subtitle">DIGESTION</span></h2> -</div> - -<p><b>The Canal.</b>—The prospect presented by a widely open mouth is -too familiar to need description, but a few details may be pointed -out. The teeth are, or should be, thirty-two in number. Starting -from the middle line of either jaw, the two first are incisors, with -chisel-shaped cutting edges. If they meet, as they ought to do, their -edges are ground flat. The third tooth is the canine, with a more or -less pyramidal crown. Then two premolars, or “milk-molars,” as they -are often termed, because they are the only grinding teeth of the -first dentition. Twenty is the full complement of teeth in a child. -Lastly, three strong grinders—the molar teeth. The third molar, or -wisdom-tooth, is evidently disappearing in the human race. In civilized -people, whose brains are large and jaws small, it does not appear -until about the twentieth year. Sometimes it tries to squeeze through -the gum of a jaw not large enough to carry it, and causes trouble by -becoming “impacted” beneath the ascending ramus. Not infrequently it -fails to appear. It may be truly said that the increasing wisdom of -the human race is responsible for the postponement of its development, -although this is hardly the circumstance to which it owes its name. A -fold of mucous membrane—the frenulum linguæ—connects the under -side of the tongue with the floor of the mouth. On either side of this may -be seen the opening of a duct common to the submaxillary and sublingual -salivary glands. The opening of the duct of the parotid gland is not so -easy to find. It pierces the mucous membrane of the cheek opposite to -the base of the second molar tooth of the upper jaw. The parotid gland -lies just below the ear, behind the jaw. The saliva which it secretes -<span class="pagenum"><a name="Page_97" id="Page_97">[Pg 97]</a></span> -is a watery fluid containing little beside salt and a weak ferment. It -serves to moisten the food as it is being crushed by the molar teeth. -The submaxillary and sublingual secretions contain, in addition to the -ferment, ptyalin, mucus which the tongue mixes with the masticated food -as it forms it into a bolus suitable for swallowing. The dorsal surface -of the tongue is covered by papillæ, which rasp the food against the -palate. Of these the greater number are pointed, or filiform. The -remainder are flat-topped, or fungiform. The two varieties may be -distinguished with a lens, especially on the sides of the tongue. -Usually the fungiform papillæ are the redder. In fever, when the tongue -is densely furred, they stand out as bright red spots. The back of -the tongue is crossed by a <b>V</b>-shaped row of papillæ of larger -size, each surrounded by a slight fossa and a vallum, and hence termed -“circumvallate.” Very minute organs of sense—taste-bulbs—stud -the mucous membrane which lines the fosse.</p> - -<p>The hard palate ends in a muscular curtain—the soft palate—the -central portion of which—the uvula—depends lower than the rest. -On either side the soft palate splits into two folds; the anterior, -continued to the side of the tongue; the posterior, to the pharynx. -These folds, since they bound the gateway into the pharynx, which is -known as the “fauces,” are termed the “pillars of the fauces.” The -tonsil lies between the anterior and posterior pillars of the fauces, -but does not appear as a prominence unless inflamed or enlarged.</p> - -<p>The pharynx hangs as a bag from the base of the skull. It, like all -the rest of the alimentary tract, is lined by mucous membrane. “Mucous -membrane” is not a happy term. It does not denote that the epithelium -secretes mucus. It may or may not possess this property. Nor does it -imply that it has a different origin from the skin—that it arises from -hypoblast, the inner layer of the rudiment from which the embryo grows. -The term is applied to all internal, and therefore moist, surfaces, -whether they arise from hypoblast, as in the case of the lining of the -greater part of the alimentary tract, or whether they are involutions -of epiblast as in the case of the mouth and also of the extreme lower -end of the alimentary tract. Almost the whole of the alimentary canal -is, in the first instance, a tubular cavity in the interior of the -<span class="pagenum"><a name="Page_98" id="Page_98">[Pg 98]</a></span> -embryo, lined by hypoblast. This cavity communicates with the -yolk-sac, but has no openings on the exterior until it joins up with -two epiblastic pits—one the stomodæum, or mouth-cavity, at the -anterior end; and the other the proctodæum, at the posterior end of -the body. The distinction between the middle closed portion of the -alimentary canal and its two secondary openings suggests morphological -speculations, into which we have not space to enter, as to the ancestry -of the vertebrates. The majority of anatomists believe that the -primitive canal is represented in the middle portion, and that, in -prevertebrate animals, it opened to the exterior in a different way. -The pharynx is 4½ inches long. It is enclosed by three thin muscles, -which overlap from below upwards—the constrictors of the pharynx. -The anterior attachment of the superior constrictor is to the jaw; of -the middle constrictor to the hyoid bone; of the inferior constrictor -to the thyroid cartilage. Above the soft palate the nasal chambers -communicate with the pharynx by the posterior nares. Below the hyoid -bone, which is easily felt in the neck as a bony arch just above the -thyroid cartilage (Adam’s apple), the windpipe, or trachea, joins the -pharynx by a single pear-shaped orifice—the rima glottidis. When we -consider the mechanism of swallowing, we shall study the arrangements -which prevent food, passed through the fauces, from entering either the -nasal chambers above or the windpipe below and in front. At the level -of the lower border of the thyroid cartilage the pharynx becomes the -relatively narrow œsophagus. This tube, which lies behind the trachea, -and slightly to its left side, passes with a straight course to the -abdomen. It traverses the chest, lying behind the heart, pierces the -diaphragm, and just beneath it joins the stomach. Its length is about 9 -inches. The stomach is a sickle-shaped bag. It has two apertures—the -cardiac orifice, or junction with the œsophagus; and the pyloric -orifice, or junction with the small intestine. It is so folded on -itself that these two apertures are not more than 4 inches apart. Its -outline may be drawn on the body-wall with a piece of charcoal from a -point an inch below and an inch to the left side of the lower end of -the breast-bone, the position of the cardiac orifice, to a point about -4 inches below the end of the breast-bone, and an inch or two to the -right side of the mid-line of the body, the position of the pyloric -<span class="pagenum"><a name="Page_99" id="Page_99">[Pg 99]</a></span> -orifice, with a slight curvature to represent the upper border; to -represent the lower border the same two points are joined by a bold -curve, bulging upwards to the nipple, outwards to the side of the body, -and downwards some distance on the abdomen (<a href="#I_PXI"><i>cf.</i> Fig. 2</a>).</p> - -<div class="figcenter"> - <a id="I_099" name="I_099"> </a> - <img src="images/i_p099.jpg" alt="" width="600" height="505" /> - <p class="center"><span class="smcap">Fig. 6.</span></p> - <div class="blockquot2"> - <p class="neg-indent space-below2">The stomach has been cut across a short distance from - the pyloric valve, and removed, to show the viscera which lie behind it. The - descending aorta and the vena cava rest upon the vertebral column. They - are crossed by the pancreas and the transverse portion of the duodenum. - The head of the pancreas is enclosed by the curvatures of the duodenum. - The ducts of the liver and pancreas are seen entering the descending - duodenum side by side.</p> -</div></div> - -<p>Such an outline represents the form and position of the stomach when -distended; but it is to be understood that its dimensions depend upon -the amount of its contents. It is capable of holding about 7 pints. -The junction of œsophagus and stomach is closed by a muscular ring, or -sphincter muscle—the cardiac sphincter; the junction of stomach and -intestine is guarded by a much stronger pyloric sphincter. The average -diameter of the small intestine is about 1½ inches. It is wide enough, -therefore, to admit two fingers. The length of the tube is about 22 -feet. Its first part is termed the “duodenum,” because its length -equals the breadth of twelve fingers—<i>i.e.</i>, about 9 inches. The -<span class="pagenum"><a name="Page_100" id="Page_100">[Pg 100]</a></span> -remainder is divided arbitrarily into jejunum and ileum. The duodenum -makes three sharp curves. First it inclines upwards and to the right, -then vertically downwards, then horizontally to the left, and finally -forwards. The ducts of the liver and pancreas open by a common orifice -into the descending portion. Its horizontal portion is bound firmly to -the vertebral column. After this the whole of the small intestine is -supported by the mesentery, a double fold of peritoneum which allows it -to hang freely in the abdominal cavity. The mesentery is attached to -the back of the body-wall. Commencing on the left side of the second -lumbar vertebra, its line of attachment inclines obliquely downwards -and to the right, across the vertebral column, for about 6 inches. -Measured from its attached edge to the edge which bears the intestine, -it has a width of about 8 inches. Its free border has, as already -said, a length of 22 feet. Its measurements being as just stated, it -is clear that it must be folded backwards and forwards upon itself, -like a goffered frill. In the right groin the small intestine joins the -large intestine, or colon. It does not, as might have been expected, -simply dilate into the large intestine, but enters it on its mesial -side, its orifice being guarded by the ileo-colic valve. In other -words, the large intestine projects downwards beyond this orifice, -as the cæcum coli. In many animals the cæcum is of great length and -capacity. In the human embryo it begins to assume a similar form; but -a very small portion only (the so-called “cæcum” of human anatomy) -dilates to the calibre of the colon. The real cæcum retains throughout -life its embryonic calibre. It has a length of about 3½ inches, and a -diameter of not more than ¼ inch. This is the “vermiform appendix,” of -ill fame, which must be looked upon as one of Nature’s misfits. Its -great liability to become inflamed is commonly explained as due to the -tendency of such articles of food as pips, the fibre of ginger, flakes -from the inside of enamelled saucepans, etc., to become lodged in its -cavity. But whether this explanation be correct or no—and there are -reasons for thinking it somewhat fanciful—it is much to be wished -that the process of evolution would hasten the disappearance of this -functionless vestige of a cæcum. As there is no tendency towards the -inheritance of characters due to mutilation, and since the surgeon’s -knife now prevents this death-trap from claiming its toll of possible -<span class="pagenum"><a name="Page_101" id="Page_101">[Pg 101]</a></span> -parents, we must look upon the rudimentary cæcum, with its liability -to inflammation, as a permanent burden on the human race. In justice -to the appendix, however, it must be pointed out that it has acquired -its criminal reputation during the past twenty years. The frequency of -appendicitis has increased so enormously during this period that it -ought to be possible to correlate its prevalence with the introduction -of the cause upon which it chiefly depends.</p> - -<p>The colon has a length of about 5 feet. Its greatest width, about -3 inches, is at its commencement, but it is everywhere much wider -than the small intestine. Whereas the wall of the small intestine is -smooth externally, the wall of the colon is sacculated. Three muscular -bands constrict it longitudinally; circular bands at intervals of -about 1 inch or 1½ inch throw it into pouches. It ascends on the -right side, lying far back against the body-wall, to which it is -bound by peritoneum, which in this part of its course covers only its -anterior surface. Having touched the under side of the liver, it loops -forwards and to the left side, crossing the middle line just above the -umbilicus. On the extreme left side it touches the spleen, getting -very near to the back of the abdominal cavity. It then descends on the -left side, again bound to the body-wall by peritoneum, although not -so closely as on the right side, until it reaches the inner lip of -the crest of the hip-bone. From here onwards the fold of peritoneum -which attaches it allows it a free movement. This portion of the large -intestine, the sigmoid flexure, may even fall over into the right -groin. Lastly it curls backwards into the pelvis, as the rectum.</p> - -<p>Movement of the contents of the alimentary canal may be favoured by -judicious pressure, or massage. From the description of the situation -of its several parts given above, it will be understood that if the -right hand be placed on the abdomen immediately beneath the ribs, with -the fingers well round to the left side, the stomach will be covered. -Pressure from left to right will tend to drive its contents towards the -pyloric valve. The small intestine is so irregular in its course as to -preclude the possibility of following it with the hand. Pressure first -on one side and then on the other, with a general tendency to work from -above downwards, tends to press forward its contents; but, owing to its -<span class="pagenum"><a name="Page_102" id="Page_102">[Pg 102]</a></span> -circular form and strong muscular walls, it is not in much need of -help. Very different is the position of the large intestine in this -respect. Its calibre is much greater, its wall is sacculated, its -contents comparatively firm. If the palm of the hand be placed above -the right groin and pressure directed upwards, the cæcum coli and -ascending colon are emptied. If pressure be directed from the extreme -right side just below the ribs, across the middle line to the left -side, the transverse colon is emptied. The descending colon needs -pressure from above downwards on the left side; the sigmoid flexure, -pressure above the left groin, downwards, and towards the middle line.</p> - -<p>The inner wall of the œsophagus is smooth, save for the wrinkles into -which it is thrown when not distended; but from the cardiac orifice -of the stomach onwards the mucous membrane of the alimentary canal -exhibits folds and other projections which serve many purposes. -They serve to delay the food, keeping it longer in contact with the -secreting surface. They increase the area pitted with tubular glands; -they increase also the area through which absorption of the products -of digestion occurs. On the inner surface of the stomach the folds -produce a reticulated pattern. In the upper portion of the small -intestine, especially the duodenum, there are prominent transverse -shelves (valvulæ conniventes). No definite folds occur below the -upper three-fourths of the small intestine, with the exception of the -constrictions of the transverse colon already referred to, which affect -the whole thickness of its wall. Throughout the whole of the small -intestine the mucous membrane projects in finger-like processes, or -villi, which give it a characteristic velvety appearance. The villi are -longest in the duodenum.</p> - -<p>Lymph-follicles occur at intervals in the intestine. In the ileum they -are collected into patches (Peyer’s patches), on the side opposite to -the line of attachment of the mesentery. They serve both for the supply -of phagocytes, which hunt any germs that have penetrated the mucous -membrane, and also as stations to which germ-laden phagocytes retreat.</p> - -<p>The wall of the intestine is composed of mucous membrane, submucous -tissue, and muscle. The mucous membrane is everywhere pitted with -tubular glands, termed in the stomach “gastric glands,” and in the -<span class="pagenum"><a name="Page_103" id="Page_103">[Pg 103]</a></span> -intestines, both small and large, “crypts of Lieberkühn.” Their -relation to the wall might be exemplified by taking a block of dough -about 6 inches thick and pushing a pencil vertically into it almost -down to the table on which it rests. The holes should be made as close -together as possible, since, especially in the stomach, extremely -little tissue intervenes between the tubes of gland-cells. If the piece -of dough were placed upon a folded cloth, the cloth would represent -the muscularis mucosæ, a layer properly regarded as a constituent of -the mucous membrane. The fibres of this coat are disposed in two or -three sheets, the fibres of one sheet crossing those of the next. By -their contractions they squeeze the ends of the crypts, and probably -wobble them about, expelling their secretion. Beneath the muscularis -mucosæ is a layer of connective tissue, the submucosa, which contains -abundant lymphatic channels, bloodvessels, and nerves. At the pyloric -end of the stomach, the tubes of gland-cells tend to pierce the -muscularis mucosæ. In the first part of the duodenum, certain tubes, -having pierced this layer, branch in the submucosa. A layer of racemose -glands is thus formed—the glands of Brunner. Outside the submucosa -is the muscular coat proper, composed of plain muscle-fibres, except -in the upper part of the œsophagus, where the fibres are striated. It -consists of an inner and an outer sheet, the fibres being disposed -circularly in the inner, longitudinally in the outer sheet, with a -slight departure from this regular arrangement in the wall of the -stomach. On its outside the canal is invested by peritoneum, a layer -of flattened epithelial cells supported by connective tissue. The -abdominal wall also is lined with peritoneum. The smooth moist surface -of the peritoneum covering the intestines glides on the peritoneum -lining the abdominal wall. Between the two is a “potential” space. In -dropsy, fluid accumulates within this space. In a healthy condition the -apposed surfaces are merely moist.</p> - -<p>The movements of the intestines are of two kinds. At all times they -exhibit swaying movements, in the production of which the longitudinal -fibres play the chief part, although the circular fibres also contract. -The object of this undulation is to thoroughly mix the contents of -the gut with its secretions. If pills of subnitrate of bismuth are -administered, and their progress observed by the aid of Röntgen rays, -<span class="pagenum"><a name="Page_104" id="Page_104">[Pg 104]</a></span> -they are seen to oscillate backwards and forwards on their way down -the canal. The slower vermicular movement which squeezes the contents -forwards is called “peristalsis.” It resembles the progressive -contraction of an elastic tube which may be effected by drawing it -through a ring, but is rather more complicated. At the point at which -it is occurring the circular coat is sharply contracted. Above this -it is also somewhat contracted; below it is relaxed. The longitudinal -fibres, using the constricted portion as a <i>point d’appui</i>, pull up -the segment of the intestine which lies immediately below it, drawing it -off the contents of the tube as a glove from a finger.</p> - -<p>When food is swallowed, it falls down the œsophagus, aided by slight -peristalsis. As soon as sufficient has accumulated on the upper surface -of the cardiac valve of the stomach, the valve relaxes; at the same -time a stronger peristalsis of the lower portion of the œsophagus -squeezes its contents into the stomach. Food remains in the stomach -until it has reached a certain stage of digestion, the chief object -of which is its subdivision into small particles. Until this stage -is reached, the pyloric valve is firmly closed. The contractions of -the wall of the stomach drive its contents round and round—down the -greater and up the lesser curvature—mixing them thoroughly with the -gastric juice (<a href="#Page_124"><i>cf.</i> p. 124</a>). As the acidity of the -mixture increases, the peristaltic contractions of the stomach become more vigorous, -until, the pyloric valve relaxing, the food is little by little driven -into the duodenum.</p> - -<p>The alimentary canal has an abundant supply of nerves from the vagus -and the sympathetic systems. It contains also within its own wall an -enormous quantity of nerve-fibres and nerve-cells. They are disposed -as two plexuses, one in the submucosa, the other between the circular -and longitudinal muscular coats. In a specimen successfully stained -with methylene-blue, they are so abundant as to give the impression -that every plain muscle-cell may have its own separate nerve-twig. -Nevertheless, the contraction of the muscle-cells may take place -independently of all nerve-influence—independently, even, of the local -mechanism, the plexus referred to above. Nicotin applied to the wall -of the intestine paralyses the local nerves; yet rhythmic contractions -still occur. They are, however, no longer progressive. They do not drive -<span class="pagenum"><a name="Page_105" id="Page_105">[Pg 105]</a></span> -the contents of the intestine forwards. Co-ordinated contraction is -observed so long as the local mechanism is intact, even though all -external nerves have been cut. The intestines have their own nerve -cells and fibres, which, acting as a linked system of reflex centres, -provide for the harmonious contraction of their walls. External nerves, -sympathetic and splanchnic, convey impulses which either intensify the -movements or inhibit them, as need may be.</p> - -<p>In the matter of its nerve-supply, the alimentary canal stands apart -from the other organs of the body. It may be supposed that it presents -a more primitive condition. Its muscular fibres have the power of -contracting spontaneously. The pressure of the contents of the tube -acts as a stimulus. When the fibres are stretched, they contract. -When the tube is dilated, its muscles endeavour to restore it to its -normal calibre. Such direct action would not, however, provide for the -forward passage of its contents. To bring about peristalsis, a nervous -mechanism is needed, as abundant and complicated as that which ensures -the progress of a slug or a worm. To deal satisfactorily with the -various contents of the tube—liquid, solid, gaseous—the mechanism -must be capable of complicated adjustments. The dilated portions of -the tube—stomach, cæcum coli, rectum—require special arrangements of -muscle and nerve. Nor is the canal altogether independent of the rest -of the body. To a large extent its work is carried on without regard -to the activities of other organs, yet it is not wholly free from the -control of the central nervous system. It is regulated by means of both -afferent and efferent nerves of the vagus and sympathetic. Even the -brain has something to say with regard to the way in which it shall -contract. It is a matter of common experience that emotional influences -may affect the movements of the stomach and intestines—“His bowels -yearned.”</p> - -<p>Normally, vomiting is due to irritation of the endings of the vagus -nerve in the stomach, although the afferent impulses may have other -sources. Touching the upper surface of the epiglottis with the finger -will provoke the reflex. So also will stimulation of the olfactory -nerves by a foul smell. In this latter case the emotion of disgust to -which the odour gives rise brings about the reflex action. A flow of -saliva precedes the act of vomiting. A deep inspiration is then taken, -<span class="pagenum"><a name="Page_106" id="Page_106">[Pg 106]</a></span> -in order that for a time the lungs may be independent of a fresh supply -of air. The glottis is closed, the diaphragm fixed. Contraction of the -abdominal wall presses the stomach against the diaphragm; its cardiac -sphincter relaxes, and its contents are squirted into the œsophagus, -which undergoes a forcible retrogressive peristalsis.</p> - -<p>It is interesting to note the difference between carnivora and -herbivora in regard to vomiting. Carnivora swallow fur and other -indigestible materials, as well as many unwholesome things which they -need to be able to return. A dog can, apparently, vomit at will. -Never, while in a state of nature, do herbivora need to return the -contents of the stomach. No provision is made for vomiting. A heifer -which has strayed into a dewy clover-field is not unlikely to die -from the effects of distension of its paunch, if relief be not given -by opening it with a knife. In a horse the cardiac sphincter is -strong, the pyloric weak. Pressure on the stomach tends to drive its -contents through the pyloric valve into the duodenum, not backwards -into the œsophagus. The stomach is not so placed as to allow of its -being compressed between the wall of the abdomen and the diaphragm. -Horses cannot vomit. It is a mistake to suppose that they suffer from -sea-sickness. In rough weather they sweat, their limbs tremble, they -go off their feed; but these symptoms are probably due to the fatigue -which results from excessive anxiety to maintain their balance, and -to fear. We can never know their feelings, but there is no reason for -supposing that they experience the sensation of nausea.</p> - -<p>Vomiting is a frequent symptom of cerebral disturbance. The -fluctuations of pressure which the brain experiences as it rocks about -on its “water-bed” within the skull is the cause of sea-sickness. Yet -the motion of a ship may produce violent headache without nausea, the -brain only, not the stomach, appearing to be troubled by the motion. -Not that headache is a pain “inside the head.” Nor is it properly -described as a pain in the scalp, although the messages which are felt -in consciousness as headache originate in the endings of the nerves -of the skin which covers the skull. The excessive sensitiveness of -these nerves is due to vaso-motor conditions, usually the dilation, -<span class="pagenum"><a name="Page_107" id="Page_107">[Pg 107]</a></span> -occasionally the constriction, of the bloodvessels of the scalp. But -the vaso-motor condition is sympathetic with the disturbance of the -brain; and the special urgency or efficiency of the messages from the -skin results from their being delivered into excited brain-tissue. -Nausea and headache are equally symptoms of the irritability of the -brain caused by the motion of the ship. In one case messages from the -stomach, in the other case messages from the scalp, acquire undue -importance, owing to the agitated condition of the brain-tissue -through which they pass. Not uncommonly the voyager, who wakes in -the morning reconciled to the changes of pressure which he has -experienced while recumbent, finds, when he stands upright, that the -base of his brain is as sensitive as ever. Visual sensations also -contribute to the brain-disturbance. So, too, do the movements of -endolymph in the semicircular canals (<a href="#Page_335"><i>cf.</i> p. 335</a>). -It is, indeed, possible that this last factor is more important than the variations -in pressure on the surface of the brain. Probably it accounts for the -after-image of rolling which almost everyone experiences for at least -a day after leaving the ship. Its cause being cerebral, the tendency -to sea-sickness can be controlled by drugs which, like the bromides, -chloral, alcohol, etc., deaden the brain.</p> - -<p><b>Salivary Glands.</b>—The secretion which accumulates in the mouth -is the combined product of the sublingual, submaxillary, and parotid -glands. It is a very thin, watery solution containing not more than 0·5 -per cent. of solid substance. If red litmus-paper is moistened with -saliva, it becomes blue, showing that the secretion is alkaline. It -contains a ferment, ptyalin, which digests starch. The action of this -ferment can be demonstrated by holding in the mouth for half a minute -some warm starch mucilage—boiled arrowroot, for example. It quickly -loses its viscidity owing to the conversion of starch into sugar. -Chemically this change may be demonstrated by adding iodine-water to -a specimen of the starch before and after action. Before the starch -is taken into the mouth the iodine turns it blue (a characteristic -reaction for starch). After it has been exposed to the digestive action -of the saliva, iodine fails to colour the mixture, which now contains -no starch. All the starch has been converted into dextrin and sugar. If -<span class="pagenum"><a name="Page_108" id="Page_108">[Pg 108]</a></span> -unboiled arrowroot is placed in the mouth, some sugar is produced, but -the process of conversion is very slow. It is almost impossible to -digest raw starch in the mouth sufficiently to render it insusceptible -to the colouring action of iodine. The sugar produced by the action of -ptyalin is of the same nature as that which appears during the malting -of barley. It is therefore termed “maltose.” It closely resembles -grape-sugar, but is not identical with it.</p> - -<p><b>The Secretion of Saliva.</b>—The accessibility of the salivary -glands, and especially of the submaxillary, has led to their being used -for a very large number of experiments. They have been studied with -the aim of coming to an understanding of the mechanism of secretion -in general. The glands consist of tubes of gland-cells, each tube -suspended in a basket of connective tissue, in a bath of lymph -(<a href="#I_038"><i>cf.</i> Fig. 3</a>). Innumerable capillary bloodvessels -traverse the lymph-bath. The arteries which carry blood to the gland are supplied -with nerves, which regulate their calibre, and therefore determine the amount of -blood which passes through the capillaries into which they break -up. The glands also are supplied with nerves which influence their -functional activity. Nutrient substances and oxygen pass out of the -blood into the lymph. Carbonic acid passes into the blood from the -lymph. Waste products are either carried away in the lymph-stream, or -make their way through the walls of the capillaries into the blood. -Many problems present themselves for solution. How does the amount of -work done by the gland affect its supply of blood? Does the quantity of -saliva secreted vary directly with the pressure of lymph in the spaces -by which the gland is surrounded? Is this pressure wholly dependent -upon the pressure of the blood? Are the substances secreted by the -gland supplied as such by the blood, or does the gland make the ptyalin -and mucus which it secretes? If it makes its secernable products, what -materials does it abstract from the blood for the purpose of their -manufacture? Does it use the whole of these materials, whatever they -may be, or does it use part only and return the residue to the lymph? -Does it make its products only when it is actively secreting, or is it -always making them, and storing them in its cells in order that it may -have a supply to discharge when called upon by the stimulation which -results from the presence of food in the mouth? Is their discharge -<span class="pagenum"><a name="Page_109" id="Page_109">[Pg 109]</a></span> -merely a washing out due to the rush of fluid which occurs when the -bloodvessels are dilated, or can the gland-cells expel their products -in response to nervous action? In what way do the nerves of the -gland influence secretion? Do they call for increased production, or -increased output, or both? These are some of the problems which the -exposed situation of the submaxillary gland allows physiologists to -tackle.</p> - -<p>By means of a very simple operation, the ducts of one or both parotid -or submaxillary glands can be brought to the skin, and made to pour -their secretions on to the surface instead of into the mouth. The flow -under various circumstances can be watched. The saliva can be collected -and measured.</p> - -<p>The nerves of the submaxillary gland are easily isolated. A nerve -leaves the seventh (or facial), crosses the drum of the ear, comes -out through a minute crevice in the skull, and runs for some little -distance as a separate nerve before it applies itself to the lingual -branch of the fifth, which runs along the side of the tongue. Owing to -its passage across the tympanic cavity (drum of the ear), it is termed -“chorda tympani.” As its fibres are very small, they can be recognized -wherever they form a part of the lingual nerve. They leave the lingual -to go to a ganglion, the submaxillary ganglion, from which the gland -is supplied. The gland also receives branches from the sympathetic -nerve which ascends the neck. The last-named branches accompany the -facial artery. Stimulation of either of these nerves causes the gland -to secrete. The flow of saliva which follows stimulation of the chorda -tympani is much more copious than that which follows stimulation of -the sympathetic, and as a rule it contains far less organic matter, -although about the same amount of mineral salts. Under normal -conditions the activity of the chorda tympani is brought into play in -a reflex manner by impulses which travel up the nerves of taste (the -lingual and glosso-pharyngeal) to the cerebro-spinal axis; but almost -any other nerve will serve as an afferent path. The gland may also, as -we shall presently explain, be called into activity by the cortex of -the brain.</p> - -<p>It is certain that in the case of the submaxillary gland secretion is -not the direct result of increased blood-pressure. It is not a case of -filtration from the blood through certain membranes and cells into the -<span class="pagenum"><a name="Page_110" id="Page_110">[Pg 110]</a></span> -salivary duct. Atropin (belladonna) dilates the bloodvessels, -increasing blood-pressure, but it stops secretion. After -belladonna-poisoning, the mouth, like the skin, is hot and dry. Other -drugs there are which provoke a certain amount of secretion, even after -the bloodvessels going to the gland have been tied. It is possible, by -stimulating the chorda tympani, to obtain a pressure in the fluid in -the duct very much greater than that in the bloodvessels which supply -the gland. Here we have clear proof that secretion is not filtration. -Filtration is the passage of fluid through a filter-bed from a higher -to a lower pressure. In filtration, moreover, soluble diffusible -salts accompany the water. The saliva contains only half as much of -these diffusible salts as the blood. Therefore the gland tissue stops -half the salts. Secretion is an active process carried out by the -gland-cells, under the influence of nerves, in opposition to the laws -of filtration. The gland-cells determine how much water shall pass -through them and what percentage of salts shall accompany the water.</p> - -<p>How does a gland-cell make the substance which it secretes? There is no -reason for supposing that the ptyalin or the mucus which the salivary -glands secrete is present in the blood, either ready formed, or, as it -were, half formed, in combinations which can be easily broken up. All -the evidence obtainable points to the conclusion that the gland-cells -take out of the lymph proteid materials from which they manufacture -the peculiar substances which they secrete. During rest, granules -accumulate in the cells. During activity they disappear. It has been -shown in the case of the gastric glands that these granules consist -of the special ferment which the gland secretes, in an inactive form. -It may be that it is combined with a substance which prevents it from -exerting its digestive action on the cells within which it is made; -damped, as gunpowder is damped during transit. Or it may be that it is -not a finished ferment; it may need a further addition to its molecule. -During activity, while the granules disappear, proteins accumulate at -the bases of the cells, giving to a tube of gland-cells the appearance -of a peripheral non-granular zone. This proteid substance must have -come from the lymph, and the inference seems inevitable that the cells -have taken into their protoplasm a supply of material which will -serve for the manufacture of additional granules. Each gland-cell is -<span class="pagenum"><a name="Page_111" id="Page_111">[Pg 111]</a></span> -therefore an independent unit. By its own activity it takes up -materials from the lymph, out of which it manufactures its own special -products. It stores its products until they are wanted. Then by its -own activity it extrudes them into the lumen of the gland-tube. It -has, indeed, been shown that, when the nerve going to a salivary -gland is stimulated, the gland shrinks, notwithstanding the great -dilation of its bloodvessels. Under the influence of the stimulation -the granules in the gland-cells imbibe water, swell up, and escape -from the cells. The cells discharge their accumulated stores, in the -first instance, more rapidly than they take up materials (even fluid) -from the blood. For its knowledge (if the term may pass) of what is -wanted the gland-cell is dependent upon messages which reach it through -the nervous system. These messages take origin in the endings of the -sensory nerves of the mouth, pass up to the brain, and are reflected -down the nerves to the gland. So accurate is the information conveyed -to the glands, that when a horse transfers the work of mastication from -one side of its mouth to the other, as it is in the habit of doing -about every quarter of an hour, the flow of saliva from the parotid -gland on the masticating side is increased; on the other side it is -diminished. Two or three times as much saliva is poured out on the one -side as on the other.</p> - -<p>Not only is the amount of saliva poured out in response to stimulation -proportional to the needs of mastication, but the kind of saliva is -adapted to the nature of the food. In a dog—and this is an observation -which can be made only on an animal which lives on a mixed diet—it is -possible to determine the amount of the two kinds of saliva secreted -and the relation of flow to food. When meat is given to the animal, the -submaxillary gland yields its secretion; when it is fed on biscuit, -abundance of the watery parotid saliva is poured forth. A mouthful of -sand also causes the parotid saliva to flow, in order that the sand may -be washed out of the mouth.</p> - -<p>More remarkable than the response to direct stimulation is the effect -produced by the sight and smell of food. When meat is shown to a dog, -submaxillary saliva begins to flow; when it is offered bread, parotid -saliva is secreted. And the activity of the glands is not merely a -nervous reflex independent of the animal’s mind. The moment the dog -realizes that it is being played with—that there is no intention of -<span class="pagenum"><a name="Page_112" id="Page_112">[Pg 112]</a></span> -giving it the coveted food—the flow of saliva ceases. An emotion -may check secretion when every physiological condition is demanding -it. This is the explanation of the Rice Ordeal. Dry rice provokes a -flow of saliva in the mouth of all save the guilty man. Response to -mental impressions is a matter of the greatest consequence in the -physiology of digestion. It holds good in the case of the secretion -of gastric juice equally with that of saliva. The sight and smell of -food sets the juice flowing into the stomach, and the more desirable -the food, the more attractive its appearance, the more stimulating -its smell, the more rapidly does the secretion flow. Here we touch -upon a theme which hardly needs exhaustive treatment. It is not -the stoutest people who eat the most, although an impartial survey -of one’s well nourished friends will show them to be persons who -“take kindly to their victuals.” A small quantity of food perfectly -digested is more nourishing than much food which the digestive organs -do not efficiently prepare for assimilation. Good digestion waits on -appetite; and appetite, in civilized man, is something more than a mere -physical need of food. The hunger which leads to the bolting of food -without pleasurable anticipation, without mastication, without any -consideration of the quality of the viands, is a harmful craving which -ends in imperfect assimilation. It is more profitable to toy with a -<i>hors d’œuvre</i> than to engulf, unthinking, a plateful of beef. But -we have said enough to suggest reflections to those who take no thought -as to what they shall eat or what they shall drink; and few who take -thought need to be convinced.</p> - -<p><b>The Stomach.</b>—The sight and smell of food, its presence in -the mouth, and the performance of mastication, which induces a secretion -of saliva, gives rise at the same time to a flow of gastric juice. -It is psychic stimulation and the act of eating which cause gastric -juice to ooze from the gland-tubes of the stomach at the commencement -of digestion, not the stimulation of nerve-endings by food which has -passed down the œsophagus. As a consequence of gunshot wounds, or -as the result of operations performed for the purpose of relieving -patients whose œsophagus has become blocked, numerous cases have been -recorded in which a fistulous opening into the stomach has made it -possible to study the interior of this organ. Such cases present an -<span class="pagenum"><a name="Page_113" id="Page_113">[Pg 113]</a></span> -opportunity of watching the digestion of various foods introduced -through the opening, and of collecting gastric juice for purposes -of analysis. A similar condition has been established in animals by -operative means. The œsophagus having been cut, and the cut end sutured -to the margins of an aperture in the skin, food taken by the mouth -escaped by this opening instead of passing into the stomach. A similar -opening was made into the stomach for the insertion of food, and for -the purpose of studying the effects of reflex stimulation of the -gastric glands. As soon as food was introduced into the mouth, gastric -juice began to flow. The advantage of this experimental method lies in -the fact that the juice secreted was a pure juice—not mixed with food, -as in all the earlier experiments in which, the stomach being opened -without diversion of the œsophagus, the presence of food within it was -the stimulus which led to secretion. No juice flowed in the absence of -stimulation; nor was the secretion normal in composition when provoked -by a mechanical stimulus, such as the tickling of the gastric mucous -membrane by a feather.</p> - -<p>My lord the stomach! He is not the only, nor is he the chief, agent -in digestion; but with him rests the decision as to whether the food -offered to the alimentary tract is suitable in quality and quantity. He -is offended if it be not offered with all the circumstance and ceremony -which becomes his rank. As an intimation that he is about to receive -food, he accepts the news from the mouth that its nerve-endings are -subject to mechanical stimulation. But the chewing of indiarubber would -produce a like effect. The stomach, therefore, confers with the organs -of taste and smell. If their report is favourable, he argues that the -substance which the teeth are crushing will justify an outflow of -gastric juice. He responds most generously when prolonged mastication -assures him that he may trust to receiving the food in a sufficiently -subdivided state. At our peril we neglect to propitiate my lord. Not -always debonair when treated with consideration, he is morose or -petulant when slighted. Never content with lip-service, he exacts the -labour of teeth and tongue and palate. The tribute we offer may be of -the best—savoury, wholesome, well cooked, well chewed—but if it -be not tendered with some degree of love, if thoughts are concentrated on -<span class="pagenum"><a name="Page_114" id="Page_114">[Pg 114]</a></span> -other things, if no attention is devoted to the meal, if no sense -of liking accompanies our offering, my lord the stomach on his part -affords the viands an indifferent reception. In consulting our own -tastes we are to a large extent consulting the needs of the stomach. -Ravenous and excessive feeding is not an exhibition of taste; it is a -return to the instinct of the savage, who was never sure that he would -get his full share, and was afraid to trust that another meal would be -obtainable when nature declared it due. Some degree of epicureanism -is favourable to digestion. The flow of gastric juice in the stomach -occurs reflexly in response to the emotion of appetite, to stimulation -of the nerves of taste and smell, to the obscure sensations which -accompany the activity of the muscles of mastication.</p> - -<p>The gastric juice secreted in a day amounts probably to about 8 or 9 -pints. To this we must add, when considering the quantity of fluid -which passes through the stomach, the saliva, which certainly reaches -as much as 2 pints, and the beverages taken with food.</p> - -<p>Gastric juice collected in the manner described above is a clear, -colourless, inodorous fluid. It is very acid, and so powerfully peptic -as to digest its own weight of coagulated white of egg. Its solid -constituents amount to 0·5 per cent. They consist of the two ferments -pepsin and rennin, with traces of proteins and mucin, and various -inorganic salts. Its acidity is due to free hydrochloric acid to the -amount of 0·2 per cent. This acid is more or less in combination with -the pepsin. In pure gastric juice hydrochloric acid is the only acid -present; but when mixed with food the juice contains other acids also, -especially lactic.</p> - -<p>When food first reaches the stomach, the alkaline saliva which -accompanies it neutralizes the acidity of the gastric juice. For some -time, probably about half an hour, the conversion of starch into sugar -is still carried on by the ptyalin of the saliva, owing chiefly to the -difficulty which the gastric juice encounters in permeating the masses -of masticated food. The <i>Bacillus acidi lactici</i> is always present in -the stomach. It converts some of the sugar into lactic acid; of this a -small quantity is further changed into butyric and acetic acids, with -the formation of carbonic acid and hydrogen gas. After a while the -lactic acid is absorbed, and hydrochloric acid alone remains. -<span class="pagenum"><a name="Page_115" id="Page_115">[Pg 115]</a></span></p> - -<p>The secretion by the gastric glands of so powerful a mineral acid -as hydrochloric has always aroused interest. How is it possible for -the gland-cells to produce it without injury to them selves, or for -the stomach to contain it without self-digestion? Many chemical and -physical theories have been advanced in the belief that they rendered -the process of its production less difficult to understand. All such -theories are, however, inadequate to explain the secretion as a -discontinuous process, which occurs only as a response to demand. That -the source of the acid is the sodic chloride which the gland-cells -take from the blood does not need assertion, but we cannot picture -the process by which this exceedingly stable compound is decomposed -otherwise than on the assumption that weaker acids, or, rather, acid -salts, are also absorbed by the cells, and that, in accordance with the -laws which govern the composition of salts in solution, an exchange -of acids occurs. If sodic chloride and any acid salt—acid phosphate -of sodium, for example—are in solution in water, the salts do not -retain their form as we know them when isolated by crystallization. -The mixture contains “free” hydrochloric as well as “free” phosphoric -acid. It may be assumed that within secreting cells a similar exchange -of acids takes place. By a process which we term “vital,” the acids are -kept apart, and the hydrochloric acid is extruded by the cells. In the -present state of knowledge this vital action is mysterious; but it is -no more mysterious than the isolation of pepsin, or any other metabolic -event which occurs within a cell.</p> - -<p>The proteolytic ferment pepsin is active only in an acid medium. Yet -apart from its digestive function as an ally of pepsin, hydrochloric -acid by itself also exerts a valuable disintegrating action on certain -constituents of the food. Possibly the most important results of the -presence of free hydrochloric acid in the great chamber into which food -is first received are due to its disinfective property. It destroys -all the putrefactive germs which accompany the food, and many germs -which, if introduced into the blood, would give rise to disease. It -also destroys the germs which multiply in the stomach towards the end -of each interval between two meals. When withdrawn from the body, -gastric juice will keep an indefinite time, if evaporation of the acid -be prevented. -<span class="pagenum"><a name="Page_116" id="Page_116">[Pg 116]</a></span></p> - -<p><b>Pancreas.</b>—In structure the pancreas presents a marked -resemblance to the salivary glands. Probably this resemblance is -merely superficial. Minute examination reveals points, apparently -of great morphological importance, in which they differ. In the -gland-tubes of the salivary glands, and, indeed, in all glands with -the exception of the pancreas, secreting cells project into the lumen. -The secreting cells of the pancreas are invested internally by a layer -of flattened scales (intra-acinar cells). They lie, therefore, between -the basement membrane which invests them externally and this second -layer of flattened cells which separates them from the lumen of the -tube. At a very early date in embryonic life the gland-cells of the -pancreas are filled with highly refracting granules. As this occurs -long before any digestive action is called for, it may be taken as -indicating that the pancreas has functions which other glands—the -salivary, for example—do not possess. These granules do not, however, -appear in all parts of the tubes. Certain portions of the tubes -remain undeveloped—fail, that is to say, to acquire a secreting -function—even in adult life. Such patches of cells, not disposed in -gland-tubes, are known as islands of Langerhans. When the pancreas is -over-stimulated by artificial means, leading to its extreme exhaustion, -large portions of its glandular substance return to this primitive -condition. The gland-cells not only discharge their stores of granules, -but they lose the greater part of their cell-protoplasm. It would seem -that, in their effort to meet the demand for ferments, they use up -their own cell-substance in their manufacture. Having exhausted their -coal, they stoke the furnace with the looms and furniture of the mill. -It may be that other glands would do the same if it were possible to -stimulate them as strongly as the pancreas can be stimulated. The -result is probably due to the extreme susceptibility of the pancreas to -the action of secretin, a substance made in the intestine. Secretin can -be isolated and injected into the blood. We shall refer again to this -chemical stimulation of the pancreas when tracing the progress of food -through the alimentary canal.</p> - -<p>The secretion of the pancreas is a clear, colourless, alkaline liquid -of syrupy consistence. The quantity of juice secreted is relatively -<span class="pagenum"><a name="Page_117" id="Page_117">[Pg 117]</a></span> -small, but the organic substances which it contains are in a -concentrated form. They constitute as much as 10 per cent. of the -pancreatic juice. Proteins are present, if the juice be fresh. If it -has stood for any length of time, they are found as peptones. The -digestive ferments of pancreatic juice are the most powerful which are -secreted into the alimentary canal.</p> - -<p><b>Bile.</b>—In its most important functions the liver has no relation -to digestion. It is a storehouse of absorbed food. This organ will -therefore be treated in a separate chapter. The bile which the liver -secretes into the alimentary canal has no chemical action on any of -the constituents of food, with the exception of a feeble tendency to -digest starch. Yet it is in some degree accessory to digestion. Poured -into the second portion of the duodenum through an orifice common to -the liver and the pancreas, it mingles with the semi-digested food, -or “chyme,” which, about two hours after a meal, passes through the -pyloric valve. Gastric digestion has converted the greater part of the -proteid constituents of the food into peptones or intermediate stages. -The proteoses or propeptones—a name is needed for the intermediate -products of proteid digestion which does not commit us to any theory -as to their chemical constitution—are quickly peptonized by the -pancreatic juice. But portions of the proteins have escaped the action -of gastric juice, or have at most been affected by its acid only; -these are precipitated by the bile-salts on the mucous membrane of -the small intestine, which is raised into projecting flanges for the -purpose of delaying the passage of the chyme, in order that it may -be thoroughly submitted to the digestive action of pancreatic juice. -Bile-salts also favour the digestion of fat, and its passage through -the intestinal wall. The action of bile-salts in spreading fats is well -known to artists. Ox-gall is smeared upon glass when it is desired -to apply oil-paints to its surface. When mixed with oil, it causes -its emulsification, or breaking up into microscopic globules. In the -absence of bile, but little fat passes into the lymph-vessels which -convey digested food from the intestine to the thoracic duct, and so to -the great veins of the neck. Its action is mechanical. It favours the -digestion of fats by rendering them easily amenable to hydrolysis by -pancreatic juice. -<span class="pagenum"><a name="Page_118" id="Page_118">[Pg 118]</a></span></p> - -<p>Bile as secreted by the liver is a clear, limpid fluid of low specific -gravity; but during its stay in the gall-bladder it is concentrated by -absorption of water, and mucin is added to it. It contains “bile-salts” -of complex constitution. These salts favour the solution of certain -by-products of cell-metabolism, cholesterin and lecithin; substances -which are formed in many cells, both in animals and plants. Cholesterin -occurs most abundantly in nerve-tissue and in blood-corpuscles. -Lecithin also is a by-product of the metabolism of nerve-tissue. -Protoplasm appears to be incapable of oxidizing these substances, as -it does other products of metabolism. Other substances of equally -complex constitution are reduced to urea if they contain nitrogen; -to water and carbonic acid if nitrogen be absent. Cholesterin and -lecithin have to be eliminated without further change. Some of the -cholesterin is excreted by the sebaceous glands of the skin. It is the -chief constituent of “lanoline” prepared from sheep’s wool; an unguent -which owes its valuable properties to the resistance which cholesterin -offers to cell action, and therefore to the action of living ferments. -Bacteria cannot turn it rancid. The sebaceous glands have the power -of directing metabolism into a channel in which cholesterin is the -chief product, but apparently all cells make it in small quantity. The -bile-salts carry cholesterin and lecithin into the alimentary canal, -from which they are not reabsorbed. Some of the bile-salts are lost to -the body, but the remainder re-enter the circulation, and recommence -their work as vehicles for these inoxidizable and insoluble substances. -In the gall-bladder cholesterin is apt to separate out from the bile -in the form of gall-stones; but whether this is due to an excess of -cholesterin in the bile, or to an abnormal, inflammatory condition of -the lining membrane of the gall-bladder, is still an open question.</p> - -<p>Bile also contains bile-pigments. Their colour varies in different -animals, and changes according as the bile is exposed to the air, or -subject to the action of reducing agents. If oxidized, the colour -is green (biliverdin); if reduced, brownish-yellow (bilirubin). -Bile-pigment is formed from hæmoglobin, the colouring matter of the -blood, after the removal of its iron. Worn-out red blood-corpuscles are -destroyed in the spleen, in the manner already described, but it is -uncertain whether the conversion of the hæmoglobin thus set free into -<span class="pagenum"><a name="Page_119" id="Page_119">[Pg 119]</a></span> -bilirubin occurs in the spleen, or whether this chemical change is -reserved for the liver. Physiologists incline to the view that the -liver is the seat of the change.</p> - -<p><b>Intestinal Juice.</b>—The mucous membrane of the alimentary tract, -as far down as the middle of the rectum, is, as previously stated (<a href="#Page_102">p. 102</a>), -studded with tubular glands. They secrete a light-yellow fluid, -alkaline in reaction, and opalescent. Its most important property is -due to a ferment which converts cane-sugar into a mixture of dextrose -and levulose, and changes maltose—the sugar produced by the action on -starch of saliva and pancreatic juice—into dextrose. It is in the form -of dextrose that sugar is carried about the body and assimilated by the tissues.</p> - -<p>Intestinal juice also contains a ferment, erepsin, which shakes to -pieces the heavy molecules of peptones and partly formed peptones. -Under its influence they break up into comparatively simple bodies -containing the radicle of ammonia. Substances containing an NH₂ -group—one H of NH₃ (ammonia) having been given up, in order that -the group may have a “free arm” with which to link on to the other -component parts of the molecule—are termed “amides.” The amides -which are most characteristic of the action of erepsin are leucin, -an amidated fatty acid; and tyrosin, an amidated aromatic acid. The -tendency of proteins to break up along these two lines—the fatty acid -line and the aromatic acid line—is of considerable interest. The one -line is represented by acetic acid, <b>CH₃COOH</b>; the other contains the -hexone radicle, <b>C₆H₆</b>. Benzoic acid, <b>C₆H₅COOH</b>, is representative of the -latter. It used to be thought that proteins which were shaken into -simple bodies such as amides were lost to the economy. Their downward -career was a foregone conclusion. There could be no arresting it -before they brought up at the bottom—as urea, <b>CO(NH₂)₂</b>—the diamide -of carbonic acid. It was even supposed that this disintegration of -proteins was a provision for getting rid of the surplus animal food -which we consume. Physiological chemists now take quite a different -view. They believe that the epithelial wall of the intestine through -which these substances are absorbed, or the liver, to which they are -carried by the portal blood-stream, has the power of recombining these -<span class="pagenum"><a name="Page_120" id="Page_120">[Pg 120]</a></span> -fragments into the complex protein edifice. It is even thought that -disintegration is a necessary preliminary to the rearrangement of the -sub-groups. A large variety of proteins is ingested as food. Many of -them, especially the vegetable proteins, are quite foreign to the body. -By the activity of pancreatic juice and erepsin, they are broken into -small and relatively stable groups of atoms, which are again fitted -together into the particular forms of protein which are of use to the -economy.</p> - -<p><b>The Story of a Meal.</b>—The chemistry of digestion will be -understood most readily if the constituents of a meal are traced from -their entrance into the mouth to their absorption through the wall of -the alimentary canal, or abandonment as indigestible.</p> - -<p>We may describe as a typical meal one consisting of bread, vegetables, -cane-sugar, meat, milk, fat, and cheese. In the mouth the various foods -are crushed and mixed with the alkaline secretions of the salivary -glands. A certain amount of the cooked starch contained in the bread -is changed into maltose. In the stomach the digestion of starch is -continued for a time, but a large part even of the cooked starch awaits -the action of pancreatic juice. A certain amount of cane-sugar is -converted into dextrose and levulose, which are rapidly absorbed into -the blood; but this action is due to hydrochloric acid, and probably -affects a comparatively small part of the cane-sugar swallowed. Fat is -quite unaltered in the stomach. All proteins are attacked by pepsin, -but some yield to digestion more readily than others. Gluten of bread, -like all vegetable proteins, is comparatively resistant; but since -it is presented to the action of pepsin in small quantities and in a -spongy form—very suitable for digestion—it is probable that most -of it is peptonized in the stomach. Chemists experimenting with gastric -juice taken from the stomach, and reproducing the conditions as to -temperature, removal of products of action, etc., as closely as it is -possible to reproduce them in the laboratory, find that the various -foods take different times to digest. The proteins of meat are more -quickly peptonized when raw than after coagulation by heat. The same -is true of white of egg. Amongst different varieties of cooked flesh, -beef is more quickly peptonized than fish. The casein of milk is more -<span class="pagenum"><a name="Page_121" id="Page_121">[Pg 121]</a></span> -quickly peptonized than any other protein; and it also is no exception -to the rule that digestibility is diminished by cooking. Similar data -may be obtained for all foods. They are no doubt useful indications of -the course of action which we may expect to occur within the stomach, -but we can never be sure that my lord will obey the ruling of the -chemist. Practice with a captive golf-ball is a useful preparation -for the game; but there are conditions on the links which cannot be -reproduced on the lawn. In an artificial stomach the clean fibre of -raw fish digests more slowly than raw beef. Even when the beef is -roasted and the fish fried or boiled in the ordinary way, the beef -disappears through the dialyser (the bag of membrane suspended in a -vessel of warm water in which experimental digestion is carried out) -more quickly than the fish. Nevertheless, the living stomach is better -disposed towards a mixed meal containing a certain weight of fish than -towards a meal in which, the other constituents remaining the same, -beef takes the place of fish. Important conclusions may, no doubt, -be drawn from observations of the time occupied in the peptonization -of pure food—<i>i.e.</i>, fibrin, white of egg, clean meat, etc.—under -conditions simulating those which are present in the stomach; but -they must be accepted with many reservations. In the stomach it is -not pure substances, but mixtures, that the gastric juice has to deal -with. And here a most important factor comes into play, to which -further reference will be made later on. The amount and quality of the -secretion of the gastric glands depends upon the nature of the food. -Hence a food, or a combination of foods, which digest readily in the -laboratory may take a long time to disappear from the stomach, and -<i>vice versâ</i>. Digestibility depends upon the nature of the food. It -depends also upon its physical state. To take simple illustrations: -Cheese contains coagulated casein, one of the most easily digestible -of proteins, but the casein is intimately mixed with fat, upon which -gastric juice can make no impression. Even when finely divided, the -particles of casein are protected from the action of the juice by -fat. In the same way the meat of pork is as digestible as mutton, but -the fat of pork is quickly melted and very liquid. In the process of -cooking the muscle-fibres become saturated with fat.</p> - -<p>It is not the function of the stomach to complete digestion. Its -<span class="pagenum"><a name="Page_122" id="Page_122">[Pg 122]</a></span> -business is to initiate it. Food which reaches the stomach in fragments -is reduced to a condition in which its digestion will be readily -completed by pancreatic juice. Gastric digestion produces a much -larger proportion of intermediate products, proteoses or propeptones, -than does digestion in the duodenum. Such intermediate products are -quickly dealt with by pancreatic juice. Artificial tests of relative -digestibility do not, as a rule, take the amount of propeptones formed -in a given time into account. When considering the digestion of a -typical meal, we must bear in mind that it is not the duty of the -stomach to pass as much sugar, peptone, and fat as possible into the -blood. In fact, very few of the products of digestion are absorbed -by the bloodvessels of the stomach. The impermeability of its mucous -membrane is shown by the fact that hardly any of the water swallowed -passes through the stomach-wall. Practically all the water ingested -leaves the stomach through the pyloric valve. Various salts, some -sugar, and peptones are taken up by the vessels of the stomach; but the -bulk of all the different kinds of food passes into the duodenum in a -semi-digested state. The function of the stomach is to carry digestion -through a preliminary stage. The process will be completed in the small -intestine. It is to be noted that, although water is not absorbed by -the stomach-wall, alcohol passes through it with great rapidity. The -same is true of the various crystalline nitrogenous bodies found in -meat-extracts, and also of the essential principles of tea and coffee, -which chemically belong to the same class. All these substances are -degradation products of proteins produced by oxidation, far advanced -along the road to urea. In this selective absorption we see proof of -the activity of the cells of the mucous membrane. They take up the -substances which it is desirable to remove from the contents of the -stomach. Some may be wanted by the body for its immediate use; others -are better out of the way, because they are prejudicial to the progress -of digestion.</p> - -<p>When contemplating the activity of the cells of the gastric mucous -membrane, we feel the need of an adjective which shall express our -recognition of the fact that they have a power which we cannot confer -upon our clumsy mechanical imitation stomach. They can discriminate. -“Vital” is the only term available, though much abused. Using it without -<span class="pagenum"><a name="Page_123" id="Page_123">[Pg 123]</a></span> -prejudice, as lawyers say, we speak of the “vital activity” of the -cells when we wish to imply that things happen in a living stomach for -which we cannot make provision in a model. Of the many substances which -make their appearance as digestion proceeds, some are absorbed, others -left in the mixture.</p> - -<p>The mucous membrane shows its power of controlling digestion in yet -another way. In the neighbourhood of the pylorus its structure is -unlike that which it presents elsewhere. The gastric glands are short, -and tend to branch. Their lining cells are all of the same kind. Over -the greater part of the inner wall of the stomach the tubes are long. -They do not branch. The cells which line them are of two kinds: small -cubical cells (the term refers to their form as seen in section), -similar to those of the pyloric glands; large oval cells, placed with -their longest axes in the same direction as the axis of the gland-tube. -These oval cells do not project into the bore or lumen of the tube, but -are displaced from it by the cubical cells. They rest on the investing, -or basement, membrane. All parts of the gastric mucous membrane secrete -pepsin, although the pyloric portion produces very little; the area -which contains oval cells alone secretes hydrochloric acid. If a -short time after a meal an extract is made from some of the mucous -membrane near the pylorus, by pounding it with salt-solution and sand -to break up its cells, this extract, when filtered and injected into -the blood, stimulates the glands of the cardiac end of the stomach. -Under its influence they pour out both pepsin and hydrochloric acid. -The extract contains a substance which acts as a chemical messenger. -It is a representative of a class of bodies which play a most -important part in co-ordinating the activities of the various organs. -Hitherto physiologists have concerned themselves with the visible or -“external” secretions of glands. They have shown how the production -of these secretions is controlled by the nervous system. Recently -they have discovered that another set of influences has to be taken -into consideration. Glands, and possibly all other tissues, take from -the blood the materials out of which they make their characteristic -secretions, or, if they do not discharge secretions, the substances -which they require for the building of their own structures, and return -to the blood “internal secretions” which act as stimuli to other tissues -<span class="pagenum"><a name="Page_124" id="Page_124">[Pg 124]</a></span> -with which they are linked in harmonious co-operation. The active -principles of internal secretions have been termed “hormones”—from -ὁρμάω, I announce. The glands of the pyloric mucous membrane secrete a -hormone which calls upon the rest of the membrane to pour out gastric -juice (<a href="#Page_89"><i>cf.</i> p. 89</a>).</p> - -<p>What induces the cells of the pyloric mucous membrane to produce the -gastric hormone? Their activity in this respect evidently depends upon -the presence in the stomach of partially digested proteid substances. -The cells judge, as it were, when these substances come into contact -with them, that there is more work for the great bag of the stomach to -do. They call upon the part which is most active in secreting gastric -juice to pour it out quickly and get the business of digestion over. -Meat-extracts, which contain the products of protein disintegration, -have a similar influence in promoting the formation of the hormone. -Hence, no doubt, the general custom, found from experience to be -beneficial, of commencing dinner with soup; although it must be -remembered that the rapid absorption of meat-extracts makes them -peculiarly valuable as restoratives. They afford very little energy, -but what they have to give is quickly placed at the disposal of the -economy. Persons whose stomachs are unduly irritable are advised to -avoid soup. It leads to undesirable activity on the part of the gastric -glands, and especially of the acid-secreting cells. Well chewed bread -also encourages the production of the hormone.</p> - -<p>Here it may be well to call attention to the evident division of the -stomach into two parts—the large bag, or cardiac portion, which hangs -down; and the smaller, funnel-shaped pyloric end, which is almost -vertical. The distinction between these two parts is faintly visible in -the resting stomach, but even opening the abdomen tends to obliterate -it. That it is much more evident during active digestion has been shown -by adding subnitrate of bismuth to the food, and throwing the shadow -of the stomach on a screen with Röntgen rays. When this is done, it is -seen that the two parts work in different ways. Food is churned round -and round in the cardiac portion, and pressed towards the pylorus. Its -fluid products, mixed with the abundant secretion of the gastric mucous -<span class="pagenum"><a name="Page_125" id="Page_125">[Pg 125]</a></span> -membrane, are wrung out of it by the pyloric funnel. They are squeezed -towards the pylorus, which opens at intervals to let them through. If -lumps of solid matter reach it, the pyloric valve closes tightly, until -the undigested food has fallen back into the dependent bag. Dyspeptics -are sometimes unpleasantly conscious of the contractions of the -pyloric funnel. In fact, putting aside pain due to gastritis, all the -discomfort of dyspepsia is felt on the right side. Flatus accumulates -beneath the pyloric valve. The valve will not open to let it pass. The -pyloric portion of the stomach contracts strongly. Notwithstanding -the general trend of movement in the opposite direction, the gases -are squeezed back into the larger bag, and escape through the cardiac -orifice.</p> - -<p>Tables have been prepared showing the length of time which various -articles of food take to digest. They are based in part upon -observations made upon the living stomach in cases in which it has -been possible to examine its contents through a fistulous opening; -in part upon the results of artificial digestions carried out in the -laboratory. It is hardly too much to say that such observations are -absolutely without value as tests of the relative digestibility of the -several articles of diet consumed as parts of an ordinary meal. The -fact that the commencement of the flow of gastric juice depends upon -mental stimuli, and its continuance upon hormones, shows how difficult -it must be to reproduce the conditions which obtain in a healthy living -body. The most wholesome of foods taken by itself may be longer in -digesting, or may produce more irritation, than many less desirable -things taken in judicious combination. Crushed chicken, hastily -swallowed, sometimes proves more difficult of digestion than meat so -cooked and served as to stimulate appetite and to demand mastication.</p> - -<p>Returning to the story of a meal, vegetables pass almost unaltered -through the stomach. Some of the scanty proteins which they contain -are peptonized, but unless they are very well masticated or cooked -until they are soft, and therefore easily pulped by the churning action -of the stomach, the gastric juice has to reach the proteins through -cell-walls. None of the digestive juices are able to dissolve the -cellulose of vegetable cell-walls. Blocks of vegetable tissue pass down -the whole length of the alimentary canal in the form in which they were -<span class="pagenum"><a name="Page_126" id="Page_126">[Pg 126]</a></span> -left by the teeth. Hence the extreme indigestibility of ill-chewed -cucumber or apple. The pyloric valve of the stomach is forbidden to -allow any lumps of food to pass until the very last stage of gastric -digestion. Pieces of ill-masticated vegetable tissue lie for a long -time in the stomach, irritating the ends of the gastric nerves, until -at last the time comes for them to be shot through the pylorus into the -duodenum. Many salts which vegetables contain, especially the earthy -carbonates and phosphates, are dissolved by the acid of the gastric -juice.</p> - -<p>Meat consists of muscle-fibres supported by connective tissue. In the -stomach the gelatiniferous connective tissue is dissolved, setting -the fibres free. Further, the fibres being surrounded by a membrane -of the same nature—sarcolemma—this is removed; and although -it may be hardly justifiable to speak of “Krause’s membranes” -(<a href="#I_218"><i>cf.</i> Fig. 10</a>) as gelatiniferous septa, the fibres are -certainly composed of segments—Bowman’s discs, sarcous elements—into which -they break up under the action of gastric juice. As a result, meat-fibre is reduced -to a finely divided granular condition. The capacity of gastric juice -for dissolving collagen (the substance of which connective tissue -is composed) may be regarded as its most characteristic, as it is -one of its most valuable, properties. Collagen, when boiled or acted -on by acids, takes water into its molecule, becoming gelatin. Under -the influence of gastric juice gelatin is rapidly hydrolysed into -diffusible gelatin-peptone. Pancreatic juice is unable to act upon -collagen, unless it has been previously boiled, or swollen by the -action of dilute acids.</p> - -<p>Fat is composed of vesicles of oil supported by connective tissue. -Gastric juice, by dissolving the connective tissue and the collagenous -walls of the vesicles, sets the oil free. The oil, even though it be as -firm as suet when cold, is liquid, or almost liquid, at the temperature -of the body.</p> - -<p>Thus, with the exception of raw vegetables, the hard fibre of cooked -vegetables, elastic tissue of meat, and a few other indigestible -substances, the meal is reduced in the stomach to a cream-coloured, -fatty, strongly acid “chyme.” In this condition it enters the duodenum, -where it at once comes into contact with an alkaline secretion. The -passage of acid chyme down this portion of the canal provokes the -discharge of gushes of bile and pancreatic juice. By precipitating -<span class="pagenum"><a name="Page_127" id="Page_127">[Pg 127]</a></span> -partially digested proteins and “acid-albumin” bile renders the mixture -thicker and sticky. It colours it yellowish-brown. Under the influence -of pancreatic juice the remaining proteins and proteoses are rapidly -converted into peptones, some of which are shaken down by the violent -action of erepsin into simpler bodies, such as leucin and tyrosin, etc. -The chyme becomes alkaline, grey, and thin. All undigested starch is -changed into maltose, and this into dextrose. Cane-sugar is converted -into dextrose and levulose. These sugars are absorbed into the blood. -Milk-sugar, if not converted into lactic acid, remains as lactose -<b>(C₁₂H₂₂O₁₁)</b>, in which condition it is absorbed without “inversion.” -Fats are split by a ferment of the pancreatic juice into fatty acid and -glycerin; some of the fatty acid combines with alkali to form soap, but -of this we shall have more to say later on.</p> - -<p>The duct common to the liver and the pancreas opens into the second -part of the duodenum. The organs which produce bile and pancreatic -juice are comparatively remote from the place where their secretions -come into contact with the food. By what mechanism are they thrown into -activity when the assistance of their secretions is required? As in -the case of the stomach, the agent is a hormone, a chemical messenger. -The hormone, termed “secretin,” is formed by the cells of the mucous -membrane of the duodenum when acid comes in contact with them. It is -absorbed by the blood, which carries it to the pancreas and the liver. -When it reaches the pancreas, it acts as a most powerful stimulant -to the discharge of accumulated ferments, and to the production of -an additional supply. It stimulates the liver to pour forth bile. At -present we are in ignorance as to the chemical nature of this hormone. -It is not a proteid substance, nor is it a ferment. If scrapings -from the mucous membrane of the duodenum be crushed with sand and -hydrochloric acid, the mixture boiled, neutralized with carbonate of -soda, and filtered, the clear, colourless liquid which results has -a powerful effect upon the pancreas, when injected, in even small -quantities, into the blood. Apparently, the cells of the duodenal -mucous membrane are constantly producing and accumulating a substance -which is converted into secretin when acted on by acid. It is not -necessary for the acid to stimulate the living cells. If the mucous -membrane is ground up with sand and salt-solution, the filtrate is -<span class="pagenum"><a name="Page_128" id="Page_128">[Pg 128]</a></span> -inactive but an active extract is obtained by treating the crushed -cells with <b>HCl</b>. It changes some substance which they contain -(provisionally termed “prosecretin”) into the efficient hormone.</p> - -<p>In the lower portion of the small intestine any maltose that remains -is converted into diffusible dextrose. A very large amount of water -has been poured into the canal in the various digestive juices. This, -together with water drunk, is absorbed in the large intestine. At the -lower end of the alimentary canal nothing remains but indigestible -substances taken with food, chiefly cellulose, and the pigments and -other bodies which, as already said, are eliminated in bile.</p> - -<p>The absorption of water is checked by the ingestion of extremely -soluble salts, such as sulphate of magnesia, the heavy molecule of -which diffuses with difficulty. We attribute the fact that sulphate of -magnesia remains in the intestine, and prevents water from diffusing -<span class="pagenum"><a name="Page_129" id="Page_129">[Pg 129]</a></span> -out of it, to its slowness in passing through a membrane, because this -is what would happen in dialysis;<a name="FNanchor_2_2" id="FNanchor_2_2"></a><a href="#Footnote_2_2" class="fnanchor">[2]</a> -but we must remember that the living wall of the intestine is not a membrane. -The cells which line the intestine take up substances far less -easily diffusible than the sulphate of magnesia which they refuse. -Nevertheless, speaking generally, it is the less diffusible salts -which act as aperients, the more diffusible which are absorbed. The -forward passage of the contents of the alimentary canal is hastened -by castor-oil. The peristalsis of the intestines is stimulated by -certain drugs, such as jalap or the burnt products of tobacco. Another -class of drugs, of which aloes is an example, increases the secretion -of the intestines, small or large. Certain purgatives, such as -calomel, podophyllin, etc., used to be regarded as cholagogues. It was -supposed that they increased the flow of bile. This is an error. Their -action is complicated, but it affects chiefly the peristalsis of the -intestine. The poor misunderstood liver still suffers from the libels -of primitive medical science. It is the most innocent of organs, in -no way responsible for derangements of digestion. It carries out its -functions without haste and without delay. With the possible exception -of salicylate of soda, no drug is known which can stimulate it to a -more rapid output of bile.</p> - -<p><b>Absorption.</b>—All the cells which line the alimentary canal are -capable of absorbing food, if it is presented to them in a suitable -form. In a suitable form means, speaking generally, in a diffusible -condition, although it must not be supposed that the epithelial cells -are incapable, under certain circumstances, of taking up non-diffusible -substances, just as a unicellular organism—an amœba—can take in food. -If soluble proteins, such as white of egg or acid-albumin, are injected -<span class="pagenum"><a name="Page_130" id="Page_130">[Pg 130]</a></span> -into the large intestine, a very considerable proportion of the -substance so injected is absorbed. It is possible, indeed, to supply in -this way the whole of the nitrogenous food needed by the system, none -entering by the mouth. If milk is injected, a certain amount of the fat -also is retained. It can be shown that such absorption takes place when -no digestion of the food occurs in the colon. The food is taken up by -the epithelial cells in the form in which it is injected.</p> - -<p>The organs specially devoted to absorption are the villi, which project -into the contents of the small intestine. Each is a conical process -about 0·5 millimetre long. The villi are longest in the upper half of -the small intestine. Below this level they decrease in number and size. -A villus is completely covered with epithelial cells of short, columnar -form. The free border of each cell is slightly hardened, forming a -disc or cap which appears striated in optical section—an indication, -as some think, that it is traversed by pores. Others hold that the -appearance of striation is due to minute cilia-like projections which -beset the free border of each cell. In worms and other invertebrates -the cells carry motile projections of not inconsiderable size, which -no doubt free their surfaces from the unassimilable matter which tends -to accumulate upon them. Possibly they help to fix particles which are -suitable for absorption. In mammals the presence of cilia has not been -demonstrated. The extreme minuteness of the striæ seems to point to -their being merely indications that the border is permeable to fluids, -including droplets of fat.</p> - -<p>The so-called basement membrane upon which the epithelial cells rest -must not be regarded as a membrane in the physical sense. Rather is -it a basket-work which supports the cells, without in any degree -limiting their power of disgorging into the lymph-spaces of the villi -the substances which they have absorbed. Within the villus, connective -tissue forms a sponge-work, the spaces of which are filled with lymph, -in which considerable number of leucocytes roam, on the look-out, no -doubt, for any germs which may make their way between the epithelial -cells. In the centre of the villus is a lymphatic radicle—<i>i.e.</i>, -a fusiform cul-de-sac—which is the dilated end of a lymph-vessel. -It, like all other lymph-vessels, is walled by flattened endothelial -<span class="pagenum"><a name="Page_131" id="Page_131">[Pg 131]</a></span> -scales. It communicates with the lymph-plexus beneath the mucous -membrane, which, again, communicates with a coarser plexus outside the -muscular coat. From the peri-intestinal plexus vessels lying in the -mesentery converge to the receptaculum chyli, the bulbous commencement -of the thoracic duct, which lies at the back of the abdomen in front -of the bodies of the vertebræ. The thoracic duct runs up the front -of the vertebral column, through the thorax, and then hooks over to -pour the fluid which it conveys into the great veins shortly before -they join the heart. After a meal containing fat the fluid in the -lymphatic vessels of the mesentery, the lacteals, has, as already -stated (<a href="#Page_43">p. 43</a>), the appearance of milk. The fat absorbed -by the epithelium covering a villus is passed on into its lymph-space. From -this into the central lacteal receptacle, thence to the submucous -and peri-intestinal plexuses, the lacteal vessels of the mesentery, -the thoracic duct. Absorbed fat does not pass through the liver, but -is carried into the heart; thence through the lungs, and back to the -heart, which pumps it to all parts of the body. In addition to lacteal -radicle; the villus contains long capillary bloodvessels, and the -arteriole and venule in which they commence and end. These traverse -the lymph-spaces of the connective tissue, which contains, not only -the fat which the epithelial cells have passed into it, but the other -products of digestion also. None of the fat traverses the walls of the -bloodvessels; but the other products diffuse from the lymph, through -the walls of the vessels, into the blood. Many nerve-fibres are found -in the core of the villus on their way to epithelial cells, or to one -or two plain muscle-fibres which are disposed in the direction of its -long axis. For each villus is a little pump. By the contraction of the -muscle-fibres it is shortened, and the fluid in its lacteal radicle is -forced into the submucous vessels.</p> - -<p>Two problems have to be considered: First, in what form and by what -mechanism are the several kinds of food absorbed? Secondly, what -becomes of them after they have been absorbed?</p> - -<p>Clearly, the epithelial cell is the absorbing mechanism. It is not -a membrane governed by the laws which regulate diffusion of fluids -through membranes, but a living cell. There is hardly any limit to its -<span class="pagenum"><a name="Page_132" id="Page_132">[Pg 132]</a></span> -power of selecting the food which it ingests. It could, and very -possibly it does, ingest albumin and fats as such. Still, the elaborate -provision which is made for converting albumin into diffusible peptone, -and cane-sugar and maltose into easily diffusible dextrose, suggests -that substances which will pass through membranes are more readily -absorbed than substances which will not. We are justified in looking -upon absorption as a physical problem up to a certain point. But we -must not dwell too much on the physical aspects of the problem. If the -absorption of food were merely a process of diffusion, an enormous -quantity of water would be required to carry the diffusible products -of digestion into the villi. The passage of the foods is aided by the -selective activity of the epithelial cells. Peptonization greatly -facilitates the work of the epithelial cells, but it is not a condition -essential to absorption, so far as soluble proteins are concerned. It -is, however, essential that the proteins should be presented to the -epithelial cells in a soluble form. They could do nothing with the -solid fibres of meat, however much they might have been disintegrated -by mastication and by the action of hydrochloric acid. It is only -after digestion by pepsin and by trypsin that all the proteins of food -are brought into solution. Digestion is needed to reduce them to a -condition in which the epithelial cells can take them up.</p> - -<p>Much thought has been devoted to the question of the form in which fat -is absorbed. Fat in the chemical sense—a pure fat, that is to say—is -a compound of a fatty acid and glycerin. Suet, lard, butter, vegetable -oils, etc., are mixtures of several fats. All consist of glycerin -united with fatty acids. The acids are stearic acid, palmitic acid, -oleic acid, and others of less importance. Fats are insoluble in water; -so also are the fatty acids. A fatty acid combined with an alkali (in -place of glycerin) is a soap. Soaps are soluble in water. If milk is -examined under the microscope, it is found to contain droplets of fat, -varying in size, but all minute. The larger droplets tend to rise to -the surface as cream, but the smaller droplets do not run together. If -milk from which the cream has been skimmed is sterilized, it retains -its normal appearance for an indefinite time. Its fat remains in -droplets. In technical language, milk is an emulsion. Theoretically oil -<span class="pagenum"><a name="Page_133" id="Page_133">[Pg 133]</a></span> -and water would make an emulsion, if the droplets of oil were rendered -sufficiently minute. Such a condition has been almost obtained by -agitating oil and water with powdered glass. But the more viscous the -medium through which oil globules are distributed, the greater is -the resistance to their fusion. If oil which has become rancid—in -which a certain quantity of fatty acid has been liberated from the -glycerin with which, in a neutral fat, it is combined—is shaken with -water containing carbonate of soda, an emulsion is easily formed. -The carbonate of soda and the fatty acids form soaps. A solution -of soap is sufficiently viscous to keep the droplets of oil apart. -Emulsification of fats occurs in the intestine. It might be assumed -that the epithelial cells ingest fat in this finely divided state. -But it must be remembered that, however minute the droplets, they -are enormously large as compared with the molecules of peptones and -sugar which the epithelial cells absorb. It is unlikely that fat is -absorbed in a manner so widely different from that in which other -foods enter the epithelial cells. Nor is it necessary to make any such -assumption. Pancreatic juice contains a ferment which rapidly splits -fats into their constituent fatty acids and glycerin. In the presence -of an alkali the fatty acids are converted into soaps. In this soluble -condition of soap and glycerin the fats are probably absorbed. As soon -as they have entered the cell, the fatty acids and glycerin reunite -to form fats, setting the alkali free. The alkali is returned to the -intestine, where it is available as a solvent of further droplets of -fat. The droplets of fat accumulate in the epithelial cells. During -active digestion they are also to be seen in the connective-tissue -cells, in the leucocytes, and in the lymph inside the lacteal vessel. -The epithelial cells extrude the oil droplets, backwards, much in the -same way as the cells of the mammary glands extrude globules of milk. -In herbivora, and in Man also so far as we can judge, the contents of -the small intestine are alkaline. Conditions are therefore favourable -for the formation of soap. But in carnivora the contents are acid -throughout the greater part of the canal. Acid, it need hardly be -stated, prevents saponification. Yet carnivorous animals have an -immense capacity for absorbing fat. Fatty acids are soluble to a -moderate extent in bile. It is possible that, fats having been split -<span class="pagenum"><a name="Page_134" id="Page_134">[Pg 134]</a></span> -into fatty acids and glycerin, the fatty acids are carried into -the cells in solution in bile. But if in carnivora bile actively -participates in the absorption of fat, there is no reason opposed to -its having the same function in Man; and, indeed, all observations -which have been made upon patients in whom the bile was, for some -reason, diverted from the intestine, and in animals in which a fistula -of the gall-bladder has been artificially produced, show that in the -absence of bile the absorption of fat is considerably decreased. Yet -there is no reason for thinking that bile is secreted for the purpose -of facilitating the absorption of fat. Just as much bile is poured -into the intestine of a cow which is feeding upon grass as into the -intestine of a pig or a dog when the animal is consuming a very large -quantity of fat. Nevertheless, it appears to be certain that, not -in carnivorous animals only, but also in herbivorous animals, the -assistance of bile is necessary for the satisfactory absorption of -fat. Doubtless the co-operation of bile and pancreatic juice is more -important to carnivora than it is to herbivorous animals, in which, -owing to the alkalinity of the contents of the intestine, all fatty -acids liberated by the action of pancreatic juice might be converted -into soluble soaps.</p> - -<p>The problem of the form in which foods enter the absorbing cells is -intimately associated with the further problem of the form in which -they leave them. In the villus, and even within the epithelial cells, -fat appears abundantly as such. If, as we have reason for believing -to be the case, it enters in the form of soap and glycerin, the -re-formation of fat is an illustration of the synthetic power of the -tissues. For the purposes of the economy it is needed as fat, and not -as the constituents of fat. There is no reason for thinking that at -any stage in its future progress it is again split into fatty acid and -glycerin.</p> - -<p>We cannot see absorbed proteins with the microscope, as we can see fat, -nor can we apply chemical tests which will distinguish between the -proteins which the cells contained before digestion commenced, and the -proteins which they have received as its result. Nevertheless, it is -certain that peptones are reconverted into proteins as soon as they are -absorbed. They are not to be found in blood or lymph. If the peptones -<span class="pagenum"><a name="Page_135" id="Page_135">[Pg 135]</a></span> -absorbed after a proteid meal remained as such after they passed -through the wall of the alimentary canal, they would produce various -undesirable results.</p> - -<p>There is some difficulty in following droplets of fat across the space -which intervenes between the epithelium of a villus and its lacteal -radicle. It has been asserted that leucocytes act as carriers, catching -the droplets as they are extruded by the epithelial cells, and bearing -them into the radicle, where they set them free. Undoubtedly, many -leucocytes are present in the lymph-spaces of a villus. After a meal -they are found charged with fat. But it is hardly in accord with what -we know of the character of a leucocyte to suppose that it will let go -fat which it has once ingested into its own body-substance. A leucocyte -is not a disinterested organism. If fat droplets are floating across -from the epithelium to the lacteal, leucocytes are pretty certain to -steal some of them. But we know of no other case in which they give up -what they have stolen, unless it be something which is injurious to -their own health. Even then they usually cling to it, whether it be a -germ or a particle of soot, until their own dissolution sets it free.</p> - -<p>Neither proteins nor sugar reach the lacteal radicle. Both these -substance are absorbed from the lymph in the tissue-spaces of the -villus by the blood-capillaries and venules which traverse them. The -veins of the intestine unite to form the portal vein, up which proteins -and sugar are carried to the liver, where they are stored, to be doled -out into the blood-stream as the tissues need them.</p> - -<p><b>Bacteria of the Alimentary Canal.</b>—The enzymes (ferments) of -the several digestive juices are not the only agents which modify the -constitution of the foods within the alimentary canal. Throughout the -whole of the tract conditions are in many respects favourable for the -growth of putrefactive organisms. Mouth, stomach, small and large -intestine, has each its special bacterial flora. It is doubtful whether -any of these organisms, with the single exception of the bacteria which -in herbivorous animals break up cellulose, are favourable to digestion. -That they are not necessary has been shown by an ingenious experiment -on new-born animals. Guinea-pigs born in an aseptic chamber, through -which filtered air was drawn, and fed every hour on sterilized milk, -throve and put on weight. When killed at the end of eight days, no germs -<span class="pagenum"><a name="Page_136" id="Page_136">[Pg 136]</a></span> -were present in their alimentary tracts. Yet in all animals under -ordinary conditions bacteria are present in great numbers, at any rate, -after the nursing period, and, for good or ill, produce important -fermentations. Only a single bacillus (<i>B. bifidus</i>), and that a -friendly germ, is, it is asserted, present in the intestines of an -infant at the breast; whereas a bottle-fed baby houses a variety of -parasites.</p> - -<p>In the stomach, sugars are changed by the <i>Bacterium acidi lactici</i> -into lactic acid, which is further split into butyric acid, carbonic -acid gas, and hydrogen. Succinic acid and other substances are also -formed. This occurs in the first stage of gastric digestion. When a -considerable quantity of hydrochloric acid has been poured out, lactic -fermentation is stopped. The small amount of gaseous products formed -normally is of little consequence; but flatulence is a most annoying -symptom of indigestion. “Put your trust in Providence, and you will -feel more cheerful after luncheon,” Dr. Jowett is alleged to have -remarked to a despondent friend. The presence of food stimulates the -stomach to contraction. Accumulated gases are expelled. Hydrochloric -acid is secreted, and puts a stop to fermentation for a time. But if -the meal be too heavy or the mucous membrane in an irritable condition, -the contents of the stomach become unduly acid in the later stages of -digestion. Other bacteria then develop, leading to fresh trouble; more -gases accumulate, and the dyspeptic’s distress is greater than it was -before. Unfortunately, antiseptics, such as creosote, and carminatives, -such as oil of lavender, oil of peppermint, or alcohol, which for the -moment give relief, increase irritability, and consequently in the -long-run make matters worse. It is the fermentation of the later stages -of digestion which causes most annoyance. Admirable as was the Master -of Balliol’s advice, it hardly took account of the fact that bacteria -which cause flatulence, with its resultant feeling of oppression, are -derived for the most part from the imperfectly digested, and therefore -actively fermenting, remnants of food which were present in the stomach -when the meal was taken. It would be far beyond the scope of this -book to consider the pathology of dyspepsia; but the study of normal -conditions reveals the fact that some amount of fermentation invariably -occurs. The <i>Bacterium acidi lactici</i> is always present in the stomach. -<span class="pagenum"><a name="Page_137" id="Page_137">[Pg 137]</a></span> -Normally its activity is arrested by the hydrochloric acid of the -gastric juice about twenty minutes after a meal. After this no further -multiplication of bacteria should occur. The presence of bacteria which -grow in a strongly acid medium usually indicates that the stomach was -not completely emptied before fresh food reached it. It may be that -the last meal was too large or the interval too short. If the mucous -membrane is in an unhealthy condition, its own secretions afford -material on which bacteria thrive. Nothing short of washing it out with -a stomach-pump will clean it up. The presence, at the time of feeding, -of food left over from the previous meal is likely to perpetuate the -unsatisfactory state of affairs. All the glands of the alimentary -tract exhibit a tendency to periodicity. Their efficiency is greatest -when activity follows a period of rest. If the stomach is not able to -expel its contents, it has not the opportunity of preparing for fresh -duties. Fat undergoes a certain amount of rancid fermentation in the -stomach. Proteins are not attacked by bacteria in the stomach unless -the condition of the organ is very unsatisfactory. The odour of the -products of their decomposition is then recognizable in the breath.</p> - -<p>Bacteric fermentations in the small intestine are unimportant under -normal conditions, with the exception of the fermentation of cellulose. -Cellulose has the same empirical formula as starch. It is completely -insoluble, and is not affected by any of the digestive juices. The -greater part of the cellulose consumed by herbivora is, however, broken -up by bacteria into acetic and butyric acids, carbonic acid, and -marsh-gas. In Man also a small quantity is similarly destroyed.</p> - -<p>In the large intestine the bacteric fermentations are not unlike those -which occur in the stomach, with, in addition, the destruction of -proteins, or of products of proteid digestion. The greater the quantity -of undigested food which reaches the large intestine, the greater is -the development of bacteria. When the stomach is dilated, the ascending -colon, and especially its cæcum, is usually dilated also. Bacteric -fermentation in the large intestine, with resulting flatulence, is -evidence of imperfect digestion, due either to an excess of food or to -<span class="pagenum"><a name="Page_138" id="Page_138">[Pg 138]</a></span> -weakness of the alimentary organs, or, as is more commonly the case, -to the combination of these two factors. The relation of fermentation -to alimentation can be shown by counting the microbes in a specimen of -the contents of the large intestine. In a particular case it fell from -65,000 per milligramme upon a mixed diet to 2,000 per milligramme upon -a diet of milk.</p> - -<p>In the world at large bacteria perform many offices of the utmost -usefulness to other living things. They fix nitrogen in the soil, -sweeten polluted rivers, reduce animal and vegetable matter to a -condition in which it is available as plant-food. Their presence -within the alimentary canal is inevitable; but it is somewhat doubtful -whether, with the exception of the fermentation of cellulose, they do -the economy any service with which it could not dispense. As parasites -of the alimentary canal, some kinds are less desirable than others. -Recently a method of limiting their variety has been introduced -and advocated with much enthusiasm, as favourable to the hygiene -of the digestive tract. In countries in which the cows are driven, -in summer, to mountain pastures, the peasants of the plains live -during their absence largely upon milk brought down at intervals, and -allowed to turn sour. Sour milk, in Bulgaria, develops a bacterium of -extraordinary vigour. It can live in a medium containing as much as -10 per cent. of lactic acid, a concentration fatal to other forms of -<i>Bacterium acidi lactici</i>. It is easily cultivated, and when ingested -continues to multiply in the alimentary canal. So peculiarly lusty is -this bacterium that it makes life impossible for other germs. As it -dies out after two or three months, it seems unlikely that a man who -swallows the Bulgarian milk-germ runs a risk of inviting a repetition -of the tragedy which followed the acclimatization of the mongoose -in Jamaica. Its supremacy has been attributed to its capacity of -developing a concentration of lactic acid too high for the well-being -of other bacteria; but it is improbable that it has the opportunity of -doing this in the alimentary canal of a person living on a mixed diet. -The extinction of other bacteria (if they are extinguished) is more -likely to be due to an antagonism of a more subtle kind, at present -inexplicable, but not without parallel. The purifying influence of the -water of the Ganges has for ages been an article of faith. Pilgrims -from fever-stricken districts bathe in it, foul it, drink it, with the -<span class="pagenum"><a name="Page_139" id="Page_139">[Pg 139]</a></span> -corpses of their fellows floating down the stream. Recently it has been -shown that this belief is not without foundation. The water of the -Ganges at Benares contains bacteria which are as tigers among lesser -vermin. The germs of cholera and typhoid fever disappear from cultures -into which these overbearing microbes are introduced.</p> - -<p><b>Conditions Requisite for Normal Digestion.</b>—When M. Chevreul, -Professor of Chemistry at the Jardins des Plantes of Paris, attained -his hundredth year, an interviewer very naturally inquired of him, -“Have you always had a good digestion?” To this the still vigorous -Professor answered: “I really cannot say, for I have never noticed.” -So long as it is well used, the stomach is an unobtrusive organ. It is -tyrannical when it deems itself the victim of inconsiderate treatment. -A study of its physiology serves to show that it will work contentedly -only upon certain clearly defined terms, of which the following are -perhaps the most important: The stomach exacts due warning that its -services are wanted. The nerves of smell and taste must announce the -approach of food and guarantee its quality. “What may I eat?” asked -a large-framed, strenuous, eager, overworked barrister of a great -physician. “Eat, sir? You may eat whatever you like. But be quite -sure that you do like it.” Wise advice. The human race would not have -developed its strong preferences for certain kinds of food if all foods -were equally suitable to satisfy its needs. Taste is not a matter of -fashion. It is the expression of the experience of mankind. Fanciful -as civilization has made us, and easily as appetite is perverted, if -we are sure that we really like, and want, a food, we may trust that -our liking will guide us as safely as it guides a buffalo or a deer. -“Eat what you like.” Eating with liking carries with it the idea -of obtaining the maximum of satisfaction from the exercise of this -necessary function. Most things which are reckoned unwholesome are full -in flavour or rich in consistency. They satisfy the palate when spread -out very thin. It is poor economy to help oneself to caviare with a -table-spoon. In the second place, the stomach must be assured that the -teeth are doing their proper share of work. Among the many half-truths -which every year are exalted to the level of a revelation or a rule of -conduct is the doctrine of the “chewers”—persons who take no meals, -<span class="pagenum"><a name="Page_140" id="Page_140">[Pg 140]</a></span> -but industriously and almost continuously masticate nuts and biscuits. -Thirdly, the meal must not be so large that the stomach cannot deal -with it “at a sitting.” In from two to three hours the last of the food -should have passed through the pylorus, allowing the stomach to rest -before it is called into activity again. As proteins are practically -the only foods which are digested in the stomach, the work required of -this organ depends upon the quantity of proteins present amongst the -constituents of a meal. Meat is the food richest in proteins, although -bread, vegetables, milk, cheese also yield them. Some people can digest -three meat meals every day; but others, probably the majority, find -that it is unwise to take any considerable quantity of meat more than -once in twenty-four hours. It is only when the cells of the gastric -glands have accumulated a store of pepsinogen-granules that proteid -digestion is vigorously carried on. Fourthly, the food must be in a -form in which it does not irritate the stomach, provoking an outflow of -acid out of proportion to the pepsin which accompanies it. Experience -alone can teach the foods which are to be avoided on this account. -But speaking generally, it may be said that the stomach resents the -presence of substances which cannot be amalgamated into chyme. Its -task is the reduction of the mixture of foods which compose a meal to -the consistence of a smooth cream. Hot buttered toast or pie-crust are -made of wholesome constituents enough, but, fat being melted into the -starch, the fragments are impermeable to the gastric juice. They act -mechanically as irritants of the mucous membrane. Again, it may be said -that “pure” foods are apt to provoke acidity. Nothing could be more -wholesome than eggs or pounded meat or custard pudding; but taken by -themselves these articles of diet over-stimulate the mucous membrane. -They need to be diluted with starch-foods, or even with cellulose.</p> - -<p>And this calls attention to the dietetic value of vegetables. -Vegetables, which consist chiefly of innutritious cellulose, distribute -the digestible constituents of a meal and increase its bulk, greatly -favouring its progress through the alimentary canal. Especially in -herbivora is it important that the bulk and looseness of the food -should be well maintained. Rabbits thrive on sugar, starch, and -<span class="pagenum"><a name="Page_141" id="Page_141">[Pg 141]</a></span> -albumin, mixed with such an absolutely indigestible substance as -horn-shavings. If the inert substance be omitted, they die of -intestinal inflammation, although fed on the same mixture of pure -foods. Other rules which govern digestion might be mentioned; and it -is needless to point out that, when the mechanism is deranged, steps -adapted to the particular malady must be taken to bring it back to a -normal condition. There is, however, one precaution upon which, in a -certain number of cases, it is impossible to lay too much stress. The -digestion of proteins is seldom carried out satisfactorily when much -sugar, and especially much cane-sugar, has been eaten at the same meal. -Excessive lactic fermentation prevents the proper peptonization of -meat. The chemistry of digestion is not sufficiently well understood -to enable the physiologist to say what is amiss; but probably -by-products of peptic digestion are produced. To many people this is of -little consequence; but to those who exhibit a gouty tendency it is, -unfortunately, a most serious matter. Civilized races are particularly -subject to the uric acid diathesis. In the course of nitrogenous -metabolism uric acid is formed in place of fully oxidized and easily -soluble neutral urea. Although the chemical sequence has not been -discovered as yet, there is no question but that imperfect gastric -digestion means the formation of uric acid, with all its lugubrious -results: malaise, neck-ache, emotional depression. Birds and reptiles -form uric acid as the end-product of nitrogenous metabolism, not urea. -So also do city-fathers, butchers, and others whose diet consists too -largely of meat. Many nervous, ill-nourished men and women tend to -do the same, however abstemious their meals. It is useless to tell -such persons to reduce the amount of proteins in their diet. Their -attempts at increasing the starch, sugar, and fat at the expense of -nitrogenous foods lead to dyspepsia, which makes matters worse. They -often find, however, that if they are careful to restrict to the -narrowest limits the amount of carbohydrates (especially sugar) which -they take in conjunction with meat, fish, eggs, or other proteid foods, -the formation of uric acid ceases. Sugar, bread, fruit, and other -carbohydrates, may be taken in abundance, and with great advantage, at -breakfast and lunch, without proteid food, if dinner consists of broth, -fish, meat, cheese, vegetables, with a minimum of bread. -<span class="pagenum"><a name="Page_142" id="Page_142">[Pg 142]</a></span></p> - -<p><b>The History of the Foods after Absorption.</b>—All foods, with the -exception of inorganic salts and salts of various vegetable acids, -fall into three classes: (1) Proteins—substances of complex chemical -constitution, containing nitrogen; (2) carbohydrates—so called because -hydrogen and oxygen, in the proportions in which they enter into the -formation of water, are united with carbon; (3) fats. Proteins of -various kinds are consumed as food. The peptones produced from them by -digestion also vary. Yet very little is known as to the differences in -physiological value which distinguish the various kinds of protein when -absorbed into the fluids of the body (<a href="#Page_134"><i>cf.</i> p. 134</a>). -All carbohydrates after digestion and absorption appear as dextrose. The various -fats preserve their individuality until they are taken up by the tissues. -When fixed in the tissues, they assume, except under somewhat abnormal -conditions, the composition characteristic of the fat of the animal -which has eaten them. If a dog which has been severely starved is fed -upon mutton-fat, it puts on in the first instance fat which resembles -that of a sheep rather than the normal fat of a dog. As soon, however, -as it is well nourished (which would never occur unless some protein -and carbohydrate were added to the mutton-fat), its fat assumes the -usual form.</p> - -<p>For practical purposes we are obliged to speak of the three classes -of food—proteid, carbohydrate, and fatty—as if there were but -one member in each class. And we have abundant evidence that such a simple -classification is fully justified. The body has so large a power of -altering chemically the nature of the food which it absorbs that it -makes little difference in the further history of the food whether the -protein supplied to it be an albumin or a globulin; the fat, stearin, -palmitin, or olein; the carbohydrate, starch or sugar.</p> - -<p>In earlier days it was customary to regard the body as the receiver of -a variety of foods which it could break down into simpler substances -by oxidation, but could not reconstruct. Plants were regarded as the -manufacturers of organic compounds, animals as the destroyers of the -complex substances made by plants. The union of molecules, synthesis, -was looked upon as the function of the vegetable kingdom. Animals built -into their tissues the products elaborated by plants; some of these -<span class="pagenum"><a name="Page_143" id="Page_143">[Pg 143]</a></span> -products they shook to pieces for the purpose of setting their energy -free; others slowly disintegrated as the result of tissue “wear and -tear.” Gradually it was realized that many chemical changes occur -in the body which cannot be viewed as merely exhibitions of its -analytical capacity. The tissues were recognized as laboratories -in which reactions occur which consist in something more than the -splitting of complex into simpler molecules. The instances earliest -understood were connected with the history of carbohydrates and fats. -In the disease diabetes an enormous quantity of sugar is excreted, -amounting in extreme cases to between 1 and 2 pounds <i>per diem</i>. When -carbohydrates are present in the food, the amount of sugar excreted in -diabetes is greater than it is when they are withheld; on an almost -exclusively proteid diet the amount of sugar excreted far exceeds the -amount of carbohydrates in the food. Another illustration of the power -of making sugar possessed by the animal economy is afforded by a dog -fed upon lean meat, and nothing else. Sugar is found in its blood, and -a store of carbohydrate (glycogen) in its liver. The formation of fat -is an instance of constructive metabolism. There is abundant evidence -that the quantity of fat produced may greatly exceed the quantity -contained in the food. Animals are fattened for the market on a diet -which contains less fat than that which accumulates in their bodies. -When nursing her young, an animal may secrete in her milk much more fat -than she obtains as such in food. It was a great mistake to suppose -that the body is dependent upon its tradesmen for fat and sugar. It -can make either of these substances out of a mixed diet in which it -is relatively deficient. It must, however, be a mixed diet. An animal -cannot live exclusively on fat or exclusively on carbohydrate. It is -impossible, therefore, for us to determine whether, if given the one -alone, it can turn it into the other. Chemists were very unwilling to -credit the body with the power of performing even the simpler of these -transformations—the conversion of carbohydrate into fat. Proteins -are essential constituents of a fattening diet. Their immensely -complex molecule has always afforded a tempting field for arithmetical -ingenuity. It is easy to remove from it the atoms needed for the -composition of fat, and yet to leave such groups of atoms as might -<span class="pagenum"><a name="Page_144" id="Page_144">[Pg 144]</a></span> -reasonably be supposed to constitute its “nitrogenous moiety.” The -hypothesis that the metabolic capacity of the body is limited to -analytical processes justified the supposition that, when more fat -is laid on than the food contains, the balance comes from proteid -substances, which split into nitrogenous and fatty moieties. It has -been shown, however, that an animal during fattening may put on -more fat than is contained as such in the food, or obtainable from -its diet, even though all the atoms of carbon and hydrogen in its -proteid food were devoted to its formation. The balance must come -from carbohydrates. Perhaps a still more striking illustration of -constructive capacity is the power of making glycerin. If a dog receive -fatty acids in its diet, it accumulates normal fats. The glycerin -which, united with fatty acids, constitutes the fat, was not contained -in its food. Starch and sugar are sources of fat. As yet there is no -evidence that fat can be converted into sugar.</p> - -<p>The chemistry of the nitrogen-containing compounds appears to present -more difficult problems. Plants build up proteins. Is the animal’s -relation to these substances limited to their disintegration? Do -proteins inevitably descend from step to step until they reach -urea? There are reasons for thinking that, even when dealing with -nitrogenous substances, the metabolic power of the body is not -exclusively analytical. The liver can make urea from ammonia-salts, -such as lactate, or even carbonate, of ammonia—substances more stable, -and therefore in the chemical sense simpler, than urea. This is an -indication, though a faint one, that the body has a constructive -capacity, a power of producing more complex from simpler substances, -even in the case of nitrogenous compounds. Beef-tea, mutton broth, -meat-extracts have long been regarded as foods of value when the -power of assimilation is low. Chemists point out that the nitrogenous -substances which these decoctions contain are so near the bottom of the -ladder that the energy set free by their further oxidation to urea is -scarcely worth consideration. They admit that their ready availability -renders them useful as restoratives, but they deny them the status of -foods, on the assumption that their further progress must be downward. -As was stated when the conversion of peptones into leucin and tyrosin -was described, evidence is beginning to accumulate which shows that -<span class="pagenum"><a name="Page_145" id="Page_145">[Pg 145]</a></span> -within certain limits, at present impossible to define, the system can -reconstruct its proteins from amides and other simple products of their -degradation.</p> - -<p>The animal economy receives, and after due digestive preparation -absorbs, three classes of food—nitrogenous, fatty, and carbohydrate. -If either of the two latter kinds be deficient in the diet, the body -can to a certain extent produce it from the other two. What is the -special value of each kind of food? What use is made of it? Before -attempting to answer these questions, we must endeavour to trace the -further history of the foods after they have traversed the wall of the -alimentary canal.</p> - -<p>After leaving the stomach and intestines, the foods follow two -different routes. Proteins and carbohydrates are carried by the portal -vein to the liver. Fats are carried by the thoracic duct to the general -circulation. An excess of fat is found in the blood in all parts of the -body after a meal rich in fat. The eventual destination and fate of -fatty foods is unknown. Under certain circumstances they are added to -the fatty deposits in connective tissue; but if no additional fat is -being laid down, they go to other tissues, in which they are oxidized -into carbonic acid and water. When the amount absorbed is excessive, -a certain quantity of fat may be stored in the liver. In the cells of -this organ it is housed for a time, in order that it may be distributed -to the tissues after they have used up the supplies which first reach -them through the general blood-stream.</p> - -<p>Proteins are completely lost to sight after they are absorbed into the -blood. They take part, of course, in the formation of growing tissue, -blood-corpuscles, skin, hair, nails. It is also common to speak of them -as making good the wear and tear of active tissues, although it is -very doubtful whether we can legitimately speak of the wear and tear -of tissues. The protoplasm which does the work of the body is not worn -out in the same way as the materials of which a machine is made. There -is no friction to rub it down. Proteins, like other foods, are used up -as sources of muscular energy and heat. Eventually they are reduced to -urea, carbonic acid, and water. Chemists naturally seek for substances -<span class="pagenum"><a name="Page_146" id="Page_146">[Pg 146]</a></span> -intermediate in constitution between proteins and urea. They assume -that the degradation of proteins will occur in regular steps; complex, -partially oxidized, nitrogenous compounds being formed first—in -the muscles, for example—to be further oxidized in the glands. The -existence in all organs of nitrogenous “extractives,” which can be -separated out when the organ is subjected to chemical analysis, seems -to justify the search for stages; but hitherto this search has been -singularly unsuccessful. Urea is the final product. It is not found -in muscle, nor, indeed, in any tissue other than the liver, which, as -already said, has the power of making it, even from salts of ammonia. -It is therefore clear that if proteins are destroyed in muscle and -other tissues, and if all urea is made by the liver, the antecedents -of urea must be carried from the muscles to this organ. The substance -which is most characteristic of muscular metabolism is lactic acid. -It is not impossible that all the nitrogenous portion of the complex -proteid molecule is reduced to ammonia (NH₃), which may be regarded as -the simplest of all nitrogenous compounds, and that this, combined with -lactic acid (C₃H₆O₃) as lactate of ammonia (NH₄C₃H₅O₃), is carried by -the general circulation to the liver, where it is converted into urea. -A considerable amount of lactate of ammonia may be injected into a vein -without any of it overflowing through the kidneys. It is all reduced -to the condition of urea, water, and carbonic acid. If the liver is -so diseased as to be functionless, or if by operative measures it is -thrown out of action, salts of ammonia are excreted by the kidneys -instead of urea. In birds and reptiles uric acid takes the place of -urea. Their livers yield uric acid on analysis. If lactate of ammonia -be injected into their blood, it is converted into uric acid, so long -as the liver is intact.</p> - -<p>We know nothing of the forms assumed by the proteins absorbed into the -blood, of the organs in which they are stored, or of the higher terms -of the series of substances through which they pass before they are -finally excreted as urea, water, and carbonic acid. No nitrogenous -compounds are found in lymph or blood which can be pointed out with -confidence as the products of tissue wear and tear. When considering -the sources of muscular energy, we shall have something more to say -regarding the part that proteins play in the economy. -<span class="pagenum"><a name="Page_147" id="Page_147">[Pg 147]</a></span></p> - -<p>If there is great difficulty in following fats and proteins after their -absorption, it is quite otherwise when we come to deal with sugar. -Carbohydrates are the great sources of energy. Muscular work may be -generated by the oxidation of either of the three classes of foods, -but undoubtedly the carbohydrate glycogen is its most constant source. -Provision is therefore made for the storing of glycogen in the liver, -and the distribution to the muscles of a regular supply. After a meal -the portal blood, on its way from the intestines to the liver, contains -a higher percentage of sugar than the blood in the hepatic vein or -in any other vessel. If sections of liver be examined after feeding, -and compared with those obtained after a period of starvation, it is -found that the cells of the well fed liver contain glancing masses -of a substance which takes a port-wine colour with iodine. This is -glycogen, or animal starch. It has the same empirical formula as starch -<b>(C₆H₁₀O₅)ₙ</b>. In the dry state it is a greyish powder, which, unlike -starch, forms an opalescent solution in cold water. Like starch, it is -non-diffusible. In the animal kingdom it stands to sugar in the same -relation as starch to sugar in plants. If a sheep be killed while it is -feeding in the paddock, and its liver removed and weighed, it will be -found that it is from one-third to one-half heavier than the liver of -a sheep of the same weight obtained from a butcher; for butchers have -the stupid practice of starving animals before they kill them. It was -long ago discovered that it is unnecessary to feed an animal for a day -or two before it is killed, and this option has been elevated into a -prohibition. A tradition has grown up that it is undesirable to give -food for some time before killing. Not only will the liver of a sheep -killed during active digestion be found to be heavier than that of a -starved sheep, but it will also prove more succulent; for it is loaded -with sugar (into which glycogen is rapidly converted after death), as -well as with proteins and fats, which are withdrawn from it when the -animal fasts. It appears that the liver cannot secure the whole of the -sugar which is absorbed after a full meal. Some of it passes into the -general circulation, and is stored in the muscles; but the liver always -maintains a considerable reserve. Even after prolonged deprivation -of food, it holds on to a certain quantity, especially in carnivora. -<span class="pagenum"><a name="Page_148" id="Page_148">[Pg 148]</a></span> -Glycogen is found in the liver of a dog after a long period of -starvation. The muscles lose during activity the glycogen which they -contain when at rest.</p> - -<p>It has already been pointed out that the body is not entirely dependent -upon external agencies for the production of the sugar which it needs. -When the supply is inadequate, it manufactures glycogen for itself -out of the other constituents of the diet. It can, indeed, make it at -the expense of its own proteins. If a dog which has been caused to do -muscular work, without a sufficiency of carbohydrate food, until (as -judged from a control experiment) all glycogen has disappeared from its -liver, be placed under the influence of a narcotic drug, which arrests -the activity of its muscles, glycogen reappears.</p> - -<p><b>Dietetics.</b>—Even those who are most ignorant of the science -of physiology flatter themselves that they have one piece of information: -“The whole of the body is renewed once in every seven years.” I cannot -trace the origin of this sapient apothegm, which for generations -has passed current. If seven weeks or seventy years were the period -allowed for the renewal of the tissues, the statement would be equally -near the truth. Judging from the rate at which they are destroyed, -it is unlikely that blood-corpuscles live for more than five or six -weeks. Hairs are shed about two years after they first appear above -the surface. On attaining this age a hair drops off and a new one -takes its place. The superficial cells of the skin are shed in great -numbers every day, and their place taken by younger cells which come -up from the deeper layers. The cells of many glands would seem to -have a comparatively short term of life. On the other hand, some -tissue-elements are far more permanent. By the time a child is a year -old all its nerve-cells are in position. They last as long as the -individual lives. When the statement with regard to the renewal of the -tissues is understood as meaning, not that the cells are destroyed and -replaced by new ones, but that within a period of seven years all the -molecules which enter into their protoplasm are extruded from the body -and replaced by molecules received as food, the assertion verges on the -transcendental. It is unlikely that we shall ever obtain data against -which it can be checked.</p> - -<p>The essential part of every living cell is its sponge-work of -protoplasm. “Bioplasm” is perhaps a better term to use when we are -<span class="pagenum"><a name="Page_149" id="Page_149">[Pg 149]</a></span> -speaking of protoplasm as a structure, since it does not suggest any -prejudice with regard to its chemical constitution. Within the meshes -of the bioplasm are nutrient materials, as yet unused, and worked up -products in various stages. It has always been taken for granted that -when treating of nutrition, we have to consider the repair of the -bioplasm, as well as the provision of raw material which it can convert -into the specific products of the cell. Suppose that the cell belongs -to the class of supporting tissues; let it be a cell of cartilage, -for example. The bioplasm manufactures a collagenous substance which -remains in and around its meshwork. If it be an epidermal cell, it -forms horny substance. If a secreting cell, it accumulates secernable -products. If a muscle-cell, it develops a large quantity of material, -which by a change in form produces movement. In this last case we -suppose that the energy set free as muscular force is due to oxidation. -More stable bodies take the place of a less stable substance. After -contraction the relatively complex contractile material is renewed from -the foods stored in the muscle-cell; or if it be not, in the ordinary -sense of the word, destroyed, if it has merely parted with certain -oxidizable constituents, it obtains a fresh supply of such constituents -from the foods which the muscle-cell contains. Even in the case of -cartilage or epidermis, we imagine that, since the matrix is “alive,” -it is always undergoing molecular change, and consequently always -requiring food. The fact that every tissue, however inert, dies when, -owing to the blocking of the bloodvessels which irrigate the part, its -supply of nutriment is cut off, justifies this belief that all living -tissue is undergoing change.</p> - -<p>When we make up a balance-sheet of the body as a whole, placing to -the debit side the food which it receives, and to its credit side the -work done in external movement and in the production of heat, we again -find reason for believing that every part of every cell is constantly -undergoing change.</p> - -<p>The balance-sheet of the body can be drawn out in either of two ways. -We can estimate the quantities of nitrogen, carbon, hydrogen, and -oxygen supplied to it in the several foods, and compare them with the -amounts of each of these four elements given off in urea, carbonic -acid, and water, making, of course, a note of the body’s balance in -<span class="pagenum"><a name="Page_150" id="Page_150">[Pg 150]</a></span> -hand at the beginning and at the end of the period of observation. Or, -we may estimate the amount of potential energy contained in the food, -and ascertain the use to which this energy is put in doing external -work, in maintaining the temperature of the body, and in warming the -breath and other excreta.</p> - -<p>If we are making up the balance-sheet of a fully-grown man, we may -take for granted that he is not making fresh tissue. During the -period throughout which he is under observation, care is taken to -avoid altering the conditions of his life in such a manner as to -lead him to develop additional muscle. If he gains in weight while -under observation, he is putting on fat. If he loses in weight, he is -sacrificing fat.</p> - -<p>The whole of the nitrogen taken in leaves the body in urea, unless, -as we have said, growth of tissue is taking place. The body has not -the same temptation to store nitrogen as it has to store carbon. -Consequently, it is very sensitive to any deficiency of nitrogen in -the diet. If food does not contain as much protein as is needed, -the deficit is made up at the expense of the tissues. It does not -necessarily follow that under these circumstances a man loses in -weight. He may be putting on fat, although losing in strength owing to -waste of muscle. For observations upon the income and expenditure of -the body to be of any value, a condition of “nitrogenous equilibrium” -must be established. The nitrogen taken in must equal in amount the -nitrogen given out.</p> - -<p>Very exact determinations of income and expenditure may be made by -placing an animal, or even a man, in a box through which air is drawn. -A record is made of the volume of air drawn through the box. The -percentages of water vapour and carbonic acid which the air contains -are estimated before it enters and after it leaves. The solid food -consumed and the urea excreted are also measured.</p> - -<p>If it is desired to measure the amount of heat given off, an animal may -be placed in a calorimeter.</p> - -<p>Even when most passive, the subject under examination, whether an -animal or a man, is expending energy in keeping the body warm, in -movements of respiration, and in shifting position. If it is desired -to ascertain the relation of oxidation to external work, it is easy to -<span class="pagenum"><a name="Page_151" id="Page_151">[Pg 151]</a></span> -devise a form of resistance, such as the turning of a wheel, or the -lifting of a weight which can be measured.</p> - -<p>In testing diets, it suffices to make sure that nitrogenous equilibrium -is maintained, and then to estimate the gain or loss in weight and the -output of energy in external work.</p> - -<p><b>The Relative Value of Foods.</b>—Dried proteins contain about -15 per cent. nitrogen, 54 per cent. carbon, 7 per cent. hydrogen, 22 per -cent. oxygen, a little sulphur, and frequently some phosphorus. A large -proportion of their carbon and hydrogen is available for combustion. -Fats contain 75 per cent. of carbon, and a considerable quantity of -hydrogen available for combustion; carbohydrates, 40 per cent. of -carbon, with hydrogen and oxygen in the proportions in which they -occur in water. If 1 gramme of protein is oxidized to the condition -of urea, carbonic acid, and water, sufficient heat is liberated to -raise the temperature of 4,100 grammes of water 1 degree centigrade. -Its calorific value is therefore expressed as 4,100 calories, the unit -of measurement—a calorie—being the amount of heat needed to raise -1 gramme of water 1°. The calorific value of 1 gramme of fat is 9,300 -calories; of 1 gramme of starch, 4,100 calories. Thus, the energy -potential in protein and in starch is the same; that in fat more than -twice as great as that in either of the other foods.</p> - -<p><b>A Normal Diet.</b>—Nitrogenous equilibrium and body-weight can -be maintained and work done on diets which vary widely in percentage -composition. This is a question which we shall consider at greater -length later on. In the meantime, for the sake of illustration, it is -necessary to formulate a diet which is fairly representative of the -selection of foods made by a man of average weight—say 70 kilogrammes -(145 pounds)—who desires to do a moderate day’s work in comfort. It -has been found to amount to about 100 grammes of protein, 100 grammes -of fat, 240 grammes of carbohydrate, all measured dry and as pure -foods. If the several elements of such a diet be multiplied by the -figures which represent their calorific value, it will be found that -the man is supplied with 2,324,000 calories. The illustration that we -have chosen is the diet of a professional man who is not engaged in -hard physical work. The pure foods would be found to the amounts stated -in 17 ounces lean meat, 4 ounces butter, and 17 ounces bread. The day’s -<span class="pagenum"><a name="Page_152" id="Page_152">[Pg 152]</a></span> -diet would, of course, be much more varied than this, but it is simpler -to express it in these terms.</p> - -<p>Such a diet would hardly answer the requirements of a man doing hard -muscular work. Experience shows that he would expect to receive a more -liberal supply of energy, and that to obtain it he would increase -slightly his allowance of proteins, and very considerably increase -the quantity of carbohydrates that he consumed. The diet of European -workmen is remarkably constant in the relative amounts of its several -constituents, no matter what their nationality or the exact form of -their work may be: Proteins, about 135 grammes; fats, 80 grammes; -carbohydrates, 500 to 700 grammes—giving a supply of energy equal to -3,500 to 4,000 kilo-calories.</p> - -<p>Speaking generally, carbohydrates are the source of muscular force, -and fats of heat. In warm climates men work on carbohydrates. The -’rickshaw men of Japan are said to eat only rice on working days, and -to reserve fish for days of leisure. The Japanese, as is well known, -consume extremely little fat. The Esquimaux and other inhabitants of -high latitudes eat immense quantities of fat. Proteins constitute the -luxurious element of a diet. Not only are they more attractive to -most palates, and therefore preferred by persons whose dietary is not -severely regulated by price, but the body prefers them. It works with -greater alacrity when supplied with more protein than, in a strictly -physiological sense, it needs.</p> - -<p>The supply of food must exceed the apparent demand. The most efficient -of motors cannot convert more than 15 per cent. of the energy potential -in its fuel into work. If a man endeavours to obtain a better result -than this from his muscular system, if he tries to make his machine do -more than 15 units of work for every 100 units of energy with which he -supplies it, he does it at the expense of his own tissues. First he -loses in weight, owing to the consumption of fat; then the excess of -nitrogen discharged over nitrogen consumed shows that he is burning -up the proteins of his own tissues. It is needless to add that the -weakness which results puts a stop to excessive work. Muscles, as we -shall find when we consider the relation of their output of work to the -energy supplied to them, can produce a much better result than the best -of engines; but we are speaking of the body as a whole, which wastes -<span class="pagenum"><a name="Page_153" id="Page_153">[Pg 153]</a></span> -energy in the movements of respiration, masticating food, shifting -position, maintaining the body temperature, etc.</p> - -<p>Health may be maintained and work done on diets which depart widely -from the one which we have selected as a standard. Darwin found the -Gauchos of South America living exclusively on meat. Nansen and -Johannsen, when seeking the North Pole, lived for months on meat and -blubber. Millions of the inhabitants of India abstain from meat and -meat-fat, their diet consisting of rice, buttermilk, and a little -fruit. In the case of all persons with whom the price of food is an -important consideration, carbohydrates are preferred to proteins and -fats. Oatmeal is very much cheaper per unit of energy than meat. A -man may be a meat-eater or a vegetarian, although he is probably -unwise in overlooking the obvious teaching of his teeth and digestive -organs, which are those of an omnivorous animal. His prehistoric human -ancestors lived chiefly on the harvest of their spears and tomahawks. -If we insist upon looking back still farther, we discern a cleavage of -the race into the arboreal fruit-eaters, which still retain pre-human -characters, and the more enterprising and energetic troglodyte hunters -from whom the human race was evolved.</p> - -<p>A man may vary his diet within wide limits. Innumerable considerations -lead certain individuals to desire to depart from the diet which we -have termed “normal”—<i>i.e.</i>, typical of inhabitants of the temperate -zone. One man rebels against the expense of living; he would fain -reduce the quantity and the cost of food. Another, having to traverse -regions in which food is scarce, wishes to ascertain the lightest, and -therefore the most portable, combination of its essential elements. -A third—and he belongs to a much larger class—tormented with -indigestion or harassed by gout, asks, “Why must I consume things which -give the stomach trouble, or produce disagreeable and incapacitating -after-effects?” Many circumstances prompt to experiments in diet. -Much latitude is undoubtedly allowed. But there are limits within -which alone health can be maintained and work done. It is of great -interest to ascertain exactly how wide these limits are; and especially -important is it to find out the lower limit, the minimum of food, and -the minimum of each particular kind of food, which will enable the -<span class="pagenum"><a name="Page_154" id="Page_154">[Pg 154]</a></span> -human machine to work. The problems involved are somewhat complicated. -If it were possible to live on a single food, it would be as easy to -ascertain the irreducible minimum as it is to find out with how much -coal or with how much petrol an engine can be made to turn a wheel. But -to support the body several different kinds of food are indispensable. -It is therefore necessary to determine, not only the minimum quantity -of the combined foods, but also the minimum amount of each kind of -food, and the effect upon the total of variations in the relative -amount of each of its several factors. The problem is complicated, but -certain limits are impassably defined. In the first place, with regard -to the total amount, the work which the body does cannot under any -circumstances be reduced below a certain level. The food consumed must -provide a supply of energy equal, at the least, to the performance of -the minimum of work. The body must receive each day food of due caloric -value. Then with regard to the amount of each several constituent. -Many considerations lead us to wish to increase one of them or to -diminish another. Some food is cheap, and other food is dear. Economic -reasons are in favour of the cheaper food. Even ethical considerations -are not without weight. We have, perhaps, a prejudice against -sacrificing life to supply the pot. We have doubts as to whether our -system can properly digest, metabolize, and excrete meat. We need an -unambiguous answer to the question, To what extent can nitrogen-foods -be replaced by carbon-foods, and <i>vice versa</i>? A cell, as already -said, consists of a framework of bioplasm bathed in cell-juice which -contains nutrient substances and manufactured products. The bioplasm -is alive; the proteins, carbohydrates, and fats of the cell-juice are -the materials with which it is nourished, and upon which it works. -Some physiologists incline to the view that non-living substances must -enter into the bioplasm before they undergo metabolism. They consider -that the molecules of the non-living substance must at the time when -they undergo a chemical change be physically and chemically a part of -the living substance. Others take the opposite view: that the living -substance does not undergo change, but brings about changes in the -non-living substance which is in contact with it, enclosed within its -meshes. This is a problem which is not likely to be solved, nor is its -<span class="pagenum"><a name="Page_155" id="Page_155">[Pg 155]</a></span> -solution of great importance in relation to the question which we -are discussing. Whichever of the two views be justified, we have to -distinguish between the bioplasm of the cell—the machine—and its -raw materials and manufactured products. The question to which we want an -answer is the following: Must the bioplasm undergo change? There seems -to be no reason in the nature of things why it should. It is not, as we -have already pointed out, subject to wear and tear. A perfect machine -would in the absence of friction, which rubs down its steel and brass, -continue to turn out its products so long as it was supplied with raw -materials and the energy needed to manufacture them. We could imagine -the bioplasm as indestructible, receiving energy from a portion of the -foods, and expending this energy in the production of chemical change -in the remainder. We could imagine that when once the tissues had -attained their full growth they would require no more protein for their -own nutrition; they would be occupied in producing heat and motion from -the non-nitrogenous foods. But observation shows clearly that this is -not the case. The force which energizes the bioplasm, enabling it to -evoke metabolism in non-living substance, is obtained at the cost of -its own destruction. The bioplasm wastes unless constantly supplied -with proteid food.</p> - -<p>Under ordinary circumstances the amount of urea excreted varies -directly as the quantity of nitrogen contained in the food. Since -urea contains 45 per cent. of nitrogen, and protein 15 per cent., -every gramme of urea excreted represents 3 grammes of dry protein -consumed; or, in terms of nitrogen, every gramme of nitrogen excreted -represents 6·25 grammes of protein consumed. If all food is withheld, -the excretion of nitrogen falls, but it never reaches zero. Many -observations have been made on fasting men. On the second day of -fasting the nitrogen excreted falls to about 13 grammes, representing -80 grammes of protein used up. It is generally thought that by the -second day all “floating proteins” are exhausted, and that therefore -nitrogenous metabolism is reduced, as it were, to a business basis. So -long as the supply of food is abundant, the body has a luxurious habit -of using proteins in preference to non-nitrogenous food. But after a -<span class="pagenum"><a name="Page_156" id="Page_156">[Pg 156]</a></span> -day’s starvation there is no longer any fancy metabolism, no -consumption of proteins as fuel when cheaper fats and sugar would -answer equally well. In the case of Succi, who fasted for thirty days, -the nitrogen excreted fell to 6·7 grammes on the tenth day, to 4·3 -grammes on the twentieth, and to 3·2 grammes on the last day. Clearly, -we have to make a distinction, when all food is cut off, between -the oxidation of the protein which, failing all other material, is -withdrawn from the tissues for the purpose of supplying the force -absolutely necessary to maintain respiration and such other movements -as are inevitable, and to keep up the temperature of the body—force -which under other circumstances might be supplied by non-nitrogenous -food—and the oxidation to which bioplasm is inevitably subject, -so long as it is alive. The oxidation of bioplasm under ordinary -circumstances of course supplies force; but it does not follow that -this is sufficient to maintain the respiratory movements and the -contraction of the heart. When a herbivorous animal is starved, it not -infrequently excretes more urea at the commencement of the starvation -period than it was excreting when well fed. Its activities did not come -to a standstill when carbohydrate food was cut off. For a time they -were maintained at the expense of its own tissues. On the other hand, -the results obtained from the observation of the man who went without -food for thirty days show that Nature is able to economize force by -reducing the metabolism of living substance below the normal. It might -be supposed that the irreducible metabolism could be ascertained by -giving a nitrogen-starved animal non-nitrogenous food, but it is found -that this scarcely affects the tissue-waste. Becoming more active, -the tissues, while saved from the necessity of supplying fuel for the -production of heat and motion, suffer more waste. Again, it might be -expected that if to an animal which had been starved for a few days, -until its urea had fallen to the starvation limit, exactly sufficient -protein were given to supply this amount, the tissues would be saved. -It is found, on the contrary, that nearly twice as much urea is -excreted as before. If the quantity of protein be steadily increased, -equilibrium is at last established, but not until the amount of -nitrogen in the protein given is two and a half times as great as the -amount excreted during the starvation period. Additional food at once -<span class="pagenum"><a name="Page_157" id="Page_157">[Pg 157]</a></span> -gives rise to additional waste. The tissues which during the period of -scarcity had reduced their oxidation to a minimum become more active at -the first hint of returning plenty.</p> - -<p>This last experiment illustrates a general law. An increase of -proteid food within certain limits increases the metabolic activity -of the tissues—provokes them to extravagance. It is possible, by -adding protein to a mixed diet which sufficed for the maintenance of -body-weight and nitrogenous equilibrium, to bring about a nitrogen -deficit and to reduce the body-weight. Or, if the body is gaining in -weight, owing to the accumulation of fat, the substitution of protein -for carbohydrate (weight for weight, since their caloric values are -the same) will lead to its reduction. It is difficult to avoid the -use of fanciful language in accounting for these results. The animal -economy is like an over-careful housekeeper, who, when meat is scarce, -doles out porridge also with a thrifty hand. When meat is plentiful -she is prodigal with every article of diet. Protein is the most costly -of foods. Any indication that it is scarce leads to a shutting-down -of activity. On the other hand, no other food is so readily absorbed -(unless the digestive organs be protein-sick); none is so quickly -incorporated in the bioplasm; none is so easy to decompose. When fed -with protein the machinery hums. The insatiable appetite for beef and -eggs which overtakes a man of sedentary habits after a long morning in -a boat or on a bicycle does not indicate that his muscular tissue is -suffering from wear and tear. It does not prove that he is setting free -energy by oxidizing proteid food. It shows that he is asking certain -tissues which are accustomed to a quiet life to exhibit prodigious -energy. They will not shake off their customary sloth unless he -stimulates them with sumptuous fare. At the end of a week he finds -that proteins are not the best fuel for steady work. If he consumes -sufficient to supply all the energy needed by his muscles, he is -hampered by a quantity of nitrogenous residues which have to be reduced -to urea and eliminated by the kidneys. He goes back approximately to -his old regimen, so far as proteins are concerned, and consumes more -carbohydrates for the supply of the force which his increased muscular -activity demands. -<span class="pagenum"><a name="Page_158" id="Page_158">[Pg 158]</a></span></p> - -<p>It is possible to live on meat alone, but the quantity required is very -great, involving the digestive organs, the liver, and the kidneys in an -excessive amount of work. On the other hand, it is possible to reduce -the consumption of proteins to a minimum by substituting for them fats -and carbohydrates. But, again, after the proper balance is disturbed, -the substitution ceases to be a simple problem in arithmetic. The -carbon-food has to be increased out of all proportion to the protein -which it replaces. If a dog which is being fed on a diet natural -to it—chiefly meat—is in a condition of nitrogenous equilibrium, -carbohydrate may be substituted for some of the meat. But from the very -beginning it is found that, if nitrogenous equilibrium is not to be -disturbed (if the dog is not to be induced to consume its own tissues), -a weight of carbohydrate must be given considerably greater than the -weight of the protein withdrawn. The disproportion increases as the -experiment proceeds, until perhaps 12 to 15 grammes of carbohydrate -have to be substituted for every gramme of protein. The proteid food -has now come down to 1·5 gramme per kilogramme of the animal’s weight. -Owing to the increase of carbohydrate, the caloric value of the total -food, nitrogenous and non-nitrogenous, is several times as great as -the animal requires. The surplus is oxidized without any equivalent in -work. At about this point the experiment is brought to an end, owing to -the failure of the digestive organs to deal with so large a mass of food.</p> - -<p>The value of gelatin as an article of diet is of interest in this -connection. Gelatin is not, strictly speaking, a protein, and it cannot -be built up into the tissues. It does not prevent, nor even delay, -starvation. Yet up to a certain point it can be used as a substitute -for proteid food. In the observation just referred to, protein might -be withdrawn at any stage, without disturbing nitrogenous equilibrium, -by substituting about 2 grammes of gelatin for every gramme of protein -withdrawn. It spares protein, although it does not take its place. -It is said that the minimum of protein necessary for the maintenance -of nitrogenous equilibrium may be reduced to about one-half by the -substitution of gelatin. This has been interpreted as indicating that -when we have reduced the oxidation of nitrogenous substance to its -smallest amount the nitrogen comes from two sources in about equal -<span class="pagenum"><a name="Page_159" id="Page_159">[Pg 159]</a></span> -proportions—(<i>a</i>) the bioplasm; (<i>b</i>) the food-proteins in -contact with it. It is inferred that gelatin, although it cannot be built -up into bioplasm, may take the place of proteins present in the -cell-juice. It appears to be impossible to starve the cell until it -consists of a bioplasm framework bathed in nitrogen-free cell-juice. -As the non-living proteins of cell-juice are removed, they are, if no -nitrogenous food be given, renewed by the breaking down of bioplasm. -When gelatin is absorbed, it takes its place in the cell-juice, and -the breaking down of bioplasm is no longer necessary. When digestion -is impaired, or vitality lowered, decoctions of meat which contain -extractives of low calorific value, useless, without synthesis -(<a href="#Page_144"><i>cf.</i> p. 144</a>), for the purposes of tissue-repair, -may to a certain extent save tissue-waste. In the same way, gelatin, which is -very rapidly digested in the stomach, may cover the consumption of proteins, -although it cannot take their place.</p> - -<p>To sum up: The requisite daily income of energy must come from both -nitrogenous and non-nitrogenous food. It is impossible to reduce the -nitrogenous factor below a certain minimum. From this minimum upwards, -until a certain level is reached, every additional unit of nitrogenous -food enables the system to dispense with more than its equivalent of -non-nitrogenous food. When the proper balance of foods is attained, -there is no waste either of labour involved in digestion, or of labour -involved in metabolism and excretion.</p> - -<p><b>The Liver.</b>—The liver weighs from 3 to 3½ pounds. It lies -beneath the diaphragm, more on the right side than on the left. Its -posterior border, which rests against the last three ribs (separated -from them by the diaphragm), is about 3 inches thick. Its anterior -border is thin, and keeps close along the line of the ribs. If the -organ is neither unduly enlarged nor squeezed out of its place owing -to the use of a tight corset, it does not project below the ribs, save -where it crosses the space between the rib-cartilages below the end of -the breast-bone. -<span class="pagenum"><a name="Page_160" id="Page_160">[Pg 160]</a></span></p> - -<div class="figcenter"> - <a id="I_160" name="I_160"> </a> - <img src="images/i_p160a.jpg" alt="" width="500" height="443" /> - <img src="images/i_p160b.jpg" alt="" width="500" height="173" /> - <p class="center"><span class="smcap">Fig. 7.</span>—<span class="smcap">Diagram of - a Lobule of the Liver<br /> divided vertically through its Axis.</span></p> - <div class="blockquot2"> - <p class="neg-indent space-below2">In its centre is a space, the intralobular vein, through which the - blood falls into a branch of the hepatic vein, on its way to the heart. - An interlobular branch of the portal vein, which brings the blood - from the digestive organs, pours it by many smaller vessels over the - surface of the lobule. It filters into the lobule through innumerable - pseudo-capillary vessels, or spaces, between the radiating columns of - liver-cells. Arterial blood is brought to the lobule by a twig of the - hepatic artery. Bile is drained away from it by an affluent of the - hepatic duct. In the lower part of the diagram seven liver-cells are - shown, forming a divided column, magnified about 300 diameters. The - cells are loaded with glycogen, and contain minute globules of fat. Red - blood-corpuscles and two leucocytes are seen between the columns of - liver-cells. One of the leucocytes has ingested two blood-corpuscles.</p> -</div></div> - -<p>The liver is supplied with blood by the hepatic artery. This vessel is -small for so large an organ. Although responsible for the nutrition -of the liver, it does not bring it the materials which are stored in -its cells. A much larger supply of blood is derived from the portal -vein, which breaks into capillaries, or, to speak more accurately, -into sinuses, or pseudo-capillaries, in the liver. The blood, whether -conveyed to the organ by the hepatic artery or by the portal vein, is -<span class="pagenum"><a name="Page_161" id="Page_161">[Pg 161]</a></span> -drained away by the hepatic veins. The plan of structure of the liver -is best understood when viewed with reference to the hepatic veins. -These, if traced backwards, are found to break up into fairly straight -vessels, each of which has a large number of lateral branches. Each -of the lateral branches is in the centre of a mass of cells, which -are packed round it in radiating columns. These masses, which have a -diameter of from 1 to 2 millimetres, are termed “lobules.” By mutual -pressure the lobules are squeezed into a pentagonal or hexagonal form. -The vein in the centre of the lobule is the intralobular vein. Turn now -to the portal vein; this is seen to break up into branches which run -between the lobules, and are therefore termed “interlobular veins.” -The branches of the hepatic artery also run between the lobules, as do -the radicles of the bile-duct. Each lobule is a liver in miniature. -The blood of the portal vein, which has come from the spleen, in -which red blood-corpuscles are destroyed, and from the stomach and -intestines, from which it has absorbed the products of digestion, is -poured over the surface of the lobule, to be filtered through into -its central intralobular vein. In its passage from the interlobular -veins (and branches of the hepatic artery) to the intralobular vein -the blood is confined to radiating capillary channels; but since these -merely prevent the escape of red blood-corpuscles without imposing any -restrictions upon the exudation of blood-plasma, the portal blood is to -all intents and purposes filtered through the columns of liver-cells. -The body-substance of the liver-cells is soft, destitute of envelope, -and capable, when free on the (warmed) stage of a microscope, of -changing in form, somewhat after the manner of a leucocyte. Such -cells have a great capacity for taking up the products of digestion. -Possibly they take up and store fats and proteins, but undoubtedly it -is their chief business to absorb sugar which accumulates as glycogen -in their substance. The glycogen is handed out to the hepatic blood -as required. The pigment which results from the disintegration of red -blood-corpuscles in the spleen is secreted, along with the bile-salts, -into minute channels, or canaliculi, which groove the flat surfaces of -adjacent liver-cells. These canaliculi converge to the bile-ducts. The -liver is therefore at the same time the storehouse of sugar which it -<span class="pagenum"><a name="Page_162" id="Page_162">[Pg 162]</a></span> -takes up from the blood when it is in excess, and passes out to the -blood when it is deficient, and an excretory organ which eliminates the -refuse of hæmoglobin. The iron derived from hæmoglobin it stores, and -returns to the blood.</p> - -<p>Another function of the liver has been referred to already. It is the -organ, and, as far as we know, the only organ, in which urea is made -in mammals, and uric acid in birds. If the liver of a freshly killed -animal be excised and a stream of blood passed through it, the blood -which leaves the organ contains urea. If a salt of ammonia, even the -carbonate, be added to the blood, it is converted by the liver into -urea. When a bird’s liver is made the subject of the same experiment, -uric acid appears instead of urea. The liver can convert many -nitrogenous substances into urea, but it seems probable that, normally, -the salt with which it has chiefly to deal is lactate of ammonia -(<a href="#Page_146"><i>cf.</i> p. 146</a>).</p> - -<p>A few words must be added with regard to the functions of the liver -during prenatal life, obscure though these functions are. The liver -develops very early, and attains a relatively enormous size. At the -third month it weighs as much as the whole of the rest of the body -(<a href="#Page_34"><i>cf.</i> p. 34</a>). Yet it cannot, one must suppose, have to -do much of the work which falls to its share in postnatal life. Food is reaching -the embryo in a constant stream, and not as the result of intermittent -meals. The embryo has no need to store glycogen; nor does its liver, -on analysis, yield much of this substance. In the embryo glycogen is -widely distributed throughout the tissues, not specially accumulated -in the liver. No digestion is occurring in the alimentary canal. Bile -is not needed to aid the hydrolysis and absorption of fats. A small -quantity of cholesterin and less lecithin is being eliminated, but not -much bile is needed to facilitate this process.</p> - -<p>A process which is proceeding at a great rate in the embryo, in -various situations, is the formation of red blood-corpuscles. In -this the liver takes part. But its duty in regard to blood-formation -is not sufficiently onerous to account for its size. The formation -of blood-corpuscles in the liver is observed with difficulty in -microscopic sections. It is therefore impossible to speak with -certainty as to the extent to which it is going on, but it may be -<span class="pagenum"><a name="Page_163" id="Page_163">[Pg 163]</a></span> -safely asserted that this function by itself cannot be held to account -for the great size of the organ in embryonic life. What other office it -fills at this period is a question which still awaits an answer.</p> - -<p>There is no more curious chapter in medical history than the story of -the views held at various periods with regard to the functions of the -liver. From being a mere mass of “parenchyma” serving as packing for -the abdominal viscera, it was elevated to the rank of Grand Purifier -of the “humours” of the body. Next, its excessive activity became -the cause of that form of dyspepsia known as “biliousness.” Still -later its want of activity was its chief vice. A “sluggish” liver was -held responsible for mental perversity and moral dulness. Calomel, -podophyllin, and other drugs were used as whips to stir it up; and the -increased secretions of the alimentary canal were mistaken for bile. -Poor patient organ! It is the still-room of the body, in which the -day’s supplies are stored, and from which they are served out, without -haste and without delay. And it makes urea. What else it does we have -yet to find out; and it is not impossible that when physiologists -have quite shaken themselves free from the explanations based upon -conjecture, which their predecessors have handed down, they may -discover that it has other duties which are not obvious, but of great -importance.</p> - -<hr class="chap" /> -<p><span class="pagenum"><a name="Page_164" id="Page_164">[Pg 164]</a></span></p> -<div class="chapter"> - <h2>CHAPTER VII<br /><span class="h_subtitle">RESPIRATION</span></h2> -</div> - -<p>Life means change. We cannot imagine its continuance without -liberation of energy. Arrest of molecular activity is death. There -is no possibility of its revival. A watch that has stopped may be -started by shaking. On the cessation of molecular activity an animate -being becomes inanimate. Dead, it is liable to further chemical -changes. Bacteria invade it. They shake down its complex unstable -compounds into simple, stable, so-called “inorganic groups”; but the -ordered combination with oxygen, which constitutes living, can never -recommence. Putrefaction may be prevented by the exclusion of germs. -The inanimate mass of organic material may remain unchanged. Its return -to life would be a miracle. From time to time a frog is found enclosed -in old red sandstone, or some other rock which for countless ages has -lain beneath the surface. The cleft through which the frog entered -a few hours or days before it was discovered is overlooked. It is -supposed to have lived “in a state of suspended animation” for millions -of years. The fact that no frogs are to be found among the fossils of -the old red sandstone is an objection too casuistical to be seriously -entertained. The physiologist’s demand to know what has become of the -mountains of solid carbonic acid, water, and urea which the frog must -have produced during its unimaginable term of incarceration is regarded -as the natural expression of his prejudice—that life cannot continue -without molecular change. And he is bound to admit his inability to -prove that it cannot. Nevertheless, his experience that, whenever and -however he may, by experimental methods, arrest change, he loses the -power of causing it to recommence justifies him in his conviction that -life is change. Even a living seed is to his mind an organism whose -<span class="pagenum"><a name="Page_165" id="Page_165">[Pg 165]</a></span> -complex constituents are slowly—however slowly—setting free energy -by settling down the steps which lead to stability and ultimate, inanimate -rest; and the only source of this energy is combination with oxygen. In -the case of a seed the oxygen need not come from without. Seeds retain -their power of germination after long occlusion in nitrogen or other -neutral gases. But all the time some change is occurring, some internal -oxidation which resolves their less stable into more stable compounds. -Otherwise they would not be alive. A physiologist is willing to believe -that this may continue for ten years, fifteen years—for any period -that the botanist tells him that he has, under verifiable conditions, -observed that it does occur; but when he is told that peas taken from -the hand of an Egyptian mummy, or seeds set free by the spades of -navvies after a far longer burial, have been found to retain their -vitality, his credulity is stretched beyond breaking-point. He cannot -imagine a change so slow as to be spread over a geological period, -still without exhaustion of all changeable compounds.</p> - -<p>The term “respiration” has been extended until it is synonymous with -“oxidation.” At one time it was supposed that the combination of oxygen -with oxidizable substances occurred in the lungs. The lungs were the -hearth of the body, to which the blood brought fuel which burned -in the air drawn into them. When it was understood that the actual -combination of combustible material with oxygen occurs, not in the -lungs, but in the tissues, a somewhat illogical distinction was made -between “external respiration”—the combination of oxygen and blood -in the lungs—and “internal respiration”—the combination of oxygen -and tissue-substances. The terms are not comparable. The taking up of -oxygen by the hæmoglobin of blood is a different process to the union -of oxygen, after the hæmoglobin has parted with it, with the carbon, -hydrogen, and nitrogen of the tissue-substances.</p> - -<p>The blood-stream carries both fuel and oxygen to the tissues, but the -fuel while in the blood is not in an oxidizable condition. The foods -are taken up by the tissues. They enter into combination with their -protoplasm. Oxygen also combines with tissue-substances. In proportion -as the tissues are active oxidized compounds are split off. They fall -into the lymph, whence they are absorbed by the blood. If they are -<span class="pagenum"><a name="Page_166" id="Page_166">[Pg 166]</a></span> -nitrogenous compounds, they are carried to the liver, formed into urea, -and passed to the kidneys for elimination. If carbonic acid, it is -carried to the lungs for exhalation. The water formed by combination of -hydrogen and oxygen may escape from the lungs, the kidneys, or the skin.</p> - -<p>Two or three pounds of mixed foods are consumed every day. By the blood -they are carried to the tissues, whence an equivalent quantity of -waste—that is to say, oxidized—material is removed. About -1½ pounds of oxygen is required to burn the day’s fuel.</p> - -<p>The problems of respiration are twofold. In the first place we have -to consider the physics and chemistry of the combination of hæmoglobin -with oxygen, and of the elimination of carbonic acid from the blood in -the lungs; secondly we have to explain the transference of oxygen from -hæmoglobin to the tissues, and the reception in the blood of carbonic -acid produced by the tissues.</p> - -<p>The apparatus by which air is brought into relation with the blood -consists of lungs and windpipe. At its upper end, where it joins the -portion of the alimentary tract common to deglutition and respiration, -the special respiratory tube is protected by the larynx. The nasal -chambers belong to the respiratory tract; the gullet, or pharynx, is -common to the two functions.</p> - -<p>The mucous membrane which lines the nose and windpipe is kept moist -in order that it may catch particles of dust drawn in with the air. -At the same time the nasal chambers serve to warm the air, and to add -moisture to it if it be too dry; for the lining epithelium of the lungs -would suffer if dry air came in contact with it. The wall-surface -of the nasal chambers is increased by the projection of folded and -chambered “turbinate bones.” The importance of warming the air before -it is admitted to the lungs is remarkably illustrated in the case of -certain sea-birds. The nasal chambers of the frigate-bird, and of some -other birds which resemble it, are exceptionally complicated. Since the -animal is devoid of any sense of smell, and the air which it breathes -must be nearly saturated with moisture, the only function which can be -assigned to these convoluted passages is that of warming inspired air. -<span class="pagenum"><a name="Page_167" id="Page_167">[Pg 167]</a></span></p> - -<p>The larynx will be more minutely described when it is considered -as the organ of voice. In connection with respiration, it must be -regarded as primarily a valve which closes the entrance to the windpipe -during swallowing. It is overhung by a leaf-like appendage—the -epiglottis—formed of exceedingly elastic tissue. It was thought until -lately that the epiglottis drops over the aperture of the larynx when -food is passing down the gullet, and springs up again as soon as the -act of deglutition is over; but recent observations have shown that -during deglutition the epiglottis is pressed against the back of the -tongue, and that the closure of the larynx is effected by its own -sphincter muscles. The mucous membrane of the larynx is extremely -sensitive to stimulation by anything which would be prejudicial to the -tissue of the lungs. When its sensory nerve—the superior laryngeal—is -stimulated, the larynx closes. It is the agent in carrying out many -reflex actions, in which not the larynx only, but also the muscles of -the chest and diaphragm, take part. For example, it immediately stops -inspiration if an irritating vapour is present in the air. It stops -respiration if any foreign body, such as a crumb of bread or a drop -of water, touches the mucous membrane. When the trunk of the nerve is -stimulated by an electric current, respiration is inhibited. Further, -under suitable stimulation the nerve brings about respiratory movements -in which inspiration is gentle and expiration sudden, violent, -convulsive. Rib-muscles and diaphragm combine to produce a cough, which -ejects the noxious body. Again, its stimulation in a different way -probably helps to produce constriction of the smaller bronchi which -regulate the amount of air supplied to the air-cells of the lungs; -although this constriction may be largely due to a reflex which starts -in the air-cells. The epithelium of the air-cells has an immensely rich -supply of sensory nerves. In some persons this protective mechanism -is very prone to overact its part. A little dust or foul gas in the -air leads to such marked contraction of the bronchi that respiration -becomes very difficult. Such an exaggerated tendency to reflex action -constitutes the neurosis, asthma. In this malady the mechanism is -unduly sensitive. Very slight stimulation leads to a maximum discharge -of impulses to the muscular tissue of the bronchi. -<span class="pagenum"><a name="Page_168" id="Page_168">[Pg 168]</a></span></p> - -<p>The trachea has a length of about 4 inches. It extends from the lower -edge of the cricoid cartilage, which is easily felt in the neck beneath -the thyroid cartilage (Adam’s apple), to the under side of the arch -of the aorta, where it divides into the right and left bronchi. The -epithelium which lines the trachea and bronchi is ciliated. The cilia -propel the secretion which accumulates on its surface upwards towards -the larynx. The wall of the windpipe is kept open by rings of cartilage -which are incomplete behind, where the trachea and œsophagus are in -contact. Rings and plates of cartilage also support the bronchi. The -bronchi divide and subdivide until their diameter is reduced to about -0·2 millimetre. Each bronchiole then breaks up into a bunch of very -thin-walled, elongated infundibula, club-shaped, and with a diameter -about five times that of the bronchiole with which they are connected. -They may be three or four times as long as they are broad. The wall -of an infundibulum is pitted like a piece of honeycomb into shallow -chambers—the air-cells or alveoli.</p> - -<p>The walls of the air-chambers, or alveoli, are formed of a membrane -upon which is spread a network of capillary bloodvessels. The -air-chambers are so closely packed together that a common wall -separates one chamber from the next adjoining. Minute bloodvessels -pierce the partitions which separate the chambers, appearing now on -one side of the wall, now on the other. The air-chambers are lined by -thin epithelial scales or tiles. The blood in the capillary vessels -is separated from the air in the air-chambers by the wall of the -capillary; by a lymph-space, probably rather potential than actual; and -by the epithelial tiles. This covering suffices to prevent the escape -both of red corpuscles and of plasma, yet offers very little resistance -to the passage of gases from the blood into the air, and from the air -into the blood.</p> - -<p>Leucocytes make their way between the tiles, and creep over their -internal surfaces, searching for cell débris or foreign matter. -Anything that they find they carry to the clumps of lymphoid tissue -which occur in the outer wall of the bronchi. In a town-dweller, -leucocytes are found in these lymph-thickets, charged with particles of -soot. They show droplets of fat and other evidences of degeneration. At -other spots are to be seen little collections of soot which have been -left behind after the dissolution of the leucocytes which brought them there. -<span class="pagenum"><a name="Page_169" id="Page_169">[Pg 169]</a></span></p> - -<p>It is not possible to make anything like an accurate estimate of the -number of alveoli in the lungs; 725,000,000 is a figure arrived at by -measuring the average cubic capacity of an alveolus, and comparing it -with the total cubic capacity of the lungs. Each alveolus supports some -forty or fifty capillary vessels. The superficial area of vascular -membrane exposed is placed at 90 square metres, or about 100 times the -area of the skin. Figures such as these convey very little meaning, but -they help one to realize the magnitude of the provision made for the -aeration of the blood.</p> - -<p>Pneumonia is a condition in which the lining of the air-chambers is -inflamed, usually, possibly always, owing to the entrance of bacteria. -Lymph exudes through the walls of the alveoli. Epithelial scales flake -off. Pus cells (dead leucocytes) accumulate in the air-chambers. -Respiration is curtailed, and dyspnœa results. After a time, if the -case progresses favourably, “resolution,” as it is technically termed, -begins to occur. The exuded substances are either expectorated or -absorbed, and the lung-tissue returns to a normal condition.</p> - -<p>Here a few words may be devoted to respiratory sounds. <i>Spirare</i> means -to sigh. Breathing received the name by which it is known in physiology -from the sound which accompanies the exit of air from the nostrils. -Since the introduction of auscultation as a means of ascertaining -the condition of the lungs, other sounds, not heard until the ear -or a stethoscope is placed against the chest, have acquired great -importance. These sounds, termed “murmurs,” may be divided into two -classes. (<i>a</i>) When the ear is placed against the windpipe, or in -the middle of the back between the shoulder-blades, a murmur is audible, -due to the movement of air through the larynx. If the larynx, the -trachea, or the bronchi contain mucus, it is a harsh, rough, bubbling, -or crackling sound. It accompanies both inspiration and expiration. -(<i>b</i>) A softer, more delicate murmur is heard when the ear is placed -against the front or the side of the chest. This is the vesicular or -pulmonary murmur. It is heard during inspiration, and is due to the -passage of air out of the smallest bronchi into the more spacious -infundibula in which they end. These two kinds of murmur must be -rigidly distinguished—the laryngeal murmur, heard in situations in -<span class="pagenum"><a name="Page_170" id="Page_170">[Pg 170]</a></span> -which no lung-tissue intervenes between the ear and the great -tracheal or bronchial tubes; and the pulmonary murmur, heard over -all regions where the bronchi are buried in lung. Healthy lung is as -bad a conductor of sound as a sponge or a wad of cotton-wool. The -laryngeal murmur is inaudible in regions in which lung lies beneath the -chest-wall. It would be far beyond the scope of this book to attempt to -describe the very varied alterations in the chest-sounds which may be -produced by disease. The student would do well to familiarize himself -with the nature of the sounds which are heard in health, and the -situations in which they are heard, in order that he may be able, in -abnormal conditions, to recognize that something is wrong.</p> - -<p>The chief departures from the normal may be grouped under the following -heads: (1) The pulmonary murmur may lose its soft, smooth, sighing -character owing to inflammation of the alveoli and infundibula. It may -be as loud in expiration as in inspiration. Only a practised ear can -estimate the significance of these changes. (2) The laryngeal murmur -may be reinforced by “râles”—a convenient term for supplementary -sounds. The source of such râles may be a cold in the chest, -laryngitis, or bronchitis of various degrees. (3) The laryngeal murmur -may be heard in situations in which lung intervenes between the ear -and the larger bronchial tubes. This can be due only to the lung being -in an abnormal condition as a conductor of sound. Instead of being -as spongy as well-made Vienna bread, its air-spaces are filled with -solid or fluid deposit. It is as firm as dough. To such a condition -it attains at the height of pneumonia—a stage termed “hepatization” -because in section it looks like liver rather than lung.</p> - -<p>Breathing is the enlargement and diminution of the chest, which causes -air to be drawn into and expressed from the lungs. The windpipe being -open, the air inside the lungs is, of course, at the same pressure as -the atmosphere. Expansion of the chest results in the equal expansion -of the lungs. Since there is no air-space between the outer surface -of the lungs and the inner surface of the chest-wall, the lungs -cannot separate from the chest-wall when it expands. But the lungs -contain elastic tissue always slightly on the stretch. If the chest be -punctured, and air admitted between the chest-wall and the lungs, the -<span class="pagenum"><a name="Page_171" id="Page_171">[Pg 171]</a></span> -lungs collapse. The expiratory movement, the contraction of the chest, -is due to the elasticity of the lungs. This tendency on the part of the -lungs to contract is sufficient in quiet respiration to restore the -chest to its usual size after inspiration, and thus to expel air. The -lungs are held open owing to the negative pressure in the space which -separates them from the chest-wall. This negative pressure has a most -important relation to their permeability by air. Imagine the condition -reversed. Picture a lung into which air is forced by a muscular pump. -After each stroke of the pump the lung would collapse. Its finest -tubes and their dilated terminations could be maintained as open -spaces, between the strokes of the pump, only by giving a considerable -thickness and firmness to their walls. Such a substantial structure -would be unfavourable to an interchange of gases between the blood and -the air. The reverse of this condition is found in Nature. The lung is -stretched from without. Its tissue, delicate as crêpe, cannot collapse -even at the end of the deepest expiration.</p> - -<p>The ribs are united by intercostal muscles, disposed in two sheets. -The fibres of the external intercostals are directed downwards and -forwards, those of the internal intercostals downwards and backwards. -In tranquil respiration the chest is enlarged by the external -intercostal muscles, which raise the ribs, and the diaphragmatic -muscle, which renders peripheral portions of the diaphragm flat. -The rôle of the internal intercostal muscles is a subject still -under discussion. For the most part, physiologists regard them as -accessory to expiration, but some hold that they combine with the -external intercostals in raising the ribs and twisting them outwards -during inspiration. The diaphragm is a partition which separates the -thoracic from the abdominal cavity. It is in the form of a vault. The -central portion of the dome is membranous, its margins muscular. Its -membranous centre is in contact with the pericardium, which encloses -the heart. The level of this part is therefore fixed, except in -forced inspiration, when it descends slightly. It constitutes a fixed -plane for the muscles of the diaphragm, which are attached below -to the vertebral column and the ribs. When the muscles contract in -inspiration, the curvature of the marginal portions of the diaphragm is -<span class="pagenum"><a name="Page_172" id="Page_172">[Pg 172]</a></span> -diminished, and the chest-cavity consequently enlarged. During -expiration the space between the muscle of the diaphragm and the -chest-wall closes up, and the lower border of the lung slips out of it.</p> - -<p>There is a marked difference in the relative extent of the costal and -diaphragmatic movements in men and women. In women respiration is -chiefly costal; in men it is chiefly diaphragmatic. In men the abdomen -moves forwards, as the diaphragm descends in tranquil breathing; in -women the chest rises. Men who wish, for the purposes of athletics, or -singing, or public speaking, to retain the power of making the most of -their chest-capacity are wise in not allowing themselves to fall into -the habit of lazy, abdominal breathing.</p> - -<div class="figcenter"> - <a id="I_172" name="I_172"> </a> - <img src="images/i_p172.jpg" alt="" width="600" height="311" /> - <p class="center"><span class="smcap">Fig. 8.</span>—<span class="smcap">The - Diaphragm and Organs in Contact with it—A, in Expiration;<br /> B, at - the End of a Deep Inspiration. Transverse Vertical Sections in the<br /> Line - of the Armpit.</span></p> - <div class="blockquot2"> - <p class="neg-indent space-below2">A, At the end of an ordinary expiration the lung does not extend below - the upper border of the eighth rib. From this level to the middle or - lower border of the tenth rib the two layers of the pleura covering - respectively the inner wall of the chest and the upper surface of the - diaphragm are in contact. B, When the lung is distended with air it - occupies the whole of the pleural cavity.</p> -</div></div> - -<p>When additional pressure is required, when respiration is forced, -various external muscles attached to the spinal column, the -shoulder-blades, and the clavicles, as well as the muscles of the -abdomen, come into play.</p> - -<p>The chest is lined and the lungs covered by a serous membrane—the -pleura. Normally there is only just sufficient lymph in the space -<span class="pagenum"><a name="Page_173" id="Page_173">[Pg 173]</a></span> -between the visceral layer of the pleura which invests the lungs and -the parietal layer which lines the chest-wall to prevent friction -during respiration. When the pleura is inflamed, one layer of the -membrane rubs against the other. In the early or “dry” stage of -pleurisy, the physician recognizes this condition by the friction-sound -which he hears on placing his stethoscope against the chest. In a -later stage lymph (pleuritic fluid) is poured out. It accumulates in -the lower part of the chest, and is recognized by the absence of the -resonant note which, under normal conditions, is given out by the chest -when percussed.</p> - -<p>The lungs are not compressed during expiration; they are not squeezed, -as a pair of bellows or a sponge may be squeezed, emptying it of its -contents. At the end of tranquil expiration the lungs still contain -about 3½ litres of air. At the top of tranquil inspiration the volume -of their contents does not exceed 4 litres. It is evident, therefore, -that air is not drawn into and driven out from the air-chambers by -the movements of respiration. The tide of air does not extend far -beyond the ends of the bronchi. The gases in the air-chambers are -exchanged with the fresh air drawn into the infundibula by diffusion. -The composition of the air which is in contact with the bloodvessels -is constant. It is about 4 per cent. poorer in oxygen and 3 per cent. -richer in carbonic acid than the outside air.</p> - -<p>Of the air drawn into the windpipe during an inspiration, about -one-third returns to the open with the following expiration; two-thirds -remains in the lungs. If, therefore, the air taken in at each tide -equals one-seventh of the quantity already in the lungs, and if of this -one-seventh two-thirds remains, each alveolus renews about one-tenth of -its air. Its contents are completely changed in ten respirations.</p> - -<p>Fresh air is composed of 21 per cent. oxygen, 79 per cent. nitrogen, -and a trace (0·04 per cent.) of carbonic acid. Forced by a syringe -through lime-water, fresh air does not produce any appreciable -milkiness, whereas air breathed through a tube into lime-water renders -it turbid owing to the formation of carbonate of lime. Carbonic acid -<b>(CO₂)</b> occupies the same volume as its oxygen <b>(O₂)</b> would occupy if -free. The oxygen which breathed air has lost slightly exceeds in amount the -<span class="pagenum"><a name="Page_174" id="Page_174">[Pg 174]</a></span> -carbonic acid which it has gained in exchange. The difference is due -to the retention of some of the oxygen for the purpose of uniting with -hydrogen to form water, and of forming urea. The proportion between -carbonic acid gained and oxygen lost, <b>CO₂/O₂</b> is termed the “respiratory -quotient.” Its value varies, of course, with diet. In a herbivorous -animal, whose food consists of carbohydrates, it departs but little -from unity; in a carnivore, which eats fat and nitrogen-containing food, -it is about 0·8.</p> - -<p>The respiratory exchange is very much smaller in cold-blooded animals -than in animals which maintain the temperature of the body at a fixed -level. In warm-blooded animals it rises as the temperature falls, falls -as it rises, the increased oxidation warming the body, the diminished -oxidation allowing it to cool; whereas in cold-blooded animals it -increases as the temperature rises, owing to the greater activity -induced by warmth, and falls as the temperature falls.</p> - -<p>The respiratory exchange is increased by muscular activity. If the -amounts of oxygen absorbed and carbonic acid given out are measured -while a man is at rest, and again while he is doing hard physical work, -it is found that during work the respiratory exchange is twice as great -as during rest. During periods of starvation the respiratory exchange -remains unaltered, since heat has to be constantly produced if the -temperature of the body is to be kept from falling.</p> - -<p>Since the purpose of respiration is to give to the blood the -opportunity of renewing its supply of oxygen, and of getting rid of the -carbonic acid with which it is charged, it might be supposed that the -respiratory exchange would be increased, so far as the intake of oxygen -is concerned, by breathing oxygen gas instead of air; but it appears -that under normal conditions nothing is gained. When an animal is -breathing air, its blood takes up all the oxygen that it wants—all the -oxygen, that is to say, for which its tissues are asking. Offering it -pure oxygen in place of mixed oxygen and nitrogen does not induce it to -take up more. The hæmoglobin is almost saturated with oxygen when the -blood leaves the lungs under ordinary conditions. In certain diseases -of the lungs, however, in which the blood becomes unduly venous, the -respiration of oxygen may be beneficial; but even in these cases the -<span class="pagenum"><a name="Page_175" id="Page_175">[Pg 175]</a></span> -results are disappointing, because the system is suffering much less -from deficiency of oxygen than from accumulation of carbonic acid. -Substituting oxygen for air does not facilitate the escape of carbonic -acid.</p> - -<p><b>The nervous mechanism of respiration</b> has been the subject -of much investigation and of many experiments, without, it must be -confessed, the development of a quite complete or satisfactory theory. -Respiration is a rhythmic process. About seventeen times in a minute -the intercostal and diaphragmatic muscles contract. Inspiration is -immediately followed by expiration, the falling movement being due, as -already explained, to the elasticity of the lungs, which are stretched -during inspiration. A slight pause intervenes between the end of -expiration and the commencement of the next inspiratory movement. -Tranquil respiration is a succession of reflex inspiratory movements, -the depth of which varies according to the needs of the body—that -is to say, according to the condition of the blood. If the need for -aeration of the blood becomes urgent, the depth of inspiration is -increased, and expiration also becomes an active movement, certain -muscles, especially those of the abdomen, being called into play. In -this condition two sets of reflex actions alternate. A large number -of nerves are concerned even in tranquil respiration. If a man in -falling “breaks his back” at the junction of the cervical and thoracic -regions, costal respiration ceases. The series of intercostal nerves -which arises from the dorsal spinal cord below the level at which -it is injured are thrown out of action. Diaphragmatic respiration -still continues, because the nerve of the diaphragm, the phrenic, -arises from cervical roots. The lungs are supplied by the vagus -nerve. This nerve joins the medulla oblongata as one of a group of -three—glosso-pharyngeal, vagus, and spinal accessory—which by a -large number of roots enter the groove between the olive and the restiform -body. The vagus is the channel along which afferent impulses from the -lungs enter the medulla. Such impulses call for respiratory movements. -Cutting both vagi, however, does not put an end to respiration. -Inspiratory movements continue, but they are much deeper and separated -by much longer pauses. Such a form of respiration is inefficient. The -<span class="pagenum"><a name="Page_176" id="Page_176">[Pg 176]</a></span> -blood is not properly aerated. The animal falls into a condition of -dyspnœa, which ends in death. When the central end of the cut vagus is -stimulated, the movements become more natural. Clearly, the respiratory -reflex is not dependent upon the vagus, since it continues after the -nerve is cut, although the impulses which pass up this nerve regulate -its rhythm. They govern the length of the inspiratory movements, cut -them short at the right moment, and secure their succession at proper -intervals.</p> - -<p>The transfer of afferent impulses into efferent channels occurs in -the medulla oblongata. Long ago it was found that if the brain above -this level be removed, part by part, respiration is not interfered -with until the medulla oblongata is injured. When a cut is made into -the floor of the fourth ventricle not far to one side of the middle -line, the respiratory movements on that side of the body cease. If the -injury be bilateral, even though very limited in extent, respiration -stops. This spot was therefore spoken of as the “respiratory centre.” -Flourens, who first discovered it, believed that it was a mere spot. -He gave to it the fanciful name of <i>nœud vital</i>. It is the place at -which the afferent nerves which call for respiration are brought -into connection with all the various motor nerves which bring about -the respiratory movements of nostrils, larynx, chest, and diaphragm. -Possibly the knife in Flourens’ incision divides the tract of fibres -which distributes afferent impulses, but whether the junction be a -defined tract or no, injury to this region of the medulla throws the -nervous mechanism of respiration out of gear. At this particular spot -lies the “centre” for respiration—the one part of the nervous system -which must be intact if the movements of respiration are to be carried -out. There is no reason for thinking that respiratory impulses are -generated at this spot. It is a centre in the same sense in which Crewe -is a centre for distributing the goods of Lancashire and other parts -of England to North Wales. The use of the term “nerve-centre” has been -very much abused. Centres were supposed to be collections of cells, -each group of which had some prerogative of initiation. Reasoning -from the analogy of human institutions, it was thought necessary that -the nervous system should be organized into departments severally -responsible for the administration of the activities of certain sets of -<span class="pagenum"><a name="Page_177" id="Page_177">[Pg 177]</a></span> -muscles: one centre controlled respiration, another the beat of the -heart, another deglutition. The centres were dependent one on another; -each regulated lower centres, and was governed by those above it, -in this bureaucratic scheme. We know nothing of any function of -nerve-cells other than that of transmitting impulses. All that we know -about nerve-cells is that they place afferent and efferent routes in -communication, and interpose resistance into nerve-circuits. Every -nerve-cell of the grey matter of the brain and spinal cord gives -off processes which ramify. The ultimate twigs into which a branch -divides are in connection with other sets of twigs derived from the -end-branchings of nerve-fibres or processes of other nerve-cells. A -nerve-fibre is but the axis-cylinder process of a nerve-cell. Impulses -encounter resistance in passing along the neuro-fibrillæ (<a href="#I_321"><i>cf.</i> Fig. 22</a>) -contained in the twig-connections of the ramifying processes of -nerve-cells. There is no reason for supposing that anything like -the same resistance is offered to the passage of impulses along the -fibrillæ where they lie within the stout branches of the cell-processes -or within the body of the cell. It is easy to make a pictorial -representation of such a mechanism. Imagine a model of the stem of a -tree made by binding together a large number of wires; its branches as -containing small groups of wires; the ultimate twigs as separate wires. -Carry wires from the roots of one tree to the branches of another. -Trees so constructed might be taken as representing nerve-cells. We -have not as yet succeeded in demonstrating the isolated neuro-fibrillæ -as they pass over from the end-twigs of a nerve-fibre to the end-twigs -of a nerve-cell branch, but we have abundant reason for believing -that they do so pass, and that the resistance to the passage of a -nerve-impulse is interposed in this neutral or junctional zone. This -resistance has to be overcome. It is overcome by the summation of -impulses. All nerve-impulses are vibratory. The first vibrations may -fail to get through; but if the vibrations continue, they exert a -cumulative effect. After a time they overcome the resistance; sensory -impulses flow through the centre into motor channels. In this way we -endeavour to explain the rhythmic discharge through the respiratory -and other centres. It has not been found possible to determine the -source of all the afferent impulses which reach the centre. Respiration -<span class="pagenum"><a name="Page_178" id="Page_178">[Pg 178]</a></span> -continues after all accessible nerves have been cut, including even -the posterior roots of the cervical nerves. Probably it is a mistake -to look for definite afferent channels in the medulla and the rest of -the brain. All parts of the body need aerated blood. From all parts, -including nerve-tissue itself, arises the demand for respiration. -Possibly nerve-centres have the power, as it were, of storing impulses, -and discharging them after the stream of fresh arrivals has ceased to -flow. They may acquire a habit.</p> - -<p>The resistance in the centre is profoundly affected by the condition -of the blood. As the blood becomes more venous, impulses pass across -the nerve connections with ever-increasing force. Kept in the first -instance to definite channels, they spread as the centre becomes more -excitable farther and farther afield, reaching one group of muscles -after another, and pressing them into the service of respiration. -When, in dyspnœa, every muscle which can in any way help the movements -of the chest is doing its best, others which are useless for this -purpose receive the reflected impulses and join in, producing general -convulsions. The increased activity of the respiratory centre which -is produced by slight venosity of the blood is shown in the rapid -and deep inspirations which are caused by violent exercise. Perhaps -it is justifiable to go a step farther, and to assert that there is -something in blood which has been rendered venous by muscular activity -which is specially exciting to the respiratory centre. If the blood -from a limb be prevented from returning to the general circulation, -by compressing or tying its great veins, and if the muscles of the -limb be strongly stimulated by an electric current, their activity, -so long as the passage through the veins is blocked, has no influence -upon respiration. But, on relaxation of the pressure on the veins, -respiration may become twice as deep and twice as frequent as it was -before the muscles were stimulated, although the limb is now in a -condition of perfect rest.</p> - -<p>What is the special action of the vagus nerve? Its superior laryngeal -branch checks inspiration and induces expiration, as already said. The -impulses which pass up its main trunk bring about ordered movements. -They are not dependent for their generation upon the condition of the -<span class="pagenum"><a name="Page_179" id="Page_179">[Pg 179]</a></span> -blood in the lungs. When the chest is filled with nitrogen, inspiration -and expiration alternate in the usual way, although the blood is -growing steadily more venous. The failure of inspiration to bring -about aeration of the blood does not lead to a prolongation of the -inspiratory effort. Inspiration is cut off and expiration established -in regular sequence. In performing “artificial respiration” (<a href="#Page_184"><i>cf.</i> p. 184</a>) -for the purpose of saving life, in cases in which respiration -has ceased owing to the lungs being filled with water, or for other -reasons, the chest is enlarged by raising the arms above the head, -and diminished by pressing the elbows against the sides. Enlargement -promotes a tendency to expiration, compression a tendency to a natural -inspiratory effort. Evidently there is a connection between the -movements of the chest and the stimulation of the respiratory centre. -If respiration is being carried on artificially, by forcing air from -a bellows into the trachea, the nostrils dilate as the chest is -distended, and contract as it is emptied, so long as the vagus nerve -is intact, just as they do in normal respiration. This shows that, -when the chest is emptied, a message is sent through to the nucleus -of origin of the nerve which supplies the dilator muscles of the -nostril. When the lungs are full, a message calls upon the nostrils to -contract. The only factor which is common to pressing in and pulling -out the ribs, and filling and exhausting the lungs with a bellows, is -the alteration in the form of the lungs which is produced by the two -methods. It is impossible to resist the conclusion that the stretching -of the tissue of the lungs stimulates the nerve-endings of the vagus. -The impulses thus induced automatically stop inspiration, and lead to -an expiratory effort.</p> - -<p>There are many indications that the nervous mechanism of respiration -is a double one, certain stimuli inducing expiration, with inhibition -of inspiration, others inhibiting expiration and inducing inspiration. -There are, however, many difficulties in the way of formulating a -satisfactory theory of the relation of these antagonistic actions. -We may frequently observe indications of such an antagonism between -the two phases of the respiratory mechanism. Cold water dashed on the -back of the head (when the head is being shampooed) induces a long -inspiration with inhibition of expiration. A blow in the pit of the -stomach “knocks all the wind out of a man.” Expiration is prolonged -<span class="pagenum"><a name="Page_180" id="Page_180">[Pg 180]</a></span> -until the lungs are unusually empty, and yet the victim of the blow -feels as if he would never again be able to draw breath.</p> - -<p><b>Modified Respiratory Movements.</b>—The object of coughing is to -expel foreign matter from the windpipe or larynx; of sneezing, to -clear the nose. The former action consists of a long deep inspiration; -the closure of the glottis; a forcible expiration. The blast of air -encountering a closed glottis acquires considerable pressure. When -the resistance of the glottis is overcome, the blast rushes through, -carrying with it mucus or bread-crumb, or whatever the substance -may be which irritated the endings of the superior laryngeal nerve. -In sneezing, the back of the tongue is thrust against the palate, -closing the aperture of the fauces. Inspiration is prolonged. A strong -expiration follows. The blast rushes through the nasal cavities. This -reflex is usually provoked by a tickling of the endings of the fifth -nerve in the nasal mucous membrane. It is also caused in many persons, -through the optic nerve, by a bright light; an apparently purposeless -reflex about which we shall have something more to say in a subsequent -chapter. Laughing and crying are modified respiratory movements as -useless, so far as any immediate purpose is accomplished, as sneezing -in response to a bright light. As means of expressing emotions they -have been cultivated by the human race. Possibly a case for crying -might be made out on physiological grounds. Under certain circumstances -it relieves a feeling of distress which, while it lasts, is detrimental -to the proper functions of the body. Laughing undoubtedly is -beneficial. The rapid movements of the chest quicken the circulation. -The shaking of the midriff favours the discharge of digestive -secretions, accelerates the movements of the alimentary canal, and -generally is beneficial to digestion. But “laugh and grow fat” is not -necessarily the order of cause and effect. An efficient digestion and -a good capacity for assimilation lead to a sense of <i>bien-être</i> which -predisposes to a merry view of life.</p> - -<p>Yawning is a deep inspiration with open mouth and larynx. It commences -usually at the end of a normal inspiration, a slight pause being -followed by further inspiration, deep and prolonged. Its commencement -seems to be due to impulses generated by the relaxation of the tone of -<span class="pagenum"><a name="Page_181" id="Page_181">[Pg 181]</a></span> -the muscle which holds up the lower jaw. The masseter goes off duty for -a moment, allowing the jaw to fall. A reflex contraction of the muscles -which open the mouth immediately follows. Muscles of the neck and head -also come into play. Not improbably the yawn ends in a general stretch. -If the origin of this reflex is obscure, its usefulness is marked. The -circulation is quickened, the blood is changed, nervous system and -muscles again become alert.</p> - -<p>“Apnœa” is the condition of arrested respiration. If a man about to -dive into the water breathe deeply and rapidly half a dozen times, he -abolishes for a while the desire to breathe. One is naturally inclined -to explain this as due to a surplus of oxygen taken into the blood, but -a moment’s reflection shows that this cannot be the cause. In the first -place, as we have already pointed out, the blood which leaves the lungs -in tranquil respiration is very nearly saturated with oxygen. It can -take up but little more. Again, the deep inspirations do not change the -air in the air-chambers; time is required for the renewal by diffusion -of their gaseous contents. It is improbable that the constitution of -the air in the alveoli is sensibly altered by a few deep breaths. -Probably the explanation is to be found in the effect upon the -nerve-centre of distention of the chest. Stretching the nerve-endings -of the vagus in the lungs inhibits inspiration. If the stimulation be -excessive, inspiration is inhibited for a considerable time. That this -is the right theory of apnœa is proved by repeatedly inflating the -lungs of an anæsthetized animal with a pair of bellows. The same arrest -of inspiration is induced whether the lungs are inflated with air or -with a neutral gas, such as nitrogen, so long as the vagus nerve is -intact. If this be cut, inflation with a neutral gas no longer produces -apnœa.</p> - -<p>“Dyspnœa” is the term applied to the complex conditions and movements -which result from deficient aeration of the blood, or, rather, from -the distribution of insufficiently aerated blood to the centres in -the medulla oblongata. The blood of the rest of the body may be in a -satisfactory condition, but if, owing to ligature of the carotid and -vertebral arteries or other causes, the blood supplied to the brain be -inadequate to its proper nutrition, the phenomena of dyspnœa are as -marked as they are when air is prevented from entering the lungs. That -<span class="pagenum"><a name="Page_182" id="Page_182">[Pg 182]</a></span> -the excitability of the nerve-centres in the brain is greatly increased -when this organ is supplied with venous blood, and that their tendency -to transmit impulses which call for respiration is consequently -exaggerated, is remarkably shown by the following experiment: Two -rabbits—A. and B.—are placed under the influence of chloroform. Their -carotid arteries are cut, and a crossed circulation established by -connecting the proximal ends of A.’s arteries with the distal ends of -B.’s, and <i>vice versa</i>. The head of each rabbit is now supplied with -blood from the heart of the other, the rest of its body by blood from -its own heart. A.’s chest is now opened, so that its lungs collapse and -cease to take part in respiration. The animal continues to make the -movements of respiration in a tranquil manner, whereas B. is thrown -into violent dyspnœa. The animal whose brain is receiving aerated blood -remains normal, notwithstanding the fact that its lungs and the rest -of its body are poisoned with venous blood. The animal whose brain is -supplied with venous blood becomes dyspnœic, although its lungs and -body are receiving pure arterial blood.</p> - -<p>There is a regular sequence in the phenomena of dyspnœa leading up -to the final stage termed “asphyxia.” If the trachea be suddenly -blocked, so that no air can pass, the respiratory movements at once -become deeper and more rapid. This condition is termed “hyperpnœa.” -In a comparatively few seconds the system appears, as it were, to -find out that inspiration is not needed. Expiratory efforts begin to -preponderate. They increase in violence. All accessory muscles are -brought into play. The cry for air is heard even by muscles which -cannot help. Muscles of the limbs contract, although their contraction -has no effect upon the capacity of the chest. Every expiratory effort -is accompanied by convulsions of a flexor type. At the end of two -minutes there is usually a sudden change. Attempts at expiration cease. -Slow, deep, infrequent inspirations take their place, accompanied by -convulsions of extensor muscles. Pupils are widely dilated, mouth -open, head thrown back. The subject is absolutely insensitive to every -kind of stimulus. The pulse shows a high arterial tension. The beating -of the heart is slow and strong. In about four minutes from the time -at which the windpipe was blocked respiratory movements cease. The -<span class="pagenum"><a name="Page_183" id="Page_183">[Pg 183]</a></span> -arterial tension falls. The heart’s action grows rapidly weaker, -although for two or three minutes longer it may still continue to -flicker. Recovery is possible until it finally gives up. After death -the right side of the heart is found gorged with blood, the left side -empty, showing that the heart had been unable to force the blood -through the capillaries of the lungs.</p> - -<p>Under all ordinary conditions the sequence of phenomena of asphyxia -is the same—a stage of exaggerated breathing (hyperpnœa), a stage -marked by the co-operation of muscles which are not called into -action in tranquil breathing (dyspnœa), followed by the condition of -asphyxia properly so termed. An animal whose supply of fresh air is -cut off passes through these three stages, whether it be enclosed in -a small space or in a very large one. It must, however, be noted that -in asphyxia several factors combine in varying degrees. Carbonic acid -is in excess in the blood, oxygen deficient. The nervous mechanism -which regulates respiratory movements is thrown out of gear. Motor -and inhibitory impulses are in conflict. It is important, if these -complex phenomena are to be analysed, that one factor only should be -altered at any given time. For example, carbonic acid may be allowed -to increase in the air while a constant oxygen tension is maintained. -Under these circumstances the dyspnœic contractions are much less -marked. No convulsions follow. The paralysing action of carbonic -acid predominates. Anæsthesia passes into complete unconsciousness. -Death is tranquil. And this, speaking generally, is what happens in -disease of the lungs. Asphyxia comes on slowly. The supply of oxygen -is undiminished, but carbonic acid accumulates in the blood, acting as -a narcotic poison which lowers the excitability of the nervous system, -suspends consciousness, and slowly brings the vital activities to a -standstill.</p> - -<p>In cases of drowning, when the lungs are filled with water, the -resistance to the passage of blood through their capillary vessels is -greater than it is when they are still filled with air. The heart is -sooner beaten in its effort to drive the blood through them. Usually it -stops in about four minutes. Yet it is difficult to say for how long -after a person has been immersed in water it may be still possible to -resuscitate him. Reports vary, owing in large measure to uncertainty as -to the exact time at which the immersed person sank and his lungs filled -<span class="pagenum"><a name="Page_184" id="Page_184">[Pg 184]</a></span> -with water. It is a wise precept that artificial respiration should -be tried in every case, without waiting a single instant to ascertain -whether the heart still beats. The first thing to do is to empty the -chest of water. Then place the subject on his back. Kneel on the ground -behind his head. Grasp an arm just below the elbow, in each hand. -Draw the arms up above the patient’s head, so that the pectoral and -other muscles drag on the ribs, enlarging the chest; then lower them, -and press them into the sides. This must be done with the natural -rhythm of respiration, and not more frequently than twenty times in a -minute. It is well if an assistant draws the tongue forward, to give -free admission to air. Presumably the slight exchange of air brought -about by mechanical expansion and compression of the chest favours -the passage of blood through the capillaries of the lungs; but the -real object of artificial respiration is to stretch the endings of the -vagus nerve, and in this way to originate impulses which will call -the respiratory centre into action. Perhaps it may not be superfluous -to point out that the failure of the pulse must not be taken as an -indication that the heart has ceased to beat. Owing to the obstruction -to the circulation through the lungs, the left side of the heart is -almost empty. Very little blood is pumped into the aorta. None reaches -the wrist.</p> - -<p><b>Exchange of Gases in the Lungs.</b>—In the lungs each red corpuscle -takes from the air a charge of oxygen which it carries to the tissues. -In the tissues the plasma of the blood receives carbonic acid, which -escapes from it when it reaches the lungs. Water dissolves oxygen and -carbonic acid. Towards animals and plants which live in it, water plays -the same rôle as the atmosphere towards dwellers on land. The quantity -of a gas which will dissolve in water is proportional to the pressure -to which it is subjected. If water were the circulating fluid, some -oxygen would enter it in the lungs; some carbonic acid would be taken -up in the tissues and liberated in the lungs. But it is clear that -the small quantity of fluid which the vascular system will hold would -be incapable of serving as an efficient medium of exchange between -the tissues and the lungs. When a given quantity of venous blood is -agitated with air, five times as much oxygen is taken up as the blood -<span class="pagenum"><a name="Page_185" id="Page_185">[Pg 185]</a></span> -could carry if the gas were simply dissolved. Both oxygen and carbonic -acid are held by the blood in chemical combination.</p> - -<p>The condition in which oxygen is carried was discovered in 1864 -(<a href="#Page_68"><i>cf.</i> p. 68</a>). From all time it had been noticed that -the blood which flows from a vein is darker and of a more purple tint than the -blood which spurts out of a cut artery. Shortly before the date mentioned above, -the spectroscope had begun to be used to distinguish more accurately -than the eye can do the groups of rays which a coloured solution -transmits. The colour of a ray of light depends upon its wave-length. -The light of the sun, when its rays are sorted by a prism, according -to their wave-lengths, shows all colours from the long waves of red to -the short rays of violet, with certain gaps. At intervals where rays -are missing, the spectrum exhibits dark bands—Fraunhofer’s lines. -The colour of a solution is measured by placing a flat-sided vessel -containing it in the course of a beam of the sun’s light, on its way -to a prism. When the rays are spread out, it is observed that certain -groups have been absorbed by the coloured fluid. The colour of the -solution is due to the rays which it transmits. It had been pointed -out in 1862 that blood diluted with water absorbs parts of each end -of the spectrum, and also two groups of rays lying between the fixed -bands of Fraunhofer which spectroscopists had labelled D and E. Stokes -observed that this is true only of arterial blood. Venous blood absorbs -a broad band in this part of the spectrum in place of the two narrow -bands. He showed that, “like indigo, it is capable of existing in two -states of oxidation, distinguishable by a difference of colour and a -fundamental difference in the action on the spectrum. It may be made -to pass from the more to the less oxidized condition by the action of -suitable reducing agents, and recovers its oxygen by absorption from -the air.” The reducing agents of which Stokes made use were alkaline -solutions of ferrous sulphate or of stannous chloride containing some -citric or tartaric acid. These sub-salts of iron and tin very rapidly -absorb oxygen from the air or from any chemical substance which parts -with it readily. With these solutions Stokes replaced the tissues. -He abstracted the oxygen from the oxyhæmoglobin; then, shaking the -solution of reduced hæmoglobin with air, he reproduced the action which -occurs in the lungs. -<span class="pagenum"><a name="Page_186" id="Page_186">[Pg 186]</a></span></p> - -<p>If the hand be held between a spectroscope and the source of light, -in such a position that the beam passes through the thin tissue of -two fingers where they are in contact, the spectrum of oxyhæmoglobin -is obtained. If now the circulation through the fingers be impeded by -putting strong indiarubber bands round them, the blood becomes venous, -and the two narrow bands of oxyhæmoglobin give place to the broad band -of reduced hæmoglobin.</p> - -<p>Although very soluble, hæmoglobin may be obtained in crystals, the form -of which varies in different animals. When obtained from human blood, -the crystals are rhombic prisms; from the guinea-pig, tetrahedra; from -the squirrel, hexagonal plates. Yet it is unlikely that the hæmoglobin -of one animal differs chemically from that of another in any proper -sense of the term. Probably the form of the crystals depends upon -the amount of water of crystallization. The apparent polymorphism of -hæmoglobin may be associated with the great size of its molecules -(<a href="#Page_66"><i>cf.</i> p. 66</a>).</p> - -<p>Even when in the crystalline form, hæmoglobin can take up oxygen; but -the difficulties which attend its purification and crystallization -render somewhat uncertain the amount of oxygen which a gramme of -crystallized hæmoglobin can absorb. In solution, 1 gramme can take up -1·34 cubic centimetres. The whole of the hæmoglobin of the body would, -therefore, if it were all in the oxidized condition, hold about 4 -grammes of oxygen.</p> - -<p>It is not with oxygen alone that hæmoglobin can combine. It can -absorb the same volume of carbonic oxide or of nitric oxide gas. Both -of these gases it holds more firmly than oxygen. Neither carbonic -oxide-hæmoglobin nor nitric oxide-hæmoglobin is of any use to the -tissues. If the blood becomes charged with the fumes of carbonic oxide -(CO) given off by a coke-fire, this gas proves extremely poisonous. -The blood does not lose it in its circuit through the body, nor is it -exchanged for oxygen in the lungs.</p> - -<p>The instability of the compound of hæmoglobin and oxygen is shown under -the air-pump. The pressure of air in the open equals 760 millimetres of -mercury. When the pressure falls to about 250 millimetres, the oxygen -is rapidly given off. This is a matter of considerable interest in its -<span class="pagenum"><a name="Page_187" id="Page_187">[Pg 187]</a></span> -bearing upon the question of the height to which it is possible for a -human being to ascend. An animal placed in a chamber from which the air -is pumped dies when the pressure falls to 250 millimetres of mercury. -It has been ascertained that a man under the same circumstances can -bear with impunity a reduction to 300 millimetres. How much lower -must the pressure fall before it proves fatal? Of three aeronauts -who ascended in the balloon <i>Zenith</i> to a height of 8,600 metres -(26,500 feet), two died. The third, Tissandier, became unconscious, -but recovered during the descent. The pressure of the atmosphere at -such a height is 260 millimetres. The greatest mountain heights yet -attained are 23,100 feet (Aconcagua, in the Southern Andes), reached by -Fitzgerald, and 23,400 feet (Trisul, in the Garhwal Himalayas), reached -by Dr. Longstaff and his companions. The pressure at this height was -320 millimetres. From these facts it is clear that mountaineers have -just about reached the limit; but since they have not as yet mounted to -a height at which the barometric pressure is less than 300 millimetres, -it is possible that slightly higher mountains are still waiting to -be conquered. At 23,000 feet the oxygen contained in arterial blood -does not exceed 10 volumes per cent. (<a href="#Page_190"><i>cf.</i> p. 190</a>). -It is therefore about half the normal amount. Hence the breathlessness and sense -of feebleness experienced by climbers. The least exertion leads to the -consumption of all the circulating oxygen. But since the effects of -want of oxygen are felt at altitudes much lower than those to which -reference has been made, it is clear that the question cannot be -regarded as simply one of physics. The nervous system suffers when an -attempt is made to do work with a deficient oxygen-supply. Violent -headache and nausea attack most persons long before a level is reached -at which the combination of hæmoglobin with oxygen ceases to be -possible. The occurrence of this “mountain sickness” reminds us that -we must not take for granted that the nervous system will continue -to do its work right up to the altitude at which oxyhæmoglobin is -dissociated. Still, the figures show that, apart from these nervous -symptoms, which disappear after a time, no serious disturbance occurs -even though the atmospheric pressure be but little higher than the -absolute minimum at which hæmoglobin combines with oxygen. -<span class="pagenum"><a name="Page_188" id="Page_188">[Pg 188]</a></span></p> - -<p>The capacity of the blood for rapidly absorbing oxygen in the lungs -and readily parting with it to the tissues is easily and completely -explained by the property which hæmoglobin possesses of forming an -unstable compound with this gas.</p> - -<p>It is quite otherwise with regard to the liberation of carbonic acid. -The problems presented by the solution of this gas in blood and its -elimination in the lungs are difficult to solve. Less than one-tenth -of the volume of carbonic acid which can be extracted from blood by -the air-pump is simply in solution. The remainder is in loose chemical -combination, the chief agents in holding it being the alkaline -carbonates which the plasma contains. With an excess of carbonic acid -they form acid carbonates, which give up carbonic acid and again become -normal carbonates in the lungs. About one-third of the carbonic acid -is, however, held by the blood-corpuscles—partly in virtue of their -alkaline carbonates and phosphates, partly in combination with their -globulin. The affinity of these several vehicles for carbonic acid is -sufficient to enable them to take it from the lymph, and to hold it -while the blood is in the veins. When they reach the capillaries of the -lungs, they part with their burden of carbonic acid to the air. It is -in connection with this renunciation that certain difficulties remain -to be explained. The carbonic acid is given up with greater readiness -than our knowledge of the chemistry of the compounds into which it -enters in the blood would lead us to expect.</p> - -<p>Why does oxygen enter blood as it circulates through the lungs, and -carbonic acid leave it? We have referred to the immense surface which -the lungs expose to air. If a soap-bubble be filled with a mixture of -oxygen, nitrogen, and carbonic acid, and if the oxygen be in smaller -proportion, and the carbonic acid be in greater proportion, than in -the air of the room, oxygen will enter the bubble, and carbonic acid -will leave it, by diffusion. If, instead of filling a bubble with gas, -we fill a bladder with water charged with carbonic acid, but destitute -of dissolved oxygen, a similar exchange with the gases of the air will -take place. It is merely a question of “gaseous tension.” The tension -of the gases in the lungs is measured by passing a small tube down -the trachea, and along one of the two chief bronchi until it becomes -blocked in a bronchus just large enough to admit it. Respiration is -carried on under normal conditions in the remainder of the lung; but in -<span class="pagenum"><a name="Page_189" id="Page_189">[Pg 189]</a></span> -the lobe which the catheter blocks diffusion from stationary air to -tidal is no longer allowed. At the same time, since the circulation is -not interfered with, the gases in the blood of the occluded lobe of -the lung are not in markedly different proportions from those in the -air-chambers of other parts. If at the end of a sufficient interval -the air of the occluded lobe is drawn off and its gases measured, -their tensions can be compared with the tensions of gases in specimens -of arterial and of venous blood. If from 10 c.c. of fluid 1 c.c. of -gas can be removed by the air-pump, the volume of gas dissolved is -10 per cent. of the volume of the fluid which dissolved it. Commonly -this is written “10 volumes per cent.” To ascertain experimentally -the tension of a particular gas in a particular fluid when dissolved -to the amount of 10 volumes per cent. at the ordinary pressure of the -atmosphere and at the temperature of the body, it would be necessary -to place it in an open vessel in air containing a sufficient admixture -of the gas to prevent its escape from the fluid. Suppose that it were -found that, when the fluid containing the dissolved gas was placed in -air mixed with the same gas to the extent of one-tenth of its volume, -the fluid neither gave up gas nor absorbed more gas, the tension -of the gas would be equal to one-tenth of an atmosphere. Since the -pressure of the atmosphere equals 760 millimetres of mercury, the -tension of the dissolved gas would be 76 millimetres. If more gas were -added to the air, more would dissolve in the fluid; if some of the -gas were removed from the air, gas would escape from the fluid. Gas -passes from the medium in which its tension is high to the medium in -which its tension is low. The tension of carbonic acid in tissues, -particularly in muscles and glands, is higher than in lymph; in lymph -higher than in blood; in blood higher than in air. Hence it passes by -these several stages from the tissues in which it is formed to the air -in the lungs. Much ingenuity has been devoted to perfecting methods -for the determination of the tension of carbonic acid in lymph and -in venous blood. Frequently results have been obtained which seemed -opposed to the doctrine that carbonic acid progresses from one medium -to another in accordance with the law of pressures; but such perplexing -results were probably due either to imperfections in method or to the -establishment of abnormal physiological conditions during the course of -<span class="pagenum"><a name="Page_190" id="Page_190">[Pg 190]</a></span> -the observations. When, for example, it was found that the tension -in lymph was less than the tension in blood, the specimen of lymph -examined was probably not in the same condition as the lymph in the -tissue-spaces where the exchange occurs. The experimenter in such -a case was in error in supposing that the specimen of lymph which -he examined contained as much carbonic acid as did the lymph in the -tissue-spaces from which the blood which he compared with it received -its supply of this gas.</p> - -<p>We have already given the figures for the composition of the air in the -air-chambers of the lungs. The figures commonly accepted as correct for -the percentages of the several gases in the blood are, at 0° C. and 760 -millimetres of mercury pressure:</p> - -<table border="0" cellspacing="0" summary=" " cellpadding="0" > - <thead><tr> - <th class="tdl"> </th> - <th class="tdc"> Oxygen. </th> - <th class="tdc">Carbonic<br /> anhydride. </th> - <th class="tdc"> Nitrogen.</th> - </tr> - </thead> - <tbody><tr> - <td class="tdl">In 100 vol. of arterial blood</td> - <td class="tdc"> 20</td> - <td class="tdc">39</td> - <td class="tdc">1-2</td> - </tr><tr> - <td class="tdl">In 100 vol. of venous blood</td> - <td class="tdc">8-12</td> - <td class="tdc">46</td> - <td class="tdc">1-2</td> - </tr> - </tbody> -</table> - -<p>This table shows the gain in oxygen and the loss in carbonic acid which -results from the passage of blood through the capillaries of the lungs. -The aerated blood returned to the heart by the pulmonary veins contains -8 to 12 volumes per cent. more oxygen, and about 7 volumes per cent. -less carbonic acid, than the blood which the pulmonary artery carries -to the lungs.</p> - -<p>As to the physics of this exchange, the air in the recesses of the -lungs contains about 16·36 per cent. of oxygen, and an amount of -carbonic acid variously estimated at from 2·57 per cent. to 3·84 per -cent. Of the 760 millimetres of mercury which the atmosphere holds up -in a barometric tube, the oxygen in the alveoli of the lungs supports</p> - -<table border="0" cellspacing="0" summary=" " cellpadding="0" > - <tbody><tr> - <td class="tdc">760 × 16·36</td> - <td class="tdc"> </td> - </tr><tr> - <td class="tdc">————</td> - <td class="tdc"> = 124·33 millimetres;</td> - </tr><tr> - <td class="tdc">100</td> - <td class="tdc"> </td> - </tr> - </tbody> -</table> - -<p class="no-indent">the carbonic acid, at the lower figure quoted -(2·57 per cent.), 19·5 millimetres.</p> - -<p>The tension of gases in arterial blood is ascertained by opening an -artery into a closed vessel which contains nitrogen mixed with oxygen -and carbonic acid at about the tensions which it is computed that they -have in the blood. If the amounts of these gases are exactly right, no -exchange occurs between the blood and the mixture of gases. The mean of -<span class="pagenum"><a name="Page_191" id="Page_191">[Pg 191]</a></span> -many observations made in this way by various physiologists is, for -oxygen in the blood 72·2 millimetres mercury pressure, for carbonic -acid 20·5 millimetres mercury pressure. At a glance it is seen -that, since the tension of oxygen in the blood never exceeds 72 -millimetres, whereas its tension in pulmonary air never falls beneath -124 millimetres, there is no difficulty in accounting for its passage -from air to blood. The position is somewhat otherwise with regard to -carbonic acid. Aeration continues in the lungs until the tension of -this gas in the blood returning to the heart does not exceed 20·5 -millimetres; whereas the tension in pulmonary air, even accepting -the lowest figure obtained by experimental means, is as high as 19·5 -millimetres. This leaves a very small margin of pressure to account for -the escape—and it is undoubtedly a rapid escape—of carbonic acid -from blood as it circulates through the lungs. As was said regarding the -fixation of carbonic acid in the blood, it is somewhat doubtful whether -the problem has been completely solved.</p> - -<p>The carbonic acid exhaled contains all the carbon of the digestible -food, with the exception of a comparatively small quantity given off in -urea. It amounts to about 900 grammes per diem.</p> - -<p>How are we to determine the quantity of air which an individual -requires? We can but make the general statement that it must be -sufficient to dilute the carbonic acid exhaled to an extent which -precludes poisoning. It is impossible to fix a limit. Breathing becomes -embarrassed, and frontal headache and other symptoms make themselves -felt when 10 per cent. of pure carbonic acid is mixed with air. Even in -so large a proportion as this, carbonic acid is not fatal to life. Yet -an atmosphere in which there is present a hundredth part of this amount -of carbonic acid, produced by respiration, is extremely injurious to -health under the ordinary conditions in which people live. It may be -asserted, therefore, that under ordinary conditions 0·1 per cent. is -the extreme limit for wholesome living. But again we are obliged to add -that air contaminated to this extent is not under all circumstances -injurious to health. The explorers on the recent Antarctic Expedition -were obliged at times to sleep three men in one sleeping-bag, with -the aperture of the bag tightly closed. The atmosphere must have been -heavily laden with carbonic acid. Dr. Wilson assures us that it was -<span class="pagenum"><a name="Page_192" id="Page_192">[Pg 192]</a></span> -impossible to keep a pipe alight inside the bag. Not that any man so -placed would desire, one would imagine, to add the combustion-products -of tobacco to those given off from the lungs! The survival of the -explorers proves that it is impossible to fix a limit of safety even -for the carbonic acid in air vitiated by respiration. It is, however, -a matter of common observation that air which is moist and warm, -owing to respiration, and tainted with the odours of humanity, is -extremely prejudicial to those who live in it. Such an atmosphere -is a favourable medium for the conveyance of germs, whether of the -common cold or of a more virulent type. At one time it was supposed -that the volatile emanations which can be condensed, along with water, -by hanging a vessel of ice to the ceiling of a crowded room, were -actively poisonous; but this statement has not been confirmed by recent -research. It is unnecessary to call any such evidence in support of the -thesis that human beings thrive better in fresh air than in foul. The -admirable results achieved by the “fresh air cure” show that there is -no degree of vitiation which can be pronounced innocuous. Nevertheless, -public opinion demands that sanitarians should give some figure as a -guide. Commonly they fix the maximum of carbonic acid compatible with -health at 0·06 per cent., the quantity of carbonic acid being taken -as the measure of all impurities present. An adult exhales about 0·6 -cubic foot of <b>CO₂</b> per hour. Fresh air already contains about 0·04 per -cent. If, therefore, the percentage is not to rise higher than 0·06 -per cent., each adult must be supplied with 3,000 cubic feet of air -per hour. With good ventilation air may be changed four times an hour, -and therefore 800 cubic feet is regarded as sufficient space for each -occupant of a room. The figure may pass. It is a reasonable basis from -which to calculate the packing capacity of a dormitory. So long as a -man has 800 cubic feet of air to himself, he may safely feel that he -has room to stretch his lungs. Dwelling on this figure may make him -feel uncomfortable when he finds himself in a railway carriage, seated -five on a side, with the windows closed. In the theatre or in church -he may doubt whether he has all the fresh air to which his humanity -entitles him. But, as a philosopher rather than as a physiologist, he -reflects that, whether on the Antarctic icecap in a sleeping-bag or -<span class="pagenum"><a name="Page_193" id="Page_193">[Pg 193]</a></span> -standing on a summit in the Alps, he takes all that he can get, for -fresh air is one of the few good things of which one can never have -enough.</p> - -<p><b>Tissue Respiration.</b>—A frog will live for seventeen hours -in an atmosphere of nitrogen. Under these circumstances it is clearly -impossible for it to take up oxygen, yet for several hours it gives -off as much carbonic acid as it would do if it were living in air. -Such an observation as this proves that oxidation does not occur in -the lungs, but deeper in the body. At one time the blood was regarded -as the seat of oxidation; the products formed by the splitting up of -proteins in the tissues were supposed to be passed into the blood, -where they came in contact with the oxygen carried by hæmoglobin. A -certain amount of oxidation does take place in the blood, as in all -other tissues, for blood is a living tissue and needs to respire. But -the oxidation which occurs in the blood is small in amount as compared -with that in the organs which the vessels traverse. Muscle and other -tissues detached from the body and free from blood give off carbonic -acid. It is possible to wash the blood out of the vessels of a frog and -to replace it with a solution of salt. In an atmosphere of oxygen such -a “saline frog” lives for a day or two, taking in the same quantity of -oxygen and giving off the same quantity of carbonic acid as a normal -frog. The oxygen is chiefly absorbed through the skin, the carbonic -acid discharged from the lung. This experiment shows that blood is not -essential for oxidation. Oxidations do not occur in the salt solution -with which blood is replaced. Taking all the evidence together, it -seems to be safe to conclude that the tissues absorb the oxygen which -the oxyhæmoglobin brings into their neighbourhood, and that they have -some capacity of storing it. A piece of detached muscle which gives off -carbonic acid in an atmosphere of nitrogen would appear to be holding a -store of oxygen, much as hæmoglobin holds it. The proof is not quite so -definite as might be desired; but we are probably justified in holding -the belief that the main part of the respiratory exchange occurs in -the tissues. Lymph dissolves oxygen which it obtains from the blood. -The tissues take it from lymph. Tissues set free carbonic acid which -lymph dissolves. Its tension being higher than in blood, carbonic acid -diffuses from lymph, through the walls of the capillary vessels, into -blood, from which it passes into the air in the lungs.</p> - -<hr class="chap" /> -<p><span class="pagenum"><a name="Page_194" id="Page_194">[Pg 194]</a></span></p> -<div class="chapter"> - <h2>CHAPTER VIII<br /><span class="h_subtitle">EXCRETION</span></h2> -</div> - -<p>Many things enter into the alimentary canal. If an analysis were -made of a day’s food and drink, from the cup of tea on waking to the -cocoa or other potion which is regarded as a necessary preliminary -to settling for the night, it would be found that a great variety of -substances were included in the food or taken as adjuvants to food. All -these things, differing widely in chemical constitution, must leave -the body. Some are not digested. They do not, properly speaking, enter -into the diet. Such are the cellulose of vegetables, especially skins, -husks, woody fibres; elastic fibres of meat; horny substances, etc. The -quantity varies greatly, according to the nature of the diet. About 2 -ounces (weighed dry) is the average. With this indigestible refuse is -included undigested food, if the diet be excessive, and a variety of -substances secreted by the liver, such as cholesterin and bile-pigment, -some residues of the secretions of the alimentary canal, and products -of bacteric fermentations. All food which is digested and absorbed is -oxidized. It leaves the body by the lungs, the kidneys, or the skin. -Foods, as already stated, are classified as proteins, carbohydrates, -and fats. The chief excreta are carbonic acid, water, and urea. -Carbonic acid makes its exit from the lungs; water from the lungs, the -kidneys, and the skin; urea from the kidneys. The three great groups of -foods and the three great groups of excreta overshadow in amount all -the other substances which pass through the system. A balance-sheet in -which proteins, carbohydrates, and fats appear on one side, carbonic -acid, water, and urea on the other, is substantially correct. The -energy which is set free by burning in a calorimeter the items entered -on the debit side, after deducting that yielded by burning the urea -<span class="pagenum"><a name="Page_195" id="Page_195">[Pg 195]</a></span> -(carbonic acid and water are incapable of further oxidation), gives a -day’s income. Other constituents of the diet are so small in quantity -as to be negligible in making up the body’s accounts. The chemical -changes which they undergo add practically nothing to its capacity for -work. Yet some of them are essential to the maintenance of health. Of -such are common salt (sodic chloride), alkaline and earthy carbonates, -sulphur, phosphorus, etc. These things, together with some products -of action of the bacteria in the alimentary canal, the final stage of -hæmoglobin, imperfectly oxidized nitrogenous substances, and other -soluble substances which enter with, or are formed from the food, -are removed by the kidneys. We speak of the elimination of waste -products, as excretion. Not that there is any physiological distinction -between excretion and secretion. Both terms refer to the selection or -production and the discharge of materials by cells. If the product -discharged has a useful function to perform—if it be a digestive -ferment, for example—it is said to be secreted. If it is of no further -use to the economy, we say that it is excreted—got rid of. In some -cases either term is equally appropriate. The sebum prepared by the -sebaceous glands is useful as a lubricant of the skin. It is thrown -off. We may speak of the glands as either secreting or as excreting -this fatty substance.</p> - -<p><b>The Kidney.</b>—From worms upwards, all animals possess organs -for the removal of waste products in solution. This statement might, -indeed, be widened so as to include animals even lower than worms. All -animals which have a cœlomic cavity—a space between the alimentary -canal and the body-wall—have organs for the removal of soluble waste. -The segmental organs of worms are obviously the same organs as the -kidneys of mammals; the latter are distinguished from their prototypes -by greater concentration of structure and specialization of function. -The kidney is the oldest of organs, if its antiquity be estimated as -the length of time during which it has had a form practically identical -with that which it now presents. The lungs are of late appearance in -the animal scale. Alimentary canal, heart, brain, have passed through -many transformations. The kidney assumed its permanent form very far -back in the history of the animal kingdom. The most primitive animal -<span class="pagenum"><a name="Page_196" id="Page_196">[Pg 196]</a></span> -which has a digestive cavity, and vessels in which the products of -digestion circulate, needs an organ which provides for the overflow -from the body-fluids of all substances which are injurious or effete.</p> - -<p>The kidney is an aggregation of long urinary tubules. The head of -each tubule is dilated into a globular capsule, into which a tuft of -bloodvessels depends. This is the sink into which the waste-water -of the blood drips. The long urinary tubules are lined with cells -well qualified by form and constitution to search the blood in the -capillaries which border them, for substances which, not being easily -diffusible, have to be forcibly dragged from it and added to the water -trickling down the pipe which connects the rain-water head with the -sewer. The hydrostatic conditions of this apparatus—the provision for -greater or less flow of blood through the tufts (glomeruli) which hang -in the capsules, and for longer or shorter exposure of the blood to -the purifying activity of the epithelium of the renal tubules—will be -described after a very brief account has been given of the structure of -the organ.</p> - -<p>The outer border of the kidney is convex, its inner border concave. The -concavity is termed the “hilus.” The central depression of the hilus is -embraced by the expanded end of the ureter—the tube which carries the -secretion of the kidney to the bladder. The renal artery and the renal -nerves enter, and the renal vein leaves, the kidney at the hilus.</p> - -<p>If a kidney be split longitudinally, it will be noticed that its outer -part, the cortex, is darker in colour than its inner part, the medulla -(<a href="#I_197">Fig. 9</a>). The glomeruli already referred to occur in the -cortex. The medulla is occupied by radiating tubules, collected into groups. -Those of each group converge towards a common duct. From twelve to eighteen -ducts open into the expanded end of the ureter, each at the apex of -a pyramid. If the section of the kidney be examined with a lens, it -will be seen that narrow rays from the medulla extend into the cortex. -The cortex is therefore made up of interdigitating pyramids of dark -substance, consisting of glomeruli and the contorted tubules, about to -be described, and of lighter substance, consisting of straight tubules -continuous with those of the medulla. -<span class="pagenum"><a name="Page_197" id="Page_197">[Pg 197]</a></span></p> - -<div class="figcenter"> - <a id="I_197" name="I_197"> </a> - <img src="images/i_p197.jpg" alt="" width="500" height="474" /> - <p class="center"><span class="smcap">Fig. 9.</span>—<span class="smcap">The Upper - End of the Left Kidney, vertically divided, and magnified.</span></p> - <div class="blockquot2"> - <p class="neg-indent space-below2">It is invested by a capsule with which, at the hilus, the - dilated end of the ureter blends. A portion of a papilla (the end of a pyramid) is - shown projecting into one of the calices into which the ureter dilates. - The peripheral portion of the kidney containing glomeruli and contorted - tubes is termed its cortex, the central portion medulla. At A is shown - a single urinary tubule. Commencing at the third glomerulus, it winds - in the cortex, descends into the medulla, turns in a loop of Henle, - again winds in the cortex, and ends in a collecting tube, which joins - a duct. The arrangement of the bloodvessels is shown at B. A straight - artery and a straight vein lie side by side. The artery gives branches - to the glomeruli. The venules from the glomeruli again divide into - capillaries, which supply the contorted tubes and loops of Henle. The - ducts are supplied by long arterial capillaries. C shows the structure - (magnified) of a glomerular tuft of capillary vessels, invested by a - capsule which closes into a contorted tube, <i>ct</i>; <i>dH</i>, a descending - limb; <i>aH</i>, an ascending limb of a loop of Henle; <i>d</i>, a duct.</p> -</div></div> - -<p>The urinary tubules are the separate pieces of apparatus of which -the kidney consists. The problems connected with a single tubule are -therefore the problems of the kidney as a whole. These structures are -all exactly alike. The description of any one of them applies to all. -Each begins as a capsule containing a glomerulus. The wall of the -<span class="pagenum"><a name="Page_198" id="Page_198">[Pg 198]</a></span> -bulb—which is merely a thin basement membrane covered by epithelial -scales—is involuted by the tuft of bloodvessels. The vessels do not -penetrate its capsule. Between the tessellated epithelium which covers -the tuft and the similar epithelium which lines the capsule there is a -space communicating by a narrow aperture with the next portion of the -tubule—termed its “contorted” part, because it is twisted about like a -tangled thread in the cortex of the kidney. The contorted tubule is of -relatively large calibre. The cells which line it are irregular in form -and indistinct in outline. The basal half of each cell, between its -nucleus and the basement membrane, is vertically striated, or “rodded,” -as it is usually termed. Such an arrangement of the protoplasm of a -cell is commonly associated with a habit of absorbing fluid. It would -seem to indicate in this case that the cells take water and various -substances dissolved in water from the direction of the basement -membrane. After a time the contorted portion of the tubule, although -still sinuous, becomes more nearly straight—the “spiral portion”—and -assumes a radial direction. In the zone between the cortex and the -medulla, the spiral portion tapers into an exceedingly slender tubule -which, after running some distance in the direction of the hilus, turns -back again towards the cortex, making a loop, known as the “loop of -Henle.” The ascending limb of this loop is of larger calibre than the -descending limb. The descending limb is lined by flattened epithelium, -each cell so thin that (in microscopic sections as ordinarily -prepared) its nucleus bulges into the lumen of the tube. The cells -of the ascending limb are more nearly cubical in form. On reaching -the cortex, the tubule again becomes contorted. The second contorted -portion narrows into a “collecting portion,” which joins a ductule. The -ductules unite together, until at last a single duct is formed which -opens at the apex of a pyramid. The cells of the ductules are cubical -or columnar. Their cell-substance is clear, whereas that of the cells -lining other parts of the tubule is cloudy in appearance.</p> - -<p>Such a tubule, viewed as a hydrostatic mechanism, presents three -portions, evidently fitted for different functions: (1) The glomerulus -is an apparatus which allows of the rapid exudation of water from -<span class="pagenum"><a name="Page_199" id="Page_199">[Pg 199]</a></span> -blood. (2) The contorted portions of the tubule present the appearance -of a secreting mechanism. The large soft, cloudy cells which line them -are eminently fitted to take from the blood, or rather from the lymph -which fills the tissue-spaces which intervene between the walls of the -capillary bloodvessels and tubules, the various substances which they -excrete. (3) The loop of Henle is a remarkable piece of apparatus, the -purpose of which has been a subject of much controversy. Looking at it -from the point of view of hydrostatics, it seems safe to conclude, from -its extremely narrow bore, that it raises the pressure of the fluid -in the glomerulus and first contorted portion; but it may have other -functions also.</p> - -<p>A consideration of the arrangement of the bloodvessels of the kidney -bears out the conclusion that the secreting apparatus is divisible into -at least two separate portions, possibly into three. The glomeruli -are supplied by short and relatively wide arterioles. Each arteriole -breaks up, as soon as it enters the capsule, into a bunch of capillary -vessels, which, in the same abrupt manner, reunite to form a venule. -On leaving the capsule, this little vein behaves in a fashion for -which the only parallel is to be found in the portal system of the -liver. Instead of uniting with a larger vein, it again breaks up -into capillary vessels, which supply the contorted tubules and loops -of Henle. The medulla of the kidney is supplied by long arterial -capillaries of the usual type. The short arterioles of the glomeruli -are controlled by nerves which, constricting them, or allowing them -to dilate—possibly by actively causing them to dilate—rapidly -diminish or increase the amount of blood passing through their tufts -of capillary vessels. Here, therefore, is a mechanism by which the -glomeruli can be suddenly flushed with blood—a condition favourable -to exudation into the urinary tubules. The interposition of a second -set of capillaries prevents this sudden flushing from unduly disturbing -the pressure in the vascular system as a whole. In the renal-portal -capillaries of the kidney the blood-pressure is fairly constant and, -presumably, low. The use of the term “renal-portal” is justifiable, not -only on the ground that the vessels of the kidney behave like those of -the portal system of the liver, but also owing to the very significant -fact that in fishes and amphibia the kidney actually has a double -blood-supply. In such an animal as the frog the glomeruli are supplied -<span class="pagenum"><a name="Page_200" id="Page_200">[Pg 200]</a></span> -with arterial, the tubules with venous, blood. The glomeruli receive -branches from the renal artery, the tubules from a portal system -derived from veins of the abdomen and hind-legs.</p> - -<p>Sir William Bowman, who in 1842 gave the first detailed description -of the microscopic structure of the kidney, concluded that, whereas -“the tubes and their plexus of capillaries are probably the parts -concerned in the secretion of that portion of the urine to which its -characteristic properties are due (the urea, lithic acid, etc.), the -Malpighian bodies [<i>i.e.</i>, the glomeruli] may be an apparatus destined -to separate from the blood the watery portion.”</p> - -<p>All physiologists are in accord in regarding the glomeruli as the -principal seat of exudation. There is great diversity of view as to the -function of the tubules. In 1844 Ludwig advanced the opinion that all -the constituents of the urine pass through the glomeruli in a large -excess of water, and that in the course of the tubules this excess of -water is reabsorbed. This theory was based, among other considerations, -upon the extreme thinness of the epithelium which covers the glomerular -tufts; he judged that water would filter through it very readily. A -large amount of experimental work has been directed to the solution -of these two problems—viz., (1) Do urea and other similar substances -pass through the glomeruli? (2) Is water returned from the tubules to -the venous system? Our views as to the functions of the kidney as a -whole will not be greatly influenced by the answers that may eventually -be given to these questions; yet their discussion is of very great -interest, owing to the nature of the evidence which may be marshalled -on either side.</p> - -<p>There is, perhaps, no other organ in the body the problems with regard -to which seem to be so nearly plain questions of hydrostatics. It is -easy to make a model of a urinary tubule and its blood-supply. If such -a model were shown to a sanitary engineer, and he were asked to explain -the working of the drainage system of the body, and especially to -answer the two questions which we have propounded, he would say that -there could be no doubt as to the part of it through which most water -enters the tube, the glomerulus. He could give no opinion as to whether -<span class="pagenum"><a name="Page_201" id="Page_201">[Pg 201]</a></span> -urea, uric acid, and other substances of a like nature, accompany the -water until he had tried the experiment of separating blood from water -containing the inorganic salts of urine by a permeable membrane—the -blood being at such a pressure as the physiologist told him he might -expect it to have in renal arterioles, the water at such a pressure as -he might expect it to have at the upper end of a urinary tubule. He -would find that urea, and still more uric acid, is very reluctant to -pass through the membrane. Again, when asked whether water, in which -urea and other things were dissolved, would leave the tubule—say from -the loop of Henle—to pass back into the blood, he would repeat his -experiment with a membrane. This time he would allow the urine and the -blood to be at the same pressure (or, possibly, would assign a higher -pressure to the former), and he would dilute the urine to make the -conditions agree with those which Ludwig supposed to exist; but his -experiment would prove to him that, unless the urine were very dilute -indeed, water would still tend to pass into it from the blood, and not -<i>vice versa</i>. And here it may be remarked that the results of these -experiments might have been predicted by calculation. When Ludwig -advanced his theory, osmosis was a mysterious phenomenon. Its laws -have since been accurately ascertained. Given the molecular weights of -bodies in solution and their degree of concentration, the direction -in which they will pass through a membrane can be predicted. The -force with which water will tend to pass from one solution to another -can be calculated. Urine as secreted contains far more urea, sodic -chloride, and other salts than blood. It has a much higher degree of -concentration. The concentration of blood is 0·55; that of urine, 1·85. -Water passes from a less concentrated to a more concentrated solution, -not <i>vice versa</i>. As a solution of a problem in hydrostatics Ludwig’s -hypothesis is untenable.</p> - -<p><b>Osmosis.</b>—Cells of all kinds, both vegetable and animal, are -limited, or surrounded by a layer of cell-substance which is firmer -than, and probably different in constitution from, the substance in -the interior of the cell. This outer layer is a living membrane. The -nutrition and growth of the cell are dependent upon the capacity of its -limiting membrane for regulating the ingress and egress of water and of -substances dissolved in water. The phenomena of osmosis—that is to say, -<span class="pagenum"><a name="Page_202" id="Page_202">[Pg 202]</a></span> -of the passage of water and of solutions through membranes—are of such -high importance in relation to the life of the tissues that it may be -permissible to make a further digression for the purpose of describing -them (<i>cf.</i> pp. 40, 128). A very simple apparatus will suffice to -exhibit a phenomenon which will give an idea of the meaning of osmosis. -If the top of a glass funnel, covered with a piece of bladder, so -fastened to its edge as to make it water-tight, be fixed in an inverted -position in a glass vessel, the glass vessel filled with water, and the -funnel filled to the same level with a solution of sugar, it will soon -be evident that water is passing through the membrane into the funnel. -The level of the sugar-solution will rise in the tube of the funnel. -If, instead of water outside the funnel and sugar-solution inside it, a -strong solution of sugar be placed in the funnel and a weaker solution -outside it, water will leave the weaker for the stronger solution, and -sugar the stronger solution for the weaker. If some of the solution in -the funnel be removed from time to time so that the pressure in it is -kept down to the same level as that outside it, water will continue -to enter through the membrane and sugar to leave the contents of -the funnel until the concentration of sugar is the same on the two -sides. The fluids will then be of identical composition, and therefore -isosmotic. In the further consideration of the phenomena of osmosis, a -distinction must be made between permeable and hemipermeable membranes. -Suppose in the first instance that a permeable membrane is used. Let -it be so placed as to separate two watery solutions of different -constitution, yet of the same osmotic pressure. By their being of the -same osmotic pressure is meant that they are of the same molecular -concentration. The liquid A contains certain salts in solution; but the -liquid B may contain the same salts in quite different proportions. -It so happens, however, that the salts are so balanced that the total -tension of the salts in A is equal to the total tension of the salts -in B. At first there may be some change in level in the two liquids, -owing to differences in rates of diffusion through the membrane of the -various salts which they contain; but after a time the levels of the -two liquids will be the same. To outward appearance, nothing will have -happened. Nevertheless, if the experiment has been continued for a -<span class="pagenum"><a name="Page_203" id="Page_203">[Pg 203]</a></span> -sufficient length of time, it will be found that great changes have -occurred in the constitution of the two liquids. At the commencement, -although their total tensions were equal, the proportions in which -the various salts were distributed in A, and therefore their partial -tensions, were very different to their proportions and partial tensions -in B. At the end of the experiment each of the several salts is equally -divided between A and B, supposing the volume of A to equal that of B. -This experiment shows that the molecules of substances in solution are -free to move. They behave like gases. Gases diffuse through a membrane -until their partial tensions are the same in the two spaces which the -membrane separates. The æther in which physicists picture gases as -dissolved offers no resistance to the migration of their molecules; -neither does the solvent—water, for example—prevent the movement -of salts which are distributed through it.</p> - -<p>One other illustration of the phenomena of osmosis will suffice to -give an idea of the laws by which they are governed. In the case just -cited the membrane was permeable to all the salts in solution. When the -phenomena of osmosis were first investigated, a distinction was drawn -between substances which will pass through membranes—crystalloids—and -substances which cannot pass—colloids. We have already had occasion -to note that, whereas albumin is a colloid which does not diffuse, its -hydrate, peptone, is a crystalloid which does. The term “crystalloid” -indicates that substances which can be crystallized are diffusible. -Substances which are diffusible are therefore allied to those which -crystallize. The nature of the membrane used to test diffusibility was -not at first taken into account. Now a distinction is drawn between -membranes which are permeable to all diffusible substances, and -membranes which are permeable to the solvent, but impermeable to the -substances which it dissolves. The latter are termed “hemipermeable.” -Imagine now that water is separated from a solution of sugar by a -membrane which stops sugar, but is permeable to water. Water will pass -through the membrane into the solution of sugar. The level of the -solution will rise. Pressure will be needed, and a very considerable -pressure, to prevent its rising—to prevent endosmosis, that is to say. -<span class="pagenum"><a name="Page_204" id="Page_204">[Pg 204]</a></span> -The force needed to resist osmosis is directly proportional to the -degree of concentration of the solution. If the solution contain 1 per -cent. of sugar, a pressure of 500 millimetres of mercury is needed; -if it contain 2 per cent., a pressure of 1,000 millimetres; if 6 per -cent., of 3,000 millimetres.</p> - -<p>In the next experiment separate two solutions, A and B, by a -hemipermeable membrane. Let A contain one salt only—X; let B contain -several salts—X, Y, Z. Water will pass from A to B, or <i>vice versa</i>, -unless the osmotic pressure of the salts which the solutions contain -is the same. The osmotic pressure will be found to be the same if -the total number of molecules dissolved in A equals the total number -of molecules dissolved in B. If in A there be N molecules of X (per -unit volume), and if in B there be nX, n′Y, n″Z, the osmotic pressure -will be the same provided n + n′ + n″ = N. This, it will be seen, -is a very different matter from equality of percentage composition. -Some molecules are light; others are heavy. The percentage weight of -X + Y + Z in B may be very different from the percentage weight of -X in A. To estimate the osmotic pressure of a mixed solution, it is -not sufficient to add together the percentages of the various salts -which it contains. “Concentration,” in the sense in which it was used -in regard to blood and urine, refers to the number of molecules of -dissolved substances in a given volume, not to their weight.</p> - -<p>It would be undesirable to attempt in this place to enter upon the -theory of osmosis. Enough has been said to suggest to the reader that -he should, when endeavouring to apply its laws to the explanation -of physiological phenomena, bear the following facts in mind: Some -membranes are permeable to water and to the crystalloids which it -dissolves; others, although permeable to water, are impermeable -to substances in solution. Some substances are diffusible through -permeable membranes; others are not. Osmosis of water occurs from the -solution of lower to the solution of higher concentration. Diffusion of -crystalloids is their escape, owing to their own molecular movements, -from a situation in which they are denser to a situation in which they -are less dense. It must be added, however, that various circumstances -prevent the reduction of the laws of osmosis to simple terms—the -tendency of salts to dissociate when in solution, their bases and acids -<span class="pagenum"><a name="Page_205" id="Page_205">[Pg 205]</a></span> -acting as independent “ions,” is an example of the complications -which produce apparent departures from these laws. It must further be -added, and with emphasis, that, important though it be that anyone who -attempts to explain the interchanges which occur between the various -fluids of the body should be conversant with the laws of osmosis, -it is impracticable, and in some cases misleading, to rigidly apply -them. Living membranes and dead membranes do not necessarily control -diffusion in the same manner. Still less do the laws which govern -diffusion through dead membranes hold good, without qualification, to -living cells.</p> - -<p>To return to the sanitary engineer whose opinion we asked regarding -the mode of working of the drainage system of the kidney. Probably he -would deny that the problems came within his province. “They are not -physical, but vital,” he would say. “I know nothing about the vital -action of the cells which line the tubule.” Objection may be taken to -the form of expression, albeit he was fully justified in declining to -discuss the question any further. He does not know enough about the -internal structure of a cell to be able to predict the phenomena of -osmosis which will occur within it. No one can say what capacity living -cells may have of taking substances from the blood, returning some of -them, and excreting others. This unknown capacity leads to results -which, when they do not appear to be in accordance with the laws of -physics, are commonly termed “vital.” The term is a stumbling-block -which has tripped up generations of physiologists. The expressions -“vital action” and “physical phenomena” have been used as if they -were antithetical, whereas all vital actions are physical phenomena. -“Vital” in this sense connotes “as yet unknown.” Yet, in truth, there -is abundant excuse for the use of a term which covers ignorance, so -long as its connotation is not extended until it assumes a positive, -antiphysical sense. “Physical” and “vital” are expressions which point -a contrast constantly present to a physiologist’s mind. He knows -perfectly well that the passage of water and salts through a membrane, -and their passage into and out of a living cell, are equally phenomena -of osmosis. But the former process he can test and measure in his -laboratory; the latter he can but observe in much obscurity in the -<span class="pagenum"><a name="Page_206" id="Page_206">[Pg 206]</a></span> -living body. He cannot make a model of a living cell. In the case -of the salivary gland, as we have already seen, living cells take -water from lymph, and discharge it as saliva in apparent opposition -to osmotic force. They reverse the direction of the flow which would -occur were lymph and saliva separated by a membrane. But a cell is not -a membrane. It is an extremely complicated structure with an elaborate -architecture of its own. As well might we compare the distribution of -water by a County Council water-cart and its passage through a brewery. -According to all the laws of hydrostatics, the water which flows into -a brewery should leave it through its drains. Its exit in barrels on -drays is antiphysical. When the physiologist can explore the living -cell, he will discover that the imbibition and extrusion of water, the -selection, retention, and discharge of salts, are phenomena as strictly -physical as their passage through a dialyser in his laboratory. In -the meantime he can but contemplate the cell with a certain degree -of awe. His best devised model of a urinary tubule may lead him into -error, for the simple reason that he cannot line it with living cells. -A living cell has a power which upsets all calculations, falsifies all -experimental findings. Its protoplasm can isolate and place out of -action any of the substances which enter it. If observations eventually -prove to us that water passes from the urinary tubules into the blood, -“in the face of osmotic force,” we shall be constrained to explain -this antiphysical phenomenon as due to the action of living cells. The -cells, we shall say, take up fluid from the urinary tubules, fix its -urea and other salts in their protoplasm, discharge its water into the -venous blood, return the urea and other salts to the urine. Given this -property of protoplasm, such a process is strictly in accordance with -physical laws.</p> - -<p class="space-above3">Enough has been said regarding the theory, or -want of theory, of the action of the kidney. Turning now to matters -of observation, it can easily be shown that the epithelium of the -tubules has the power of excreting into the urine highly complex -materials which diffuse with difficulty. If a substance soluble in -blood, but insoluble in urine, an alkaline salt of indigo, for example, -be injected into the vascular system, it is rapidly excreted by the -<span class="pagenum"><a name="Page_207" id="Page_207">[Pg 207]</a></span> -kidney. The indigo is precipitated even before it comes in contact -with the acid urine. If the animal be killed a short time after the -administration of the indigo, the contorted portions of its tubules and -the ascending limbs of the loops of Henle are strongly coloured blue. -An ammoniacal solution of carmine may be used for a similar experiment; -but the results are not nearly so sharply limited to the large-celled -portions of the tubules. Even the glomerulus is coloured red, a fact -which has been interpreted as showing that, although the greater part -of the carmine is excreted into the tubules, some of it accompanies the -water which exudes from the blood through the glomerular tufts.</p> - -<p>The practical identity in structure of the kidney in birds and -reptiles and mammals would seem to have an important bearing on this -controversy. The urinary excretion of birds consists almost exclusively -of uric acid. As seen under the microscope, it is a semi-solid white -deposit, made up of crystals, supposing no special precautions have -been taken to obtain it fresh. The water, pigment, and salts which are -essential elements of the excretion of mammals are practically absent. -Yet the kidney of a bird presents the same arrangement of glomeruli -and tubules as the kidney of a mammal, although the glomeruli are -relatively smaller. Uric acid diffuses with great difficulty. If it -is, so to speak, washed through the glomeruli, and the water which -dissolved it reabsorbed by the tubules, an enormous quantity of water -must pass through the kidney in order that it may carry the uric acid -in its stream. If uric acid be excreted by the epithelium of the -tubules, it is difficult to account for the presence of glomeruli, -since no water leaves the kidney. Crystals of uric acid are to be seen -in a section of the kidney, not only in the cells of the tubules, but -also in the glomeruli; but it may well be that in both situations -crystallization has been induced during the preparation of the section. -It jars an histologist’s conception of the constitution of a secreting -cell to contemplate the formation within its network of protoplasm, and -the extrusion from it, of sharp-angled crystals. As a matter of fact, -it is not in its crystalline form that uric acid is excreted by birds, -but as quadri-urates—<i>i.e.</i>, salts containing only one-fourth of their -“normal” complement of base; crystalline spheres or amorphous deposit, -not angular crystals. These quadri-urates decompose very quickly, -<span class="pagenum"><a name="Page_208" id="Page_208">[Pg 208]</a></span> -setting free crystals of uric acid. It must be confessed that, in -whatever way one attempts to account for the excretion of uric acid -by birds, the similarity of structure of their kidneys and those -of mammals is difficult to reconcile with the wide difference in -consistency and in chemical composition of the excrement.</p> - -<p>Reflecting upon all the evidence bearing upon the mechanism of the -mammalian kidney, the majority of physiologists come to the following -conclusions: The greatest outflow of water occurs in the glomeruli. The -water is accompanied by salts, including a small quantity of urea. The -contorted and spiral portions of the tubule and the ascending limbs of -Henle’s loops add to the urine the remainder of the urea, together with -various bodies still less readily diffusible.</p> - -<p>It may be that the chief function of the loops of Henle is to oppose -resistance to the passage of fluids, thus heading up the secretion, and -favouring the osmosis of water into it from the blood of the glomerular -capillaries. It is possible that the calibre of the slender descending -limbs is influenced by external pressure, their partial occlusion being -increased, and the pressure in them raised, when the organ is very -active and its intermediate zone turgid with blood.</p> - -<p>Various drugs influence the secretion of the kidney. In some cases -their action seems to be mainly hydrostatic. They change the rate of -flow by altering blood-pressure. Digitalis increases the force of -the heart. The heart beating more strongly, blood-pressure rises. -Higher blood-pressure is accompanied by a more copious secretion. -This action of digitalis is far more marked when the heart is out of -order than when it is healthy. In heart-disease the blood-pressure is -unduly low, and the tissues become water-logged in consequence. When -the blood-pressure is restored and a brisker capillary circulation -established, water and waste products, which have accumulated in lymph, -pass, as they ought to do, into the veins. Carried into the general -circulation, they overflow from the kidney.</p> - -<p>It is a little difficult to realize the abundance of the body-fluids. -From one-quarter to one-third of the whole body-weight is due to lymph, -using this term in its most general sense. The waste products of -tissues collect in the lymph. The blood circulating through capillary -<span class="pagenum"><a name="Page_209" id="Page_209">[Pg 209]</a></span> -vessels which traverse lymph-spaces takes up water and waste products. -Its just composition is maintained by the eliminating activity of the -kidneys.</p> - -<p>Even in the diuretic action of digitalis we see indications of -something more than an alteration of the hydrostatics of the -blood-supply of the kidney. The brisker circulation carries -waste products to the liver; the liver transforms nitrogenous refuse -into urea; urea stimulates the renal epithelium. It would be a mistake -to lay too much stress upon the direct effect of the drug upon the -blood-pressure in the kidney. Other illustrations throw the mere -hydrostatics of the problem into the background. Adrenalin (extract of -suprarenal capsule) causes a severe contraction of the small arteries, -which raises the general blood-pressure considerably; but the increased -blood-pressure is not accompanied by diuresis, because the glomerular -arterioles share to a full extent, perhaps to a disproportionate -extent, in the general constriction. In migraine and certain other -disorders it frequently happens that the blood-pressure in the aorta -is unduly high, yet very little fluid enters the renal tubules. If -a “saline diuretic,” potassic nitrate, sodic acetate, or some other -drug of the same kind, be administered, a copious flow is established, -the blood-pressure is relieved, the distressing symptoms disappear. -Then, again, certain diuretics, such as “sweet spirits of nitre,” -tea, gin, etc., may bring about a flow out of all proportion to the -alteration they produce in the hydrostatics of the circulation. The -diuretic action of these various drugs is clearly due to increase -in permeability of the renal epithelium. And, of all stimulants to -secretion, urea, the natural stimulant, is the most effective. If a -kidney be removed from the body, a cannula inserted into its artery, -and defibrinated blood caused to circulate under pressure through the -organ, water may or may not drip from the ureter. On addition of urea -to the blood, a copious excretion is set up. In explaining the mode of -working of the kidney, as, indeed, in explaining that of every other -organ of the body, the mechanical aspects of the problem must be kept -in the background. When we are contemplating the plan of construction -of the kidney, the hydrostatics of the circulation attract attention; -but alterations in hydrostatic conditions are not the initiating cause -of a greater or less flow of urine. The chemical condition of the blood -<span class="pagenum"><a name="Page_210" id="Page_210">[Pg 210]</a></span> -circulating through the kidney is the initiating cause. When the -presence in it of urea demands a more copious flow, the hydrostatic -conditions are adjusted to this need. In the case just cited of the -isolated kidney, it might be urged that the flow caused by urea is a -mechanical effect. The cells of the contorted portions of the urinary -tubules remove urea from the blood. They secrete it into the tubules. -The solution of urea, being headed up towards the glomeruli, owing to -the resistance offered to its passage down the tubules by the narrow, -descending limbs of Henle’s loops, surrounds the capillary tuft. Urea -rapidly attracts water from the blood. A copious flow is the result. -But it is just this contrast between the capacity of removing urea -possessed by living cells, and the passage of urea in solution from -one side to the other of a membrane, which justifies the retention of -the expression “vital.” Mechanical conditions are those which we can -imitate in a model; vital conditions, those which at present we are -unable to reproduce.</p> - -<p><b>Nitrogenous Waste.</b>—Meat, fish, eggs, milk, vegetable-albumins, -are the sources of nitrogen. The kidney is the organ which eliminates -it from the body. Since all nitrogenous food which is digested is -eventually reduced to simple, soluble compounds which appear in the -urine (the quantity thrown off in perspiration is so small as to be -negligible), the proportion which the nitrogen of the urine bears to -the nitrogen in the food is a measure of the efficiency of digestion. -A certain quantity of the nitrogen eliminated is in the form of uric -acid, creatinin, and other compounds of a like order; but these less -oxidized substances, though always present in some degree, are not, -in Man and other mammals, the normal end-products of nitrogenous -metabolism. Urea is the final and simplest product. It is therefore -sufficient to estimate the quantity of urea excreted, and to compare -the nitrogen which it contains with the nitrogen ingested in the form -of “animal food.” About nine-tenths of the nitrogen ingested should -be accounted for by urea. When alimentation is excessive or digestion -imperfect, the proportion is less than this; some nitrogenous food is -not absorbed; some that is absorbed is imperfectly oxidized.</p> - -<p><b>Urea</b> is characteristically an animal product. Inorganic -chemistry deals with stable, organic chemistry with unstable, -<span class="pagenum"><a name="Page_211" id="Page_211">[Pg 211]</a></span> -compounds. Not that there is any boundary between inorganic and organic -chemistry. They are merely terms which it is convenient to use to -indicate the groups of atoms which occupy the chemist’s attention at -the time. Nor is stability an attribute of certain groups, instability -an attribute of others. Stability is relative, not absolute. But -admitting these terms as convenient indications of degree, it may -be said that inorganic chemistry has to do with such substances as -carbonates, nitrates, ammonia; organic chemistry, with compounds -in which carbon is not satisfied with oxygen, as it is in carbonic -acid; nitrogen not satisfied with oxygen, as in nitric acid, or with -hydrogen, as in ammonia. Carbonic acid (anhye)drid has the formula CO₂; -ammonia, the formula NH₃. Urea is a combination of the two compounds. -It is carbonic acid in which one (divalent) atom of oxygen is replaced -by two (monovalent) atoms of ammonia. It is ammonia in which two -(monovalent) atoms of hydrogen are replaced by one (divalent) atom of -carbonic acid.</p> - -<table border="0" cellspacing="0" summary=" " cellpadding="0" > - <thead><tr> - <th class="tdc">Carbonic<br /> anhydride </th> - <th class="tdc" colspan="3">Urea</th> - <th class="tdc"> Ammonia</th> - </tr> - </thead> - <tbody><tr> - <td class="tdc"> </td> - <td class="tdc"> </td> - <td class="tdc"> </td> - <td class="tdc">NH₂</td> - <td class="tdc"> </td> - </tr><tr> - <td class="tdc"> </td> - <td class="tdc"> </td> - <td class="tdc">/</td> - <td class="tdc"> </td> - <td class="tdc"> </td> - </tr><tr> - <td class="tdc">CO₂</td> - <td class="tdc">CO</td> - <td class="tdc"> </td> - <td class="tdc"> </td> - <td class="tdc">NH₃</td> - </tr><tr> - <td class="tdc"> </td> - <td class="tdc"> </td> - <td class="tdc">\</td> - <td class="tdc"> </td> - <td class="tdc"> </td> - </tr><tr> - <td class="tdc"> </td> - <td class="tdc"> </td> - <td class="tdc"> </td> - <td class="tdc">NH₂</td> - <td class="tdc"> </td> - </tr> - </tbody> -</table> - -<p>Urea is an amide—carbonic diamide. It very readily takes -water into its molecule, changing into carbonate of ammonia. -<b>N₂H₄CO + 2H₂O = (NH₄)₂CO₃</b>. This change is rapidly brought -about by the influence of bacteria in urine exposed to the air.</p> - -<p>In thinking of the transformations which proteid substances -undergo in the system, it is legitimate to regard their nitrogen -as from the first united with hydrogen in the form of ammonia. -Not that the grouping is so simple as this. An albumin is not -an amide. But in the dance of atoms of its great molecule as -it progresses through the system—forming part of the blood, -taken up by the cells as floating protein, incorporated in the -protoplasm of the cells, shaken into smaller aggregates in the -muscles—nitrogen and hydrogen are partners. They leave the -body hand in hand. Gusts of oxygen atoms enter through the -lungs; use blood-corpuscles as carriages; dismounting, they -traverse lymph, forcing their way into the interior of the cells; -they join in the dance. With their strong arms they detach -carbon atoms and hydrogen atoms from the huge albumin chain. -<span class="pagenum"><a name="Page_212" id="Page_212">[Pg 212]</a></span> -As carbonic acid and water they bear them to the lungs. But nitrogen -clings to hydrogen. Oxygen cannot detach its grasp. Out of the molecule -of albumin this firmly united couple slips, without contributing -anything to the energy which moves the body and keeps it warm. Nitrogen -is not a source of energy. It even saves a portion of the hydrogen of -albumin from combustion. Urea burnt in a calorimeter has a balance of -energy to give up.</p> - -<p>Many attempts have been made to ascertain the stages through which -proteins pass on their road to urea. The search for intermediate -compounds is probably futile, since there is no sufficient reason -for supposing that proteins disintegrate in stages, each a step less -complex than the food and a step nearer to urea. Every nitrogenous -extractive found in the tissues is, of course, on its road to urea. It -will be removed as urea, unless indeed, like uric acid or creatinin, -it has to be excreted without further change. But it appears to be -impossible to discover in the tissues any nitrogenous compounds which -occur in sufficient quantity to justify us in regarding them as -inevitable halting-places on the downward road (<a href="#Page_146"><i>cf.</i> p. 146</a>).</p> - -<p>The metabolism of albuminous substances, like other oxidations, takes -place chiefly in muscles. Very little is known regarding the nature of -the products. Urea is not amongst them. Whatever they may be (<a href="#Page_267"><i>cf.</i> p. 267</a>), -they are carried to the liver, in which they are turned into urea.</p> - -<p>The metabolism of the body is not normally derived from the oxidation -of nitrogenous foods. Failing a sufficient supply of other kinds of -food, they may be used as sources of energy; but we must picture them -as splitting into carbonaceous and nitrogenous portions. If, after the -reserve of glycogen in the liver has been brought low by abstention -from carbohydrates and fats, nitrogenous food is consumed, and the -muscles are then called upon to do severe work, the amount of carbonic -acid and water given off rises at once. The excess of urea derived from -the nitrogenous food which was destroyed for the purpose of liberating -the energy which the muscles expended makes its appearance some time -later. If the diet contains a sufficiency of carbohydrates, muscular -work does not increase urea. The output of urea is exceedingly steady. -<span class="pagenum"><a name="Page_213" id="Page_213">[Pg 213]</a></span> -It is not increased by muscular work, nor diminished, beyond a certain -limit, by absence of food. The tissues are constantly throwing off -nitrogen-containing molecules, which, if the body is not to waste, must -be as constantly renewed.</p> - -<p><b>Uric Acid.</b>—When nitrogenous metabolism has reached the bottom, -when albuminous substances have been shaken into the simplest and most -stable compound or compounds which the muscles are capable of making -(we know not whether the end-products be one or many), they are carried -to the liver by the blood. The mammalian liver converts them into urea; -the liver of birds and reptiles changes them into uric acid. Uric -acid is not, however, completely absent from the urine of carnivorous -animals. In Man the amount excreted is about 0·8 gramme per diem, but -subject, even in perfect health, to considerable variations (0·2 gramme -to 1·4 gramme). There is no reason for thinking that uric acid is made -in the liver of mammals. On the contrary, it seems to be either an -end-product of the disintegration and oxidation of leucocytes (<a href="#Page_53"><i>cf.</i> p. 53</a>), -or, like certain other more complex nitrogenous compounds which -appear in very small quantities in the urine, the relic of albuminous -food which has missed the broad down-path, via muscles and liver, -to the kidney. It is a troublesome burden for lymph and blood, and, -unfortunately, the kidney finds difficulty in throwing it out. Uric -acid has a pernicious way of accumulating in tissue-spaces, producing -all the malevolent symptoms of gout. During an acute attack of gout the -quantity of uric acid in the system may be largely increased. It may be -so abundant in the blood that, when a sample is allowed to cool, uric -acid begins almost immediately to crystallize out. Speaking generally, -it is right to ascribe gout to an over-production of uric acid; -but it must be remembered that the balance between elimination and -production is very delicately adjusted. During an attack of gout the -amount excreted in the urine is not increased; frequently it is less -than usual. The clearing up of the attack is accompanied by abundant -excretion of urates, or lithates (λίθο, stone), as they used to be -called, because the “stones” which are found in the bladder consist -largely of uric acid. From this it appears that faulty distribution -and inadequate excretion have more to do with the development of the -<span class="pagenum"><a name="Page_214" id="Page_214">[Pg 214]</a></span> -symptoms of gout than over-production. In a previous chapter (<a href="#Page_140">p. 140</a>) -we gave as the predominant cause of gout acid fermentations -in the stomach. It does not, by any means, follow, however, -that we were right in correlating imperfect digestion with an -excessive formation of uric acid. It may well be that the gouty -symptoms to which hampered peptic digestion gives rise are -due in larger measure to a disturbance of the composition of -the body-fluids which renders them unfit to carry uric acid to -the kidneys in such a form, or in such relation to the fluid in -which it is dissolved, as will insure its escape into the urinary -tubules. The interference with the efficient working of the -system caused by accumulation in it of uric acid gives a particular -interest to all that is known regarding the nature and -origin of this substance.</p> - -<p class="space-below1">Uric acid has the formula <b>C₅H₄N₄O₃</b>. It -is a more complicated and a more stable body than urea. The deposits -of guano in Peru contain uric acid (the excrement of birds) which -has remained practically unchanged for years—for centuries, -perhaps. Its chemical nature is not completely understood. It can be -readily made to yield urea; and it can be formed by conjugating urea -with a nucleus derived from lactic acid (<a href="#Page_13"><i>cf.</i> p. 13</a>). -Its formula is therefore commonly represented as that of a diureide—a substance -containing two urea radicles:</p> - -<table class="large_font_bold" border="0" cellspacing="0" summary=" " cellpadding="0" > - <tbody><tr> - <td class="tdc" rowspan="5">CO</td> - <td class="tdc" rowspan="5"><img src="images/cbl-5.jpg" alt="" width="30" height="107" /></td> - <td class="tdc"> </td> - <td class="tdc">HN—</td> - <td class="tdc">CO </td> - <td class="tdc"> </td> - </tr><tr> - <td class="tdc"> </td> - <td class="tdc"> </td> - <td class="tdc">|</td> - <td class="tdc"> </td> - <td class="tdc"> </td> - </tr><tr> - <td class="tdc"> </td> - <td class="tdc"> </td> - <td class="tdc">C</td> - <td class="tdc">—NH</td> - <td class="tdc" rowspan="3"><img src="images/cbr-3.jpg" alt="" width="16" height="57" /></td> - <td class="tdc"> </td> - </tr><tr> - <td class="tdc"> </td> - <td class="tdc"> </td> - <td class="tdc">|</td> - <td class="tdc"> </td> - <td class="tdc"> CO</td> - </tr><tr> - <td class="tdc"> </td> - <td class="tdc">HN—</td> - <td class="tdc">C</td> - <td class="tdc">—NH</td> - <td class="tdc"> </td> - </tr> - </tbody> -</table> - -<p class="space-above1">But notwithstanding this inclusion in its -molecule of two radicles of urea, it is safe, when one thinks of the -contrast between urea and uric acid, to lay stress, in the case of the -former, on the binding of nitrogen to hydrogen; in the case of the -latter, on the binding of nitrogen to carbon.</p> - -<p>Uric acid is soluble with difficulty; it crystallizes in rhombs. It -forms salts, normal and acid. Those which appear in the urine are -always acid salts. As a treatment for “stone,” lithia water has long -had a reputation which it probably deserves, the acid urate of lithium -<span class="pagenum"><a name="Page_215" id="Page_215">[Pg 215]</a></span> -being the most soluble salt of uric acid which the kidney can secrete. -When uric acid is in excess in urine, brown crystals of uric acid are -deposited as “gravel” soon after it is passed. Even when not in excess, -uric acid crystals appear after a sufficient time. In other cases uric -acid, when in excess, is thrown down in the form of a cloud of acid -urates of sodium and other bases, which renders the urine turbid. These -urates are redissolved when the water is warmed.</p> - -<p>The more fortunate of human beings need never concern themselves -with the chemical history of uric acid. It is always present in -their body-fluids. It is excreted by the kidney. Its formation is -of no greater interest than that of creatinin and other nitrogenous -compounds which escape the almost universal reduction to urea. Persons -who have a uric acid diathesis are in a very different plight. Every -scrap of evidence bearing upon its origin is of supreme importance. -Unfortunately, the evidence collected as yet is scanty, and its -application for remedial purposes impracticable.</p> - -<p>The only disease in which uric acid is invariably in excess is -leucocythæmia. This is a condition or habit marked by the presence -in the blood of a very great number of white blood-corpuscles and -a paucity of red ones. The connection between this disease and the -production of uric acid is made plain by certain experiments in diet. -If flesh which contains relatively a large proportion of cell-nuclei -is eaten, the uric acid excreted is markedly increased. Sweetbread, -especially “neck sweetbread”—<i>i.e.</i>, thymus gland—is a -mass of comparatively small cells with large nuclei. If thymus gland be -substituted for all other meaty foods, the quantity of uric acid -appearing in the urine is doubled. A large increase in the quantity of -ordinary meat or fish consumed also increases uric acid, because all -meat-fibres contain nuclei. If egg-albumin be taken instead of meat, -uric acid is not increased. A sudden excess of muscular work leads to -an increase in uric acid, owing presumably to the unusual activity -of the tissues. This used to be very noticeable in the case of young -men during the first few days of “training” under the old system; -but it may have been due to the generous consumption of chops and -steaks, rather than to the increase in physical work, and consequent -destruction of tissue. Nuclei contain nucleo-proteins, which split into -<span class="pagenum"><a name="Page_216" id="Page_216">[Pg 216]</a></span> -proteins and nuclein. Chemically, it is reasonable to attribute to -nuclein the parentage of uric acid; a plausible line of descent can be -traced. The association of leucocythæmia with the production of uric -acid is probably due to the destruction of leucocytes which are present -in abnormal numbers (<a href="#Page_53"><i>cf.</i> p. 53</a>).</p> - -<p>Such is the evidence at present in the hands of physiologists. -Naturally, physicians have endeavoured to turn it to account. -Patients have been recommended to avoid animal foods which contain -nucleo-proteins—to take, instead of meat and fish, eggs, milk, -cheese, vegetable-albumins. Certain physicians contend that such a -diet is followed by the happiest results; others, equally competent, -and perhaps less biassed by “medical theory”—the most dangerous of -handicaps for anyone who practises an art which must ever remain -empirical—are satisfied that equally good results are obtained -by excluding from the diet eggs, milk, and cheese. Physiological -discoveries suggest treatment. Modern medicine is in the fullest sense -applied physiology. But treatment based upon theory must be controlled -by unprejudiced observation. It is possible that the gouty diathesis -may be held in check in certain cases by the exclusion from the diet -of certain kinds of nitrogenous food. The experience of generations -has taught us that the injudicious use of such articles of diet as -fruit, pastry, sugar, which do not contain nitrogen, is the main factor -in inducing an attack of gout; that imperfect digestion, sluggish -circulation, insufficient activity on the part of the kidneys, lead to -the accumulation in tissue-spaces of the <i>fons et origo malorum</i>. Even -sweetbread, which with the precision of a chemical experiment increases -the production of uric acid by a healthy person, is not necessarily -found unwholesome by those who are inclined to gout. It is amongst -the most digestible of all meat foods, and easy digestion covers a -multitude of metabolic sins.</p> - -<hr class="chap" /> -<p><span class="pagenum"><a name="Page_217" id="Page_217">[Pg 217]</a></span></p> -<div class="chapter"> - <h2>CHAPTER IX<br /><span class="h_subtitle">THE CIRCULATION</span></h2> -</div> - -<p>The blood circulates in a closed system of tubes, continuous from the -heart back to the heart. The walls of these vessels separate the blood -from the tissues. Nowhere, except in the spleen, does it come into -contact with any cells other than the lining cells of the vessels in -which it flows, and the exception made by the spleen is more apparent -than real. The spleen (<a href="#Page_79">p. 79</a>) is a kind of sponge invested -with a firm capsule. Small arteries discharge their blood into its spaces; small -veins collect it. But the organ is essentially a part of the vascular -system. Its spaces take the place of the capillary vessels which -connect arteries with veins in other situations.</p> - -<div class="figcenter"> - <a id="I_218" name="I_218"> </a> - <img src="images/i_p218.jpg" alt="" width="600" height="487" /> - <div class="blockquot2"> - <p class="center"><span class="smcap">Fig. 10.</span>—<span class="smcap">The Heart - cut in the Plane of its Long Axis, and the Vessels which open into and - out of it.</span></p> - <p class="center">Chordæ tendineæ attach the margins of the auriculo-ventricular valves - to musculi papillares which project from the inner aspect of each - ventricle.</p> -</div></div> - -<p>The blood makes a double circuit. From the right heart it passes -through the vessels of the lungs. Returning to the left heart, it is -driven through the body. Although the heart consists of two separate -pumps, it makes but a single organ. Its division into right auricle and -ventricle and left auricle and ventricle is but slightly indicated on -the surface. In most invertebrate animals the two pumps are distinct. -In some the lung-heart and the body-heart are on opposite aspects of -the body. But one must not, when thinking of the morphology of the -vertebrate heart, picture it as formed by the juxtaposition of two, -originally separate, pumps. Truly, in its very earliest stage of -growth, it is represented by two tubes which lie, in the embryo, far -apart. But these, before we can speak of the existence of a heart, fuse -into a single tube, with four contractile bulbs in series. As the heart -develops, the dilatation at its hinder or venous end and the dilatation -at its anterior or arterial end disappear. A partition is formed which -divides the two middle bulbs into right and left auricle and right and -left ventricle respectively. Immediately after birth the lungs are, for -<span class="pagenum"><a name="Page_218" id="Page_218">[Pg 218]</a></span> -the first time, distended with air. Up to that particular minute they -have had no functional use. Nothing would be gained by compelling all -the blood of the body to traverse the vessels of the embryo’s lungs. -Until birth, therefore, the inter-auricular septum is perforate. The -blood takes a short-cut, through the foramen ovale, from right auricle -to left. But by birth-time a curtain has grown down on the left side of -the foramen. When the lungs are expanded by the forcible enlargement -of the chest-cavity which contains them, their bloodvessels are -distended by the same extensile force. Blood is sucked into them from -the right side of the heart. A difference in pressure on the two sides -is established. A condition is set up which is favourable to what may -almost be termed the adherence of the flap which hangs down on the -left side of the foramen ovale. The growth of its margin very rapidly -obliterates the hole. Occasionally the closure of the foramen is not -<span class="pagenum"><a name="Page_219" id="Page_219">[Pg 219]</a></span> -complete. A child grows up with a perforate inter-auricular septum. If -the aperture be very small it causes little inconvenience. Shortness -of breath and blueness of lips indicate its existence if it be large -enough to lead to deficient aeration of the blood.</p> - -<p>The two sides of the heart being quite separate, it is clear that all -blood ejected by the right ventricle into the lungs must return to the -left auricle, to be driven by it round the body. Yet it does not follow -that the heart must at each stroke drive exactly the same quantity of -blood into the pulmonary artery and into the aorta. On the average, -each of the two sides ejects the same amount—about 3 ounces. Nor does -it follow that as much blood is lodged in the lungs as in the whole of -the rest of the body. The amount varies, but on the average the lungs -contain not more than one-fifteenth of the whole blood. The heart may -be likened to two cogwheels; the blood-stream to a chain, folded into -a figure eight, against which the cogwheels work. Synchronously each -cogwheel lifts a link, the right one of the smaller, the left one of -the larger loop. Any given link returns to its starting-place in half -a minute. Such an illustration gives an idea of the arrangement of -the circulation as a whole, although the motion of a fluid is widely -different from the motion of a chain.</p> - -<p>If, the jugular vein of the neck being cut, a colouring matter—such, -for example, as ferrocyanide of sodium or methylene-blue—is injected -into its central end towards the heart, it begins to appear in the -blood which issues from its distal end in half a minute. In this -short space of time it has passed through the right heart, through -the lungs, through the left heart, and through the vessels, arteries, -capillaries, and veins, of the head. Half a minute is therefore the -“circulation-time.” Not that all the blood-corpuscles of the body -make the circuit as rapidly as this. The time taken depends upon the -particular route they follow in the greater or systemic circulation. -Some traverse the vessels which supply the walls of the heart itself—a -short journey; others go down to the foot and up again. But the average -circulation time does not exceed a minute or a minute and a half. It -is particularly in the veins of the liver and other abdominal viscera -that blood tends to linger. Usually half the blood of the body, or even -<span class="pagenum"><a name="Page_220" id="Page_220">[Pg 220]</a></span> -more, is lodged in these capacious reservoirs. It is thanks to their -capacity for storing blood that a supply is provided adequate to meet -any special demand. If a man runs a few hundred yards, two-thirds of -the whole blood of the body is transferred to his limbs. It is quickly -withdrawn from the abdominal vessels when it is wanted elsewhere; -but, failing an efficient cause for removing it, its accumulation -induces lethargy. Even tight-lacing has been defended by an eminent -physiologist on the ground that it prevents accumulation of blood in -the abdomen. But tight-lacing diminishes the capacity of the chest, -hampers the action of the heart, checks the circulation, distorts -the abdominal viscera, and generally deforms and jams the domestic -machinery, even though the professor be right in his view that visceral -compression may favour alertness of mind. More by token, it interferes -with this admirable adjustment by which blood is distributed to the -various parts of the body in proportion to their needs. The brain is -the only organ which has any difficulty in securing all it wants, -and its claim to so much blood might be disputed. Nature has not -provided for long-continued passivity of the body associated with -strained activity of mind. When the stimulus to mental activity is not -unreasonable, most “nervous” people are apt to discover that their -brains are better supplied with blood than is good for their health.</p> - -<p>The effect upon the distribution of blood throughout the body of -squeezing the viscera is experienced after taking a deep breath and -contracting the muscles of the abdomen. The contents of the abdomen are -compressed between the depressed diaphragm and its muscular wall.</p> - -<p>Certain other forces co-operate with the beat of the heart in causing -blood to circulate. Two such factors are especially deserving of -attention. In the first place, the movement of blood in veins is -largely dependent upon external pressure. The veins are valved at -frequent intervals, the folds in their interior being of course -directed towards the heart. Any external force which empties a section -of a vein drives blood forward. A “good stretch” brings the lateral -pressure of contracting muscles to bear upon the walls of the veins -which lie between them, or beneath. More blood is delivered to the -heart. The exercise of standing erect in the attitude of attention, and -<span class="pagenum"><a name="Page_221" id="Page_221">[Pg 221]</a></span> -then slowly raising the arms until the thumbs meet above the head, -and slowly lowering them again, has a remarkable effect in quickening -the circulation—increasing the blood-supply of the brain. Changes -of posture, by relieving pressure on subcutaneous veins, removes an -impediment to the flow of blood.</p> - -<p>The second of the factors to which we have referred as adjuvant -to the heart’s action is the negative pressure of inspiration. In -explaining the effect of this force upon the circulation, the relation -of the lungs to the thorax must be taken into account. The box in -which the lungs are enclosed is too big for them; nevertheless, -being extensible and elastic, they always fill it. They follow its -movements when in inspiration the muscles between the ribs enlarge -it, and when in expiration it diminishes again. No air or fluid, save -the moisture which lubricates the surface of the pleura, reducing -friction, occupies the (potential) space between the lungs and the -chest. But the moment the chest is punctured the lungs collapse. -Air is sucked into the pleural cavity. The lungs fill the chest -only so long as there is neither air nor fluid between it and them. -Lung-tissue is extremely delicate. Each air-cell is a cup of thin -membrane holding together a basket-work of capillary vessels. So long -as the chest-wall is stationary the negative pressure in the pleural -cavity has no effect upon these slender tubes. But when the chest -expands, the capillaries are between two minus pressures, the pull of -the chest-wall and the resistance offered to the entrance of air into -the lungs by the passages through which it has to pass. The calibre of -the lung-capillaries is increased, just as it would be increased were -they hanging in an air-pump while the piston was drawn out. More blood -passes to the left heart through the wider capillaries. Ejected into -the aorta, it raises the pressure in the arterial system. A record of -the pressure in an artery shows a rhythmic rise for each heart-beat. It -shows also a rise with inspiration and a fall with expiration. These -larger undulations correspond with the movements of the chest, although -they are necessarily somewhat late on respiration, for the first effect -of the dilatation of the capillaries is to cause them to hold more -blood and to deliver less. The first effect of expiration, on the other -hand, is to urge on the blood which the dilated vessels contain. In any -<span class="pagenum"><a name="Page_222" id="Page_222">[Pg 222]</a></span> -case a single beat is needed to throw into the aorta the blood which -has been received by the right auricle.</p> - -<p>The expansion of the chest influences the flow of blood in yet another -way. The heart and the great vessels which join and leave it are -themselves enclosed within the chest, subject to the negative pressure -produced within that cavity by the elasticity of the lungs. The lungs -pull upon the pericardium, the membranous covering of the heart. When -this pull is increased owing to the forcible expansion of the chest, -blood is sucked into the great veins, just as air is sucked into the -windpipe. The thick-walled aorta, containing blood at high pressure, -does not feel the effect of slight variations in the pressure round -it. The soft-walled veins are expanded during inspiration to a not -inconsiderable degree. What relief a deep yawn gives by hastening -a languid circulation! Leaning over an account-book late in the -afternoon, every condition is unfavourable to the flow of blood. It -accumulates in the legs and in the abdomen. The head is thrown back -and the mouth opened wide, while the chest expands in a long deep -inspiration. Down on the liver, stomach, and intestines presses the -flattened diaphragm, squeezing their blood towards the heart. The -negative pressure within the chest sucks this up, and draws down the -blood contained in the great veins of the neck. The capillaries of the -lungs are widened, allowing blood to pass more quickly from the right -side to the left side of the heart. The heart responds to the call upon -it, throwing all that it receives into the aorta. Only a great effort -of the will had kept the pale brain at work; in the attic it suffers -more than organs on the lower storeys from insufficient pressure. For -a short time after the yawn it finds itself nourished with an adequate -supply of blood.</p> - -<p>The negative pressure in the thorax is considerable at all times. -If a manometer—a <b>U</b>-shaped tube with mercury in its loop—be -connected with a cannula passed through the wall of the chest, the -difference of level of the mercury in the two limbs of the <b>U</b> is -a measure of the force with which the lungs are endeavouring to shrink -away from the chest-wall. Even at the end of expiration the mercury -in the limb next the chest stands about 6 millimetres higher than the -mercury in the outer limb. During a deep inspiration the pressure in -the chest falls 30 millimetres below the atmospheric pressure. Hence a -<span class="pagenum"><a name="Page_223" id="Page_223">[Pg 223]</a></span> -problem is presented of which no completely satisfactory solution has -yet been given. How comes it that lymph is not sucked into the pleural -cavity? In health there is no more pleural fluid than just suffices to -keep the membrane moist. The endothelial cells which cover the surface -of the pleura resist further exudation. Valves in the lymphatic vessels -prevent backward flow. Yet in disease, when the pleura is inflamed, -lymph pours out quickly, often to be reabsorbed with equal rapidity -when the pleurisy subsides. This flow uphill, from a lower to a higher -pressure, can be explained only as a phenomenon due to the “secretory” -capacity of endothelium. As an answer to the hydrostatic problem this -is hardly satisfactory.</p> - -<p>The circulation of the blood is the result of the difference between -the pressure in the vessels through which it leaves the heart, and that -in the vessels through which it is returned. The pressure in the aorta -amounts to about 200 millimetres of mercury. In the venæ cavæ it is -nil, or, owing to the aspiration of the thorax, less than nil.</p> - -<p><b>The Heart.</b>—Inspection of the liver, the spleen, or the kidney -helps but little to the comprehension of the mechanism of these organs. -It is quite otherwise in the case of the heart. Its mechanics being -comparatively simple, physiology is concerned with measurements, with -the conditions under which it can and cannot work, and with the action -upon it of the nervous system and of drugs. The heart of any mammal -will suffice for anatomical study. A sheep’s heart is about the same -size as that of a man, and exactly similar, save in minute particulars, -which do not appreciably affect its mode of working.</p> - -<p>The heart is a hollow muscle, composed of minute contractile cells. -Each cell is a cylinder, about twice as long as it is broad, with an -oval nucleus in its centre. There is no impropriety in speaking of the -heart as a single muscle. Muscles which we can move at will, “voluntary -muscles,” consist of fibres, each from 1 inch to 2 inches long, and -of about the thickness of a piece of thread (<a href="#I_262">Fig. 16</a>). -Every fibre is surrounded by a membranous sheath, its sarcolemma, which -completely isolates it from the others. Each has its separate nerve-supply. -A voluntary muscle-fibre is a cell-complex. The single embryonic cell -which grew into the fibre underwent nuclear division until hundreds of -nuclei were formed, but its cell-substance was not divided into -<span class="pagenum"><a name="Page_224" id="Page_224">[Pg 224]</a></span> -territories appertaining to the several nuclei. In heart-muscle, on -the other hand, nuclear division has been followed by cell division; -but minute protoplasmic bridges are left between the cells. The whole -of the heart-substance is thus in structural continuity. The cells are -not invested with sarcolemma. As the result of this arrangement, an -impulse started in one part of the heart spreads over the whole, with -certain limitations as to the directions in which it is able to travel, -whereas in voluntary muscle a separate impulse must be delivered to -each fibre. The wave of contraction commences in the great veins, the -venæ cavæ and pulmonary veins, near their junction with the heart, -spreads from cell to cell throughout the auricles, and onwards down -the ventricles to the apex of the heart. The substance of the heart -has not, however, a homogeneous appearance. Its cells are collected -into fascicles, which lie in various planes and cross the axis of -the heart at various angles. In a boiled sheep’s heart it is easy to -separate one fascicle from another, and to distinguish the sheets into -which the fascicles are collected. The four valves of the heart lie in -almost the same plane. They are supported by a fibrous plate divided -into four rings (<a href="#I_225">Fig. 11</a>). Most of the fascicles are attached -to this plate, though some which encircle the auricles are independent of it. -With one or with both ends attached to the plate, fascicles loop over -the auricles. They run down the ventricles with a twist from right to -left. Those on the surface turn in at the apex of the heart, and run -up the inner surface of the ventricles. Some of them go to form the -free columns which are found on the inner surface of the ventricles, -pointing towards the valves—musculi papillares. The fibrous plate -which supports the valves cuts off almost all of the muscle which makes -the walls of the auricles from that which constitutes the ventricular -walls; but a thin sheet is continued from the inner surface of the -auricles down the interventricular septum. To a considerable extent the -walls of the two auricles and of the two ventricles are respectively -continuous, insuring synchronous contraction.</p> - -<p>The arrangement of the fascicles accounts for the changes in form which -the heart undergoes when it contracts. Systole commences in the cardiac -ends of the venæ cavæ and pulmonary veins. They empty the last of their -<span class="pagenum"><a name="Page_225" id="Page_225">[Pg 225]</a></span> -blood into the auricles, and close to prevent regurgitation, their -mouths not being valved. Then the auricles quickly shrink in all -dimensions, and as soon as their contraction is at its height the -ventricles contract, while the auricles relax. The ventricular wave -runs from base to apex too rapidly to be followed with the eye, -and ends, owing to the involution of the fascicles, in the musculi -papillares. As soon as ventricular systole has commenced, the auricles -relax. After emptying their contents into the aorta and pulmonary -artery, the ventricles relax, their contraction giving way first at -the apex, and being longest held at the base. Then follows a pause -(diastole), during which both auricles and ventricles are flaccid. If -the pericardium is open, the heart is seen to become round instead of -oval in transverse outline during systole. It shortens. Its apex twists -a little to the right, and projects forward. But if it is within its -pericardium the shortening is not accompanied with any displacement of -the apex. Instead of the apex mounting, the base descends. The front -of the right ventricle, at some little distance from the apex, presses -the chest-wall forwards in the fifth intercostal space, about an inch -to the inner side of a line falling vertically through the nipple. This -pressing forwards is felt as the “impulse of the heart.”</p> - -<div class="figcenter"> - <a id="I_225" name="I_225"> </a> - <img src="images/i_p225.jpg" alt="" width="600" height="433" /> - <div class="blockquot2"> - <p class="center"><span class="smcap">Fig. 11.</span>—<span class="smcap">A Section - approximately at Right Angles to the Long Axis of the Heart, exposing - the Four Valves which lie very nearly in the Same Plane.</span></p> - <p class="center">The semilunar valve which guards the aperture of the pulmonary artery - is the nearest to the breast-bone.</p> -</div></div> - -<p>The contraction of the heart is not a see-saw of auricles and -ventricles. During diastole blood is falling from the veins through the -<span class="pagenum"><a name="Page_226" id="Page_226">[Pg 226]</a></span> -auricles into the ventricles. In a sense, the auricles are not -necessary parts of the double pump. They collect blood while the -ventricle is contracting, thus preventing it from heading up in the -veins. They save time. Their contraction completes the filling of the -ventricle, so that the instant the ventricular contraction begins blood -enters the aorta and pulmonary artery.</p> - -<p><b>The Valves.</b>—If ever expressions of admiration were appropriate -in a treatise on the animal body, such preface might be permitted to -a description of the cardiac valves. Which means no more than this: -Men make pumps. Therefore they are in a position to appreciate the -mechanism of the heart. We cannot admire what we do not understand. -If we made secreting organs or self-contracting springs, glands and -muscles would evoke our commendation. We should recognize that Nature’s -apparatus is even better adapted to its work than any that men can -make. This is the admission which is forced from us when we study the -heart.</p> - -<p>The apertures connecting auricles and ventricles are extremely wide, -allowing the contents of the former to be emptied into the latter -almost instantaneously. If we attempted to make a pump fulfilling this -condition, we should find that it failed in several respects. In the -first place, the rush of fluid from the one chamber into the other -would press the flaps of the valves back against the wall of the second -chamber. They would cling to the wall, and would not float up quickly -into place when the second chamber was squeezed. Let us call the two -chambers A and V for brevity’s sake. When V contracted, some of the -fluid would be thrown back into A, because, the resistance in that -direction being lower than the resistance offered by the column of -fluid above the pump (the resistance in the aorta is very high), the -contents of V would rush past the margins of the A-V valve. This would -happen even though its flaps were not pressed back against the wall. -Further, at the height of contraction the membranous valve would bulge -backwards into A, making a cup towards V which V could not empty. In -the heart these difficulties have been overcome.</p> - -<p>The tricuspid valve, which separates the right auricle from the right -ventricle, has three flaps. The mitral valve, on the left side of the -heart, has but two. The flaps are composed of tough membrane, but are -comparatively thin. The following direction for deciding at an autopsy -whether or not they were healthy at the time of death was given many -years ago by a surgeon of repute: “You ought to be able to see the -<span class="pagenum"><a name="Page_227" id="Page_227">[Pg 227]</a></span> -dirt under your thumbnail when you place it beneath one of the flaps.” -Surgery has improved in cleanliness as well as in other ways; indeed, -the possibility of advance has been due to the recognition of the need -for transcendental cleanliness. But this is a digression. The margins -of the flaps are crenulated. Threads—chordæ tendineæ—are attached -to them like the stay-ropes of a tent. At their other end these tendons -are attached to the musculi papillares already mentioned. The bunch -of tendons from each papillary muscle spreads, to be inserted into -the contiguous margins of two flaps. We have mentioned some of the -difficulties which have been overcome in the construction of the pump. -(1) The flaps do not flatten back against the wall of the ventricle -during systole of the auricle. It must be remembered that during -diastole of both chambers blood is flowing through the auricle into -the ventricle. The latter being partly filled before systole of the -auricle commences, the flaps are floated up. This is greatly favoured -by the form of the inner wall of the ventricle. It is not flat, but -raised in pillars—columnæ carneæ. The spaces between these pillars -cause backwash currents, which lift the flaps and help to bring them -into apposition as soon as systole of the ventricle commences. (2) No -blood which has entered the ventricle is thrown back into the auricle. -The valve “balloons” over the blood in the ventricle before the -contraction of the auricle has ceased. The thin margins of its flaps -come together with great rapidity. The tendinous cords holding their -edges on the ventricular side, they meet, not edge to edge, but folded -flap to folded flap. (3) The valve does not bulge into the auricle. On -the contrary, at the height of systole it is pulled into the ventricle -by the contracting musculi papillares. As the ring to which the valve -is attached is diminished in size, by the contraction of the base of -the heart, which continues, it will be remembered, until after the apex -has begun to relax, the edges of the flaps are folded farther and still -farther over by the pull of the musculi papillares, and the blood is -squeezed out from between the wall of the ventricle and the indrawn valve. -<span class="pagenum"><a name="Page_228" id="Page_228">[Pg 228]</a></span></p> - -<p>The “semilunar valves,” which close the apertures into the aorta and -pulmonary artery, have each three flaps. The aortic semilunar valve, -which has the higher pressure to bear, shows its characteristic -features in a rather more marked degree than the other. Each of its -three flaps is a half-cup. At the centre of the margin of the half-cup -is a small fibrous nodule. The edge of the cup on either side of this -is very thin. Fine elastic fibres radiate from the nodule to all parts -of the flap. The wall of the aorta shows three bays, or “sinuses,” -one behind each flap. Hence, when the valve is forced by the rise of -pressure in the ventricle, the flap is not flattened back against the -wall of the aorta. There is always a certain amount of backwash in the -pocket behind it. The instant the pressure in the ventricle begins -to fall, the three flaps come together with a click, so smart as to -be plainly audible over most of the front of the chest. The click is -the “second sound” of the heart. The auriculo-ventricular valves also -make a sound when they close; but this “first sound of the heart” -has a different character. It is prolonged, soft, low-pitched. It is -customary to represent the sounds by the syllables “lūbb dŭp—lūbb -dŭp,” the pause during diastole being of about the same length as the -sounds when the heart is beating with its normal rhythm. The duration -of systole is little affected by variations in the rate of beat. It -is diastole that is shortened or prolonged. The second sound is due -entirely to the closure of the semilunar valves. It is heard most -clearly when the stethoscope is placed over the region where the aorta -comes nearest to the wall of the chest—at the second rib cartilage on -the right side of the breast-bone. The first sound is loudest near the -apex of the heart. It is generally agreed that it is not wholly due to -the closure of the auriculo-ventricular valves, but possesses a second -constituent. Some persons assert that they can with the ear distinguish -the clearer valvular sound at the commencement from the general -rumble which overtakes it. The main part of the sound, if it have -two constituents, or the whole sound, if there be no distinguishable -valvular constituent—observers differ—is just the noise of a distant -cab (<i>bruit du cab</i>) or the waves on a far-off beach; it is the sound -which the ear picks up from any irregular mixture of tones which it -<span class="pagenum"><a name="Page_229" id="Page_229">[Pg 229]</a></span> -cannot analyse. It is the resonance-tone of the ear. That the -membranous valves play the leading part in producing the first -sound cannot be doubted, whether by their first closure or by their -subsequent vibration. We should be inclined to attribute to them -the whole performance, were it not that the first sound, or at any -rate a sound, is heard during the beating of a bloodless heart. If -an animal be killed and the heart removed from its thorax with the -utmost despatch, it will beat for about a minute while lying in the -palm of one’s hand. When a stethoscope is applied to the ventricle, -a “first sound” is heard. This was described as a muscular sound, -owing to a misconception. It is similar to the sound which is heard -when a stethoscope rests upon a contracting biceps. Until recently -the voluntary contraction of a muscle was believed to be vibratory—a -tetanus. The sound corresponds to a rate of about thirty-six vibrations -to the second. There being reasons for thinking that muscle contracting -naturally does not vibrate as fast as this, the sound was interpreted -as the first overtone of the muscle-note. Muscle was said to vibrate -eighteen times a second. The similarity of the first sound of the -heart and the ordinary muscle-sound led physiologists to infer that -the contraction of the heart also was a tetanus. But this was a -mistake. Neither voluntary muscular action nor the contraction of -the heart is an interrupted contraction in this sense. In the case -of the musculature of the heart especially, contraction is a steady -shrinking, followed by a steady relaxation. The sound produced by the -bloodless heart is due to the various displacements which occur when it -contracts. Its interior is very irregular, with its columns, papillary -muscles, tendinous cords, valves. The displacement of these various -structures is responsible for the noise.</p> - -<p>The sounds of the heart afford to the physician a means of ascertaining -with the utmost nicety the condition of the valves. If the sounds -are altered from the normal in the least degree, the valves are not -healthy. Alteration of the structure of a valve is in ordinary parlance -heart-disease. It is usually indicated by an addition to the normal -sound. Such addition is termed a “murmur”; in French, <i>un bruit de -souffle</i>. Either term is somewhat misleading to the tyro. We remember -<span class="pagenum"><a name="Page_230" id="Page_230">[Pg 230]</a></span> -a fellow-student to whom our chief had in vain expounded the nature of -a murmur. “Surely, Mr. S., you can hear the murmur in this case.” We -others could hear it as we stood around the bed. After listening for -a minute, S. replied: “I think I could hear it, sir, if the heart -wasn’t making such a thundering noise.” The thundering noise was the -murmur. It is the business of the physician to recognize that there is -a departure from the normal, to analyse its character, to determine -the time at which it is heard in relation to the cardiac cycle, and to -locate the place on the chest where it is heard most loudly. He is then -in a position to state which of the valves is affected and what is the -nature of its lesion. Is it a lesion obstructing an orifice, or is it -causing regurgitation of blood? Or is one of the valves, as is commonly -the case in heart-disease, imperfect in both respects?</p> - -<p>A murmur, in the strictest sense, is a sound added to a heart-sound. -It is due in all cases to vibration of a fluid column (“fluid vein” is -the term in physics). When fluid passing under pressure along a tube -of a certain calibre enters a tube of smaller calibre, no vibration -occurs. When it passes from a tube of smaller calibre into a larger -tube or space, it is thrown into vibration. Under normal conditions no -vibration occurs in the heart. The auriculo-ventricular orifices are so -large that auricle and ventricle form a single cavity when the valve is -open. The ventricles drive the blood into tubes of smaller dimensions -than themselves. These are not the conditions which set up vibration -in a fluid column. But if one of the orifices is constricted, owing to -thickening or partial adhesion of its valve, the fluid column vibrates -on entering the space beyond it. The sound is propagated forwards, -beyond the constriction, not behind it, and transmitted to the wall -of the ventricle, aorta, or pulmonary artery, as the case may be. -When either of the auriculo-ventricular orifices is constricted, the -vibration of the fluid column can be felt as well as heard. The finger -placed against the chest-wall at the spot where the impulse of the -heart occurs is sensible of a thrill. The vibration may occur whilst -blood is flowing <i>through</i> an auricle into a ventricle, before the -auricle contracts. In time, it is presystolic. The murmur produced by -regurgitation into an auricle is synchronous with systole. The murmur -due to regurgitation into a ventricle past an incompetent semilunar -valve is postsystolic. -<span class="pagenum"><a name="Page_231" id="Page_231">[Pg 231]</a></span></p> - -<p>We have said that the heart is so formed that no vibrating fluid -vein is produced when it is functioning normally. Murmurs are due to -alterations in the valves which are visible after death. This statement -needs modification. Not infrequently functional murmurs are heard, -which disappear again after a time—in a few weeks, or even days, -perhaps. The explanation of murmurs of this class is very difficult. -They are heard most frequently in anæmic persons, and appear in these -cases to be due to the heart having shrunk, owing to the blood in -circulation being deficient in quantity, until the cavities of the -ventricles have a smaller diameter than that of the great arteries into -which they expel their contents.</p> - -<p>Such is the explanation of the physical cause of murmurs given by -Chauveau and Marey, the physiologists who have paid most attention to -this subject. But it must be remembered that the valves which, when -diseased, are the sources of the murmurs are membranous structures. It -may be that fluid veins would be produced by them if they were rigid -ledges which jutted into the blood-stream; but, being membranous, they -are capable of vibration. Certain physicists are of opinion that a -murmur is caused, not by the vibration of a fluid vein, as such, but by -the vibration of the membranous structure which impedes the passage of -the fluid. The physics of the problem is of little consequence to the -physician. The murmur is produced at the spot where a diseased valve is -situated, and is propagated forwards. It enables him to ascertain with -accuracy what is amiss with the heart.</p> - -<p><b>Bloodvessels.</b>—The greater circulation occurs through a closed -system of vessels which unite the left ventricle with the right -auricle. The aorta gives off lateral branches. Its branches branch. -Subdivision continues until the vessels are just wide enough to allow -blood-corpuscles to pass in single file, or but little wider. When -a bough of a tree divides, the united cross-sections of its twigs, -their soft bark being stripped off, may be a little larger than the -cross-section of the bough; but the disparity is usually small. -The united cross-sections of the smaller arteries is considerably -greater than that of the trunks which give origin to them. By the -time the capillaries are reached, their total bed—their united -cross-section—is about 640 times as great as that of the aorta. -<span class="pagenum"><a name="Page_232" id="Page_232">[Pg 232]</a></span> -This estimate is based upon the diminution in the rate at which blood -flows through the vessels. The velocity with which a stream flows -through a channel varies as the cross-section of the channel. In a -capillary vessel the blood flows at the rate of from 0·5 millimetre -to 1 millimetre per second. In the aorta the velocity is about 320 -millimetres per second. In the re-formation of the venous system a -converse process of reduction occurs, but not with anything like the -same rapidity. The united calibre of the two venæ cavæ, in which the -reduction is complete, is about twice that of the aorta. From this it -follows that the veins hold much more blood than the arteries; and -since veins are more easily distended, the amount that they can hold -varies within wide limits. They constitute to some extent a reservoir -for blood.</p> - -<p>The capillary vessels are the tubes of the circulatory system in which -blood comes into use. On the average they are about 0·5 millimetre -long. Through them the blood flows slowly. Through their walls alone -is there any exchange worth mentioning between the blood within the -vascular system and the lymph by which it is surrounded. Interest -therefore centres in these vessels. Their walls are formed of -endothelial tiles. In the centre of each thin transparent tile is a -boss, where its lens-shaped nucleus is situate. The outline of the tile -is sinuous. Its margin dovetails with the margins of those adjacent to -it. Oxygen and carbonic acid, nutrient substances and waste products, -pass rapidly through the endothelial cells. Leucocytes have the power -of pushing the cells aside, in order that they may make their way out -of the blood into the lymph which fills the tissue-spaces. With the -exception of the lens and cornea of the eye, cartilage, and the various -epidermal structures, all tissues are traversed by capillary vessels. -It is not difficult to calculate the number of such vessels in the -body exclusive of the liver and the lungs. The diameter of the aorta -is 28 millimetres, that of a capillary about 0·008 millimetre. The -cross-section of all the capillaries added together is 640 times that -of the aorta, as already stated.</p> - -<p>Many schemata have been devised to illustrate the vascular system; -but all are misleading, inasmuch as they fail to give any idea of the -extent to which the subdivision of its vessels is carried. If the -<span class="pagenum"><a name="Page_233" id="Page_233">[Pg 233]</a></span> -water-pipes supplying a town branched until the original conduit was -represented by five to six thousand million little pipes, the friction -which the pumping-station would have to overcome would be very great. -But little force would remain in the water when it reached the smallest -pipe. Still greater is the resistance to the flow of blood, which -is slightly viscous, and contains solid corpuscles, which increase -friction. Two thousand miles of capillary tubing in the body of a man, -without reckoning the vessels of his liver and lungs!</p> - -<div class="figcenter"> - <a id="I_233" name="I_233"> </a> - <img src="images/i_p233.jpg" alt="" width="600" height="270" /> - <p class="center"><span class="smcap">Fig. 12.</span>—<span class="smcap">A - Portion of the Wall of a Small Artery cut transversely and highly - magnified</span>.</p> - <div class="blockquot2"> - <p class="neg-indent space-below2">Its inner coat consists of a lining sheet of epithelial scales - supported by connective tissue and a strong elastic membrane. This - membrane is perforated with holes which place the lymph-spaces on - its two sides in continuity. The middle coat is composed of plain - muscle-fibres and patches of elastic membrane; the outer coat of - elastic fibres, mostly longitudinal, and connective tissue.</p> -</div></div> - -<p>Water is supplied to houses in rigid tubes. Arteries are elastic, and -their elasticity is self-regulating. The cause of this will be apparent -if a section of an artery is examined. It contains much elastic tissue. -It also contains plain muscle-fibres. The smaller the artery, the -greater is the amount of muscle relatively to the other constituents of -its wall. The wall of a vein contains very little muscle, and not much -elastic tissue. The muscle of all arterial walls is in a chronic state -of tone. To some extent the degree of tone is varied automatically. -Pressure within an artery acts as a stimulus to the muscle-fibres of its -wall. Any increase leads the fibres to contract more strongly. Any -diminution induces them to relax. The arteries resist distension; -they do not narrow to any great extent when pressure falls. But more -important than this automatic mechanism for maintaining a uniform -<span class="pagenum"><a name="Page_234" id="Page_234">[Pg 234]</a></span> -pressure in the capillaries in general are the changes of pressure -in particular localities, brought about by the mediation of -vaso-constrictor and vaso-dilator nerves. In almost all organs and -parts of the body the automatic tone of arteries is enhanced by -impulses which flow continuously down vaso-constrictor nerves. These -impulses start from, or, to speak more accurately, pass through, the -vaso-motor centre in the medulla oblongata. From every part of the body -impulses ascend to this centre, urging it to keep up the blood-pressure -by universal constriction. Yet no separate organ would be interested -in sending such a message if it were not open to it to ask at the -same time that the constriction of its own vessels might be relaxed. -Hence it may be said that every individual in the community is crying -out for universal economy, with more generous treatment of himself. -The response made by the State to the latter part of his demand is in -proportion to the vehemence with which it is presented.</p> - -<p>If the spinal cord of an animal be cut across near the medulla -oblongata, respiration being maintained by pumping air into and out of -the lungs, the heart continues to beat with undiminished force, but -the pressure in the large arteries falls to one-third of its normal -height. Constricting impulses no longer pass down the spinal cord from -the vaso-motor centre. This experiment also illustrates the truth of -the statement that models of the vascular system—arrangements of pumps -and indiarubber tubes—are more likely to mislead than to inform. In -an artificial schema the relaxation of the constriction of the small tubes -on the proximal side of the capillary vessels would reduce friction. -Fluid would reach the capillaries in larger quantity, and pass through -them more quickly. The pressure in the tubes which represented veins -would consequently approach more nearly to that on the arterial side. -But when the spinal cord is divided the pressure falls in the veins, -as well as in the arteries. This is due to another factor, and one of -very great importance in the regulation of the circulation. The blood -from the digestive organs is collected by the “portal system” of veins. -These do not join the inferior vena cava; they go to the liver, where -they again break up into capillaries. It is not until after this second -distribution through minute vessels that the blood is re-collected by -<span class="pagenum"><a name="Page_235" id="Page_235">[Pg 235]</a></span> -the hepatic veins and forwarded to the heart. As in the case of the -arteries, the portal system of vessels is controlled by the nervous -system. When the spinal cord is divided they also dilate. The whole -vascular system becoming more capacious, blood-pressure falls in veins -as well as in arteries.</p> - -<p>When the digestive organs are active, other parts of the body are kept -short of blood. It chanced to the writer, in his student days, to -spend the early summer in Paris, with a big healthy Yorkshireman as -companion. We dined together each night at one of the restaurants of -the Palais Royal <i>à prix fixe</i>. After dinner, with British regularity, -my friend called for the <i>Times</i>. Then followed a short period of -placid reading, interrupted by the remark: “How cold it is!” Half an -hour later, giving himself a shake: “Suppose we go and dine somewhere -else?” His well-ordered digestive organs had made short work of the -two-franc dinner. They had been ably supported by the vaso-motor system -of nerves which provided them with the bulk of the blood, while limbs -and skin ran short.</p> - -<p>Vaso-constrictor nerves leave the spinal cord by the roots (called -“rami communicantes”) of sympathetic ganglia. Beyond the ganglia they -apply themselves to the large arteries whose course they follow. The -constrictor nerves for the face and neck leave the spinal cord within -the chest by the roots of the first four thoracic nerves. They do not -at once apply themselves to the great artery of the head. Until the -upper part of the neck is reached, they traverse the ganglionated -sympathetic cord, which lies behind the carotid artery and internal -jugular vein. If in a rabbit this cord be cut, the vessels of its -ear dilate, as evidenced by the rosy blush which is observed when a -light is held behind it. If the upper part of the sympathetic cord be -stimulated, the ear grows pale. The redness of the ear remains for -many days after section of the nerve; but gradually the engorgement -diminishes, and the vessels acquire the power of automatically -regulating the flow.</p> - -<p>The classical experiment with the rabbit’s ear suffices to show the -relation of bloodvessels and nerves which holds good for all areas of -the skin. The condition of the skin is the chief factor in regulating -the temperature of the body. In a cold atmosphere its vessels are -severely constricted to limit loss of heat. When one passes into a warm -<span class="pagenum"><a name="Page_236" id="Page_236">[Pg 236]</a></span> -room the constriction is relaxed. The skin is flushed; heat is thrown -off by radiation. The sweat-glands secrete water, which is evaporated -by the heat of the skin. Constriction and remission of constriction are -the processes which diminish or increase loss of heat.</p> - -<p>This mechanism is different in the case of glands and some other -structures which, when active, require an abundant supply of -blood. Such organs are provided with vaso-dilator in addition to -vaso-constrictor nerves. The most conspicuous example of this is to be -seen in the case of the submaxillary gland. The nerve to this gland -runs for some distance as an isolated thread—the chorda tympani. -Stimulation of the chorda tympani has the double effect of dilating -the arteries of the gland and of causing it to secrete. But the -administration of atropin prevents secretion. Vaso-dilation is then the -only visible effect. Stimulation may increase sixfold the outflow of -blood from the veins of the gland. It rushes through with such rapidity -that it retains its bright arterial hue. The gland also receives a -twig from the sympathetic cord in the neck, which, as already stated, -controls the vessels of the face. By stimulating the one nerve or the -other the physiologist can at will increase or diminish the amount of -blood flowing through the submaxillary gland. Stimulating any sensory -nerve causes in a reflex manner an increased outflow of constrictor -impulses from the centre in the medulla oblongata to all parts of -the body, with the exception of the part to which the sensory nerve -appertains. Its own constituency receives an increased supply of blood. -It is not difficult to appreciate the importance of this double action. -A part is injured. The restrictions placed upon its supply of blood are -suspended. Lest its increased consumption should lead to a general fall -in pressure, all other parts have their supply curtailed. The effect -is even more pronounced than this. The whole blood-pressure is raised -above its ordinary level. The flow of blood to the injured part is -therefore greater than it would be were relaxation of its arteries the -only change.</p> - -<p>The most important of all constrictor nerves are the splanchnics which -control the supply to the stomach and intestines. When these nerves -are cut, the digestive organs become engorged to such an extent that -a pronounced fall of the general blood-pressure is the result. Their -stimulation renders the digestive organs anæmic. We have already shown -<span class="pagenum"><a name="Page_237" id="Page_237">[Pg 237]</a></span> -that the relaxation of vaso-constriction occurs in a reflex manner. -The reflex relaxation of the splanchnic area is a matter of great -importance, because it can be brought about by stimulation of one of -the sensory nerves of the heart. The higher the blood-pressure, the -harder the heart would work if left to itself. It is an impetuous -organ, always trying to quicken its pace and to increase the force -of its beat. Excessive zeal would get it into trouble if severe -precautions were not taken to hold it in check. True, it is encouraged -by certain “accelerator nerves”—sympathetic filaments which leave -the spinal cord by the anterior roots of the second and third thoracic -nerves; but the influence which the accelerators exert under normal -conditions is not, it would seem, very pronounced. The nerves which -restrain the heart are much more in evidence than those which urge -it on. The arrangements for diminishing the work of the heart are of -two kinds. In the first place, branches derived from the vagus act -as a continuous check. From a certain spot in the medulla oblongata, -the cardio-inhibitory centre, impulses are always descending to slow -the heart. They are of reflex origin, but a high blood-pressure in -the centre increases the facility with which they are transmitted. -Some of these stimuli originate in the heart itself, ascending and -descending the vagus nerve. The remainder come from various sources. -A severe injury to any part of the body slows the heart. Injury -to the intestines, such as occurs in peritonitis, is particularly -effective in increasing vagus inhibition. Slowing of the heart lowers -blood-pressure. When both vagi are cut, the heart begins to gallop -whatever may be the pressure against which it has to work.</p> - -<p>A sensory nerve of the heart, termed the “depressor,” is the chief -agent in lowering blood-pressure. Its course is not the same in all -animals, but it runs more or less in conjunction with the vagus. -Usually it joins its superior laryngeal branch. Impulses which ascend -this nerve inhibit the constriction of the splanchnic vessels. They -open a floodgate which brings down the general pressure. The severe -pain and extreme distress of angina pectoris are the cry of the heart -when blood-pressure is too high—when it feels unable to work against -it. This was recognized by physiologists long before a remedy was -known. A systematic search was instituted for a drug which could be used -<span class="pagenum"><a name="Page_238" id="Page_238">[Pg 238]</a></span> -with safety to lower blood-pressure. The discovery that the inhalation -of amyl nitrite answers this purpose and fulfils this condition was the -result.</p> - -<div class="figcenter"> - <a id="I_238" name="I_238"> </a> - <img src="images/i_p238.jpg" alt="" width="500" height="473" /> - <p class="center"><span class="smcap">Fig. 13.</span>—<span class="smcap">Manometer - for measuring Blood-Pressure.</span></p> - <div class="blockquot2"> - <p class="neg-indent space-below2">A <b>U</b>-tube contains mercury, on which floats a - rod supporting a scratching point, which makes a “tracing” on blackened paper - wrapped round a revolving drum. Between the manometer and the cannula which is - introduced into the central end of a cut artery is a three-way cock, - which leads to a pressure-bottle containing a half saturated solution - of sodic sulphate. This solution prevents blood from clotting. Before - it is connected with the artery the apparatus is filled from the - pressure-bottle. The cock is then turned into the second position, - and the bottle raised until the mercury in the manometer stands at a - level somewhat higher than that which it may be expected to attain - under the influence of blood-pressure. The cannula being then inserted - into an artery, the cock is turned into the third position, which - places the manometer in connection with the blood, and excludes the - pressure-bottle. As the mercury is a little higher than blood-pressure, - some of the sodic sulphate solution enters the artery, but no blood - enters the cannula. The scratching point, rising and falling with every - variation in blood-pressure, makes a record on the soot-blackened - paper, which is subsequently removed from the drum, and varnished.</p> -</div></div> - -<p>When we consider the hydrostatics of the circulation, it becomes -evident that changes in the force with which the heart beats, -and changes in the calibre of the bloodvessels, work together -in determining blood-pressure. Both vessels and heart contract -automatically—the former continuously, the latter rhythmically. The -heart of a frog, if it is enclosed in a moist chamber, beats for a long -time after its removal from the animal. Even when cut in pieces, in -certain ways, the separate pieces beat. A strip from the ventricle of a -tortoise’s heart, kept gently stretched by the weight of a light lever -attached to one of its ends, continued to contract rhythmically for -<span class="pagenum"><a name="Page_239" id="Page_239">[Pg 239]</a></span> -forty-eight hours. When the heart has come to a pause, it cannot be -started again by stimulating any nerve. It has in the most marked -degree its own views as to the rapidity and force with which it ought -to beat. But within certain limits it is under nervous control. The -accelerators hasten it, to its own detriment. They belong to the -division of katabolic nerves—a name given them to indicate that they -waste the tissues, impoverishing their condition. The vagus nerve slows -the heart. It protects it from itself. Its action is anabolic. The -condition of the heart is improved under its influence. If it has been -kept in check for a time by stimulation of the vagus, the heart beats -more strongly when this nerve ceases to act than it did before it was -induced to rest.</p> - -<p>The arteries also are under the influence of two antagonistic sets -of nerves. Those which increase their tonic contraction are almost -universal in their distribution. It may be that those which actively -check it are equally widespread, but the evidence is not altogether -free from ambiguity. On certain organs—such as the salivary glands, -already instanced—which require great variations in the amount of -blood supplied to them, the influence of dilator nerves is very marked. -The simplest hypothesis as to the mode of action of vaso-constrictor -and vaso-dilator nerves leaves the initiative with the muscle-ibres -of the vessel-wall. The distending internal pressure of blood is the -stimulus which induces the muscle to contract. In some invertebrate -animals—the snail, for example—if blood be prevented from entering -the heart, so that there is no distending pressure, the heart stops. -In higher animals the heart has acquired a habit of contracting, which -keeps it going in the absence of its proper stimulus. The two classes -of nerves exercise opposing influences on the muscle. Vaso-constrictor -nerves increase the excitability of its fibres; vaso-dilator nerves -diminish it. Only thus can we explain their action on a common -basis. A good deal might be said as to the reasonableness of such -an explanation. Our views as to the relation of nerve-influence and -muscle-contraction are apt to go astray, owing to the fact that -generations of physiologists have observed the phenomenon of a spasm of -a muscle following on a sudden stimulus to a nerve. The two events are -evidently related. The stimulus appears to set up a new condition in the -<span class="pagenum"><a name="Page_240" id="Page_240">[Pg 240]</a></span> -nerve—to initiate a process which was not occurring before the -electric current was passed through it. The muscular spasm equally -appears to be an isolated event. As usual, we are misled by the analogy -of human inventions. We compare the nerve-impulse to the fall of a -hammer, the muscle-spasm to the explosion of gunpowder. We forget that -nerve and muscle are in permanent connection; that the impulse is a -sudden exaggeration of an influence which the nerve is continuously -exerting, the contraction an exaggeration of metabolic changes which -are constantly occurring in muscle. (See in this connection the -explanation of muscle-tone, <a href="#Page_273">p. 273</a>.) In the case of plain -muscle, nerve stimuli do not cause contraction; they merely increase the excitability -of the muscle. It may be more difficult for us to figure to ourselves -the way in which dilator nerves diminish excitability; but the -existence of such an anabolic influence is beyond the reach of doubt. -Heart and bloodvessels are part of the same system. The heart has its -accelerator and inhibitory nerves, the bloodvessels their constrictor -and dilator nerves. For both vessel-wall and heart the stimulus to -contraction is the distending pressure of blood—although it is not -altogether necessary that this stimulus should be acting at the time. -Sympathetic and vagus nerves can to a certain extent control the -beating of a bloodless heart. The heart-tissue has acquired the habit -of beating, and the habit of listening to advice conveyed to it through -these nerves.</p> - -<p>The self-adjustment of the blood-tubes to the pressure to which -they are exposed is exhibited in the adaptation of their degree of -contraction to the position of the body—to the weight, that is to say, -of the column of fluid which they have to support. Everyone has played -the game of “right hand or left.” When the hand is held above the head -the blood leaves it, and the hand becomes cold; but if there be need -for adjustment, and time is given for the mechanism to come into play, -it works to perfection. When we are standing erect, there is neither -too much blood in the feet nor too little in the head. But after a -fortnight in bed a convalescent finds, the first time that he stands -upright, that his legs are quickly engorged—his slippers after a few -minutes feel too tight for him—whereas the brain becomes so anæmic -that he turns giddy, or even faints. -<span class="pagenum"><a name="Page_241" id="Page_241">[Pg 241]</a></span></p> - -<p>Numberless illustrations of vaso-motor action are met with in daily -experience. It is a curious fact that the nerves which control the -calibre of the bloodvessels tend to overact their part. When an organ -demands more blood, it is supplied at the expense of the rest of the -body, and especially of the parts most nearly adjacent. This is partly -a mechanical effect. If all the houses in a terrace are supplied with -water from a common main, the bursting of a water-pipe in one of them -will reduce the supply of its neighbours more than it will reduce the -supply of houses in distant parts of the town. But vaso-motor nerves, -in their compensating adjustment, go farther than this. A thimbleful of -blood removed by a leech produces an effect upon an underlying engorged -organ altogether out of proportion to the hydrostatic requirements -of the case. “Cupping” the loins diminishes the congestion of the -kidneys. This is the explanation of the curative efficacy of various -agents which, with improvements in surgery and the introduction of -more reliable drugs, have almost disappeared from the surgeon’s -armamentarium—scarification, blisters, setons, and the like. Such -methods have been relegated to veterinary practice.</p> - -<p>There is a marked tendency to see-saw between the skin and the mucous -membrane of the alimentary canal. During active digestion, when the -“splanchnic area” is full of blood, the skin is cold. Hot fomentations, -by dilating the vessels of the skin, diminish congestion of the -alimentary tract. An inflamed throat is relieved by a compress round -the neck. Conversely, it must be admitted that, in certain persons, -slight constriction of the vessels of the skin induces inflammation -of the mucous membrane. This is one reason for the almost universal -dread of draughts. A draught cools a limited area of the skin. Some -of us cultivate a love of draughts. They are the sensible evidence -of the entrance of fresh air. Yet we admit reluctantly that certain -fragile mortals are not altogether fanciful in supposing that a draught -may give them a catarrh or a toothache. If asked why they object to -draughts, many persons answer that they “are afraid of catching a -chill”—carrying us back to the time before the clinical thermometer -was invented; to days when the shivering fit, or “rigor,” which first -calls attention to the fact that the temperature is already two or -<span class="pagenum"><a name="Page_242" id="Page_242">[Pg 242]</a></span> -three degrees above the normal, was supposed to be the commencement -of the illness. The patient imagined that the “chill” caused him to -shiver, and that if he had not “caught” it he would not have been ill. -The substitution of the term “cold” for “rheum,” naming the malady -after one of its prominent symptoms, has done much to perpetuate this -superstition. “Chill” is a word we scarcely dare to mention. When -doctors could no longer attribute to witchcraft the occurrence of -disorders for which they had no other explanation, they invented the -luminous theory that inflammatory diseases—especially those of the -stomach, liver, and lungs—were produced by “a chill.” At one time all -diseases which were not evidently infectious were caused by chill. The -discovery of germs and the recognition of their maleficent activity -has stripped this cloak of ignorance off almost every case of abnormal -tissue-metabolism. It is recognized now that the germ <i>is</i> the disease, -not the effects which the germ produces. Pneumonia is impossible in -the absence of the pneumococcus, however severe the chill to which the -patient was exposed when out in the cold and wet. Consumption is the -effect produced by the tubercle bacillus. If there are no bacilli, -there can be no consumption. Yet these two diseases illustrate the -possibility of the use of the term “chill” without impropriety. The -coccus of pneumonia may frequently be found in the mouth of a healthy -person. If everyone with whom the tubercle bacillus has at some time -come in contact were inevitably its victim, no human being would be -free from phthisis, if any still survived. There are conditions of -health, or rather of unhealth, in which the economy is less resistant -than usual to the germs. Apparently the vaso-motor disturbances of -internal organs caused by the cooling of the surface of the body, if it -occur when health is otherwise depressed, contributes to the production -of such a state.</p> - -<p>The vaso-motor system is influenced by emotions. It is a little -difficult to express accurately the relation between emotion and -vaso-motor change. Some psychologists regard the vaso-motor change as -the emotion. “All emotions,” says a prominent exponent of this view, -“are wholly due to excitation of a particular kind of the vaso-motor -centre.” The person about to be subject to an emotion of shame, anger, -fear, disgust, recognizes a fact or circumstance, or conjunction of -<span class="pagenum"><a name="Page_243" id="Page_243">[Pg 243]</a></span> -circumstances, which justifies the emotion. (We are assuming that -emotions may be justified; that the intellectual appreciation of -a situation and reasoned decision regarding the action which it -demands is not sufficient.) This recognition as an intellectual act -of the higher brain is accompanied by certain forms of enhanced -activity or inhibition of activity of the vaso-motor centre in the -medulla oblongata which cause changes in the degree of contraction -of the bloodvessels of certain organs. The vascular changes produce -an alteration in the state of the organ which is reflected in -nerve-currents sent back to the brain, providing the background of -feeling which constitutes emotional tone. We are not prepared to -endorse this extreme view of the nature of an emotion. A maiden’s -blush is not an emotion of embarrassment or shame. It is its harmony. -Her mind plays the air. The sensations which originate in the flushed -skin of the face sustain it with their accompaniment. The emotional -tone keeps attention fixed on the fact or circumstance which led her -to conclude, by the exercise of her reason, that she was placed in an -awkward situation. This fixing of attention is frequently so pronounced -as to inhibit all other intellectual action. The maiden is less quick -than she would have been, had the emotion not glued her thoughts -together, in recognizing the readiest means of extricating herself -from embarrassment. All nerves found within the chest and abdomen -were in very early times termed “sympathetic.” The cord in the neck -was the “little sympathetic.” The name explains itself; but it will -be understood that it implied much more in the days when the liver, -spleen and heart were supposed to pour out emotions than it does now. -The vagus nerve was termed the “middle sympathetic.” Shame inhibits the -activity of the vaso-constrictor nerves of the face; dilation of the -vessels which they supply is accompanied with constriction of other -cutaneous nerves. Kipling must, we think, have embellished Nature -when he represents the very unimpressionable hero of Lungtungpen as -admitting “I niver blushed before or since; but I blushed all over -my carkiss thin.” Usually the carmine of the face contrasts with the -pallor and coldness of the hands. Still, we are not prepared to assert -that it is impossible, under circumstances as trying as those in which -<span class="pagenum"><a name="Page_244" id="Page_244">[Pg 244]</a></span> -Private Mulvaney and his companions were placed, for all the cutaneous -constrictor nerves to let go their grip at the same time. Terror -heightens the control of the vaso-motor centre over the vessels of the -skin; it increases vagus inhibition of the heart. Even disgust evoked -by a revolting sight or a foul smell may call the vagus so forcibly -into action as to bring the heart to a standstill.</p> - -<p><b>The Pulse.</b>—The arterial system is always distended. The -pressure in the largest arteries amounts to about 140 millimetres -of mercury. The source of pressure is the beat of the heart pushing -the blood forward against the resistance offered to its flow by the -smallest vessels. At every stroke another 3 ounces is added to the -already overfull vessels. In the aorta, therefore, the blood moves -forward with jerks, but by the time it reaches the capillaries the -intermittent accessions of force have been taken up by the elastic -walls of the vessels and returned to the stream in the form of constant -pressure. In the very smallest arteries the blood flows in a steady -stream. If the corpuscles in a capillary vessel are watched under the -microscope, they show no variations in rapidity synchronous with the -beat of the heart. The “pulse” in the larger arteries is the push -given to the column of blood by the sudden contraction of the left -ventricle. Its propagation along the arteries will be understood if it -is remembered that the blood is contained within elastic tubes. The -first effect of the ejection into the aorta of an additional quantity -of blood is the distension of its wall. The wave of distension travels -down all the arteries of the body with gradually decreasing force.</p> - -<div class="figcenter"> - <a id="I_244" name="I_244"> </a> - <img src="images/i_p244.jpg" alt="" width="600" height="188" /> - <p class="center"><span class="smcap">Fig. 14.</span>—<span class="smcap">Sphygmograph.</span></p> - <div class="blockquot2"> - <p class="neg-indent space-below2">A, An ivory button which is pressed on the skin over the radial artery - by a metal spring. B, A continuous screw which works against the - cogwheel C. By rotating B, the lever D is raised to a position in which - its point scratches the travelling-plate E (covered with blackened - paper). F, A box containing clockwork which moves E. G, A screw by - means of which the pressure of the spring is adjusted to the force of - the pulse.</p> -</div></div> - -<p><span class="pagenum"><a name="Page_245" id="Page_245">[Pg 245]</a></span></p> - -<div class="figcenter"> - <a id="I_245" name="I_245"> </a> - <img src="images/i_p245.jpg" alt="" width="450" height="630" /> - <p class="center"><span class="smcap">Fig. 15.</span></p> - <div class="blockquot2"> - <p class="neg-indent space-below2">A, A cardiogram, or tracing of the impulse of the heart, recorded on - a blackened plate borne on the end of a vibrating tuning-fork; <i>a-b</i>, - the systole of the auricles; <i>b-e</i>, the ventricular systole. From <i>c-e</i> - the heart is shrinking as blood leaves it by the aorta and pulmonary - artery. B, C, D, E, F, Sphygmograms. B is shaded to show the portion - of the pulse-wave which corresponds to the systole of the heart. - C, High-tension pulse of vigorous health. D, Low-tension pulse. E, - Dicrotic pulse of fever. F, Hog-backed pulse of hardened (atheromatous) - arteries.</p> -</div></div> - -<p><span class="pagenum"><a name="Page_246" id="Page_246">[Pg 246]</a></span> -Much may be learned from the pulse with regard to the condition of -the vascular system, although it is impossible to balance the effects -of the several factors which go to the production of its various -modifications. The character of the pulse depends upon the vigour -with which the heart is beating, the efficiency or otherwise of the -cardiac valves, the quantity of blood in circulation, the suppleness -of the arterial walls, the degree to which they are contracted, the -resistance offered by the smaller vessels. Departures from the normal -may take the direction of unduly high tension or of unduly low tension. -In place of the sudden rise and more or less gradual fall, with the -slightest possible roughness due to secondary waves, which constitutes -a healthy pulse, the rise may be shorter, its subsidence prolonged. -This is a high-tension or hard pulse. The pressure in the arteries -is unduly high, or the walls of the vessels are not as elastic as -they should be. Considerable pressure is needed to obliterate such a -pulse—<i>i.e.</i>, to prevent it from passing on beneath the finger. -As the converse of this condition, the difference between the beginning of -the pulse and its end may be very marked, the vessel suddenly dilating -and as suddenly collapsing. But little pressure is needed to stop such -a low-tension pulse from passing beneath the finger. Usually it has a -distinct secondary or dicrotic wave. Some tactile education is needed -by the finger that aspires to read the pulse. It was hoped that the -personal equation would be of less importance if mechanical records -were substituted for statements as to the impression produced upon the -observer. Various forms of sphygmograph (σφυγμός, pulse) have been -invented for this purpose. The form commonly used (<a href="#I_244">Fig. 14</a>) -consists of a metal spring which is adjusted so that a button beneath its free -end presses on the radial artery at the wrist. The force with which -it presses is regulated by a screw. At each pulsation its free end is -lifted by the distension and rounding of the artery. Its movement is -transmitted by means of a continuous screw, attached to it vertically, -to a cogged wheel, which in its turn raises a lever. The end of the -lever scratches blackened paper fastened on a plate moved by clockwork. -Records made in this way are useful for future reference. They are not, -however, so valuable as it was anticipated that they would be. The form -<span class="pagenum"><a name="Page_247" id="Page_247">[Pg 247]</a></span> -of the tracing depends to so large an extent upon the amount of -pressure exerted by the spring, and the amount of pressure must -be adapted to the vascular tone in every case. Some of the most -interesting tracings are obtained from old people affected with -atheroma of the arteries. This is a condition in which, owing to -old-standing inflammation of the subepithelial coat of the vessels, -the arteries have lost their suppleness. They are hard and inelastic. -Instead of showing the normal steep face of the pulse-wave rising -abruptly to its highest point, the tracing rises vertically for a -short distance, and then slopes upwards. The wave is flat-topped or -hog-backed.</p> - -<p>All pulses are dicrotic, although the dicrotism may not be sufficiently -pronounced to be felt with the finger. The notch which divides the -primary from the secondary wave is produced by the closure—that is -to say, by the falling down of the aortic valve. The wave from the -commencement of its ascent to the dicrotic notch corresponds to the -period during which blood is passing from the heart into the aorta. -This part of the tracing represents systole of the ventricle after the -semilunar valve has been forced. It is the push given to the bottom of -the column by the additional 3 ounces of blood thrust into the aorta. -The effort of the ventricle then comes to an end. The pressure beneath -the semilunar valve is less than that above it. The valve closes. If -the blood were contained in an open tube, the wave would now end, -save for secondary oscillations, due to inertia of the fluid. But the -arterial system is practically closed owing to the fineness of the -tubes into which it ultimately divides. Its walls are elastic. They -distend, taking up the pressure and returning it again in the second -half of the wave. In fever, after the consumption of alcohol, and in -other conditions in which the finest bloodvessels are dilated, the -division between the two parts of the wave is very marked. Dicrotism -is plainly felt. We have used the expression “finest vessels” rather -than “capillaries,” because the ascription to the capillary vessels of -all peripheral resistance has led to misunderstanding. Resistance is -offered throughout the whole vascular system, with the exception of the -largest veins. It is greatest in the small arteries, capillaries, and -small veins. It is so adjusted as to fall to zero just before the blood -reaches the heart.</p> - -<hr class="chap" /> -<p><span class="pagenum"><a name="Page_248" id="Page_248">[Pg 248]</a></span></p> -<div class="chapter"> - <h2>CHAPTER X<br /><span class="h_subtitle">MUSCLE</span></h2> -</div> - -<p>Living matter, protoplasm, is irritable. It responds to influences -impressed upon it by its environment. An effective influence, termed -a “stimulus,” produces a change in protoplasm at the spot at which -it acts. From this spot the change spreads outwards as an “impulse.” -Protoplasm is said to “conduct.” A stimulus may be likened to a -blow given to a fixed but elastic mass; an impulse to the vibration -which travels outwards from the spot struck. Unfortunately, the term -“stimulus” is used both for the stick that strikes, the stimulator, -or stimulant, and for the blow that is struck; but breaches of logic -seldom lead to confusion in an experimental science. The context -indicates the particular application of the term. The manifestation of -stimulation is a physical or chemical change—most obvious when it -is one of form. This change of form may occur at the spot stimulated, or -may be deferred to a distant part to which the impulse is conducted.</p> - -<p>In opening the study of muscle and nerve we need to form a conception -of the nature of these three functions—irritability, conductivity, -and changeableness of form. Not that the functions are as distinct as the -ideas to which the three terms give rise. They are three aspects of a -common function; although this is a reflection which will carry more -weight when the ways in which protoplasm reacts to external forces have -been considered.</p> - -<p>A stimulus may be mechanical, something in the nature of a blow which -displaces the particles of protoplasm; or it may be chemical or -thermal, disintegrating a portion of its substance; or electrical, -divorcing the ions of its molecules. Only the last in any way resembles -<span class="pagenum"><a name="Page_249" id="Page_249">[Pg 249]</a></span> -a natural stimulus; since electrical stimulation alone can be repeated -without the substance stimulated showing any evidence of injury in -the process. Mechanical, thermal, chemical stimuli destroy a portion -of the protoplasm upon which they act. Yet even the weakest of -electric currents is a gross disturbance as compared with natural -stimuli, such as touch, warmth, sound, light. The essential and most -distinguishing quality of living matter is its return to its original -state immediately after stimulation. It does not even wait until the -stimulator has ceased to act. An effective influence is a sudden change -in the environment. It is answered by a sudden response, followed by a -return of protoplasm to the state in which it was before the impact of -the external force. The change progresses through the protoplasm as a -transitory alteration of state, the particles concerned in conducting -it returning to their original condition the moment it has passed. -No non-living matter responds to force in this way. If a stone is -dropped into a pond, a wave circles outwards from the spot it strikes; -but this is a wave of displacement, not a change of state. Suppose -the pond contained a solution of sugar which the impact of the stone -changed into vinegar, and that the zone of vinegar spread outwards, -the liquor returning to the condition of sugar and water as it passed. -Here we should see some analogy to the progress of an impulse. But no -non-living matter behaves like this. A product of the laboratory may -be so unstable as to explode when shaken, passing on the slightest -provocation into a more stable state. It does not return after the -explosion to its previous strained condition. Having thrown away its -energy, it continues on a lower plane. Protoplasm parts with energy -to recover it again. It returns to instability after assuming a more -stable form.</p> - -<p>If we are to form a conception of the cause of the irritability of -living matter, we must have a mental picture of the physical conditions -which distinguish life from death. All matter is in a state of motion. -It consists of separate molecules, each moving in its orbit with vast -rapidity. A molecule is a cluster of atoms. The dimensions of its orbit -depend upon the number and weight of the atoms in its cluster. If we -could watch the dance of the molecules of proteins and other substances -<span class="pagenum"><a name="Page_250" id="Page_250">[Pg 250]</a></span> -into which protoplasm breaks up on dying, we should see each separate -cluster executing the figure appropriate to its mass, indifferent to -the movements of neighbouring groups. But if living protoplasm were -of the company, the scene would be one of vastly greater animation; -for now it is the ambition of our dancers to form a single group. To -this they can never attain. There is a physical limit to the number -of dancers who can hold together while the music carries them in wide -sweeps backwards and forwards across the floor. At every gust of -wind which bursts through an open doorway a group breaks, to clasp -hands again as the wind subsides. Protoplasm is always on the verge -of instability; always snatching at additional atoms which it draws -within its ring; always shaking off other groups of atoms because -the ring is too large to hold together. Touch it, and it falls into -simpler combinations. Kill it, and it becomes a mixture of organic -and inorganic compounds which we know and can name. But as long as it -is alive—as long as it is protoplasm, that is to say—integration -and disintegration are occurring. Simultaneous complication and -simplification <i>is</i> life. The protoplasm-molecule, if we dare to think -of it as a molecule, in the sense in which a chemist uses the term, -is always changing. It is its variability which makes stimulation -possible. Irritability is a tendency to dissociation under the -influence of an external force, with reassociation when the force -ceases to act.</p> - -<p>The molecules which protoplasm gathers into itself may be classified -under the headings oxygen, foods, water, and inorganic salts. It is the -two latter which most affect its state, conferring upon it the capacity -of exhibiting the phenomena of life. Water and the ions of salts -dissolved in water, electrolytes, are linked to the other elements of -its groups. Striving to find room for more molecules of water and more -ions, protoplasm expands. It becomes more mobile and more irritable; -for irritability and mobility vary as the number of these extraneous -groups of atoms which protoplasm is in a position to let drop. As -an impulse travels through it they lose their hold, recovering it -as they pass the impulse on. This progress towards expansion is the -lifeward tendency; the quickening of activity which leads also to the -incorporation of additional atoms of nitrogen-containing substances, -and consequent growth. -<span class="pagenum"><a name="Page_251" id="Page_251">[Pg 251]</a></span></p> - -<p>The opposite tendency is deathwards. Protoplasm drops extraneous groups -of atoms; retires into itself; loses irritability; settles down to rest.</p> - -<p>The molecules of proteins exhibit a property which appears to pertain -in some degree to living matter also. When their relation to the water -in which they are dissolved and the electrolytes which it contains is -disturbed, they appear to go out of solution, they coagulate. This -disturbance is brought about in all proteins by heat; in some it is -the result of altering the amount of salt in the water in which they -are dissolved. Coagulation in protoplasm is the prelude to death; but -it would appear that a step in this downward path is taken whenever -an impulse is conducted. Coagulation is due to the clustering of the -molecules of a protein. When protoplasm drops electrolytes and water -its molecules cluster in some degree, regaining their independence and -reattaching their accessory groups of atoms as the cause which drove -them to make for safety passes by.</p> - -<p>Our conception—not of life, but of “the physical basis of life,” may -be very wide of the mark. The account given above is intended as little -more than a hint of the lines along which thought is travelling at the -present time. The reader must not regard it as a serious attempt to -present in detail the views of any of the workers who are endeavouring -to apply the results of recent discoveries in molecular physics to the -solution of problems in the chemistry of living matter. There can, -however, be little doubt but that we are on the eve of further advances -which will secure data upon which it will be legitimate to construct -hypotheses. At present it would be unreasonable to do more than -indicate the direction from which it may be hoped that light will shine.</p> - -<p>A stimulus is a change of circumstance rather than a transient -disturbance. When an electric current is thrown into it, protoplasm -dissociates—parts with something. It instantly reassociates. The -continued passage of the electric current does not maintain it in a -dissociated condition. When the current is cut off, the sudden change -again acts as a stimulus. Within limits, the efficiency of an electric -stimulus varies as its suddenness. Similarly with all other stimuli to -which protoplasm responds: crushing, burning, chemical decomposition, -<span class="pagenum"><a name="Page_252" id="Page_252">[Pg 252]</a></span> -are effective at the moment of their occurrence. When they generate a -succession of responses, it is because they continue to produce changes -in the protoplasm. Their continued action does not, under ordinary -circumstances, prolong the response.</p> - -<p>Response to stimulation travels as an <i>impulse</i> through protoplasm. -An impulse is commonly likened to a wave, but enough has been said -already to prove that the simile is misleading. It is not of the same -nature as the wave which a stone starts on the surface of a pond, a -pulsation of sound through air or water, an undulation of light or heat -in the æther. These various kinds of waves are waves of displacement, -a swing first to one side and then to the other. An impulse traverses -protoplasm, whether it be the apparently diffuse protoplasm of a -leucocyte or the severely oriented protoplasm of a nerve or muscle, as -a change which may be described as chemical, with reservations as to -the meaning allowed to this term. We may without impropriety represent -the fall (dissociation) and subsequent rise (association) graphically -as a wave; but even then it is but a half-wave, and inverted. It is -a very different thing to the onward progression of an accession of -force, with which it is not infrequently confused.</p> - -<p>All protoplasm is not equally susceptible of stimulation. Probably it -is safer to put this in a different form. Protoplasm is not everywhere -equally exposed to stimulation, nor is it when especially exposed -to stimulation in one way equally accessible to all other effective -forces. A sense-organ is a collection of cells in which protoplasm is -so disposed as to be susceptible to a certain kind of stimulus. It is -a “receptor” for a particular force. At the same time it is essential -to its efficiency that it should be insusceptible to other forces. The -protoplasm in certain of the sense-organs of the skin dissociates when -compressed, in others when warmed. The cells of these receptors have -a certain structure which exposes their protoplasm in such a manner -that it cannot escape dissociation when, in the one case, the cells are -squeezed, or when, in the other case, they are heated. The ear contains -sensory cells so constructed that the protoplasm which they contain -dissociates when affected by pulsations of sound. In the receptors -of the tongue and the nose protoplasm is exposed to the influence of -<span class="pagenum"><a name="Page_253" id="Page_253">[Pg 253]</a></span> -chemical stimuli; in the eye it is exposed to the dissociating action -of light.</p> - -<p>Protoplasm is responsive to external force. It conducts the impulses -to which stimulation gives rise. Eventually the impulses, which -travel along strands of tissue highly specialized for the purpose -of conduction—nerves—reach collections of protoplasm which are so -disposed that when they dissociate energy is set free. A comprehensive -term is much needed for the connotation of this third essential -property of protoplasm, the capacity of liberating energy which -characterizes “effectors.” An external force, so small in intensity -as to be negligible when we are dealing with the body’s accounts, -acts upon the protoplasm of a receptor. A change in state results. -The change is conducted to an energy-liberating organ. This organ is -supplied with blood which brings it food. Food is its store of energy, -the raw material from which it manufactures its ammunition. When an -impulse reaches an energy-liberating organ its protoplasm dissociates. -But here the protoplasm is so disposed—the cells which contain it have -such a form—that when it dissociates a change in the cell follows; -it alters in shape, or it discharges into its environment heat, or -electricity, or light. The dissociation and reassociation of the -protoplasm of an effector involves chemical change. Molecules of water -and of carbonic acid are cast off. The energy sacrificed in letting -matter fall into these very stable forms is the energy made visible, as -it were, in lifting a weight or dispersing heat. It must be replaced if -the organ is to retain its power of acting when next an impulse reaches -it. To replace it, protoplasm takes up food and oxygen from the blood.</p> - -<p>The liberation of energy which occurs when a muscle contracts is not -a special phenomenon—something which does not occur when the muscle -is at rest. It is an intensification of a process which is always -taking place. The substance of muscle, like that of nerve and every -other tissue, is always combining with oxygen and giving off water and -carbonic acid. When we are auditing the body’s accounts, we enter so -and so much food and oxygen on the debit side, we credit it with the -same weight of water and carbonic acid; or we debit it with the energy -<span class="pagenum"><a name="Page_254" id="Page_254">[Pg 254]</a></span> -potential in the food, and enter to its credit the mechanical work done -and the heat set free by the oxidation of this food. Food is the petrol -the combustion of which causes the movement of the car. The external -force which stimulates a receptor is too insignificant in amount to be -carried to account. Physiologists neglect it, just as engineers neglect -the energy liberated by the sparking-plug which ignites the petrol, -when they are estimating the efficiency of a motor.</p> - -<p>Compared with the amount of energy actually received from the -environment when a sensory cell of the eye or ear is excited, the -energy needed to start an artificial impulse in a nerve is relatively -enormous; yet a well-known comparison of the energy conveyed to the -nerve in a certain experiment with a nerve-muscle preparation from a -frog, and the energy expended by the muscle in contracting, brings home -to our minds the fact that it is impossible to carry even this item to -account. The energy furnished to the nerve from an electric condenser -measured 0·001 erg; the energy expended by the muscle reached 100,000 ergs.</p> - -<p>It is easy to determine the amount of mechanical work which results -from a given expenditure of energy. By alternately flexing and -extending the joints of his legs, a man lifts his own weight up a -hill of a certain height. The work can be measured in foot-pounds or -in kilogrammetres. But this by no means accounts for all the energy -potential in his food. A still larger amount is expended for the -purpose of keeping the body warm, or, not improbably, making it too -warm; in either case generating heat which is dissipated into the -atmosphere. When a machine is being planned, attention is concentrated -upon the problem of how to get the largest result in work for a given -quantity of fuel. Fuel costs money. All energy dissipated as heat -is wasted. Every ounce saved makes for economy. Engineers therefore -speak of the “efficiency” of an engine as the relation between the -work actually done and the work which would have been done if no -energy had been wasted. In the best steam-engines it stands at about -1 to 10. Since the chief function of muscle is to do mechanical work, -physiologists are apt to adopt the engineer’s point of view. But in the -case of muscle this is justifiable only in a limited degree. The body -of a warm-blooded animal is maintained at a temperature higher than -<span class="pagenum"><a name="Page_255" id="Page_255">[Pg 255]</a></span> -that of the surrounding air. Muscles are the chief producers of heat. -If they turned all the energy which they receive into work, they would -be inefficient as regards this very important function. Yet even from -the engineer’s point of view muscles are more efficient than the best -of engines.</p> - -<p>It is almost impossible to determine with accuracy, in regard to -isolated muscles, the amount of food taken up from the blood, and the -return in work by the muscles of the energy potential in the food. -Calculations have to be based upon observations of food consumed, gain -or loss of body-weight, work done by a man or an animal during a period -lasting for several days. We shall consider the evidence obtained -in this way in a subsequent section (<a href="#Page_149">p. 149</a>). But whether -we study isolated muscles or the body as a whole, the relation between work and -heat varies within wide limits. So wide, indeed, are the variations as -to justify the conclusion that there is no necessary relation between -the two phenomena. Muscles develop heat when they are quiescent. -Activity is accompanied with an increased evolution of heat; but, if it -be desirable, the evolution of heat is reduced until it is, relatively -to the output of work, much smaller than in the case of any engine -which has yet been made. It is sufficient in this connection to state -that, under certain conditions, the return in work may amount to about -one-half. The comparison with an artificial motor, of whatever kind, -breaks down. In an engine combustion develops heat, heat causes steam -or gas to expand, the expanding gas pushes a piston. In muscle certain -of the carbon, hydrogen, and oxygen atoms contained in protoplasm -combine to form water and carbonic acid—compounds too stable to be -reassociated with the remaining atoms of the protoplasm-molecule. -They are replaced by complex, energy-yielding substances—foods—and -by oxygen, carried in the blood. Their displacement brings about a -change in the form of the molecules which involves, owing to their -peculiar orientation, a change in shape of the muscle as a whole. -Such an explanation is, perhaps, more exact than our knowledge at -present warrants; or rather let us say, since we do not know what the -expression “the form of a molecule” means, it has an appearance of an -exactitude which does not characterize it. It is merely intended to -help the reader to realize the hopelessness of attempting to compare -<span class="pagenum"><a name="Page_256" id="Page_256">[Pg 256]</a></span> -muscle with any mechanical contrivance. In the boiler of a steam-engine -heat is applied to water until its molecules cannot remain in so -close a state of aggregation. Their orbits are greatly increased. The -cause of the thrust given to the piston of an engine is the increased -amplitude of movement of the molecules of steam behind it. In a -combustion engine a mixture of petrol and air is ignited. Energy is -set free by the resolution of unstable petrol into stable water and -carbonic acid. This energy heats the gases, causing them to expand. -Waste of energy as heat is inevitable in a machine which depends for -its motive-power upon the translation of molecules. The source of -muscular force (if it be not intramolecular change) is certainly not, -directly, increased amplitude of molecular swing.</p> - -<p>But we must not conclude, as we are tempted to do, that muscle is -capable of liberating as mechanical work the whole of the energy -supplied to it in food, seeing that its activity is always accompanied -by evolution of more heat than can be attributed to friction. If -the bulb of a thermometer be inserted into a group of muscles, the -instrument shows a marked rise of temperature when the muscles -contract. Even though the temperature of the chamber in which an animal -is placed is equal to its own, the animal makes more heat if compelled -to work, notwithstanding the fact that the consequent rise of its -body temperature may prove fatal.</p> - -<p>Nor can muscles dispense unlimited heat without doing mechanical work. -If I am too cold, the obvious means of getting warm is jumping about. -There appears to be a level of heat-production which cannot be exceeded -without movement. When more heat is called for than quiescent muscles -can produce, they exhibit flickering contractions, shivering, without -moving the limbs. The signal for increased production is given by -the skin. The skin is sensible of the amount of heat which is being -lost. Exposure to cold air makes one shiver, by suddenly withdrawing -heat. But an increase of temperature in the blood behind the skin -has an exactly similar effect. In the first stage of fever, when the -temperature of the body has risen two or three degrees, and before -the system has become accustomed to this state of affairs, the skin -<span class="pagenum"><a name="Page_257" id="Page_257">[Pg 257]</a></span> -announces to the muscles that heat is being rapidly lost. A severe -shiver, termed a “rigor,” is the result. At the same time loss of -heat by evaporation is checked, just as it is when the skin is cold. -The sweat-glands are rendered inactive. A phenomenon which marks the -nightly fall of temperature in consumptive patients is the sudden -return of activity in these glands.</p> - -<p>Muscle when most highly developed has an extraordinarily definite -structure. It is minutely subdivided into units which appear, looked -at separately, simple in design. We are tempted to believe that the -explanation of the way in which each of these units works is not far to -seek. It is disappointing to be obliged to admit that, notwithstanding -all the thought which has been devoted to the problem, we are as far -as ever from a definitive solution. We understand the principles on -which steam-engines, combustion-engines, electric motors are planned. -We compare muscle with each of these mechanical contrivances in -turn, expecting to discover the principle of its construction. Many -ingenious hypotheses have been formulated; but the fact that some of -these are mutually destructive shows clearly enough that as yet no -approach to certainty has been made. Probably the fundamental error -lies in attempting to compare muscle with a mechanical contrivance. The -apparent simplicity and regularity of structure of “striped muscle” -misleads us. We ought to have commenced our investigations at the other -end of the scale of mobile tissue—to have begun with semifluid and -apparently homogeneous animal matter, working upwards to the tissue -which, being limited to the one function of movement, and movement in -one direction only, has, as it were, crystallized along the lines of -force.</p> - -<p>All protoplasm is mobile. Its particles move one on another. Hence -follows either circulation of the living matter within the cell or -change in shape of the cell. The two phenomena are identical in nature. -Circulation is best studied in a large-celled, transparent part of a -plant. A filamentous water-weed is suitable for the purpose. If this -be examined with a microscope while still alive, its cells are seen to -contain a watery juice enclosed in spaces of denser cell-substance. -Bridges of cell-substance span the spaces. The particles of which these -<span class="pagenum"><a name="Page_258" id="Page_258">[Pg 258]</a></span> -bridges consist are in a state of constant streaming motion, which has, -it is needless to say, no effect upon the shape of the cell (<a href="#Page_9"><i>cf.</i> p. 9</a>).</p> - -<p>The unicellular animal amœba, leucocytes, and certain spores of plants, -are devoid of cell-wall (<a href="#Page_28"><i>cf.</i> p. 28</a>). Their soft protoplasm -is not limited by a rigid case. When it streams, the form of the cell is -changed. True, we must not think of the body-substance of an amœba as -homogeneous. It exhibits an internal structure. Yet its architecture -is not, so far as we can see, sufficiently fixed to restrict the -directions in which it can stream. Any change of shape is possible. -We cannot find in Nature an isolated clump of living protoplasm; nor -do we suppose that, if we found it, it would prove to be homogeneous. -It appears to be necessary that protoplasm and metaplasm—the terms -have no chemical significance; “primary” and “secondary,” or “chief” -and “subsidiary” would be equally distinctive—should be intermixed. -Streaming is apparently due to alterations in the surface relations of -the two substances.</p> - -<p>In multicellular animals certain elongated cells are arranged in -groups, with their long axes all pointing in the same direction. They -can change in shape, diminishing in length, with equivalent increase in -breadth. Since all the cells of a group undergo this change of form at -the same time, the result is an alteration in the shape of the animal -of which they are a part. Applying the experience which we have gained -in studying the movements of unicellular organisms, we conclude that -these elongated cells are composed of two substances—protoplasm and -metaplasm. The restriction of their capacity for altering their shape -to one direction indicates that their protoplasms and metaplasm are not -indifferently mixed. The two substances set in lines in the direction -of the long axis of the cell. Hence, when streaming occurs—when the -force which keeps the molecules of protoplasm and of metaplasm in their -respective rows is relaxed—the lines thicken. The cell broadens, -with an equivalent diminution of length.</p> - -<p>Muscle-fibres exhibit all degrees of specialization. The simplest, -“plain muscle-fibres,” are found in the wall of the alimentary canal, -of bloodvessels, of ducts, in the tissue of the spleen, in the skin, -<span class="pagenum"><a name="Page_259" id="Page_259">[Pg 259]</a></span> -and elsewhere. Each fibre is a fusiform cell. Save for its central -nucleus and a little granular protoplasm in which the nucleus is -embedded, the cell may show no architectural features. But in most -varieties of plain muscle, and especially in that of the alimentary -canal, the substance of the fibres is striated longitudinally. This is -visible evidence of the orientation of the molecules of protoplasm and -metaplasm in the direction of the long axis of the fibre. It shows that -the streaming of particles occurs along these lines. It is, as it were, -a diagram of the lines of force.</p> - -<p>Heart-muscle has been described already (<a href="#Page_224">p. 224</a>). -Its striation, which is both transverse and longitudinal, is so delicate as almost -to defy microscopical analysis. The transverse striæ are the darker and more -distinct. But close examination shows that the transverse striæ do not -indicate the direction in which the particles of cell-substance are -oriented. They are oriented longitudinally. The cell is a bundle of -rods of substance A, embedded in substance B. The transverse markings -are very thin lines which cross the bundles at right angles.</p> - -<p>The third variety of muscle is the kind by which locomotion is -effected. It is present in large masses—all the red tissue to which -the term “meat” is commonly applied. It accounts for about 35 per cent. -of the body-weight. This kind of muscle is not composed of single -cells, but of compound cells, or cell-complexes, termed “fibres.” A -fibre may attain a length of upwards of 2 inches, with a breadth of -about ¹/₅₀₀ inch. In most cases the fibres are attached by one end to -a bone, by the other to a tendon; and since they are shorter than the -muscle as a whole, the tendon commences as a membrane which covers the -surface of the muscle, sloping to it from the bone to which by their -other ends the fibres are attached. A fibre is developed from a single -cell. The cell elongates, its nucleus divides, and the daughter-nuclei -divide until several hundred have been formed; but cell division -does not follow. The result is a cylindrical mass enclosed within a -delicate membranous sheath, the sarcolemma. In the early stages of its -development its nuclei are in the axis of the fibre, but subsequently -they are displaced outwards. In the most highly specialized muscle, -known as the “white” variety, they lie just beneath the sarcolemma -(<a href="#I_262"><i>cf.</i> Fig. 16, B</a>). -<span class="pagenum"><a name="Page_260" id="Page_260">[Pg 260]</a></span></p> - -<p>The feature of this type of muscle is its transverse striation, almost -mathematically regular. Commonly striated muscle is spoken of as -“voluntary,” because, for the most part, it is under the control of -the Will; but the term, in so far as it implies a connection between -structure and mode of actuation, is misleading. Transverse striation -is evidence of capacity for rapid action. The muscles which the Will -directs exhibit promptitude; but striated muscle, which is not under -the direction of the Will, is found in certain situations—<i>e.g.</i>, -the upper part of the œsophagus. Conversely, many animals can voluntarily -call into action muscle which is not striped. A turkey erects its -feathers by setting in motion little groups of “plain” fibres, which -pull on elastic tendons attached to the tips of the buried ends of -their shafts. Plain muscle contracts less promptly and relaxes more -slowly than the striped variety. Cardiac muscle is quicker in acting -than plain, but does not hold the contraction so long.</p> - -<p>All striped muscle is not equally rapid. Two varieties are -distinguishable: “white fibres,” which respond suddenly to a single -stimulus and quickly relax; “red fibres,” which respond in a more -leisurely way, but remain contracted longer. In some muscles these two -types of fibre are intermixed. Others are wholly red or wholly white. -Everyone is familiar with the contrast which the white flesh of a -turkey or of the domestic fowl presents to the red flesh of game-birds -and birds of prey. In the breast of a blackcock a sheet of white -muscle overlies a mass of red. When the bird is cooked the difference -in colour is strongly marked. Of the two muscles which, in a rabbit, -correspond to our muscles of the calf, the superficial, gastrocnemius, -is white; the deeper, soleus, red. The former acts over both knee -and ankle joints; the latter over the ankle only. The muscle which, -acting over a longer range, has to contract more quickly is white; the -shorter, more slowly acting muscle is red. Experiment shows that red -and white muscles are distinguished by a difference in the promptitude -with which they respond to an electric current. It shows, too, that the -white muscle is exhausted sooner than the red. It cannot give so many -successive responses to stimulation without a rest. We shall find, when -we are considering the minute structure of striped muscle, a difference -<span class="pagenum"><a name="Page_261" id="Page_261">[Pg 261]</a></span> -between its two varieties which we can correlate with their different -modes of action. All human muscles belong to the red kind.</p> - -<p>The most efficient muscle-fibres in the animal kingdom are found in -insects. This will not surprise anyone who thinks of an insect’s power -of movement. If a man could jump as many times his own height as a flea -can, he would clear the dome of St. Paul’s. An ant can drag an object -sixty times as heavy as itself, with no wheels beneath it to diminish -friction. Under the same conditions a horse cannot drag much more -than its own weight. A dragon-fly, it is asserted—although we have -not met a man who guarantees that he has made the observation—will -support its heavy body in the air by the rapid vibration of its wings -for four-and-twenty hours without alighting. The chirp of a cricket is -produced by the rubbing together of its hind-legs. A mosquito sounds -its war-cry much in the same way. The pitch of the note proves that -the insect’s muscles are contracting and relaxing at least 300 times a -second. None of these figures must be applied without qualifications in -estimating the relative strength of insect and human muscle. Weight for -weight, the muscle of a flea is not so much stronger than ours as the -figures might lead one to infer. To ascertain the numerical relation, -it is necessary to compare the total cross-section of the two chief -segments of a flea’s leg with the cross-section of the extensor muscles -of a man’s thigh and calf, and a man’s weight with the weight of a -flea. Nevertheless, after all deductions have been made, a considerable -balance of superiority lies with the insect as regards the strength -of its muscles, their rapidity of contraction, and power of repeating -contraction without fatigue. An insect’s muscle is the most suitable -that can be obtained for microscopic examination. Its pattern is larger -and more distinct than that of other animals. That the pattern should -be larger is not quite what might have been expected. It would not have -surprised us had we found the pattern finer in the more effective type.</p> - -<p>Nothing is easier than to mount a specimen of insect-muscle. The large -water-beetle (<i>Dytiscus marginalis</i>) is an excellent subject. It is so -easily handled. Having cut off the animal’s head, a leg is pulled out -from the thorax. It is split open with a penknife, and a little of the -<span class="pagenum"><a name="Page_262" id="Page_262">[Pg 262]</a></span> -muscle is dug out from within its hard case, placed on a clean slide, -and covered with a cover-slip. If the preparation has been made quickly -and cleanly, the muscle remains alive for five or ten minutes. Not only -can it be studied, with the microscope, unaltered by reagents, but -under the most favourable circumstances the progress along its fibres -of waves of contraction can be watched. The structure of the fibres is -more easily made out if a little salt-solution or white of egg is added -to the preparation.</p> - -<div class="figcenter"> - <a id="I_262" name="I_262"> </a> - <img src="images/i_p262.jpg" alt="" width="600" height="267" /> - <p class="center"><span class="smcap">Fig. 16.</span>—<span class="smcap">A, A - Minute Portion of an Insect’s Muscle-Fibre, highly magnified.<br /> B, White - Fibre of Mammalian Muscle.</span></p> - <div class="blockquot2"> - <p class="neg-indent space-below2">A, The nuclei are in the core of the fibre. B, The nuclei lie - immediately beneath the sarcolemma. The disc on the left of this fibre, - and the fibril on its right, show the two ways in which striated - muscle-fibres tend to cleave. The dark line, or row of dots, is known - as Dobie’s line, or Krause’s membrane. The figures are severely - diagrammatic.</p> -</div></div> - -<p>Striped muscle is crossed by bands, dim, bright, and dark. The sequence -is as follows: Starting with the very thin dark line, which often -appears as a row of dots, the next band is bright; then comes a dim -band about twice as broad as the bright one; then another bright -band. This sequence is repeated with extreme regularity from end to -end of the fibre. Usually the bands cross the whole breadth of the -fibre, although occasionally it is divided by longitudinal lines into -parts in which the stratification is shifted a little backwards or -forwards. A segment of a fibre comprises the substance between two -dark lines—<i>i.e.</i>, two bright bands with a dim one between them. -If the muscle has been hardened in one of the fluids commonly used for -the purpose of preparing tissues for the microscope, with its two ends -fixed, say, by binding them to a piece of a match, so that it could not -shrink, a thin clear line appears crossing the middle of the dim band. -This seems to show that the fibre is not made up of single dim discs -<span class="pagenum"><a name="Page_263" id="Page_263">[Pg 263]</a></span> -between two bright discs, but of couples, comprising half a dim disc -and a bright disc. The thin dark lines indicate that the fibre is -divided into compartments by transverse septa, which are probably -reticulated. The appearance of a transverse line of dots, in place of -a continuous line, is due to the existence of very fine longitudinal -markings (it is unsafe to give them a name which connotes structure). -Where the longitudinal lines cross the transverse lines, the optical -effect is the appearance of a dot.</p> - -<p>If pieces of muscle are placed in a solution of osmic acid, they become -hard and brittle, and their markings are accentuated. Muscle from -the claw of a crab or a lobster is very suitable for this purpose, -owing to its exceptional freedom from connective tissue. After this -hardening the fibres are easily separated with the aid of needles into -fibrils immeasurably slender. An isolated fibril shows with extreme -distinctness the alternation of dark, bright, dim, bright, dark -markings already described. The appearance of a cross-section of a -fibre also proves that it is a bundle of fibrils. The cut ends of the -fibrils appear as dots surrounded by homogeneous substance. In this -respect there is an important difference between red muscle and white. -In the red fibres the fibrils are fewer and thicker than they are in -white, and the embedding substance is more abundant. It is generally -assumed that the homogeneous substance, sarcoplasm, is the nutrient -protoplasm of the fibre, the fibrils the contractile elements. The -more complete the differentiation of the fibre into fibrils, the more -rapid is its action; the more abundant the sarcoplasm, the greater its -capacity for continued work.</p> - -<p>If a living muscle-fibre is observed while a wave of contraction -is passing down it, the ends of the fibre being free, so that its -shortening is not prevented, it is noticed that the widening of the -fibre is accompanied by the thinning, even to obliteration, of the -bright bands. The dim discs extend laterally, without any noticeable -diminution of their thickness. It looks as if the bright discs, or -something contained in the bright discs, were absorbed into the -dim discs. The fibre is, as we have already pointed out, striated -longitudinally. The striation is more clearly visible in the dim discs -than it is in the bright ones. That the dim disc has an architectural -structure absent from the bright disc is placed beyond doubt when a -<span class="pagenum"><a name="Page_264" id="Page_264">[Pg 264]</a></span> -muscle-fibre is illuminated with polarized light. The dim disc is then -found to be doubly refracting; the bright disc is not. When the prism -in the tube of the microscope is placed with its axis at right angles -to the axis of the prism which intervenes between the source of light -and the stage of the microscope, a succession of bright bands is seen -corresponding to the dim bands seen with unpolarized light. The rest -of the fibre is invisible, because it has not the property of twisting -the undulations of light which the lower prism has set all in the same -plane. Various hypotheses as to the cause of contraction, or, to speak -more correctly, as to what happens during contraction, have been based -upon the thinning of the bright discs. It is assumed that the dim discs -have a definiteness of structure which the bright discs do not possess. -They are thought of as being traversed by pores, or as consisting of -short rods. Microscopists who take the latter view believe that during -contraction the more fluid substance, sarcoplasm, which occupies the -bright bands is drawn into the dim bands between the short rods, or -sarcostyles, which are consequently separated more widely.</p> - -<p>No tissue could be more unsuitable than muscle for microscopic -examination; for none other offers the same optical difficulties. -This will be evident to anyone who considers the description already -given of the markings which it exhibits. Whatever may be the true -interpretation of these markings, it is clear that they point to -an almost infinite multiplication of minute elements adjusted with -absolute accuracy side by side and end to end. A cylinder filled with -these transparent objects has to be viewed by transmitted light. The -elements, whatever may be their nature, refract light in different -degrees. It is impossible to eliminate the effects of internal -reflection, refraction, and interference of waves of light. The most -alluring hypothesis must be accepted with a considerable amount of -reserve. Any fact which seems to militate against it must be taken into -consideration. The view set forth above, in general terms, is very -attractive to everyone who wishes to bring muscle within the category -of machines. Suppose we accept the hypothesis that the dim band is a -plate made of sarcostyles surrounded by sarcoplasm then the impulse -<span class="pagenum"><a name="Page_265" id="Page_265">[Pg 265]</a></span> -which reaches a fibre causes an alteration in the surface relations -of the rods to the substance in which they are embedded. Molecules -of fluid from the bright bands are drawn in amongst them; the rods -are pushed farther apart; the fibre broadens with a corresponding -diminution in length. This brings muscular contraction into the -category of the phenomena which play the most important rôle in -bringing about the varied activities of the animal mechanism. -Contraction is due to osmosis.</p> - -<p>The separation of muscle into fibrils after hardening does not seem -to bear out either the rod or the pore hypothesis of the structure of -the dim disc. It must be remembered, however, that before the fibrils -are teased apart the substance of the fibre has been coagulated. The -fluid in the bright disc may thus have become as much a part of the -fibril as the rod in the dim disc. The longitudinal striation of plain -muscle and the appearance of continuous fibrillation in heart-muscle is -more difficult to reconcile with the hypothesis that striped muscle is -composed of interrupted rods.</p> - -<p>Muscle transforms the energy supplied to it by the blood into -mechanical work. It is doubtful whether any hypothesis as to -structure will help us to an understanding of the way in which this -transformation is effected. Explanations are seductive, but all -attempts at explaining the connection between molecular change and -change in shape must be viewed with suspicion. It is quite clear that -muscle as a motor is not to be compared with any form of motor with -which we are acquainted. It is also clear that the theory of muscle -must be applicable to all its varieties—striped, cardiac, and plain. -It must cover the alterations in form of an amœba and the streaming -movements of protoplasm within a vegetable cell. Probably it must -extend farther, and cover the discharge of electricity by an electric -organ and the emission of light by the lamp of a firefly. We are on -ground so treacherous that we are not sure whether, in crossing it, we -may lean with confidence on the laws of thermodynamics; and doubt as to -the applicability of these laws to living tissue almost upsets one’s -mental balance. Until we have evidence to the contrary, we are bound to -exclude such a misgiving from our minds. If we allow it to influence us -at all, it is merely to the extent of causing us to hesitate to assume -<span class="pagenum"><a name="Page_266" id="Page_266">[Pg 266]</a></span> -that the explanation of muscular contraction can be based upon an -analogy between muscle and any known mechanical contrivances for -generating power, not even excluding apparatus designed for the purpose -of measuring osmotic force.</p> - -<p>If living muscle is frozen, pounded with snow containing 0·6 per cent. -of sodic chloride, and placed upon a filter, a fluid plasma passes -through the filter as the mixture thaws. Like blood-plasma, it clots -spontaneously—without, however, so far as is known the intervention of -a ferment.</p> - -<p>All muscles become rigid after death, owing to the coagulation of their -plasma. It used to be thought that contraction was a stage towards -rigidity—a stage from which muscle, so long as it is alive, recovers. -This view was based upon the fact that exhausted muscle—such, for -example, as that of a hare which has been coursed—becomes rigid -much sooner than rested muscle. But this phenomenon has a different -explanation. The setting of muscle in rigor mortis is due to the -development of lactic acid (one of the waste products of active -muscle). The more there is of this ready formed at the time of death, -the more quickly does coagulation of muscle-plasma occur. The formation -of lactic acid is due to deficiency of oxygen. So long as muscle -obtains as much oxygen as it wants, its metabolism is complete. The -oxidized products which it loses are water and carbonic acid. This is -true also of the changes which occur after death. If a strip of muscle -is hung in an atmosphere of oxygen, it forms no lactic acid, and it -does not become rigid. If, on the other hand, the supply of oxygen has -run short before death occurred, rigor mortis sets in very quickly. -A dead frog takes a long while in becoming rigid, and its rigidity -is transient. Until the moment of death the frog is taking up oxygen -through its lungs, and even after death it probably takes it, as it -does when it is alive, through the skin. A fish becomes rigid very -quickly. For some time after it is caught it continues to live; but, -being unable to breathe in air, every molecule of oxygen which was in -its body when it left the water is used up before it dies.</p> - -<p>In the human body rigor mortis usually sets in from two to four hours -after death, and lasts about two days; but both the rapidity of its -appearance and its duration depend upon various circumstances. As -<span class="pagenum"><a name="Page_267" id="Page_267">[Pg 267]</a></span> -muscles become rigid they contract, moving the limbs, and the -shortening is more extensive than mere coagulation of muscle-plasma -would account for. It is evident that a process similar to functional -contraction precedes coagulation. Many a watcher in the chamber of -death has been startled by the shaking of the bed. Even a sound -resembling a sigh may be caused by contraction of the muscles of the -chest. Placing his hand over the region of the heart, the attendant -finds the body warmer than it was when life became extinct, for much -oxidation has since taken place.</p> - -<p>What chemical changes occur in muscle when it contracts? What is the -chemical source of its power? Carbonic acid is given off. This is -the only product which we can collect and measure; but it is taken -for granted that hydrogen atoms also combine with oxygen, forming -water. There is no reason for thinking that nitrogen is removed from -the molecules of its protoplasm with any greater rapidity during the -activity of muscle than when it is quiescent (<a href="#Page_212"><i>cf.</i> p. 212</a>). -Is the oxidation immediate and complete, or does it occur in stages? For -many years attention has been directed to lactic acid, partly because -this substance is found in muscle which has been made to contract -under experimental conditions, partly because, on theoretical grounds, -glycogen (animal starch) is looked upon as the most important of -muscle-foods. Lactic acid—<b>C₃H₆O₃</b>—has the same percentage composition -as glycogen—<b>C₆H₁₂O₆</b>. Its formation from glycogen merely involves -a rearrangement of atoms. It has been supposed that lactic acid is -formed in the first instance, and then, if the supply of oxygen be -sufficient, oxidized to carbonic acid and water. But this hypothesis -may be resisted on various grounds. Undoubtedly, lactic acid appears -when oxygen is deficient. Under all circumstances and in all tissues a -certain amount of it is formed. There are reasons for thinking that it -carries away the nitrogen which is wasted, as lactamide. But it does -not follow that under normal conditions, when muscle is abundantly -supplied with blood, lactic acid appears in any greater quantity during -activity than during rest. The hypothesis is due to the misconception -which we have already endeavoured to correct. It is difficult to get -<span class="pagenum"><a name="Page_268" id="Page_268">[Pg 268]</a></span> -away from the steam-engine analogy. A steam-engine is made of iron and -brass. These materials are subject to wear and tear; but they are not -the source of its power. Its power is due to the combustion of fuel. -Muscle, physiologists formerly said, is made of protoplasm. This wears -down when it works, setting free creatin and other nitrogenous débris. -Its fuel is glycogen. This is not the way, however, in which the matter -is now regarded. Protoplasm is not the machine only, but also the -source of power. Glycogen is not burnt in a framework of protoplasm. -When muscle contracts, protoplasm casts out <b>CO₂</b> and <b>H₂O</b>. -Glycogen is the food readiest to restore to it the atoms which it has lost.</p> - -<p>Another consideration opposed to the hypothesis of the conversion of -glycogen into lactic acid is the uselessness of such a transformation -from a physical point of view. The stability of the atoms of <b>C₃H₆O₃</b> is -so little greater than that of the atoms of <b>C₆H₁₂O₆</b> that practically -no energy is set free when the one substance changes into the other. -We cannot, however, overlook the fact that the formation of acid may -be a means of profoundly altering the state of the colloid substances -dissolved in cell-juice. The casein of milk coagulates when milk turns -sour. The neutralization of a faintly alkaline solution of a protein -(and muscle is faintly alkaline) will throw it out of solution. The -appearance of lactic acid may be intimately associated with movement of -protoplasm, and yet the change of glycogen into lactic acid not be the -source of the energy which muscle expends.</p> - -<p><b>Fatigue.</b>—For its continued activity muscle needs an adequate -supply of food and oxygen. If the blood which distributes food is -circulating properly, and the liver, the great depot of food, is -well stored, fresh supplies are brought to the muscles as they are -needed. There are muscles—those of the eye and of the heart, for -example—which never become exhausted. However continuous their -activity, they take food from the blood as rapidly as they waste it; a -statement which, perhaps, needs qualifying by the addition, “so long as -the work exacted of them is such as may be reasonably expected.” If, in -a picture-gallery, one keeps the eyes elevated for an hour or more a -headache follows. Our eye-muscles have taken over their duties on the -understanding that we look down or straight forwards far more often -<span class="pagenum"><a name="Page_269" id="Page_269">[Pg 269]</a></span> -than we look up. If a long-sighted child is required to focus his -eyes upon a printed page without the aid of spectacles, not headache -merely, but actual disease of the brain, may be the result. The -ciliary muscle within the eyeball, which effects accommodation of the -eye for near objects, is unduly strained. Even the use of our modern -type, with its vertical height greater than its breadth, which has -taken the place of square Roman letters, is probably related to the -development of astigmatism of the lens, and thus indirectly a cause of -headache. It is asserted on high authority that vertical astigmatism, -the commonest form, is not present in the eyes of children before they -learn to read. Headache is an exaggeration of the feeling of fatigue. -It may be interpreted as the brain’s expression of unwillingness to -be made to work; a protest always to be listened to, notwithstanding -that it does not necessarily follow that unwillingness to work is the -result of overwork. Constipation, irritation of the sensory nerves -of the stomach, overdosing of the brain with alcohol, and many other -causes, may, through the vaso-motor system, set up the conditions -which normally result from activity unduly prolonged. The fact that -a central disturbance, headache, results from undue muscular work -calls our attention to the double nature of the mechanism concerned -in movement. Muscles are set in motion through the intervention of -the nervous system. After they have worked to an unusual extent the -nerve-centres connected with them grow tired. This, at least, is a -legitimate inference from the fact that headache occurs when certain -muscles of the eyeball have been subjected to an improper strain. But -it must be remembered that the muscles of the eyeball never tire. They -do not, like other voluntary muscles, give notice that they are in need -of rest. It is not so clear that the central mechanism is in any way -involved in the fatigue which is produced by excessive use of arms or -legs. The muscles of the limbs (and the central nervous system) are -protected by the sensations which originate in muscles when they are -overworked. The fact that a weary man can, if a great emergency demands -activity, use his muscles with as much vigour as if he were fresh from -bed, has been cited as an argument in favour of the view that fatigue -is of central origin; but it is an argument which works both ways. A -<span class="pagenum"><a name="Page_270" id="Page_270">[Pg 270]</a></span> -strong emotion causes a fervent response from the nervous system. Tired -muscles contract energetically when the impulses which reach them are -sufficiently urgent.</p> - -<p>Nothing so definitely removes muscle from the category of machines -as its liability to fatigue. To speak of a muscle as tired is, of -course, to transfer to an object a term which is applicable only to a -phenomenon of consciousness; but it is necessary, unless a cumbrous -expression is to be used, to designate thus the effect upon the muscle -of prolonged activity. The petrol may be low in the tank, but the -quantity burnt in the cylinder at each stroke is not reduced. If an -isolated muscle is repeatedly stimulated by an electric current of a -certain strength, the response which it makes improves for the first -two or three induction shocks; then it begins to weaken. At each -succeeding spasm the muscle shortens a trifle less than before. More -remarkable than the diminution in the amount of work done by a muscle -which is growing tired is the prolongation of the time taken both -in contracting and in relaxing. Further, it has been shown that the -fatigue which accompanies the contraction of an isolated muscle is -not a condition dependent upon the shrinking of the store of energy -which it possessed when it was first thrown into activity. Muscles -undisturbed as to blood-supply, and contracting under the direction -of the Will, also exhibit it. Speaking generally, it may be said that -the tiring of muscle is not so much due to the exhaustion of its store -of food as to accumulation of products of action. Vigour is restored -to a tired muscle by passing through its bloodvessels a stream of -salt-solution, which brings it no food, but washes away some of its -waste. But the problem is far more complex than this. The machinery -is not simply clogged with the products of its own activity. If the -blood of a tired animal is injected into the vessels of one that is -rested, the muscles of the latter exhibit the phenomena of fatigue. -Evidently muscle is self-protective. During activity it prepares -a “fatigue-substance” which poisons its own nerve-endings, making -them worse conductors from nerve to muscle of the commands which -descend from the brain. Not only does the fatigue-substance dull the -nerve-endings in the particular muscle which has contracted, but, being -distributed by the blood to the whole body, it produces a general -<span class="pagenum"><a name="Page_271" id="Page_271">[Pg 271]</a></span> -effect. If the legs have been severely worked, they exhibit fatigue in -the highest degree; but after a long walk the arms also are less ready -and less capable than the state of their nutrition warrants.</p> - -<p>The condition of stiffness experienced for a day or two after excessive -exercise is due to various causes in combination. The fact that it may -be remedied by encouraging the circulation through the muscles most -affected, as by hot baths and massage, tempts us to assign it also in -large measure to accumulation of products of action; but the means -taken to reduce stiffness favour the nutrition of the muscles both by -giving them more food and by carrying off their waste.</p> - -<p>Equally remarkable with the self-protective disposition of muscle, -which forbids it to give, except at the instance of increasingly urgent -messages from the central nervous system, more than a part of the work -of which it is capable, is its preparation for meeting an increased -demand. It grows with use. Running increases the girth of the leg by -developing especially the muscles of the calf. Raising weights enlarges -the muscles of the shoulder and arm. Use-growth may reach inconvenient -proportions. Nothing is more noticeable during the training of young -athletes, whose nutritive responsiveness is at its height, than their -liability to pass through a stage in which they are “muscle-bound.” -Their legs grow bigger, but their pace falls off.</p> - -<p>The development by means of exercises of a strong muscular system has -received much attention during recent years. Our ancestors cultivated -strength and agility in certain movements without paying much attention -to the muscles by which the movements were performed. It is fashionable -nowadays to lay stress upon the importance of maintaining an abundant -musculature, because of its relation to general “fitness.” The balance -between muscular activity and the organic functions which is observed -by everyone who takes an active holiday proves beyond doubt that -the nutritive condition of the various glands and of the heart and -bloodvessels is in some degree dependent upon the condition of the -muscles. Possibly they secrete into the blood other “messengers” in -addition to fatigue-substance—messengers whose call wakes up the -organs of digestion. The man who is so fortunate as to be able to use -<span class="pagenum"><a name="Page_272" id="Page_272">[Pg 272]</a></span> -his muscles in the open air has no need of exercises in his bathroom. -Failing out-of-door opportunities, much can be done by the systematic -use of the various muscles working against resistance. It is alleged, -and we are not disposed to dispute the justice of the contention, that -movements made with the fullest degree of mental concurrence have -a more rapid effect upon the growth of muscle than actions more or -less unconscious. Muscle and nerve are parts of a single mechanism. -It may be that fixing the attention on an exercise, and watching its -performance in a looking-glass, aids the nutrition of muscles by -increasing the influence of their nerves, possibly by improving the -nutrition of their nerve-centre. Unfortunately, this is one of many -theories which hardly come within the reach of a control experiment. -Could one concentrate attention on the movements of the right arm, -then absent-mindedly repeat them with equal vigour with the left, -it might be possible to ascertain whether there is anything in this -idea. Two other contentions with regard to the best way of performing -movements, with a view to the promotion of muscular growth, appear to -be justified by their results. Working against a moderate or light load -is said to be more effective than putting muscles to a severe strain. -A small number of maximal contractions, it is said, induce more rapid -growth than many partial shortenings. According to this scheme, when a -particular muscle needs strengthening, because in a certain action it -is to be the chief performer, it is made to bring its two ends as near -together as the plan of its attachments allows. Maximal shortening is -apparently favourable to blood-supply and otherwise promotes nutrition.</p> - -<p><b>Tone.</b>—Hitherto we have spoken of quiescence and activity, -as if muscle were doing nothing when not visibly contracting. A wrong -impression may be engendered by these terms. Muscle is never idle. -During sleep, and still more when a person is under the influence of -anæsthetics, the muscles approach the condition of machines at rest. -But again the language of the workshop is inapplicable. When a headless -frog is hanging from a hook its legs are slightly bent. All its muscles -are weakly contracted, if we understand by contraction a condition in -which the length of muscle is less than it would be were it not alive. -But the flexors are tenser than the extensors, hence the crooking of -<span class="pagenum"><a name="Page_273" id="Page_273">[Pg 273]</a></span> -hip, knee, and ankle. If the sensory roots of the sciatic nerve are -cut, the leg straightens out. So long as the nerve was intact the -weight of the limbs acted as a stimulus to sensory nerve-endings, -causing a reflex “tone” of the flexor muscles via the spinal cord. The -tone of the extensor muscles was less because they were not stretched -by the weight of the limbs. Every joint is under the influence of -antagonistic muscles which are perpetually watching one another. When -the limb is extended the flexors become anxious. When it is flexed the -extensors get ready for a spring. Only when it is half flexed is there -anything approaching to a truce. And this in most cases is the position -of greatest comfort. But even when most at rest, muscles still possess -a certain degree of tone. The tendency to shortening in one set causes -it to pull against, and thereby increases the tone of, its opponents. -When a muscle contracts it does not lift a loose bone. It has to -overcome the tone of the muscles which would cause a movement in the -opposite direction. And here another adjustment comes into play. The -same gross stimulus which leads to the contraction of A starts impulses -of a finer kind for B, directing it to relax its tone. We have seen -how the heart and bloodvessels are under the influence of two sets of -nerves of opposite sign—anabolic, diminishing irritability; katabolic, -increasing it. All muscles are under similar management; but we can -rarely detect the influence of the anabolic, inhibitory nerves, the -brakes, because the katabolic display is overwhelmingly conspicuous. -We must be content with two experimental demonstrations. An animal’s -hamstrings have been cut; the flexor muscles of its thigh are therefore -severed from their attachments below the knee. The tone of the extensor -muscle keeps this joint extended. If now the pad of the foot be -tickled, the flexor muscles contract, just as they would do if they -were still able to carry out the reflex action of raising the foot. -They cannot do this, because their tendons are divided; nevertheless, -the knee bends owing to reflex relaxation of the extensor muscles. -Still more striking evidence of reciprocal contraction and relaxation -is afforded by the claw-muscles of a crayfish. A weak stimulus to -its nerve causes the claw to set open; a stronger stimulus causes it -to close. Both these movements are due, not to “contraction,” but to -<span class="pagenum"><a name="Page_274" id="Page_274">[Pg 274]</a></span> -change of tone. Under certain conditions, a current passed through -its abductor muscle, the claw being open at the time, causes closure -by inhibiting the tone of this muscle. In this case the stimulus acts -directly on the muscle, producing an effect which is opposite to the -one we are accustomed to associate with stimulation; in place of -contraction, relaxation.</p> - -<p>Contraction of muscle, moving something, impresses one as a positive -phenomenon. Relaxation seems negative—the undoing of contraction—and -to a very large extent this attitude of mind is justified. Return of a -muscle to its full length is due either to stretching by the weight it -has lifted, or to the antagonism of other muscles. An isolated muscle -lying on a pool of quicksilver does not return to its full length -after it has contracted. But it is necessary to banish the machine -idea. A machine gives out all the energy it has in store. Muscle is -extremely parsimonious. No stimulus can induce it to part with more -than a fraction of its energy. Recovery is as definite a function -as disturbance. A machine starts when a crank is moved, stops when -it is replaced. Muscle has a certain degree of automatism, although -its tendency to act on its own account has been almost completely -transferred to the governing nervous system. Muscle and nerve work -together, and the efficiency of muscle depends upon the maintenance -of its relations with its nerve. If the nerve is cut, the muscle -atrophies. We will not stop to consider whether wasting may be properly -attributed to disuse, or to vaso-motor changes. In its lowest form -nervous influence shows itself in the regulation of the nutrition of -muscle. A somewhat more forcible exhibition of control is seen in the -regulation of tone. The maximum is reached when a wave of undoing which -has passed down a nerve infects the protoplasm of muscle with the same -tendency to disintegration. The muscle-substance explodes. The muscle -shortens.</p> - -<p>Remarkable evidence of the existence of muscle-tone is afforded by the -<b>knee-jerk</b>. Place a person on an upright chair, with his legs -crossed, muscles lax, foot hanging free. With a paper-knife or the end -of a stethoscope, or even the hand used edgewise, tap the ligament -which connects his knee-cap with his shin. The tap is instantly -followed by a jerking forward of the foot. The deep muscles of the -<span class="pagenum"><a name="Page_275" id="Page_275">[Pg 275]</a></span> -thigh, vastus, and crureus, have contracted. This phenomenon is easy -to account for. When we are standing upright, the trunk is supported -on three joints, of which one—the hip—is a perfect ball and socket, -and the other two—knee and ankle—are of the same order so far -as the absence of any provision for locking them is concerned. If the -muscles on the front and the back of the leg did not constantly adjust -our balance, by swaying the trunk forward when it falls back, and -pulling it back when it sways forward, the joints of the leg would -double up beneath us. A photographer knows how little confidence is to -be placed in a man’s assertion that he is able to stand still. This -see-saw of alternate contraction and relaxation is kept up by means -of nerve-impulses which ascend from the nerve-endings surrounding the -separate bundles of tendons, or from the Pacinian bodies which are -found in abundance in the neighbourhood of tendons and ligaments, or -from the elaborately twisted nerve-fibres found in muscle-spindles, or -possibly from all three classes. Muscle and tendon are richly supplied -with sense-organs susceptible to pressure and stretching. There is an -abundance of nerve-endings to choose from. The slightest change in -their tension, whether due to the muscle’s own contraction or to the -action upon it of other muscles or weights, is recorded not only in -the spinal cord, but also in the cortex of the cerebellum, and, if the -contraction is an act of volition, in the cortex of the great brain. -Although it was skin which was tapped, skin-nerves have nothing to -do with the jerk. It was the result of the slight sudden stretching. -In short, the tone-mechanism has been fooled. Notice the position of -the leg. The knee is semiflexed; the foot is hanging free. There is -nothing for the extensor muscles of the thigh to do. Now, if ever, they -are justified in dozing. It is not to be wondered at that the sudden -stretching of the ligament takes them off their guard, or that on -waking they give a quite unreasonable start. The phenomenon is, as we -asserted, easy to account for. It would also be easy to explain, if it -were not for the extreme rapidity with which the jerk follows the tap. -The interval is about one-hundredth of a second. This is thought to be -too short to allow an impulse to ascend a sensory nerve, pass through -<span class="pagenum"><a name="Page_276" id="Page_276">[Pg 276]</a></span> -the cord, and descend a motor nerve. It is true that these reflexes -of adjustment must stand on a different level to other reflexes. The -tone-impulses which cause them are incessantly patrolling to and fro -from sense-organs to nerve-endings. The paths they follow must be the -most open in the nervous system. Receptors and effectors must, in an -electrician’s phrase, be incessantly switched on; or, to express the -analogy more accurately, the flexor and extensor tone mechanisms are -incessantly and reciprocally switching each other on and off. It must -be confessed that it is very difficult to explain the knee-jerk if it -be not a reflex action, but, as has been supposed, a direct response of -the thigh muscles to their own stretching. The latter hypothesis does -not appear to be reconcilable with its dependence upon the maintenance -of the nervous connection of the muscles with the spinal cord. It -cannot be elicited unless the “spinal arc” is intact. It ceases after -the severance of either sensory or motor roots. Nor will it occur if -the supply of blood to the lower end of the spinal cord has been cut -off. Still more difficult is it to explain its extraordinary sympathy -with everything that happens in the whole nervous system, if the -impulses which cause it do not pass through the spinal cord. By a very -simple mechanical arrangement it is possible to record the amplitude -of the knee-jerk. The foot moves a lever which writes on a travelling -surface. The jerk is elicited by the hammer of a clock strapped to the -shin. In this way it is possible to extend the period of observation -over several consecutive hours, the subject becoming completely -oblivious of the movement his foot is making once a second, if it be -screened from his view. In deep sleep the jerks stop; but the subject -may doze, and still jerk follows tap. And the record made by his foot -mirrors all the changes in his nervous system. If he clench his fist, -the movement is reinforced, as it is when a child cries, a lamp is -lighted, his ear itches. There is music in an adjoining room. His foot -is the baton which beats <i>fortissimo</i> to Wagner, and is lulled to -<i>piano</i> by the “Lieder ohne Wörte.” On a bright day this spinal pulse -throbs gaily. It is indolent in dull, depressing weather. The knee-jerk -is the physician’s guide to the condition of the nervous system.</p> - -<p><b>Elasticity of Muscles.</b>—Muscles are very extensible, and -after stretching return to their original length. Their elasticity is a -<span class="pagenum"><a name="Page_277" id="Page_277">[Pg 277]</a></span> -quality of great practical importance. It enables them to meet sudden -resistance without rupture, as when a man alights from a height. At -the moment when the feet touch ground elasticity dissipates the shock. -The stretching of the muscles then leads reflexly to the increase of -their tone. Here we see an advantage in the short reaction-time of -the knee-jerk. Tone comes into play long before impulses generated by -contact of the sole of the foot with the ground have had time to reach -the brain, or even to induce reflex contraction through the spinal -cord. The elasticity of the muscles is also of use in the performance -of certain sudden actions. A pea is flicked across the room by pressing -the thumbnail against the pad of a finger, or a finger against the -thumb, and releasing it with a jerk.</p> - -<p>An electrical change accompanies an impulse in its passage down a -nerve, and a wave of contraction in its passage along a muscle. In -1788 Galvani observed that the hind-limbs of a frog, suspended by -a metal hook to metal railings, twitched when the wind blew them -against the bars. The hook passed through the lumbar plexus of -nerves. He recognized that the cause of the twitch was the closing of -a circuit. The birth of dynamic or galvanic electricity dates from -this observation; and ever since this phenomenon was first observed -the electric changes in nerve-muscle preparations made from frogs’ -legs have been favourite subjects of research. Many observations with -regard to nerve-conduction and muscle-contraction may be made, and many -experiments performed, without special apparatus. A frog having been -killed by cutting off its head, or by placing it beneath a tumbler -with a wad of cotton-wool soaked in chloroform, the skin of the leg -is removed, displaying the khaki-coloured muscles, bluish tendons, -and bright white threads of nerve. A stretch of the largest nerve of -the back of the thigh, the sciatic, is isolated. All the muscles of -the thigh are then cut away and the bone nipped across just above the -knee. The bones below the knee are removed, the superficial muscle of -the calf, the gastrocnemius, being allowed to hang free, its bifid end -attached to the fragment of thigh-bone. Its lower end terminating in -the tendo Achillis, with its insertion into the prominence of the heel, -is left intact. The bone is fixed in a clamp. A light lever made from a -wooden spill is suspended from the tendo Achillis. The nerve may then -be stimulated in various ways: by crushing in a pair of forceps, burning -<span class="pagenum"><a name="Page_278" id="Page_278">[Pg 278]</a></span> -with a heated needle, touching with a drop of glycerin or a strong -solution of salt. But of all methods of stimulation, the best is the -current from an induction coil. Since it does not injure the nerve, it -can be applied as often as may be desired. The amateur provided with an -induction coil is in a position to study the relation between stimulus -and response. He can vary the strength of the stimulus and vary the -weight which the muscle has to lift. He can observe the progressive -onset of fatigue, and otherwise gain much information regarding the -behaviour of muscle as an isolated piece of apparatus.</p> - -<p>It is the ambition of the expert to obtain absolutely correct records -of the time-phases and of the changes in electric potential of nerve -and muscle under varied experimental conditions. For this purpose he -needs the finest apparatus which instrument-makers can furnish, and the -knowledge and dexterity requisite for its employment. Consider, for -example, the record of the change of form. A nerve-muscle preparation, -obtained by the method already described, is arranged so that the point -of the lever scratches on a rapidly travelling blackened surface. As -the muscle contracts it makes a “tracing.” A tuning-fork vibrating -at the rate of, say, 400 times a second also scratches a tracing on -the same travelling-plate. It is easy to time the several phases of -contraction and relaxation by comparing them with the undulations made -by the tuning-fork. By means of an induction shock a single impulse -is generated in the nerve and a single spasm evoked in the muscle. -Our tracing shows that the spasm lasts about one-tenth of a second, -and that about half this time is occupied by contraction, and half by -relaxation. But the ascending curve is usually a little steeper than -the descending curve, and the apex a little nearer to the commencement -of ascent than to the termination of descent. An electric signal marked -the instant at which the current was sent into the nerve. The time -taken by the impulse in travelling from the spot where the electric -current entered the nerve to its junction with the muscle can therefore -be estimated. The contraction begins so much sooner or later, according -as the shock is delivered nearer to, or farther from, the muscle. By -shifting the electrodes up and down the nerve, the rate at which the -<span class="pagenum"><a name="Page_279" id="Page_279">[Pg 279]</a></span> -impulse travels is directly measured. After the time that the impulse -took in reaching the muscle has been allowed for, there still seems to -be an interval before the muscle begins to shorten. This was termed -the “latent period,” under the impression that some time is actually -lost in turning the nerve-impulse into a muscle impulse. The impulse -was supposed to be latent in the end-plates of the nerve. Various -hypotheses were formulated as to the nature of the transformation. -The progressive improvements in apparatus and methods is testified -by the diminution in this latent period as given in the text-books of -successive decades. It is now put at ¹/₄₀₀ second, and is regarded by -most physiologists as a delay due to the inertia of the muscle. Owing -to its elasticity, the molecular change in muscle does not immediately -affect its shape. When the latent period appears to be longer—say -¹/₁₀₀ second—the balance is due to the inertia of the recording -apparatus. Usually the curve shows a rise lasting ⁴/₁₀₀ second and -a fall occupying ⁵/₁₀₀, due to the fact that inertia of muscle and -apparatus delays the commencement of the rise, but does not hasten the -termination of the fall.</p> - -<p>When an impulse is generated artificially by an induction shock, a -single spasm or twitch is the result; but in Nature contraction is -never limited to a single twitch. Impulses descending from a motor -nerve-cell to a muscle are always rhythmic. They follow at the rate -of eight or ten a second in human nerves; and since in our muscles -contraction and relaxation take longer than in a frog, a second impulse -reaches the muscle before the effect of the first has passed away. -The muscle has not had time to relax, when it is again called upon to -contract. Hence a summation of contractions. The muscle continues to -shorten until the maximum of contraction is reached. This condition is -termed “tetanus,” to distinguish it from a single spasm. In fullest -contraction the length of a muscle may be diminished by one-half, or -even by two-thirds.</p> - -<p>It would be impossible to treat of the <b>electrical phenomena</b> -displayed by nerves and muscles without presupposing some acquaintance -with the methods and laws of physics. As this is contrary to our -understanding with our readers, we must be content with the statement -of a few salient facts. At the moment when an impulse is passing along -<span class="pagenum"><a name="Page_280" id="Page_280">[Pg 280]</a></span> -a nerve, or a wave of contraction along a muscle, the electric -potential of the active part of the structure, whether nerve or -muscle, is different from that of the not-acting parts on either side -of it. A battery in its commonest form is a glass vessel containing -sulphuric acid in which a plate of zinc and a plate of copper are -immersed. The zinc is electro-positive as regards the copper. In a -muscle the contracted portion is electro-positive as regards the parts -uncontracted. The degree of positivity can be measured by connecting -the muscle at two spots with the two wires of a galvanometer. When -one wire makes contact with the contracted portion, and the other -with a part which is not contracted, a current passes through the -galvanometer, causing its needle to swing; and since the wave of -contraction is not stationary, but passes down the muscle, the current -is subsequently reversed. The wave, as it were, first tilts up one end, -and then, passing on, tilts up the other, letting down the first. The -contracted spot is electro-positive to the spot not contracted, and -then the latter, contracting, becomes electro-positive to the former, -which has relaxed. The needle of the galvanometer swings first to the -left, then to the right. The importance of this method of investigation -lies in the fact that the electric variation exactly represents, both -in time and in intensity, the change which is occurring in nerve and in -muscle. By following it, we can ascertain the rate at which an impulse -travels down a nerve. We can determine its length and its “form.” -Represented on paper, it is a wave. This wave travels in warm-blooded -animals with the rapidity of 35 metres in a second. When it reaches -a muscle, its rate—that is to say, the rate at which the wave of -contraction invades the muscle—is 6 metres in a second. The time -during which any particular level in the muscle remains contracted in a -single spasm, under the influence of an artificial stimulus, is about -0·05 second. The length of the wave is 300 to 400 millimetres. These -measurements give us a very clear idea of the events which occur in a -nerve-muscle. An impulse picked up by a motor cell in the spinal cord -runs down its axon—termed later a nerve-fibre—with great rapidity. -Even the most distant muscle is reached in less than one-thirtieth of a -second. From the end-plate of the nerve it travels in both directions -along the muscle-fibre—or group of fibres, since each nerve divides -<span class="pagenum"><a name="Page_281" id="Page_281">[Pg 281]</a></span> -into branchets for thirty to forty muscle-fibres—with reduced -velocity. Every particle of each fibre rises and falls; but, seeing -that the wave of contraction is much longer than the fibre, the whole -fibre is in a state of contraction at the same time, although not with -equal vigour throughout its whole length.</p> - -<p>We cannot dismiss the further consideration of the electric phenomena -of nerves and muscles without some inquiry into their meaning. It is -evident that they are intimately related to the molecular changes which -constitute an impulse. But at present the physics of the phenomena are -beyond our grasp. We may speak in a general way of dissociation of -ions; but we do not really know what is happening at the spot which is -in a state of impulse. We cannot bring the transformation which it is -undergoing into line with chemical and physical transformations which -we understand. Probably the electrical phenomena which mark it are not -peculiar to muscle and nerve. All living changes of state are of the -same nature. Cellular activity, or protoplasmic activity, to use a -better term, wherever it occurs, is accompanied by electrical change. -But it so happens that nerve-substance and muscle-substance have a -definite orientation which gives to the electric force a cumulative -effect. In a liver-cell it is dispersed in all directions. In a muscle -the change of potential at one particle is added to the change at -the next, until the sum of all these changes, transmitted along the -length of the fibre, is sufficiently large to deflect the needle of a -galvanometer. Owing to its summation it attracts our attention.</p> - -<p>Although they cannot tell the true significance of the electromotive -change which marks the passage of an impulse, physiologists are in a -much better position now than formerly to controvert certain popular -misconceptions. There is no such thing as “nerve-force” in the vulgar -sense. A nerve does not transmit energy to a muscle. The muscle -obtains the energy which it dispenses when contracting from the foods -with which the blood supplies it. The nerve transmits an excitation. -Over-excitability is not a sign of strength, but of weakness. Nor is -an impulse in a nerve an electric current. It may be generated by an -electric shock, but a chemical stimulus is equally as effective. The -slow rate at which it travels, as compared with electricity, puts it -<span class="pagenum"><a name="Page_282" id="Page_282">[Pg 282]</a></span> -altogether out of comparison with an electric current. Its relatively -rapid progress, on the other hand, equally excludes the hypothesis that -it is a movement of ions, as that phenomenon is observed in solutions -of salts.</p> - -<p>What is the nature of the process by which energy is conveyed along -a nerve? When speaking of the passage of impulses from receptors to -the central nervous system, and through this to effectors, we have -used the vague expression “molecular change,” to avoid the necessity -of being more precise. But the problem is of such profound interest -that we look with eagerness for any hint of the direction from which -light will eventually be thrown upon it. Recent discoveries regarding -the nature of electricity, combined with investigations at present in -progress as to the physical constitution of proteid substances, give -more than a hint. Hitherto the choice has lain between a chemical -and a physical explanation; now the border-line between chemistry -and physics, always wavering, has disappeared. The hypothesis that -an impulse is a progression of chemical change has meant in the past -that the “wave” was due to the oxidation of substances contained in -nerve, with liberation of <b>CO₂</b> and <b>H₂O</b>. Various considerations -render such metabolism of the substance of which nerve-fibres are composed -improbable. In the first place, nerve-cell bodies contain a store of -material, tigroids (<a href="#Page_320">p. 320</a>), which is recognizably drawn -upon during nervous activity. It would appear, therefore, to be the tigroids, -and not the substance of the nerve-fibre, which supply the energy -transmitted along a nerve. Then, again, the axon of a nerve-fibre, -enclosed as it is in a tube of fat, is peculiarly ill-placed for the -reception of the nourishment which would be needed to make up for -waste, if its metabolism be fluctuating and at times excessive. Nor -have nerves more than a very meagre blood-supply. Secondly, observation -does not give any support to the hypothesis of fluctuating metabolism. -A nerve does not give off more <b>CO₂</b> when active than when passive. -Nor does it become acid. Thirdly, nerves, or, to be quite accurate, -medullated nerves, are indefatigable. Their capacity for conduction is -not diminished by previous use, as it would be were it dependent upon -their reserve of nutriment. These various considerations rule out a -<span class="pagenum"><a name="Page_283" id="Page_283">[Pg 283]</a></span> -“chemical” explanation of the old-fashioned type. It is premature -to do more than outline the “physical” theory which seems destined -to take its place; and the reader will perhaps forgive if, for the -sake of clearness, the case is put with unjustifiable definiteness -and simplicity. Proteid substances are constituted of clusters of -molecules. The form of the clusters depends upon the salts (or, -more precisely, the ions) with which they are associated, and the -associations depend upon the electric charges which the ions carry. -In resting nerve-protoplasm the clusters are small, and, since the -total surface-area of a number of small spheres is greater than the -surface-area of the same weight of matter when condensed into large -spheres, there is, so to speak, more surface for the ions to cling to. -Conversely, when the ions leave the small clusters, the latter are -not protected from the influence of mutual attraction. They fuse into -larger clusters. Fusion is carried to its extreme limits when a protein -coagulates. A nerve-impulse is a “wave” of partial coagulation. The -positive electricity generated in a cell-body by the metabolism of its -tigroids repels the positively charged ions which cling to the nearest -protoplasm-clusters in the axon. Like acrobats swinging from trapeze to -trapeze, each flight of ions dispossesses the ions from the clusters in -front of it; and in this way the disturbance progresses down the axon -as an electric wave.</p> - -<p>Thus we interpret the shadow cast by a theory of which either of -several pioneers who are diligently climbing may at any time obtain -a view. The conductivity of protoplasm (and what is true of its -conductivity will be found to hold good equally for its irritability -and changeableness of form) is due to the readiness with which its -molecules enter into unstable associations with electrolytes. The -instability of these associations is related to the tendency of -the molecules to cluster. An impulse is passed along a nerve as a -displacement of ions; the ions being transferred from one molecule, -or group of molecules, to the next. Such an explanation of an impulse -involves no chemical breakdown of nerve-substance during its passage -along a nerve. It transfers the metabolism which liberates energy -(reinforcing the impulses which have originated in sense-organs) to the -nerve-cell bodies. It is based upon certain experimental data which -appear to have been established; but, like all other hypotheses which -<span class="pagenum"><a name="Page_284" id="Page_284">[Pg 284]</a></span> -are intended to account for physiological phenomena, this one must be -brought to the test by varying the conditions under which impulses pass -along nerves, and ascertaining whether the consequent alteration in the -force, rate, and other attributes of the phenomena are in accordance -with physical laws. In applying these tests to the activities of -protoplasm, we are, however, met by an insuperable difficulty. The -matter which transmits nerve-impulses is alive. We have no laboratory -standards by which to judge whether the changes in conduction which -are produced by changes in the conditions of the conductor are, or -are not, consonant with physical theory. It is with protoplasm that -we are dealing, and not with a mixture of proteins in solution. If -we surround a nerve with nitrogen, it loses its conductivity in five -hours, to recover it when oxygen replaces the neutral gas. This has -been regarded as proving that metabolism of the nerve is necessary for -the transmission of impulses. But conductivity is a phenomenon of life. -Deprivation of oxygen for five hours must bring the nerve-substance -to the verge of death. It might be argued that the retention by the -nerve for so long a time of its power of conducting impulses shows -that its metabolism is not a cause of the phenomenon. Again, it has -been shown that warming the nerves of cold-blooded animals greatly -increases the rapidity of conduction. It is more than doubled in the -nerves of the “foot” of a slug-and a similar increase has been proved -for the nerves of a frog-by a rise of temperature of 10° C. Reflecting -on the results of this experiment, a physicist would exclaim: “Then an -impulse is a wave of chemical change. A rise of 10° C. increases the -rate of chemical processes from two to three times; whereas no known -physical process is accelerated by more than 5 to 15 per cent.” But the -physiologist remembers that a rise of temperature of 10° C. increases -all the activities of a frog. He is hardly prepared to say that its -greater vivacity may not be the expression of more rapid oxidation; -but he sees no fore-ordained balance of vital enterprise and chemical -change. He is, or ought to be, extremely suspicious of any explanation -which appears to over-ride physical laws; yet, at the same time, he -is aware that until he has more accurate knowledge regarding the -constitution of protoplasm he will not be in a position to understand -<span class="pagenum"><a name="Page_285" id="Page_285">[Pg 285]</a></span> -how physical laws apply. The protoplasmicity of protoplasm is increased -by warmth. What change of molecular constitution does this imply?</p> - -<p>The view that in a muscle molecular change gives rise to an electrical -change, which in turn produces the change in form, has been very -widely held. The hypothesis was based on observations which seemed -to show that the electric variation travels a little ahead of the -wave of contraction; but every improvement in recording apparatus has -diminished this apparent want of synchronism. There can be little doubt -but that the lagging behind of the wave of contraction is due to the -inertia of the muscle and of the recording apparatus. Molecular change -and electric variation are simultaneous. If this be true, the electric -change cannot be regarded as the cause of the molecular change, in the -sense, at any rate, in which they used to be considered as cause and -effect.</p> - -<p>The <b>power of muscle</b> varies as its cross-section. For human -muscles the maximum lift amounts to from 7 to 10 kilogrammes for each -square centimetre. This is a large figure, but it must be remembered -that, owing to the arrangement of the bones as levers, most muscles -act at a great mechanical disadvantage. The greater the difference -in distance from the fulcrum between the point of application of the -force and the point of incidence of the weight, when the force acts -nearer to the fulcrum than the weight, the greater is the mechanical -disadvantage. The greater also is the rapidity with which the weight -is lifted. What is lost in strength is gained in swiftness. Contrast -the slow steps of a negro, whose long heel separates the point of -application of the power (tendo Achillis) from the fulcrum (the -ankle-joint), with the springy movements of a European. A European -needs, and as a rule has, a better developed calf, which allows him -his more sprightly gait, without sacrificing his carrying power. Our -preference for slender wrists and ankles is not purely æsthetic, unless -we admit, as may be maintained, that all natural canons of taste rest -upon utility. Slimness of joints means nimbleness. A few muscles act -directly, without loss of power—as, for example, the masseter, which -lifts the lower jaw (hence a grand capacity for cracking nuts)—but -most muscles move levers of considerable length. Compare with the -<span class="pagenum"><a name="Page_286" id="Page_286">[Pg 286]</a></span> -masseter the biceps and brachialis which lift the forearm. Their -tendons are inserted into the radius and the ulna at a distance from -the elbow-joint which is about one-tenth as great as the distance from -it of a weight held in the hand. Their united cross-section is about -16 square centimetres: (16 × 10) / 10 = 16. One cannot hold out in the -hand, the elbow being pressed against the side, so that these muscles -alone are acting, a greater weight than 16 kilogrammes (34 pounds), -although the muscles are exerting a traction ten times as great as -this. The strength of muscle when pulling straight is well illustrated -by the thick white mass in the centre of an oyster. It keeps the shell -closed until a force equal to 1,300 times the animal’s weight has been -applied. This muscle also affords a good illustration of the part -played by reflex contraction in opposing stretching—the reaction by -which tone is maintained. Anyone who inserts an instrument, such as the -end of a screwdriver, between the slightly open valves of an oyster -lying under water will find that he needs to give it an exceedingly -smart twist if he would catch the muscle asleep. Stretching it causes a -reaction proportional to the stretching force.</p> - -<div class="figcenter"> - <a id="I_286" name="I_286"> </a> - <img src="images/i_p286.jpg" alt="" width="600" height="349" /> - <p class="center space-below2"><span class="smcap">Fig. 17.</span>—<span class="smcap">Biceps - Muscle in Action.</span></p> -</div> - -<p>The fact that the output of energy by muscle is proportional, within -certain limits, to the work to be done, is brought out even in -laboratory experiments. A nerve-muscle preparation teaches that the -amount of work is not a function of the stimulus. Within certain limits -a stronger stimulus evokes a higher and stronger lift; but the stimulus -remaining the same, the work done by muscle (<i>i.e.</i>, the product of -weight multiplied by height) is, up to a certain optimum, increased by -increasing the weight. Often a very light load is not lifted as high by -<span class="pagenum"><a name="Page_287" id="Page_287">[Pg 287]</a></span> -a nerve-muscle preparation as a slightly heavier one. No satisfactory -theory of this reaction to load has yet been formulated. Explanations -have been put forward, but they merely substitute one unknown for -another, a not uncommon drawback to explanations.</p> - -<p>Muscles are strongest when at their full physiological length. As he -dips an oar into the water a man exerts the greatest force of which he -is capable, provided that he is not guilty of “missing the beginning.” -Hands over the stretcher, body between the knees, ankle, knee, hip, -fully flexed, arms straight—all his strongest muscles are at their -greatest physiological length. Rowing is an exercise which has no -rival. Every muscle in the body, from little toe to little finger, -comes into play under the conditions which suit it best. And not less -admirable is the effect upon the abdominal muscles during recovery at -the end of the stroke; and the rhythmic movement which encourages deep -and measured respiration.</p> - -<p>The greatest output of work is obtained when muscles contract against -a progressively diminishing load. Towards the end of the lift the -load must be small, if contraction is to be carried to its extreme -limit. The provision for this is well seen in the case of the muscles -of the arm when lifting a weight up to a position above the head. A -portmanteau is held in the hand. Its handle is gripped by flexing the -fingers. And here it may be noted that, since the range of movement of -a muscle varies as its length, the thumb and fingers are not worked -only by muscles contained in the palm of the hand. Fingers are bent -and wrist flexed by muscles of which the origin is carried up even to -the lower end of the humerus. As the portmanteau hangs by the side, -biceps and brachialis are at their fullest length. Suppose it to be -necessary to place it on a cab. These muscles begin the work under the -best conditions. They could not, however, lift the portmanteau far did -not the muscles of the shoulder displace the elbow from the side, so -that at the end of their pull, the forearm being almost vertical, the -muscles of the arm have little more to do than to move the hand inwards -towards the head, in preparation for the extensor thrust. The secret of -getting the greatest amount of work out of any particular muscle lies -in securing for it the due co-operation of other muscles. -<span class="pagenum"><a name="Page_288" id="Page_288">[Pg 288]</a></span></p> - -<h3><span class="smcap">Electric Organs.</span></h3> - -<p>Muscle disperses energy in the forms of mechanical work, heat and -electricity. Its structure, as already pointed out, is peculiarly -favourable for the display of electromotive force. In certain fishes -muscle is so modified as to give an electric discharge without -developing mechanical work. The production of an electric change is a -by-phenomenon of muscular activity. It becomes the sole function of -an electric organ. If the skin be removed from the tail of a skate, a -cylindrical column of brawny tissue about the size of a finger will -be found embedded amongst the muscles near its root on either side. -These are electric organs, although so weak that it is barely possible -to feel the shock which they give in a live fish. The nearly allied -Torpedo of the Mediterranean has far more powerful batteries. They are -situate near its gills, occupying the whole thickness of the fish from -skin to skin. When the back of a torpedo is pressed, it discharges a -current of 30 volts, or even more. Still more violent are the shocks -given by an eel—Gymnotus—which haunts the tributaries of the Amazon, -a terror to all who have to cross their fords on foot; or the African -fish, Malapterurus. The current which these animals develop attains an -intensity of 200 volts. With the exception of those of Malapterurus, -all electric organs are modified muscle, and closely similar in -structure. The organs of Malapterurus appear to be modified glands. -The skate’s electric organ may be taken as typical of the rest. When -sliced with a knife, it is seen to be divided by firm connective tissue -into minute chambers. These chambers are piled into hexagonal columns, -which lie lengthwise in the organ (they are set dorso-ventrally in -Torpedo). Each chamber contains a jelly-like substance which embeds an -electric disc. The disc divides the chamber into a smaller anterior and -a larger posterior compartment. Each chamber is supplied with several -nerves which ramify into innumerable twigs on the front surface of -the disc. The development of the disc must be considered for a moment -if its structure is to be understood. It starts life looking as if it -would grow into a voluntary muscle-fibre. A nerve joins it, forming an -end-organ in the usual way. Then the end-organ increases its spread -<span class="pagenum"><a name="Page_289" id="Page_289">[Pg 289]</a></span> -unduly, while the rest of the fibre fails to grow. The structure -becomes toadstool-shaped, with the nerve arborizing on the seat of -the stool. The front aspect of the disc, therefore, corresponds to -a nerve-ending in a muscle. Its middle layer indicates clearly that -the fibre makes an abortive attempt to develop cross-striation. It is -laminated, the laminæ strangely contorted; in section they appear, -not as plain lines, but as rows of dots, evidently a suggestion of -longitudinal striation. The posterior layer of the disc consists of -granular protoplasm drawn out as a number of short backwardly directed -<span class="pagenum"><a name="Page_290" id="Page_290">[Pg 290]</a></span> -tongues, and one long process, the stem of the stool. No structure -could be more suggestive of the function of the organ; but no one -has as yet succeeded in catching the suggestion and pressing it -into a definite explanation of the way in which it works. Certain -physiologists, laying great stress on the fact that the functional -connections between an electric organ and its nerves are not easily -interrupted by the administration of curari, atropin, and other drugs, -which block the passage of impulses from nerves to muscles, look upon -the nerve-layer of the disc as the generator of electricity, and the -rest as an accumulator or resonator, which stores, or exaggerates, -the electric charge. Others consider that the portion of the disc -which is altered muscle-fibre—the middle, or middle and posterior -layers—generates the electromotive force, the nerve simply calling -it into activity. All agree that a brief interval (about 0·003 second) -elapses between the arrival of the nerve-impulse and the discharge of -an electric shock. This “latent period” may be used as an argument in -favour of either view. It would be in harmony with the general account -which we have already given of protoplasm as a liberator of energy -to suppose that a nerve-impulse, having reached a disc, immediately -infects the protoplasm of the disc, inducing molecular commotion, -and that the ions move in such directions as to disturb the electric -equilibrium of the disc, its front surface becoming in relation to the -back as zinc to copper in a battery. The current generated in the fish -is in the direction from head to tail. It is certain that the change -does not occur until an impulse reaches the organ. The organ is not -charged by the nervous system during a period of inactivity, and then -discharged by a releasing impulse. This is sufficiently evident from -the fact that when a piece of the organ, with its nerve, is removed -from the fish, although much sooner exhausted, it responds like a -nerve-muscle preparation to repeated stimulation.</p> - -<div class="figcenter"> - <a id="I_289" name="I_289"> </a> - <img src="images/i_p289.jpg" alt="" width="600" height="540" /> - <p class="center"><span class="smcap">Fig. 18.</span>—<span class="smcap">Electric - Organ of a Skate in Longitudinal Section—<br />A, Slightly, B, Highly - Magnified.</span></p> - <div class="blockquot2"> - <p class="neg-indent space-below2">A shows the compartments into which septa of fibrous - tissue divide the organ. In the centre of each compartment is a disc formed - from a modified muscle-fibre. Nerves ramify in abundance on its - anterior surface. B, a minute portion of a disc. At the top are seen - nerve-fibres in delicate nucleated sheaths; then follow the nucleated - layer with which they come in contact, the contorted laminæ which - represent the striations of the muscle-fibre, the granular nucleated - substance of its posterior layer, some connective tissue, a capillary - bloodvessel containing oval nucleated corpuscles. In a tissue space, a - single coarsely granular leucocyte is to be seen.</p> -</div></div> - -<p>The usefulness of a torpedo’s electric organs is unmistakable. They are -powerful enough to paralyse every animal that touches its back, whether -foe or little fish suitable for food. But of what service is its feeble -battery to a skate? This and the allied question as to the advantages -which can have accrued to the ancestors of the torpedo who first began -<span class="pagenum"><a name="Page_291" id="Page_291">[Pg 291]</a></span> -to change innocent muscle into a weapon of offence are usually answered -by pointing to the liability of flat fish lying on the bottom of the -sea to become resting-places of parasites, corallines, and other -fixed growths. Very mild shocks would suffice to disturb the peace of -would-be settlers. In the same way, the electric organs of fresh-water -fish may, when rudimentary, have protected the skin from invasion by moulds.</p> - -<h3><span class="smcap">Luminous Glands.</span></h3> - -<p>If it be difficult, when considering the dispersal of energy as -mechanical work, heat, or electricity, by living tissues, to bring the -phenomena into line with those of which physics takes experimental -cognizance, how are we to approach the problems involved in the -generation of light? Yet the photogenic property of protoplasm is -widely distributed. Protozoans and various other invertebrate animals -cause the so-called phosphorescence of the sea. The abysmal depths -of ocean are lighted by forests of luminous polyps, and traversed by -fishes whose heads are furnished with lamps. By her own light the -female glow-worm enables her winged mate to keep his tryst. Fireflies -(Lampyrus) flash amongst the orange-trees of Italy, and blaze -(Pyrophorus) beneath the mangoes of Ceylon.</p> - -<p>Luminous organs vary too widely in structure to allow us to pick out, -as in the case of electric organs, the features which are common to -them all. In Pyrophorus the organ is a double mass of cylindrical -cells near the tip of the abdomen. The cells are set vertically to -the surface, and are supported by a tubular membrane. Their substance -contains a kind of fat. Beneath them there is a layer of cells, not -luminous, but evidently a part of the photogenic apparatus, containing -chalky granules. The organ is well supplied with nerves and with -respiratory tubes (tracheæ).</p> - -<p>More interesting than its structure is the study of the peculiar -character of the light which the organ emits. It gives a spectrum which -extends from the red (beyond Fraunhofer’s line B of the solar spectrum) -to the first blue rays (F). It shows no lines. Green rays appear only -when the light is bright, and then they are the brightest of all the -rays. The light is practically destitute of actinic or chemical rays. A -<span class="pagenum"><a name="Page_292" id="Page_292">[Pg 292]</a></span> -photographic plate may be exposed for several minutes, almost without -changing, to the light of a firefly bright enough to enable one to -read with ease in a dark room; whereas light of equal brilliance from -any other source would change it in the fraction of a second. Nor are -heat-rays mixed with the light. Measurements show that the activity -of the photogenic organs does not give rise to any greater rise of -temperature than would occur in the case of any other gland.</p> - -<p>The contrast between the emission of light by an animal and its -production in any other manner is very striking when the physical -evidence, or want of evidence, of what happens in the protoplasm which -produces it is considered. The fact that no heat accompanies the light -precludes us from attributing it to oxidation. If a firefly is enclosed -in a vessel of oxygen, its lamp burns no brighter—clear evidence -that its luminosity has nothing in common with the burning of a match -or the glowing of a stick of phosphorus. Nor is the lamp put out when -the insect is suddenly exposed to great cold (-100° C.). It continues -to shine until the cold kills it. There is no relation between the -luminosity of a firefly and the phenomenon termed “phosphorescence” -by physicists. Sulphide of calcium—the substance used for rendering -matchboxes visible in the dark—returns light which it has absorbed. -A firefly’s power of emitting light is in no wise affected by keeping it -for a long while in the dark.</p> - -<p>Like all other events in vital chemistry, the generation of light by -protoplasm is due to a process of fermentation. The luminous organs -may be crushed, and the mixture of fermentable substance and ferment -extracted with water. The extract is luminous. If an extract is -prepared rapidly, and evaporated to dryness <i>in vacuo</i>, the residue -glows when moistened with water. That two substances are present in the -extract, one (luciferin) fermentable, the other (luciferase) a ferment, -is proved by the following experiment: A certain quantity of extract is -divided into two portions. One part (A) is allowed to glow until its -capacity for emitting light is exhausted. The other portion (B), as -soon as it is separated, is heated to 55° to kill the ferment. B still -contains luciferin; A contains luciferase, although all its luciferin -has been used up. Recombined, the extract is luminiferous.</p> - -<hr class="chap" /> -<p><span class="pagenum"><a name="Page_293" id="Page_293">[Pg 293]</a></span></p> -<div class="chapter"> - <h2>CHAPTER XI<br /><span class="h_subtitle">THE NERVOUS SYSTEM</span></h2> -</div> - -<p>Twenty-five years ago a new process was introduced for colouring the -elements which by their combination make up the nervous system. With -its aid anatomists discovered the inadequacy of their conceptions -of nerve-cells. It was already known that a nerve-fibre—that is to -say, its essential part, its core—is a part of a cell, the body and -other parts of which are situate within the brain or spinal cord, or -in one of their dependents, a ganglion. But the new method showed the -nerve-cell as more elaborate in form than anything which had been -imagined hitherto; and since the word “cell” was often loosely used -when the cell-body alone was referred to, it seemed worth while to give -the unit of structure a new name. The term “neurone” was introduced -to emphasize its functional individuality. The nervous system is an -association of neurones.</p> - -<p>By the extremely simple expedient of placing a small block of -nerve-tissue in bichromate of potassium, and then transferring it to -nitrate of silver, jet-black pictures of nerve-cells are obtained -showing with amazing completeness all the details of contour of their -bodies and all the intricacies of branching of their limbs. The most -surprising feature of the process is the absence of confusion in its -results. Dyes were in use which stained one kind of cell better than -another, or picked out a particular part—usually the nucleus—of -every cell. If the chrome-silver process had acted in the same way, a dense -black preparation in which no details could be distinguished would have -been the result. But instead of treating all cells alike, the process -blackens one cell here and another there, leaving hundreds or thousands -untouched. It shows no preference for any particular kind of cell. In -<span class="pagenum"><a name="Page_294" id="Page_294">[Pg 294]</a></span> -one section large cells are picked out, in another small ones; in a -third no nerve-cells are blackened, but connective tissue is brought -into view. When the block of tissue soaked with bichromate of potassium -is immersed in a solution of nitrate of silver, the chromate escapes -from it into the surrounding liquor much more quickly than the nitrate -gets in; and when at last the nitrate of silver enters, it finds that -some of the cells have fixed the chromate in their substance. This -retained chromate combines with silver. The product is rapidly reduced -to a black subchromate. No explanation of the fixing of the chromate -by individual cells has yet been offered. It is a remarkable fact that -another process which similarly makes choice amongst the elements -has since been introduced, giving even more valuable results. Pieces -of fresh tissue are placed in a very dilute solution of methylene -blue. When staining is satisfactory, nerve-cells alone take up the -dye. The selection of individual nerve-cells is not carried so far as -it is by the chrome-silver method, but it is exhibited to a certain -extent. It is probable that nerve-cells live (in a physiological -sense) longer than other tissue-elements. Methylene-blue contains some -easily removable oxygen of which the oxygen-starved nerve-cells take -advantage. The reduced methylene-blue remains in their substance, so -that when the preparation is reoxidized by exposure to air the pattern -of the nerve-cells is rendered conspicuous. When a few cells are -selected, it is, presumably, because they were the only ones alive at -the time when the dye entered the tissue. Preparations made from the -wall of the alimentary canal seem to justify this simple explanation. -They show patches in which muscle-fibres are stained, patches in which -there is no staining, and intermediate zones in which nerve-cells are -coloured and muscle-fibres are not. But the hypothesis is inadequate -to meet all cases. When first employed, the blue was injected into -the animal in successive doses until it killed it. The staining was -believed to occur <i>intra vitam</i>. Subsequently it was found that its -application to fresh tissue, or, for certain results, to tissue which -has been kept for some hours, is equally effective.</p> - -<p>Without an understanding of the nature of the two new processes, and of -the character of the results which they yield, it would be impossible -<span class="pagenum"><a name="Page_295" id="Page_295">[Pg 295]</a></span> -for the reader to realize the extraordinary advance in our knowledge of -the finer structure of the nervous system which has marked the period -during which they have been employed.</p> - -<p>The chrome-silver process is the more useful for the central nervous -system. Methylene-blue gives better results with tissues containing -minute nerve-cells and the branches of nerves. The latter method has -revealed such a profusion of nerve-twigs as would never have been -suspected but for its use. Consider, for example, the lining epithelium -of the lungs (<a href="#Page_168">p. 168</a>). Every one of its flattened cells -has its own nerve twig or twigs. They lie between the cells. They give branchlets -which enter them. A similar statement might be made regarding the -richness of the nerve-supply of the muscle-fibres of the alimentary -canal, or of the cells of glands, and possibly of other tissues. -Each fresh success achieved in the application of the method makes a -further revelation of the abundance in which nerves are distributed, -increasing our sense of the dependence of all vital processes upon -nervous control, and our appreciation of the unifying and integrating -importance of the nervous system.</p> - -<p>The term “neurone” is used by certain writers with a view to -emphasizing their belief, not in the functional individuality alone -of the unit of structure, but also in its anatomical isolation. The -peculiarity of the methods of coloration which we have described lies, -as already pointed out, in their selecting the cells which happen to -be in a particular nutritive condition, and ignoring their neighbours. -Hence pictures of separate and discrete units are obtained. This proves -the nutritive autonomy of the cells, but it does not necessarily follow -that A is not structurally connected with B, and B with C. Impulses are -passed along the chain from A to C. Functionally, therefore, they are -linked together; but until the question as to the way in which contact -is established is settled, it is as well to think of the neurones as -anatomically discrete.</p> - -<p>It would be impossible in this book to describe all the varieties of -neurone, for nothing is so characteristic of these elements as their -enormous range both in size and form. It may be truly described as -having no limits. Each of the two electric organs of Malapterurus is -governed by a single neurone. Its cell-body is a fifth of a millimetre -<span class="pagenum"><a name="Page_296" id="Page_296">[Pg 296]</a></span> -or more in diameter—large enough to be seen with the naked eye—and -traversed by capillary bloodvessels. The axon of this nerve-cell—its -single nerve-fibre—ramifies to supply a separate branch to each of the -2,000,000 chambers of the electric organ, and each branch breaks up -into a bunch of twigs within the chamber. Contrast with such a giant -cell as this one of the granules of the retina or cerebellum, the -smallest cells to be found in the body, yet each a perfect neurone, -exquisitely elaborate in form.</p> - -<div class="figcenter"> - <a id="I_296" name="I_296"> </a> - <img src="images/i_p296.jpg" alt="" width="600" height="172" /> - <p class="center"><span class="smcap">Fig. 19.</span>—<span class="smcap">A - Nerve-Fibre consisting of A, the Undivided, Fibrillated Axon of a - Nerve-Cell, with its Various Wrappings.</span></p> - <div class="blockquot2"> - <p class="neg-indent space-below2">In segment 1 the wrappings comprise B, a tube of phosphatic fat - (myelin), interrupted at H, a node of Ranvier; C, a delicate membrane - (sarcolemma); D, connective tissue; E, the rind of the axon; F, a - tubular space containing lymph, between the axon and its sheath of - myelin; G, nucleus of an enwrapping cell. At I the myelin is seen to be - divided into overlapping conical rings. 2, The medullated nerve-fibre, - running an isolated course, is merely enclosed in a tube of connective - tissue containing lymph. 3, As a “grey” or “non-medullated” fibre, the - axon has lost its myelin sheath.</p> -</div></div> - -<p>As types for description we may take one of the motor cells of the -spinal cord and a granule of the cerebellum. Every nerve-fibre which -supplies a group of voluntary muscle-fibres is a thread drawn out from -a large cell-body which lies in the grey matter of the spinal cord or -of the axis of the brain. The fibres pass out in the anterior root of a -spinal nerve or in a cranial nerve. The cell-body may have a diameter -of as much as 100 µ (1 µ = 0·001 millimetre). In shape it is like a -very irregular starfish, owing to its being continued into several, -usually four or five, thick tapering branching limbs or processes, -known as dendrites, in addition to its slender thread-like axon. From -its origin in a cell-body to its destination in a muscle—it may be a -few inches, or it may be a yard away—the axon is an unbroken thread. -A short distance from the cell-body it enters a tubular sheath, which -protects and insulates it, recalling the covering of gutta-percha in -<span class="pagenum"><a name="Page_297" id="Page_297">[Pg 297]</a></span> -which the wires of a telegraph cable are enclosed. The sheath is of a -phosphatic fat, invested and held in place by a delicate transparent -membrane, neurilemma. Beneath this membrane nuclei occur at regular -intervals, and midway between each two nuclei the sheath is cut across -by a septum. Such interruptions or nodes show that the sheath is not a -part of the nerve, if the term is used in the most restricted sense. -Each internode is a cell which has been wrapped round the nerve for its -protection. The axon with its sheath is spoken of as a nerve-fibre. -A large number of nerve-fibres bound together by connective tissue -constitute a nerve. In some cases the axon before it leaves the -spinal cord, but after it has entered its myelin sheath, gives off -one or two lateral branches (“collaterals”), which return to arborize -in the grey matter of the cord. It does not appear that they are -always present in the case of the motor neurones of the spinal or -cranial nerves—probably they are usually omitted—but collaterals -are important features of the large neurones of the cortex of the -cerebrum and cerebellum (Figs. 23, 24). Usually two, three, or four -such branches start off at right angles from the axon, and after a time -turn back towards the surface, dividing into a few extremely slender -branches. Their purpose is an enigma. Possibly they bind a group of -cells together in functional unison. Such an explanation would seem -reasonable in the case of an arrangement of collaterals on the plan we -have just described; but in various situations in the brain cells are -seen of which the axons, instead of becoming nerve-fibres, break up -completely into collaterals, which branch repeatedly. -<span class="pagenum"><a name="Page_298" id="Page_298">[Pg 298]</a></span></p> - -<div class="figcenter"> - <a id="I_298" name="I_298"> </a> - <img src="images/i_p298.jpg" alt="" width="600" height="401" /> - <p class="center"><span class="smcap">Fig. 20.</span>—<span class="smcap">A Ganglion of a Leech.</span></p> - <div class="blockquot2"> - <p class="neg-indent space-below2">Pear-shaped cells are set round a felt-work of nerve-fibrils - (neuropil). A neuro-sensory cell is shown with one fibre directed - peripherally, branching on the surface; and one directed centrally, - ramifying in the neuropil. Several very slender fibrils from the - neuropil pass up the stalk of each ganglion-cell. They join a network - near its surface. This net is connected by radiating fibrils with - a coarser net which surrounds the nucleus. From the central net a - relatively stout fibril passes to muscle-fibres.</p> -</div></div> - -<p>By various methods it may be shown that dendrites, cell-body, and -axon contain fibrils (<a href="#I_321">Fig. 22</a>). These neuro-fibrillæ lie -parallel to one another in the axon. Where it divides they are distributed -amongst its branches. Possibly they also branch. In the neurones of -Malapterurus, already referred to, this would appear to be inevitable. -The discovery of neuro-fibrillæ seemed to carry us a step nearer to -a comprehension of the physics of nervous conduction. They clearly -indicate that particles of the substance of a nerve-fibre are -oriented in the direction in which impulses pass. It is a structural -differentiation similar to the fibrillation of muscle, and probably -of the same order—a response to the same demand. But when we examine -the arrangement of the fibrils in a cell-body and its dendrites, the -appearances which we discover serve to perplex us. They complicate -instead of simplifying our mental picture of the conduction of -nervous impulses. The coarsest and most distinct neuro-fibrillæ are -to be found in annelids, the ganglion-cells of a leech, for example, -affording excellent preparations. These cells are pear-shaped, with a -single stalk. As is usual in invertebrate animals, they do not exhibit -separate dendrites and axon, but dendrites and axon pass out from the -cell in the common stalk. The bodies of the cells are set round a -felted mass of nerve-filaments, into which their stalks break up. Just -beneath the surface of the stalk of one of these cells two or three -very fine neuro-fibrillæ are to be seen. A single, much coarser fibril -occupies its axis. The fine fibrils join a network at the periphery of -the cell-body. The thick fibril is connected with a coarser network -which surrounds the nucleus. Radiating threads unite the finer with the -<span class="pagenum"><a name="Page_299" id="Page_299">[Pg 299]</a></span> -coarser net. It has been suggested that afferent impulses ascend the -fine fibrils, pass from the finer to the coarser net, and take their -exit by the thick fibril, which can be traced into a motor nerve. -Such a transit could not, so far as one can imagine, have any effect -upon the distribution of the impulses which pass through the neurone; -besides, there are reasons for believing that the course taken by -impulses which are delivered to the ganglion by sensory nerves is -determined by the felt-work in its centre, the neuropil. It is probable -that during their passage through the cell-body impulses acquire the -energy requisite to discharge the muscles to which the motor-fibre -carries them. In vertebrate animals, sensory nerves are branches of -neurones of which the cell-bodies lie in cranial or spinal ganglia. -They resemble the ganglion-cells of the leech in as much as they -are unipolar; both branches, the one which collects impulses from -sense-organs, and the one which distributes them to the spinal cord, -come off from the cell in a common trunk which afterwards divides, -although the unipolar condition of the cell of the spinal ganglion is -not primitive, but acquired. In the earliest stages of its growth the -cell is bipolar. Its two ends subsequently grow together for a certain -distance, the common portion being the vertical limb of the <b>T</b> -(<a href="#I_304"><i>cf.</i> Fig. 21</a>, which shows the growth of -a granule of the cerebellum). The body of the cell contains a network -not unlike the network of the leech. It is probably related to what -may be termed the charge of the neurone, the development of a suitable -degree of force in the impulses which pass through it.</p> - -<p>The neuro-fibrillæ of a large nerve-cell, such as a motor cell of -the spinal cord, are exceedingly slender (<a href="#I_321">Fig. 22</a>). -They branch and reunite. A certain number gather towards the axon; but the -majority pass through the cell from one dendrite to another, or from one -branch of a dendrite to another branch. It is very tempting to suppose -that neuro-fibrillæ are connected with conduction. When first discovered -they were regarded as conducting strands; but it is evident that -they are not comparable with telephone wires or other isolated or -separate conductors. There are good reasons for regarding dendrites -as collecting processes, taking up impulses from the end-twigs of -the nerves which branch in the grey matter around them, passing them -through the cell-body into the axon. The continuation of neuro-fibrillæ -<span class="pagenum"><a name="Page_300" id="Page_300">[Pg 300]</a></span> -from dendrite to dendrite seems to be irreconcilable with the -hypothesis that they are disposed in the lines of conduction.</p> - -<p>In common with those of various other types of neurone, the dendrites -of spinal motor cells are beset with “thorns.” These projections are -not rugosities or serrations, but short, delicate threads which stand -out at right angles from the dendrites (<a href="#FRONTIS"><i>cf.</i> Fig. 1</a>). -About a dozen years ago, the writer made a careful investigation of these structures; -at a time when most anatomists regarded them as artifacts. He found -that their claim to be regarded as parts of the neurone is as good -as that of its axon or its dendrites; although never seen on certain -types of cell, the thorns, of cells which carry them, are perfectly -definite in arrangement and spacing. In some kinds of cell they are -more numerous, in others less. Neuro-fibrillæ, as we now know them, had -not been discovered at the date when this investigation was undertaken; -but on various grounds the conclusion was arrived at that thorns are -the cell-ends of fibrils which pass from the end-twigs of arborizing -axons into dendrites. Upon this conclusion was based an hypothesis -of conduction which is here submitted, not because there is not much -to be said against it—or, at any rate, many a hiatus in knowledge -to be filled—but because it happens to be the writer’s own. The -chrome-silver and methylene-blue methods which reveal the existence of -thorns do not stain neuro-fibrillæ. They colour the soft protoplasm -in which fibrils are embedded. By modifying the chrome-silver method -in every way which still allows a result to be obtained, it was found -that thorns sometimes appear as comparatively long slender filaments, -at others as shorter filaments ending in minute knobs, or as filaments -bearing two or three dots; or finally no filaments are visible, but -the dots are in the position which they would occupy if fibrils were -present, but not stained. From this it was argued that the soft -protoplasm which during life surrounds the filament as a continuous -film, either falls back towards the cell after death or is made to -shrink into the cell by reagents. This accounts for the appearance -of rod and knob. What is supposed to happen may be illustrated by -dipping a wire in treacle. At first, when the wire is withdrawn, it is -surrounded with a film. Then the film gathers into droplets. It was -suggested that the entrance of impulses into dendrites, their conduction -<span class="pagenum"><a name="Page_301" id="Page_301">[Pg 301]</a></span> -across the space which separates the end-twigs of axons from the -dendrites into which their impulses pass, is by means of the thorns, -although the thorns are not in themselves conductors. Conduction -occurs only when films of cytoplasm surround the thorns. The first -effect of impulses is to call out the films, in the same kind of way -that a current of electricity converts a row of falling drops into a -continuous stream. A succession of impulses, by adding to the number of -the filaments which are enveloped in cytoplasm, or by increasing the -amount of cytoplasm investing certain groups of filaments, increases -the openness of the path. Sleep is a condition in which all paths are -open. Hence no impulses are effective. Wakefulness, alertness, depends -upon the closing of all paths save those which are actually in use. -We may go further. The power of concentrating attention is the power -of limiting the spread of nerve-impulses in the brain. Alcohol opens -extra paths; the concentrated effort which was making progress with -a problem becomes more diffuse. The first effect appears in greater -brilliance of thought, gained at some sacrifice of cogency. Unexpected -analogies are discovered. Imagination takes a wider range. But as the -dose is increased, a condition akin to sleep is set up. Nerve-impulses -become ineffective because, many paths being open, they do not attain a -sufficient intensity in any set of paths. These few illustrations are -given for the sake of showing the need of a theory of the opening and -closing of paths. It is not suggested that they favour the particular -hypothesis here set forth as to the structural arrangement which -provides the paths and regulates their accessibility.</p> - -<p>Recent discoveries in the finer structure of the central nervous -system have provided many problems which at present appear insoluble. -One of the discoveries most difficult to make use of in constructing -theory is the existence of extracellular or pericellular nets, which -have the appearance of extraordinarily delicate cases of wire-netting -immediately surrounding the nerve-cells. It is somewhat remarkable that -the spacing of the nets is often very similar to, if not identical -with, the spacing of thorns. While some anatomists look upon the nets -as nervous, others regard them as pertaining to the connective tissue -<span class="pagenum"><a name="Page_302" id="Page_302">[Pg 302]</a></span> -of the nervous system. At present it is not known how impulses -get across from the finest visible twigs of arborizing axons to -the dendrites of the neurones which they influence. The wealth -of structural detail which recent research has revealed is an -embarrassment to anyone who tries to devise a scheme. Not improbably, -pericellular nets are intermediate factors in the exchange; or, if -not the nets, the structures whose existence is indicated by the -appearance of the nets. In the case of many of the finer markings which -staining methods bring into view, it is impossible to say whether they -indicate the presence during life of the structure as it appears to -be, or whether the markings are due to coagulation of plasma or to -strain caused by shrinkage in coagulating agents. In a sense this is -not of much consequence. Coagulation in a uniform pattern would mean -the existence of an architectural substructure which determines the -pattern. We may be looking at the cake or at the tin the cake was baked in.</p> - -<p>There is a danger of seeing too much in a nerve-cell when examining -it under the highest powers of the microscope, and of endeavouring to -picture in too much detail the arrangements which regulate the flow -of impulses. Its markings are so complicated as to suggest to the -mind of the observer that it is itself a microcosm—a nervous system -in miniature. Neuro-fibrillæ appear to offer many alternative paths -within the cell. It is unlikely that such a way of looking at the unit -of structure is the right one. A certain motor cell of the spinal cord -is connected by its axon with thirty or forty separate muscle-fibres; -but there is no reason for thinking that the fibres ever contract save -as a single group. The axon consists of parallel fibrillæ, but these -do not appear to be needed as separate conductors; an impulse travels -down the fascicle. It does not appear to be necessary in the case of -a motor cell, and presumably the statement holds good for the large -cells of the cerebellum and cerebrum to picture any arrangement for the -simultaneous conduction in its axon of several impulses, or for the -conduction of one impulse along one of its fibrillæ and a different -one along another. What is necessary is that this particular efferent -path Z should be accessible from every other part of the nervous -system—from A to Y. If, merely for the sake of filling the space -which would otherwise be blank in the mental picture, we imagine a -<span class="pagenum"><a name="Page_303" id="Page_303">[Pg 303]</a></span> -pericellular net connected by thorns with the body and dendrites of -the nerve-cell Z, then the net is the meeting-ground of all the routes -through which Z is called into action. A nerve-wave from any of the -neurones A to Y, breaking upon this net, passes along the thorns into -the protoplasm of Z.</p> - -<p>In size a granule of the cerebellum presents a marked contrast to a -motor cell of the spinal cord; yet it is formed on essentially the -same plan. From its minute round body (about 8 µ in diameter) four or -five slender dendritic processes are drawn out. Each dendrite ends in -a little bunch of twigs, resembling fingers curved over the palm. Its -single slender axon runs towards the surface of the cortex. As the -granules lie at a considerable depth, this course is, for those which -distribute to the most superficial layers, a long one. They pass from -the granular to the molecular layer between the big cells of Purkinje. -When the axon has reached a certain level in the molecular layer, it -divides into two threads which run for a great distance, right and left.</p> - -<p>The granules of the cerebellum have a curious developmental history. -Every neurone in the body has a lifelong existence. Except for the -rare accident of its destruction by disease it occupies its station to -the hour of death. But at the time of birth many neurones are still -immature. Not all the granules of the cerebellum have yet assumed -their permanent form or situation. Beneath the pia mater there is -still a layer of minute undifferentiated cells. These, as they grow -into granules, elongate, in the first instance, into long spindles. -Subsequently they sink down through the molecular layer and between -the cells of Purkinje, leaving the poles of the spindle as the right -and left divisions of the axon (<a href="#I_304">Fig. 21</a>). It is interesting -to learn that such a migration is possible. It is also of interest to find -that a tiny granule of the cerebellum goes through the same stages in -attaining its adult form as one of the large cells of a spinal ganglion.</p> - -<div class="figcenter"> - <a id="I_304" name="I_304"> </a> - <img src="images/i_p304.jpg" alt="" width="600" height="448" /> - <p class="center"><span class="smcap">Fig. 21.</span>—<span class="smcap">The - Growth and Migration of Granules of the Cerebellum.</span></p> - <div class="blockquot2"> - <p class="neg-indent space-below2">Half a dozen nuclei of as yet undeveloped granules are - seen lying beneath the pia mater. From this level to the bottom of the drawing - granules are shown in successive stages of growth. These developing - granules, selected from various preparations of the cortex of the - cerebellum, were drawn from nature.</p> -</div></div> - -<p>There are many different types of neurone. Any attempt to describe -them, or to give an account of the various details of structure which -recent improvements in technique have enabled anatomists to observe, -would fill a lengthy treatise; and would, moreover, be beside our aim, -which is limited to obtaining such an idea of the unit of the nervous -<span class="pagenum"><a name="Page_304" id="Page_304">[Pg 304]</a></span> -system as will enable us to form a conception, however crude, of the -way in which it works. From the brief account that has been given, it -will be evident that anatomists are approaching to an understanding -of the mechanism. It will also be evident that they have already more -information than they can apply. They are cognizant of many details of -structure which they cannot interpret in terms of function; and at the -same time are aware of wide gaps in their knowledge regarding facts -which are essential to the construction of any scheme. This much is -clear: A sense-cell on the surface or beneath it is touched (probably -entered) by the ultimate twig of the outer limb of a neurone whose -cell-body lies in a spinal ganglion, while its inner limb, as a fibre -of a posterior root, enters the spinal cord. In the spinal cord the -root-fibre splits into an ascending and a descending division which -rain branches into the grey matter over a considerable area above its -<span class="pagenum"><a name="Page_305" id="Page_305">[Pg 305]</a></span> -point of entrance, and a smaller area below it. The finest twigs of -these branches are to be seen in the vicinity of the cell-bodies and -dendrites of certain other neurones. The axons of these second links -arborize in a similar way in the vicinity of large motor cells, whose -axons in turn become fibres of anterior roots. (For simplicity’s -sake no reference is made to hosts of other neurones which link the -ganglion-cell and the motor cell to other cells higher in the cord or -brain.) An impulse generated in the sense-cell on the surface of the -body runs up the root neurone into the cord, where the ultimate twigs -of the posterior root-fibre offer it a wide choice of distribution. -Following the path of least resistance, it passes into neurone No. -2. Again, the arborization of No. 2 offers it alternative paths. It -makes a choice which lands it in No. 3. No. 3 passes the impulse on -to the muscle-fibres with which it is connected. Three points are -especially worthy of attention: (1) The impulse has a wide (literally, -an unlimited) choice of routes. The skin of the finger is touched. Any -muscle may respond, although resistance is so graded as to cause the -impulse to seek in the first instance the group of muscles which is -most often required to act in consequence of stimulation of the finger. -This means, we may suppose, that it follows the chain which, having the -smallest number of links, offers least resistance. If it cannot get -through to these muscles, owing to the fact that other impulses, acting -simultaneously, either increase the resistance in this particular path, -blocking its way, or reduce the resistance in an alternative path, it -spreads farther afield. (2) Owing to the ramification of the root-fibre -which conveys it to the cord, an impulse is not limited to a single -line of distribution. It reaches many secondary links. It may therefore -influence various effector neurones simultaneously. For example, a -stimulus which calls extensor muscles into action, at the same time -inhibits their flexor antagonists. (3) The path which it finally takes -is accessible to all other impulses. Its root neurone was peculiar to -itself. Link No. 2 was more or less a common path. Neurone No. 3 is -open to every impulse which traverses the nervous system.</p> - -<p>Anatomy justifies the construction of the scheme just outlined. But -there are many points regarding structure upon which a physiologist -<span class="pagenum"><a name="Page_306" id="Page_306">[Pg 306]</a></span> -desires information, many details that he wants to see filled in. -How is the impulse passed from the arborization of axon No. 1 to -the dendrites of neurone No. 2? By what structural arrangement is -resistance introduced, and how is it regulated, if it varies? Supposing -the resistance to be higher in one path than in another, or supposing -that more force is needed to enable an impulse to invade a wider field, -how is additional energy supplied? To the first question no answer can -be given at present—the mechanism by which impulses are transferred -from one neurone to another is unknown; yet it is convenient to find -a name for the junction of axon-endings and dendrites. It is termed a -“synapse,” on the understanding that the word involves no hypothesis -as to its structural nature. It is generally held that resistance is -introduced into nerve-circuits at synapses; although this again is -a provisional statement. The phenomena for the explanation of which -the idea of synaptic resistance was introduced, may be accounted for -on a purely anatomical basis of distribution. The extent to which -one neurone influences another may depend upon the size of the brush -of fibrils with which its axon touches it. If a certain force is -needed to discharge a neurone, a nerve-current must either have a -sufficiently high potential when it reaches it, or it must act upon it -for a sufficient length of time. There is little to choose between the -arguments which place the resistance at the synapse and those which -transfer it to the nerve-cell body.</p> - -<p>As a mechanism the nervous system is unthinkable, unless we picture -its units as independent, yet capable of forming associations; as -functionally discrete, yet entering into functional continuity. When -acting, they act as chains. Impulses run from link to link, from -the end-twigs of an axon of one cell to the dendrites of the next. -Neurones are so arranged as to make it impossible for impulses to -escape backwards out of dendrites into axon-twigs. In this respect the -system is valved. But there is no reason for thinking of the substance -of the neurone as polarized in any way. The physical accompaniment of -an impulse—the electric variation—travels with equal facility -up and down its axon.</p> - -<p>There is no evidence of any specificity of neurones; on the contrary, -it is clear that impulses of every kind—that is to say, from every -<span class="pagenum"><a name="Page_307" id="Page_307">[Pg 307]</a></span> -source, for we recognize no specificity of impulses—can travel -equally well through neurones of all forms. At every junction, in -passing through each synapse, they are delayed. It takes at least 0·01 -second (less if the knee-jerk be a true reflex action) for a message -delivered to the cord by a sensory root to reach a motor root. This -hundredth of a second—the sum of the delays entailed in fording two -or three synapses—is regarded as the minimum reflex time. To it -must be added, in considering any particular reflex action, the time -taken in travelling up sensory and down motor nerves. Delay indicates -resistance. If a sensory stimulus be not sufficiently pronounced to -provoke a reflex action, the reflex may be obtained on intensifying -it. Prolonging or repeating the stimulus—really the same thing, since -sensory impulses are rhythmic, not continuous—has a far more potent -effect than increasing its force. The resistance of synapses gives way -after a number of impulses have bombarded them. The desire of brushing -a fly from the skin, if resisted, becomes intolerably urgent after a -time. A persistent outflow of impulses produced by the irritation of -a spot in the cortex of the brain overwhelms the nerve-muscle system -in an epileptic fit. The following is an experiment illustrating the -spread of impulses from their customary path to another less often -used: A piece of blotting-paper, wet with vinegar, is placed on the -inner side of the thigh of a brainless frog. There is no use in trying -the experiment on a frog which retains its brain; the substitution -of one action for another would be an exhibition of the adaptation -of means to end—a demonstration of the animal’s right of choice. -Besides, the frog might choose not to act, and so the experiment would -fail. The brainless frog wipes off the blotting-paper with the foot of -the same side. This foot is then fixed so that the action cannot be -performed, and the blotting-paper replaced. After a longer interval the -frog removes it with its other foot. Evidently it is more difficult -for the impulses generated by the irritation which the vinegar causes -to get across the cord than it is for them to reach motor neurones on -the same side. Evidently, too, the continued irritation of the vinegar -adds to the travelling power of the impulses. They are strengthened -until they are capable of overcoming the resistance in the longer path. -“Resistance in conductors” and “potential of current” are terms with -<span class="pagenum"><a name="Page_308" id="Page_308">[Pg 308]</a></span> -which the study of electricity has rendered us familiar; but it must -be evident from the experiment just described that these terms are -not really applicable to nervous phenomena, convenient though they -may be for use in an allegorical sense. Holding the foot does not, by -any mechanism which we can recognize, switch off the shorter circuit, -yet the impulses abandon it for the longer path. There is no evidence -of a struggle to free the foot that has been fixed, coincident with -the spread of impulses, as they gather sufficient strength to reach -the nervous mechanism of the other leg. The right foot not being -available, the impulses <i>choose</i> the route to the left foot. Any -attempt to explain this in terms of resistance and potential involves -the formulation of a number of subsidiary hypotheses; easy to devise, -no doubt, but stultifying to the explanation exactly in proportion as -they complicate it. Yet the hypothesis of lines of greater and of less -resistance (keeping as far away from electrical analogies as possible) -is essential to any explanation of nervous phenomena, and is, moreover, -justified by the evidence available. There are two causes in chief -upon which it depends: (1) The greater the number of neurones in a -linear chain, the greater is the number of synapses to be traversed. -If A, B, C are in the same circuit, the sum of their resistance has to -be overcome. (2) The greater the number of neurones amongst which a -nerve-current has to be subdivided, the smaller the charge available -for each of them. Imagine</p> - -<table class="large_font_bold" border="0" cellspacing="0" summary=" " cellpadding="0" > - <tbody><tr> - <td class="tdc"> </td> - <td class="tdc">B</td> - </tr><tr> - <td class="tdc">A </td> - <td class="tdc"> </td> - </tr><tr> - <td class="tdc"> </td> - <td class="tdc">C</td> - </tr> - </tbody> -</table> - -<p class="no-indent">so placed as to divide B and C, the charge delivered by A between. -This arrangement has, probably, an anatomical expression which accounts for -the relative ease or difficulty of a path, even on the supposition -that impulses do not open out as they advance—do not spread along -all the branches into which an axon divides—but keep to a given -line. The axon of neurone A divides, to branch about B, C, and D; but -its representation in the several pericellular nets (the expression -may pass for the sake of the simplicity which it introduces into the -picture) is unequal. In the vinegar experiment the impulses delivered -to the spinal cord by the root-ganglion neurone A pass to neurone B of -the posterior horn. B’s axon arborizes more freely about the cell-body -<span class="pagenum"><a name="Page_309" id="Page_309">[Pg 309]</a></span> -of neurone C in the anterior horn of the same side than it does about -neurone D in the anterior horn of the opposite side. Hence the impulses -generated by the vinegar stimulate C, sufficiently to discharge it, so -long as that road is open, more quickly than they stimulate D. That C -should be dischargeable only so long as the foot is free implies that -the activity of the neurone is in some way conditioned by its relation -with the muscles which it innervates. When the foot is held this -relation is interfered with, giving to the impulses generated by the -continued action of the vinegar time to overcome the resistance of D.</p> - -<p>The simile of the opening up of paths is fairly applicable to the -results which follow the use of artificial stimuli. Neurones seem to -link up in series under the influence of the impulses which bombard -them, popping like fireworks united by a common fuse.</p> - -<p>Experimental evidence points to the following conclusions: (1) -Resistance is offered at a synapse. This resistance must be overcome -before an impulse can get through from neurone 1 to neurone 2. (2) -The impulse does not, properly speaking, pass from 1, through 2. It -infects 2, causing it to discharge a fresh impulse. (3) Time is of the -essence of this process. Either the impulses head up at the synapse, -or, passing through into the neurone, they produce a cumulative effect -within it, which provokes it to discharge. (The latter hypothesis, -which is the less likely of the two, transfers the resistance from the -synapse to the neurone to be infected.) These conclusions are based -upon experiments of the following kind: The minimal stimulus which -will evoke a reflex action is determined. A stronger stimulus is then -applied. The reflex occurs more promptly, and is more pronounced. But -on further increasing the stimulus, it is found that the limit of -effectiveness is soon reached. The proportional relation of response to -stimulus is much less evident than it is when the experiment is tried -with a nerve-muscle. Choosing a reflex action easily provoked, the -afferent path is stimulated with an electric current interrupted fifty -times a second. The impulses which flow down the efferent path to the -muscle follow one another at the rate of about ten a second. A column -of nerve-fibres within the spinal cord is stimulated fifty times a -second. Again, the discharge into anterior roots has the natural rhythm -of about ten. The cortex of the “motor area” of the great brain is -<span class="pagenum"><a name="Page_310" id="Page_310">[Pg 310]</a></span> -stimulated with a rapidly interrupted current. The muscles which it -governs contract with their natural rhythm. The cortex is sliced away, -and the stimulus applied to the white matter beneath. A similar result -is obtained. Evidence such as this points to an independence of action -on the part of the neurones which one can express only in terms of -resistance and explosion. But there is another line of thought which -leads to the development of a picture of the working nervous system -which seems at first sight incompatible with the one that we have -sketched. The phenomenon of the knee-jerk (<a href="#Page_274">p. 274</a>) reveals -a nervous system so intimately linked together, so homogeneous, so mobile, -that no event, however trivial, occurs in any part without sending a -vibration throughout the rest. Instead of a multitude of batteries -enveloped in a labyrinth of wires interrupted by myriads of switches -which are crackling on and off, the image of a sheet of water better -figures our conception—a material so frictionless that it is a-ripple -from side to side and end to end, from the most distant rivulet which -feeds it to the farthest trickle in which it drains away. It is a -fluid in a state of infinite commotion, the movements of its particles -varying in amplitude from tremulous quiverings which scarcely frost -the silver of its surface to waves which, breaking on the muscular -system, throw it up in heaps. The vinegar experiment seems to demand -a scheme of batteries and wires. The knee-jerk points to a continuous -conducting medium. Other phenomena suggest the superposition of the two -pictures; the conception of a nervous system consisting of a uniform -medium conducting, not indifferently in all directions, but with such -freedom that from our point of view the paths are infinite in number; -and within this conducting medium nerve-cell bodies and their processes -which collect and distribute groups of vibrations sufficiently strong -in combination to produce visible effects. In order that one of these -neurones may be stimulated to discharging-point, the medium by which it -is surrounded must be thrown into such a state of agitation as suffices -to infect it. The considerations which point to the formulation of -this double or superposed scheme are such as follow: The passage of -tone-impulses does not appear compatible with the ideas we have formed -on other evidence of synaptic resistance and neuronic discharge. They -<span class="pagenum"><a name="Page_311" id="Page_311">[Pg 311]</a></span> -are too feeble for such a mechanism. The short “reflex time” of the -knee-jerk points to the passage of the agitation up a sensory root to -the spinal cord, and through a non-resistant medium to the environment -of the motor cells which it discharges, missing the neurone or neurones -which intervene in the case of ordinary reflex actions. This is an -illustration of the way in which tone-impulses, which we imagine as -conducted by the non-resistant medium, pass over into discharges which -produce visible effects. Again, the phenomena of <b>inhibition</b> -appear to require the supposition of extra-neuronic conduction. -Whenever a reflex path is in use, all other paths in its neighbourhood -are closed. The passage of impulses leading to a particular reflex -action is favoured by the suppression of conduction in its vicinity. -When A is talking to D through the nerve-telephone, B and C are -compelled to hold their peace. Inhibition is a phenomenon of universal -occurrence. In relation to various actions, it is sufficiently -pronounced to be visible in the effects which it produces. A simple -experiment will illustrate this. Holding water in the mouth has no -effect upon respiration, but during the act of swallowing respiratory -movements are suspended. Whilst the swallowing reflex is occurring -the respiratory reflex is inhibited. This might be attributed to the -volitional control of respiration, and certainly when attention is -being directed to the process volition plays a large part. But if a -finger is placed on the pulse, it is possible to detect that, during -the act of swallowing, the pulse quickens, owing to the suppression of -the slowing action of the vagus upon the heart. Here is a case in which -inhibition is in no degree a voluntary action. Nor is it of any value -as an adjunct to the particular reflex with which it is associated. It -is an illustration of the universal rule that activity of any one spot -in the nervous system is the cause of the quieting of the surrounding -area. Impulses which reflexly check the heart cannot get through the -medulla oblongata whilst the swallowing impulses are traversing it. -Inhibition has been described as a drainage of nerve-force into the -active area. On the structural side it seems to require the conception -of an extra-neuronic substance which, agitated in the vicinity of -the cells which are to be discharged, is brought to rest around -neighbouring cells. The promulgation through the nervous system of the -<span class="pagenum"><a name="Page_312" id="Page_312">[Pg 312]</a></span> -state which, when it reaches the centres of consciousness, produces -pain also seems to call for an hypothesis of extra-neuronic conduction.</p> - -<p>Any reference to <b>pain</b> in a work on physiology needs a few -words of preface, since popularly the term “pain” is used in various -senses. When I see pink geranium and nasturtiums growing in the same -flower-bed, I may exclaim: “It is positively painful.” The want of -harmony, and at the same time the insufficiency of contrast, of chalky -pink and translucent orange, jars my æsthetic sense. Dislikes, however -well founded, are ruled out in thinking of the physiology of pain. -Further, in defining pain, we must be careful to isolate the real -thing, and not to confuse it with sensations which seem to lead up to -it. If, putting my finger in a pair of pincers, I touch it as lightly -as possible, the first sensation is one of contact; a little harder, -and it becomes a sense of pressure; harder still, and all sense of -contact or pressure is lost in pain. It is usual to regard pain as -sensation carried to excess. But neither is this physiological. An -excessively bright light or an excessively loud sound is disagreeable. -It causes a sudden movement for the purpose of avoiding it—just such -a movement as one would make if one touched a red-hot poker—but it is -not, strictly speaking, painful. Not uncommonly in cases of accident -or disease of the spinal cord a sharp distinction is drawn between the -sense of touch and the capacity for experiencing pain. Below the injury -the patient retains his sense of touch undiminished in acuteness, but -no blow, or cut, or burn, causes him any pain. The pain caused by -squeezing the finger in a pair of pincers is not, therefore, an excess -of touch sensation. Pain begins to be experienced in the skin just when -the object applied to it is affecting it to an extent which might do -harm. If the point of a needle touches it, it causes pain as soon as -the pressure is a trifle less than that needed to pierce its surface. A -hot object begins to hurt when the temperature reaches 48° C.—almost -enough to coagulate the tissue fluids. Pain is not a discriminative -sensation. If I hold my arm out at right angles, I am conscious for -the first few minutes of its weight, and have, besides, some sense of -the traction exerted by the muscle of the shoulder. At the end of ten -minutes these sensations are merged in pain, and for some time after -lowering the arm the shoulder-muscle aches, much as it does in -<span class="pagenum"><a name="Page_313" id="Page_313">[Pg 313]</a></span> -rheumatism. Pain is an effect upon consciousness, which absorbs, -engulfs, and therefore obliterates sensation. To use an ancient phrase, -“It is less that I feel pain than that I am pain.” If we speak of the -capacity for pain as a sense, we may call it for the purpose of our -present argument the “sense of damage.” The nerves of the skin are -acutely affected by any agent which is likely to do harm. It is their -business to convey to the central nervous system an influence which -so affects it as to set up in consciousness the condition of pain. -Sensations of damage evoke reflex movements by means of which the part -of the body likely to be injured, or the whole body, is removed to a -safe distance. It being the duty of the skin to give this warning, a -service of nerves sensitive to noxious agents has been developed which -scouts in co-operation with the services devoted to the recognition -of physical contact and heat and cold (<a href="#Page_425"><i>cf.</i> p. 425</a>). -If, imagining that the fire has not been lighted, I touch an almost red-hot stove, -I acquire quite a considerable amount of information of which I am able -to make use. I gain an accurate notion of the situation of the stove, -and I put the right part of my finger in my mouth. The skin sends to -the brain the ordinary sensations of touch and pressure before the -condition of pain is established. In seeking for a definition of pain, -we must eliminate the two attributes which have characterized all the -forms of stimulation which we have considered up to the present time: -(1) The tendency to provoke movement; (2) the supply of information. -If I am suffering from a whitlow, the last thing that I am disposed -to do is to jerk my finger about. Although it enhances the urgency -of skin-reflexes, pain, in general, inhibits movement instead of -provoking it. This is well illustrated in pleurisy. So long as a man is -healthy he is quite unconscious of the fact that at each respiration -the lower part of the lung slides on the lining of the chest-wall; -but commencing inflammation on the surface of one of the lungs causes -intense susceptibility to friction, and the pain produces an effect -which the man is quite unable to produce by an effort of will; it stops -the movements of the chest on the damaged side. Pain is inhibitory, not -stimulant. It is not, properly speaking, a sensation. Frequently being -mixed with sensational elements, it conveys topographical information; -<span class="pagenum"><a name="Page_314" id="Page_314">[Pg 314]</a></span> -but pure pain approaches in quality the nebulous sense of distress of a -patient who, when asked where he felt it, replied: “Nowhere; but there -is a deal of it in the room.”</p> - -<p>Sufferers describe pain in figurative language, as “burning,” -“stabbing,” “throbbing,” “aching,” and so forth. Two persons afflicted -with the same lesion, the same source of pain, use approximately the -same terms. Hence we cannot say that pains do not differ in character. -But this is not a sufficient reason for assigning any specific quality -to pain. It varies in severity, in continuity or intermittence, in -suddenness of onset, in the sensations which accompany it, in the -emotional tone to which the disturbance of the organ from which it -proceeds gives rise, in the tenseness of the part affected and its -consequent sensitiveness to a throbbing pulse. All these things make -a complex of pain plus sensation, which causes toothache to differ -from headache, and both from the pain of burned skin. But they do -not give specific qualities to different varieties of pain. This -being the case, there is no need to presume the existence of special -nerve-endings for the reception of pain, or of a special region of the -cortex of the brain for its reception. On the contrary, the evidence -is conclusive that the nerve-fibres which serve the more highly -specialized senses, which have well-defined connections in the cortex -of the brain, do not convey the influence which enters consciousness as -pain. It is the innumerable nerves which have no specialized receptors -that take up pain. The afferent nerves of the viscera—the vagus and -sympathetic—convey no impulses which enter consciousness, so long as -the tissues which they supply are healthy. They have no representation -in the cortex. The organs with which they are connected (with trivial -exceptions, easily accounted for) are absolutely insensitive to -injury. Before the virtues of chloroform were known—in the days when, -however severe the operation, the patient had to nerve himself to -bear it without an anæsthetic—surgeons proved that the liver or the -intestines, or practically any other viscus, may be cut or cauterized -without the patient being aware that it is being touched. The same is -equally true of the brain itself. But if damage in a viscus is set up -gradually, its nerves convey to the central system an agitation which -has the most pronounced results upon consciousness, and on the way -<span class="pagenum"><a name="Page_315" id="Page_315">[Pg 315]</a></span> -profoundly affects the reflex actions which the spinal cord can carry -out, and also its capacity as a conductor. Once in his life, perhaps, -a man passes a gall-stone; for generations such a thing may not have -happened in his family. Yet the man finds that he is provided with a -nervous apparatus which conveys to consciousness intensest pain.</p> - -<p>It is difficult to think of pain as travelling along nerves in the form -of rhythmic impulses, similar to those which produce in consciousness -the effects which we have distinguished as sensations. A few lines -above we stated that no impulses which affect consciousness normally -travel up the vagus or the sympathetic nerve, limiting the term -“impulse,” perhaps unjustifiably. The vagus conveys an influence which -enters our experience, as hunger. Probably other states of feeling -for which we have no names, which resemble pain and hunger and their -opposites, are set up through the agency of visceral nerves.</p> - -<p>Fifty years ago attention was called to the difficulty of finding -pain-paths amongst the white tracts (nerve-fibres) of the spinal cord. -It is as difficult to point them out now as it was then; but the -inference that pain travels up the grey matter has given way to the -“neurone theory”; under a misapprehension as the writer holds. Pain -travels slowly. If one happens to notice a person who unsuspiciously -touches a hot surface, one observes that an interval elapses between -contact of his finger with the iron and the exclamation with which he -“relieves his feelings.” It amounts to more than a second—if the iron -is not very hot, to several seconds—whereas the “reaction-time” for -touch is only one-seventh of a second. The slowness of movement of -pain through the nervous system can on the neurone theory be explained -only on the hypothesis that it travels from link to link along a very -long chain of very short neurones. That pain is a state of the grey -matter rather than a succession of impulses, and that (within the -cerebro-spinal axis) the state is transmitted through an extra-neuronic -medium, seems a simpler explanation.</p> - -<p>The state set up in the segment of the cord in which afferent fibres, -conveying pain from viscera, embouch affects its conductivity. It -subdues reflex action through the segment, and at the same time -<span class="pagenum"><a name="Page_316" id="Page_316">[Pg 316]</a></span> -facilitates or reinforces the transmission of sensory impulses towards -the seat of consciousness. This shows itself in the apparent increased -sensitiveness of the skin of the area of the surface supplied by -the posterior root which joins the segment of the spinal cord into -which the pain influence is also being poured. For example, afferent -sympathetic nerves from the cardiac end of the stomach join the sixth -and seventh thoracic spinal nerves. Other afferent fibres run up the -vagus to the medulla oblongata. When the cardiac end of the stomach -is diseased, pain is referred to the skin area supplied by the sixth -and seventh dorsal roots. The ordinary inevitable stimuli acting upon -this area cause pain. Experimental stimuli which elsewhere would be -felt as touch or warmth are painful. The impulses to which they give -rise pass through pain-agitated segments of the spinal cord. The vagus -nerve carries its pain influence to the medulla oblongata. Now, it -happens that the sensory nerve of the face—the fifth—spreads -for a considerable distance up and down the axis of the brain. The fifth -nerve in consequence pours its sensory impulses into a region which is -pain-agitated by those fibres of the vagus which come from the cardiac -end of the stomach. Hence disease of that organ gives rise also to an -“illusion” of pain—pains and illusions of pain are philosophically -indistinguishable—on the surface of the head. The viscera, having no -direct access to consciousness, appear by deputy. When the stomach -is distressed, it makes its appeal to the whole body politic for -considerate treatment through certain nerves which have the privilege -of appearing at Court. The message is misread as coming from the front -of the chest—“heart-burn”—or from the shoulder, or from the scalp, -or from the other skin areas which these nerves serve. When the liver is -in trouble, consciousness, having no knowledge of its whereabouts—is -it the business of hand and eye to explore another man’s liver, or -incumbent upon the mind to accept their findings?—infers that the cry -comes from the shoulder. Nor have the tissues beneath the root of the -nail, or the muscle of the shoulder, or the pulp of a tooth, any direct -representation in consciousness; but since the pain-condition in the -grey matter converts it into a microphone, messages from neighbouring -structures which otherwise would fail to arouse attention, after -<span class="pagenum"><a name="Page_317" id="Page_317">[Pg 317]</a></span> -traversing the pain-segments of the nervous system, ring out clearly, -and hence the mind locates approximately the “pain” of the whitlow, the -muscle-ache, the decayed tooth. Sufferers from toothache are familiar -with the phenomenon of the spread of pain from a definite spot to the -whole jaw or the whole side of the head, dependent upon the spread -of the pain-agitation from the segment of the axis of the brain in -which the dental nerve ends to neighbouring segments. Our ability or -inability to localize a pain does not depend upon the presence or -absence of pain-nerves, but upon the existence or non-existence of -nerves coming from the same organ, or from its neighbourhood, and -capable of conveying impulses to the seat of consciousness. In passing -through the part of the spinal cord or of the axis of the brain which -is disturbed by the influence exercised by a damaged organ, silent -impulses acquire force sufficient to render them audible, and combine -with the pain to produce a feeling which consciousness can analyse, to -a certain extent. Informed as to its whereabouts by these accentuated -sensations, consciousness recognizes a sense of pain limited in its -topographical extension.</p> - -<p>Sneezing when a bright light falls upon the eye is a curious -illustration of the exaggeration of the effectiveness of sensory -impulses when they happen to be poured into an agitated segment of -grey matter. About one person in every three is affected in this way. -A friend of the writer, who was particularly sensitive, rising in the -night because he heard his child cry, three times lighted a candle -and three times sneezed it out before he could watch the application -of match to wick without suffering from a nerve-storm. Some nervous -dogs—especially fox-terriers—are very liable to this neurosis. -Many persons who do not sneeze feel, when the sunshine stimulates their -retinæ, a tickling in the nose. Again the illusion is to be traced to -the door of the fifth nerve—the sensory nerve of the whole of the -face. The nose is the true tip of the body. Morphologically it is -anterior to the eyes. Just as the fifth nerve extends its distribution -to the nose, so also its root-fibres extend their connection within -the axis of the brain forwards, until they traverse the mid-brain, the -primary centre of the optic nerve. A bright light, by stimulating the -optic nerve, sets up a commotion in the mid-brain. The ordinary -<span class="pagenum"><a name="Page_318" id="Page_318">[Pg 318]</a></span> -every-moment impulses from the nose, carried by the fifth nerve to -this region, ought not to appear in consciousness at all; but owing -to the excited condition in which they find the grey matter they -assume an importance which does not belong to them, and discharge -the reflex action of sneezing, just as they would do had one taken -snuff. Several lessons are to be learned from this phenomenon—as, -for example, one which cannot be too often impressed, that the impulses -which appear in consciousness (or, more accurately, the impulses to -which attention is directed) are but a most insignificant fraction of -those delivered by sense-organs to the central nervous system. The -impulses which give rise to the sensation of tickling in the nose -are not exceptional impulses which happened to be started when the -light fell on the eye. They were reaching the brain in a steady flow -before the agitation of the mid-brain gave to them exceptional force. -No consideration regarding the working of the nervous system has a -more important bearing than this. We cannot picture to ourselves the -activity of the sensory nervous system. Our experience is limited -to the scattered sensations which we <i>perceive</i>. Are the sensory -nerve-endings incessantly responding to external forces, throwing -an almost continuous procession of impulses up each of the millions -of nerve-fibres which connect them with the central system? Such a -conception is probably nearer to the truth than the conception which -we should develop if we trusted to experience. Yet even experience -tells us that an infinity of messages is delivered to the brain, of -which consciousness takes no account. Changing trains at a roadside -station in France, my attention was attracted by an electric bell on -the platform, which was ringing continuously. “Why does the bell ring?” -I asked the station-master. “To make known that everything goes well,” -was the response. “If it stops, something is wrong.” “But do you not -become so accustomed to it that you cease to hear it?” “Yes, truly; -it rings day and night. One does not pay attention to it until it has -stopped.” Sensory impulses generated by the contact of my skin with -the chair that I am sitting on are incessantly ringing the bell of -consciousness. I should notice them immediately if they stopped. As it -is, they do not attract my attention until they ring a little louder -<span class="pagenum"><a name="Page_319" id="Page_319">[Pg 319]</a></span> -than usual, or until some particular group, owing to unrelieved -pressure, produces a cumulative effect. Another lesson; that the -condition of the nervous system, and therefore its conductivity, is -determined at any given moment by the sensory impulses which are -reaching it. We cannot describe the effect of a bright light as pain, -yet it agitates the grey matter, altering its state, in the same way as -the nerve-inflow which we recognize as pain. A wet rag on the forehead -does not assuage a headache by cooling the brain (<a href="#Page_106"><i>cf.</i> p. 106</a>). -The headache is “in the scalp.” The cool wet rag diminishes the dilation -of the bloodvessels of the forehead, and quiets the impulses from the -skin which are pouring into a tract of grey matter pain-agitated by the -influences ascending a visceral nerve—usually the vagus.</p> - -<p>It is necessary to warn the reader that a reversion to the old idea of -“conduction through grey matter”—<i>i.e.</i>, otherwise than by a chain -of neurones—is unorthodox. It is set forth here because it seems to -the writer that the various phenomena which have to be accounted for -fit in best with the hypothesis of a double path. If evidence of the -anatomical possibility of extra-neuronic conduction is asked for, it -may be pointed out that the chrome-silver and methylene-blue methods, -upon which our knowledge of neurones is based, do not, in the very -nature of the case, show that grey matter consists only of neurones -and their obvious branches. As they select particular elements of -structure, we can never by their use alone know what they fail to -show. Attention may also be called to the fact that the same staining -process which reveals pericellular nets (<a href="#Page_301">p. 301</a>) shows -also a structure resembling a network in the substance which intervenes between -them. Truly the method is a rough one. It may well be thought that the -nitric acid used to fix the tissue may cause strange coagulations with -solution of uncoagulated substance; but, as was remarked with regard -to the pericellular nets, regular patterns indicate architectural -differentiation. But whether these nets do or do not give hints as -to the nature of the conducting medium, there is no difficulty in -finding sufficient material, after all the substance entering into the -formation of the conducting neurones, as we imagine them, has been -accounted for. <i>Ex hypothesi</i>, the conducting material is provided by -the fibrils of the sensory nerves in their extensions beyond the limits -<span class="pagenum"><a name="Page_320" id="Page_320">[Pg 320]</a></span> -to which the deposit of subchromate of silver extends, when -the chrome-silver method of displaying neurones has been used. -Sensation-impulses enter neuronic chains. The condition which, when it -affects the seat of consciousness, is known as pain, progresses up the -vertebrate neuropil.</p> - -<p>Energy is developed within the nervous system. The <b>force of -impulses</b> is adjusted to the resistance which they have to -overcome. Stimulation of the millions of twigs of the vagus nerve in -the lungs brings about the gentle movements of ribs and diaphragm -which constitute peaceful respiration. A crumb of bread touching the -mucous membrane of the larynx stimulates a few of the endings of the -same vagus nerve. Like an avalanche, the impulses gather head as they -advance, causing, not the diaphragm and intercostal muscles alone to do -their utmost, but calling into action half a dozen accessory muscles of -respiration. It is difficult to account for this reverberation of the -messages which clamour for the ejection of the crumb of bread without -figuring them as spreading from neurone to neurone, urging each in turn -to deliver its maximal discharge.</p> - -<p>Neurones are provided with material which serves as a store of energy. -In their cell-bodies, including their dendrites, are to be seen coarse -granules of nucleo-protein, which, being fitted in between groups of -neuro-fibrillæ, assume an angular form. They are known as Nissl’s -corpuscles, or are termed “tigroids,” owing to the spotted appearance -which they give to the substance of a cell. If the nerve-cells of birds -be examined just after they have alighted from a migratory flight, the -granules are found to be few and small. In a bee returning to the hive -at evening with its last load of pollen, they are smaller than they -were when it commenced its morning’s work. They disappear in certain -pathological conditions, and under the influence of various drugs; and -since their presence is revealed by staining, their disappearance is -spoken of as “chromatolysis.” -<span class="pagenum"><a name="Page_321" id="Page_321">[Pg 321]</a></span></p> - -<div class="figcenter"> - <a id="I_321" name="I_321"> </a> - <img src="images/i_p321.jpg" alt="" width="500" height="495" /> - <p class="center"><span class="smcap">Fig. 22.</span>—<span class="smcap">The - Body of a Motor Neurone.</span></p> - <div class="blockquot2"> - <p class="neg-indent space-below2">In its centre is a large clear spherical nucleus, with - a nucleolus. The body-substance is prolonged into five dendrites and an axon. - Neuro-fibrillæ are seen in dendrites and axon. They traverse the body - of the cell in all directions, in little bundles which are separated by - angular granules of stainable substance (tigroids).</p> -</div></div> - -<p>The wasting of tigroids during functional activity proves clearly that -nerve-cells do work, in the physical sense. Energy is expended in -transmitting messages from receptor to effector, from sensory cell to -muscles, from recipient nerve-ending to glands. Have nerve-cells any -privileges or duties? Their functions, so far as we have considered -them hitherto, are automatic, from a mechanician’s point of view. -Their situation and connections determine the direction in which they -conduct, and the degree in which they reinforce stimuli impressed upon -the nervous system by the environment, including what may be termed -the internal environment, food in the alimentary canal, secretions -in ducts, and so forth. Have the cells any directive or executive -functions? There is no evidence that they have; nor, it must be added, -is there any line of reasoning which leads inevitably to the conclusion -that they have not. Remembering that, until recently, it was the -custom to solve all obscure problems and to shelve all difficulties -by conferring human attributes upon nerve-cells and collections of -nerve-cells, termed “centres,” a physiologist admits the negative with -<span class="pagenum"><a name="Page_322" id="Page_322">[Pg 322]</a></span> -reluctance. The unconscious argument in the past used to run somewhat -thus: “I decide to act or to abstain from action. The nerve-cell is -the mechanism by means of which I decide. Therefore the nerve-cell -decides.” (In the past a distinction was drawn between the cell-body -and its processes, but that, we now see, was absurd.) It is very -difficult to relinquish completely this attitude of mind. I feel, I -remember, I will. There must be a <i>something</i> which feels, remembers, -wills. But a physiologist finds in the nervous system no evidence -of a capacity for any function other than that of conduction, with -adjustment of the force of current. He can no more discover feeling, -memory, or will in a chain of neurones than he can find music in a -violin. He hears the strings singing in the breeze. He can twang them -with an electric shock. But he has no vision of ghostly performers, -no glimpse of the conductor’s baton. Yet he knows, as every sane man -knows, that the neurones are the instruments played in the orchestra of -mind. He knows that, while all are sounding, some are muted, in order -that the others may produce a dominant effect. He knows, too, whenever -he decides to continue writing or to close his notebook, that the -conductor is raising the baton or allowing it to sink by his side.</p> - -<p>A neurone or nerve-cell is a transmitting link. It is scarce a thing -to wonder at that physiologists, having wrestled successfully with the -superstition of the “pontifical nerve-cell,” are unwilling to reinstate -it even as doorkeeper in a free church. It may be that it exercises -some discretion in admitting impulses, but until its authority as a -guardian of the path which stretches behind it has been established, -it is better to regard it merely as a door which swings open whenever -pressed with sufficient force.</p> - -<p>Is it possible to classify neurones according to their function? -They can be classified according to size, and, with some degree of -completeness, according to form. But if, as we believe to be the -case, size and form are governed by purely physical requirements, the -divisions into which the cells fall have no physiological significance. -The motor cells of the spinal cord and axis of the brain are large and -irregular in shape. Their dimensions are clearly dependent upon the -size, thickness rather than length, of the nerve-fibres which are drawn -<span class="pagenum"><a name="Page_323" id="Page_323">[Pg 323]</a></span> -out from them. They discharge impulses to groups of voluntary -muscle-fibres at a considerable distance. Small cells could not do the -work. Precisely similar reasons can be given for the large size of the -cells of Purkinje in the cerebellum, which transmit the elaborated -product, as we may term it, of this organ to the great brain; and -for the dimensions of the large pyramids of the great brain, which -convey its decisions to the spinal cord. The small pyramids of the -cortex of the great brain distribute the first crude impressions of -sensations to neighbouring (association) areas of the cortex. A cell of -Purkinje (<a href="#I_339">Fig. 23</a>) has a more complicated, and at the same -time a more regular, form than any other nerve-cell. It resembles an exceedingly -richly branched espalier pear-tree, set at right angles to the narrow -convolutions of the cerebellum; a disposition easily accounted for, -when the structure of the cortex of this organ is considered. Its -outer layer in which the espalier processes ramify is traversed -longitudinally by an infinity of nerve-threads, the bifurcated axons of -granules. These granules are small neurones which take up impulses from -afferent (“mossy”) fibres, and distribute them to the dendrites of the -Purkinje cells—each collecting from a few fibrils only of the sensory -channels. (The word “sensory” is used to indicate that sense-organs -are their provenance, and not that their messages become sensations.) -The numerous spreading branches of a Purkinje cell, disposed in a -transverse plane, are obviously arranged to hold up and keep apart -these myriads of longitudinal threads. A cerebral pyramid is shaped -like a fir-tree. It is placed in a definitely stratified layer. By -its branches it collects impulses from the superficial strata, which -it transmits through its stem to the white matter beneath the cortex. -The various parts of the central nervous system have work of different -kinds to do, and we find interposed in the circuits which compose the -several parts cells of various types. We speak of the large cells as -“motor,” the granules as “sensory,” the small pyramids as “association” -cells—such terms indicating the positions which they occupy in the -arcs, but not defining their functions. Of specialization of function -the physiologist cannot obtain a hint. He cannot classify nerve-cells -in groups concerned in reflex action, in feeling, in remembering, in -willing, in thought. On the contrary, he can assert with confidence -that such distinctions are not to be drawn. -<span class="pagenum"><a name="Page_324" id="Page_324">[Pg 324]</a></span></p> - -<p>In various situations in the central nervous system a certain type -of cell is found for which, in the present state of knowledge, it -is impossible to account. We mention these cells lest it should be -inferred, from what has been said above, that all neurones can be -fitted into a simple scheme of conducting arcs. In the spinal ganglia -there are neurones whose axons divide to form “baskets” around other -ganglion-cells. In the cerebellum there are similar cells, the axons of -which divide into branches, which break up to encase Purkinje-cells. -Cells of the same kind are found in a few other situations. In some -cases the end-branches which enter into the formation of the baskets -are few in number, and thick and clumsy. They grasp the body of the -cell which they surround, with gouty fingers, as it were. In other -cases the basket is a tangle of fine threads. It is difficult to see -what rôle cells of this kind can play in conduction. From the olfactory -and optic centres nerve-fibres extend outwards to the olfactory bulb -and retina. Here again is an arrangement which does not fit in with any -scheme. We might multiply examples. But enough has been said, perhaps, -to convey the impression which we wish to leave, that, although -experiment abundantly proves that the nervous system consists of an -association of sensori-motor conducting arcs, and although anatomical -investigation demonstrates the existence of chains of neurones which -take part in the formation of such arcs, it is impossible to reduce -the system to schemata or to prepare diagrams in which all structural -elements are, even hypothetically, fitted into place.</p> - -<p>It may be convenient at this point to call attention to the differences -which distinguish the <b>sympathetic system</b>—the ganglia and nerves -of the viscera and bloodvessels—from the system devoted to bringing -sense-organs into connection with the skeletal musculature which we -have chiefly considered hitherto. The fibres of the posterior root -of a spinal nerve which convey impulses from the skin and muscular -sense-organs, and the fibres of its anterior root which convey impulses -to skeletal muscles, have a similar diameter of about 15 µ. In addition -to these, the roots contain fibres which carry impulses from and take -them to the viscera. Those which bring impulses from the viscera vary -greatly in thickness, some being as large as the other sensory nerves -<span class="pagenum"><a name="Page_325" id="Page_325">[Pg 325]</a></span> -of the posterior root. The diameter of the fibres which go to the -viscera is not more than one-fifth as great as that of the other fibres -of an anterior root. Similar slender fibres are found in the vagus -nerve. If all organs are removed from an animal’s chest and abdomen, a -string of small pearl-like ganglia, united by a longitudinal cord, is -seen lying on either side of the bodies of the vertebræ, one ganglion -for each segment. This string of ganglia is termed the “sympathetic -chain” (<a href="#Page_243"><i>cf.</i> p. 243</a>). The small medullated fibres -of the anterior spinal roots join these ganglia. Some of them arborize about their -cells; some pass by them to arborize in ganglia which lie farther -afield, on the course of the great bloodvessels and within the viscera. -The axons of neurones whose cell-bodies are within a ganglion break up -into bunches of non-medullated fibres. In this way the fibres of the -sympathetic system are increased in number. Each of its neurones is a -multiplying and distributing station. There is no evidence that it in -any way serves as a “centre,” takes part in reflex action, or otherwise -usurps the functions of the grey matter of the spinal cord. Nerve-cells -are thickly strewn between the mucous membrane and the muscular coat, -and again between the two layers of the muscular coat of the alimentary -canal. It is not so certain that this system has no “central” -functions. The remarkable degree in which the wall of the intestines -retains its capacity for co-ordinated movement, after all nerves which -reach it from the ganglia and through the vagus have been cut, suggests -that the plexus of nerves within it does act to some extent as a reflex -centre. If we leave the case of the intrinsic nervous system of the -alimentary canal open, awaiting further proof, there is no reason for -looking upon the sympathetic system as in any degree independent of -the spinal cord and brain. It does its work on a large scale, and its -work is of a low order. Nature does not need to connect up the viscera -and bloodvessels with the central nervous system by means of fibres -as thick as those used for skeletal muscles. It is more convenient to -provide for the multiplication of the nerves—which must be extremely -numerous, owing to the relatively minute size of the muscle-fibres for -which they are destined—outside the central system than it would be to -include the necessary distributive cells within it. Again, we find that -a nerve-cell, when we see it at close quarters, shows no evidence of -<span class="pagenum"><a name="Page_326" id="Page_326">[Pg 326]</a></span> -administrative capacity. Although of a different shape, a ganglion-cell -of the sympathetic system is as large and as complex in form and -structure as a pyramidal cell of the cortex of the brain; yet the work -which it does is of a purely mechanical order. It receives, reinforces, -transmits impulses which reach it from the central nervous system.</p> - -<p>The often-repeated statement that a nerve-fibre is a drawn out process -of a nerve-cell body has prepared the reader to anticipate that it -dies when cut off from its central connection. When the axon is dead, -the sheath which invests it rapidly loses its tubular character. If -the situation of the cell-bodies of a nerve be known, it can be at -once foretold on which side of the cut <b>degeneration</b> will occur. -Suppose that the median nerve has been severed at the wrist. All -nerve-fibres on the distal side of the wound must atrophy, whereas none -of the fibres on the proximal side will be affected. The motor fibres -have their cell-bodies in the spinal cord, the sensory in the spinal -ganglia. Degenerations following lesions in the central nervous system -have taught pathologists more about the course of the fibres in the -white matter than any other class of observations. Degeneration above -the lesion is spoken of as ascending, below as descending—not that it -progresses upwards or downwards. It occurs throughout all the stretch -of the fibre which has been isolated from its cell-body at the same -time, or nearly so. The thought that impulses can no longer ascend or -can no longer descend, as the case may be, has given sanction to the -expressions “ascending” and “descending” degeneration.</p> - -<p>Restoration to functional activity of tracts of fibres which have -degenerated in the brain or spinal cord never occurs, but severed -peripheral nerves <b>regenerate</b>. Not that fibres join cut end -to cut end, however clean the wound. A wound in the wrist which has -divided the median nerve may heal in a few days “by first intention,” -so far as other tissues are concerned; but the patient does not for -two or three months recover the power of using the muscles of the hand -which the nerve supplied or the sense of touch in the area of skin to -which it was distributed. The ends of the axons on the proximal side of -the wound have to grow downwards to establish new connections in the -<span class="pagenum"><a name="Page_327" id="Page_327">[Pg 327]</a></span> -muscles and in the skin. The interval which elapses between the healing -of the wound in the wrist and the restoration of sensation and power of -movement is occupied in their downgrowth.</p> - -<p>The re-connection of regenerated nerves with their terminal apparatus -presents to the mind a curious problem. There is no evidence that as -function is re-established the brain has to re-learn the situation -of the sensory spots on the skin, or to re-acquire skill in using -the muscles which again come under its control. From the moment that -the outgrowing nerves have recovered their terminal connections skin -and muscles have their right representation in the brain, however -much the two cut ends may have been twisted in their relation one to -another. It seems inconceivable that each nerve-fibre can find its -way to its original station; but if it does not, our conception of -the mode of working of the nervous system still needs much refining -from the telephone-exchange analogy by which we naturally help out our -explanations. If a telephone cable has been severed, it can be made -useful again only in one of two ways. Either the two segments of every -wire that has been cut must be reunited, or the subscribers’ numbers -must be redistributed.</p> - -<p>The experiment of uniting the proximal segment of one nerve with the -distal segment of another of a quite different function gives results -which have an even more disconcerting effect upon our theory of the -nervous system. The sympathetic cord of the neck and the vagus nerve -lie very close together, alongside the carotid artery. The vagus is -both afferent and efferent. The sympathetic is wholly efferent—<i>i.e.</i>, -it conducts impulses, which enter the sympathetic chain within the -thorax, in the direction of the head. If both nerves are cut, and -the end of the vagus turned round, so that it is in apposition with -the upper end of the sympathetic, its regenerating fibres make their -way along the sympathetic cord, headwards, to the superior cervical -ganglion. They arborize about the bodies of its ganglion-cells, just -as the sympathetic fibres used to do. The vagus is a nerve of many -functions. Amongst others, it inhibits the contraction of the heart, -constricts the bronchi of the lungs, dilates the bloodvessels of the -intestines, and helps in regulating the movements of these viscera. -After it has taken the place of the upper segment of the sympathetic it -<span class="pagenum"><a name="Page_328" id="Page_328">[Pg 328]</a></span> -dilates the pupil, constricts the bloodvessels of the ear, erects the -hairs of the head, as if to the manner born. To take another example, -in a monkey the two nerves supplying respectively certain flexor and -certain extensor muscles of the forearm were cut, and their ends -crossed, so that flexor nerve-fibres grew down to extensor muscles, -and extensor fibres to flexor muscles. There was no bungling of reflex -actions or of voluntary actions when the new roads were first used. The -monkey did not jerk its hand open when it tried to scratch or to grasp -a nut.</p> - -<p>When experimental data first began to accumulate, physiologists -drew diagrams and made models of the nervous system in which they -represented it as composed of conducting arcs. The arcs were superposed -to indicate that they were of various grades—spinal for ordinary -reflexes, bulbar for co-ordinated actions, through the grey matter in -the centre of the great brain for “ideo-motor” actions, through the -cortex of the great brain for voluntary acts. They spoke of authority -and responsibility, comparing the nervous system to an army or a club. -It is premature to attempt a theory of the nervous system compatible -with recent discoveries regarding its structure and mode of working, -but it is clear that the diagrams and metaphors to which we have just -referred were misleading. In place of attempting to disarticulate -the machine, we ought to emphasize its structural unity. The results -obtained by uniting heterologous nerves cannot be explained by -reference to a model made of wires and pieces of cork. They do not -fit in with any organization of human units or with any postal system -or telephonic apparatus for transmitting news. Probably the lines of -thought which will prove most fruitful are somewhat as follows: (1) An -efferent discharge occurs as the result of the opening of a circuit -from a muscle back to the muscle. Afferent impulses—call them sensory, -on the understanding that this does not imply that they appear in -consciousness—are ceaselessly flowing from receptors to effectors in -the muscle. A sensation—in the case of skeletal muscles usually a skin -sensation—reinforces them to discharging-point. If the spinal cord has -been severed from the brain, the up-and-down flow does not reach beyond -its grey matter. It is short-circuited. If the brain is in normal -connection with the spinal cord, sensory impulses travel upwards to its -<span class="pagenum"><a name="Page_329" id="Page_329">[Pg 329]</a></span> -cortex (without, save in exceptional instances, arousing consciousness, -or, as we should prefer to express it in this connection, without -attracting attention) to a degree which varies with the several classes -of receptor and with the animal. A monkey reduced to the condition -of a “spinal animal”—<i>i.e.</i>, with its spinal cord severed from its -brain—is less competent than a dog, and a man is far less competent -than a monkey. In other words, a man habitually uses his brain more -than does a monkey, and a monkey more than a dog. The proportion which -brain-weight bears to body-weight roughly indicates the part the brain -plays in conducting the traffic of the body. (2) Communication within -the nervous system is almost unrestricted. If, before the median nerve -was divided at the wrist, receptor A usually initiated a current which -passed through the circuit to effector X, and receptor B to effector Y, -and if the new fibres which grew downwards lost their way so that the -one which used to receive messages from A attached itself to B, and the -one which used to transmit commands to X attached itself to Y, A is not -thereby cut off from X, or B from Y. Such a mechanical association is -restricted to our diagrams. It does not enter into Nature’s plan. The -spinal cord is not scored with unchangeable paths. A messenger from A -could always reach either X or Y. It was not the path, but the struggle -with competing messengers, which directed him to X.</p> - -<p>When we endeavour to picture the mechanism of the nervous system, we -find ourselves faced by phenomena which appear irreconcilable. One set -of observations leads to the conception of closed paths; another set -points to an open conductor. The experimental crossing of nerves to -which we have just alluded shows that the nervous system is adaptable, -to a degree which seems extraordinary to anyone who attempts to compare -it with any of Man’s devices for establishing communication. Paths -appear to make themselves. On the other hand, the more important, and -therefore dominant, reflex actions, such as swallowing, breathing, -the maintenance of position, are due to the union of receptors and -effectors by lines which are either reserved for their sole use, or, if -shared by other currents, it is on the understanding that they have a -first and altogether prepotent claim. No competing impulses can divert -<span class="pagenum"><a name="Page_330" id="Page_330">[Pg 330]</a></span> -them or block their way. All reflexes which in the history of the race -have established their right to dominance not only seize and hold a -route through the nervous system, to the exclusion of all competitors, -but, as we have already shown in the case of the swallowing impulse, -the traffic in neighbouring routes is suspended for their benefit. -At the other end of the scale we find reflexes which may be termed -“occasional,” in that, although of frequent occurrence, they exhibit -illimitable variability in form. Occasional reflexes require, as a -preliminary to their transmission, that the afferent impulses which -give rise to them should secure for a time the exclusive use of the -motor neurones by which they are carried out. The receptors bring the -motor neurones into tune with themselves, and while in tune they will -respond to impulses from no others. But the tuning lasts for a short -time only. Either receptor or neurone, or both, soon tire. There is no -danger of a particular reflex being prolonged to the detriment of the -organism as a whole. As an illustration of an occasional reflex, we may -cite the scratching movement of a dog. Its skin is punctured by a flea. -It scratches the place. A second flea bites it somewhere in the same -neighbourhood. The dog does not shift its hind-foot so as to scratch -midway between the two bites. It finishes out one scratch before paying -attention to its second tormentor. The exact position to which the -hind-foot is raised depends upon the position of the irritant; and -since this may be shifted over a very considerable surface, the form of -the reflex varies equally widely. Each of the very numerous receptors -in the skin tunes a slightly different group of motor neurones; and -since a second irritant may reinforce the first, instead of making an -alteration in the group of neurones which the reflex is discharging, it -is clear that there is no fixed path uniting receptor A with neurones -X, Y, Z and receptor B with neurones W, X, Y. If, however, the second -irritation occurs at a spot lying at a considerable distance from A, -in place of reinforcing the scratching movement which A has set going, -it weakens and shortens it. The receptor C, which is calling for the -discharge of a markedly different set of motor neurones, tends to -inhibit those which are already active. These results are tested with -precision upon a “spinal dog” and with the aid of an electric needle, -the other pole from the battery being a large flat plate placed in -<span class="pagenum"><a name="Page_331" id="Page_331">[Pg 331]</a></span> -contact with the animal’s body. The conception of definite paths, -to which the contemplation of permanent reflexes gives rise, is -inappropriate to occasional reflexes. The latter show so wide a range -of variability and adaptability as to prove that a given receptor -may bring any of a great variety of groups of motor neurones into -connection with itself; just as a given group of neurones may be played -upon by impulses from a great number of different receptors. We have -called it a tuning of the motor neurones. One metaphor is as good as -another. The physical process which in the brainless frog underlies the -preparation for discharging motor neurones in the spinal cord, on the -same side as the leg on which vinegar is placed, so long as that leg -is free, and on the opposite side, when that leg is fixed, is unknown. -We seem to catch a glimpse of a doubleness of action, receptors in -the muscles combining with receptors in the skin in determining the -paths along which impulses shall be reflected—the efficient muscles -sensitizing their own neurones to the tuning influence of impulses from -sounding cutaneous nerve-endings. But it is impossible to formulate a -working scheme in the present state of knowledge.</p> - -<p><b>Sense-Organs and Nerve-Centres.</b>—A vast amount of labour has -been devoted to the study of the external form of the central nervous -system and to unravelling its internal structure; to plotting out its -various groups of nerve-cells, to disentangling its innumerable tracts -of fibres. The surface of the brain and spinal cord has been mapped -and measured. Every millimetre of its substance has been cut into -sections on the micro-tome. Organs which, fifty years ago, appeared -too complicated for investigation have been described in the minutest -detail. An immense accumulation of data is available for purposes of -reference; yet anyone who submits the theory of the nervous system as -it is held at the present day to a general review must allow that the -results of anatomical research enter but little into its construction. -The reason for this is not far to seek. As knowledge has advanced, the -apparent, or rather the expected, complication of the system has given -place to ideas of unity and simplicity. Its external configuration and -the varied arrangement in “nuclei” of its nerve-cells may, without -impropriety, be described as accidental. The form of the body and the -<span class="pagenum"><a name="Page_332" id="Page_332">[Pg 332]</a></span> -consequent location of the clients of the nervous system determine -the disposition and degree of concentration of its various business -centres. It shows, when followed throughout the whole animal kingdom, -extreme variability of its constituent organs, with absolute uniformity -of plan. Indeed, from the physiological point of view the term “organ” -is scarce admissible. It implies diversity of function in too high a -degree. The several parts into which the central nervous system is -obviously divisible co-operate so intimately as to preclude us from -thinking of them as separate organs.</p> - -<p>If the citadel of the central nervous system is to be captured, all -lines of approach must be tried. Its outward form must be studied, -its minute structure examined with the microscope, its modifications -in various animals compared, its development followed, its reactions -to artificial stimuli tested, its pathological deficiencies and -vagaries watched. Yet, of all the means which have been made use of in -attempting to penetrate its secrets, the study of its history, by the -methods of comparative anatomy and embryology, has probably contributed -most to the development of sound ideas regarding the manner of its -working. The first differentiation visible in the blastoderm—the globe -of cells into which the ovum divides and out of which the embryo is -built—has relation to the formation of the nervous system. If the -earliest stages of its growth are followed, and the different phases -through which it passes are compared with the forms which it assumes -permanently in lower animals, the plan or type upon which it is -constructed shows up distinctly. Looking down the line to the earliest -vertebrata, we can discern clearly the form of nervous system possessed -by their prototype. Not that this “ideal ancestor” ever existed. -Experience teaches that it is unlikely that any animal that ever lived -was absolutely regular and symmetrical in all its parts; nevertheless, -the type can be presented in a perfectly regular scheme. The ideal -ancestor of the vertebrata was segmented, like a caterpillar or a worm. -Its mouth was not at the anterior extremity of the body, but two (or -more) segments behind it. Every segment bore a sense-organ (at one -period two sense-organs) on either side. Beneath each sense-organ there -was a clump of “grey matter.” Each segment also contained (although not -<span class="pagenum"><a name="Page_333" id="Page_333">[Pg 333]</a></span> -at the earliest epoch) two clumps of nerve-cells and neuropil in a -more central situation. These “ganglia” were united by longitudinal -and transverse commissures. They received the axons of the cells which -lay in the clumps beneath the sense-organs. They gave axons to various -muscles. Such is the type out of which the modern nervous system has -developed: two separate sense-organs and a complete nervous system for -each segment, the sense-organs connected with the ganglion of the same -side, the ganglia of the two sides bound together across the middle -line, and each row of sense-organs and each row of ganglia united -by longitudinal commissures into a chain. From the nervous system -as we see it now the majority of these segmental sense-organs have -disappeared; but the mode of formation of the cerebro-spinal ganglia -shows that they are the clumps of nerve-cells which lay beneath the -vanished organs. In the nose and the eye the grey matter retains -its original situation in the immediate vicinity of the receiving -epithelial cells—as the olfactory bulb and the deeper (anterior) -layers of the retina. The ganglia of the auditory nerve lie within the -bones of the ear. Spinal ganglia are close to the spinal cord. Auditory -and spinal ganglia contain only the cell-bodies of the first collecting -neurones (sensory nerves) together with certain curious bracketing -cells already referred to (<a href="#Page_324">p. 324</a>), all the other constituents -of the peripheral clumps of grey matter which are found in the olfactory bulb -and retina having been withdrawn from the spinal ganglia into the axis -of the brain and spinal cord.</p> - -<p>The sense-organs in front of the mouth have had from the beginning -an immense advantage over the others as observing-stations. Whereas -the body-organs collected information regarding the things with which -the animal came in contact, and consequently specialized in touch, -pressure, temperature, and, in the case of fishes, sensitiveness to -the chemical constitution of the medium in which the animal lived, -the head-organs specialized in responsiveness to forces acting from a -distance—particles suspended in air, vibrations of light, pulsations -of sound. Sensitiveness to touch, if it is to be useful, must be widely -distributed. The body-organs therefore broke into scattered groups of -sense-cells. Touch-spots are scattered all over the surface, although -they are set much closer together in the areas of skin which are -<span class="pagenum"><a name="Page_334" id="Page_334">[Pg 334]</a></span> -usually the first to come into contact with external objects than they -are elsewhere. The efficiency of the sense-organs of the head—nose, -eye, and ear—depended upon their remaining compact. Progress in -animal life, as we understand it—the rise from lower to higher -forms—has depended upon increasing integration of the body and -co-ordination of its functions. The nervous system is the agent which -has accomplished this unification. Each step in advance has depended -upon the provision of more nerve-tissue for the lacing together of -the various parts. We have seen already (<a href="#Page_329">p. 329</a>) how -intimate is the union of receptors and effectors of every kind via the spinal -cord and brain. The overwhelming predominance in the direction of -action of the nose, the eye, and the ear has led to the accumulation -in their vicinity of the ever-increasing grey matter. The cerebral -hemispheres, or “great brain,” are pouched outgrowths from the first -pair of ganglia directed towards the olfactory pits. The original eyes -bore a similar relation to the second pair of ganglia—the epithet -“original” implying that the eyes which we now use are not the organs -with which our prevertebrate ancestors saw. First one of the original -eyes disappeared, and then the other. The vestige of the second is -still to be seen in the “pineal body” which is found on the dorsal -side of the brain of every vertebrate animal—in a mammal deeply -hidden in the cleft between the cerebrum and cerebellum. In place of -the pineal eyes two other sense-organs have specialized as eyes. They -are constructed on a different plan, being, to put it shortly, pineal -eyes turned inside out; for whereas in the pineal eyes, as in most of -the eyes of invertebrate animals, the rods and cones, which are the -cells of the retina sensitive to light, are directed forwards towards -the lens, the rods and cones of our permanent eyes are directed away -from the source of light. This change has made it possible to provide -more abundantly for their nutrition, and hence a greater power of -discriminating separate points in space and of distinguishing colours -is conferred upon them. The substitution of other sense-organs for the -original eyes has complicated the pictures which are presented to us by -a brain in its successive stages of growth; but it does not prevent us -from recognizing the general plan. Probably the secondary eyes, like -their predecessors, belonged to a pre-oral segment. The sense-organs of -<span class="pagenum"><a name="Page_335" id="Page_335">[Pg 335]</a></span> -a segment behind the mouth developed into ears; and the ear was in -its earliest phases, and still is, something more than an organ of -hearing. Its semicircular canals give information of displacements -in space. Knowledge of the position of its body is, to a fish, of -far more importance than its ability to hear breakers on the rocks. -Three looped tunnels, opening at either end into a common chamber, are -hollowed in the bone which contains the ear (<a href="#I_411"><i>cf.</i> Fig. 38</a>). -Placed at right angles one to the other, they occupy all three dimensions of -space. Open a notebook until, one of its covers lying horizontally, -the other is vertical, and place a sheet of paper vertically against -the bottom of the pages. A curved line drawn on each of these three -surfaces will represent the three semicircular canals. Arrange another -notebook in the same way, and let the two rest on the table with the -two vertical covers inclining one to the other, anteriorly, at an -angle of 90 degrees. The six surfaces will be in the planes of the six -semicircular canals. Within each bony canal is a membranous tube, to -which nerves are distributed, filled with fluid. When the position of -the head is changed, the fluid within the membranous tubes slides on -their walls. It is left behind at the moment the movement commences. It -overtakes its receptacle when the movement stops. The stimulus received -by the nerve-endings is recognized as indicating an alteration in the -orientation of the head. If the movement of the fluid is violent, -as when one waltzes, the loss of the sense of position disconcerts -the brain to such an extent that giddiness results. For a time the -quiet assurance upon which so much depends, that one knows how the -body stands in relation to its surroundings, gives way to a chaos of -sensations. From the nature of the case, the information which the -semicircular canals afford relates to change. They give no help in -ascertaining the position of the head when it is at rest. This must be -the reason, although the connection is not very clear, for the waning -of the effect in consciousness when stimulation is prolonged, and also -for the very marked after-sensation. At the commencement of a voyage -attention may be unpleasantly attracted to the rolling of the ship. -After a few days it ceases to be noticeable; yet when the voyager, -the night after landing, wakes in the dark, he finds his bed-room as -unsteady as his cabin. Rising hurriedly, the attempt to adjust his -<span class="pagenum"><a name="Page_336" id="Page_336">[Pg 336]</a></span> -position to the heaving floor (we speak from personal experience) -may result in a heavy fall. Although this phenomenon must be classed -with other “after-sensations,” it is so prolonged as to suggest that -consciousness, having become accustomed to a world which causes a -backward and forward flow of endolymph, misinterprets the absence of -sensation as indicative of change.</p> - -<p>Taste is, practically, a special kind of smell. A fish’s olfactory -membrane, taste-buds, and chemical organs “of the lateral line” serve -the same sense, although, no doubt, they are applicable to the analysis -of different forms of matter in solution.</p> - -<p>Our ideal prevertebrate has now left its primitive undifferentiated -condition. In front of its mouth it bears organs with which it searches -the world. Close behind the mouth are its auditory and orienting -organs. The rest of the surface of the body is endowed with the -capacity of recognizing “taste,” temperature, and contact. Smell, -sight, and orientation determine the development of the brain.</p> - -<p>The cerebrum which has eventually become, as the seat of consciousness, -and hence the apparatus of mind, the dominant factor in the nervous -system, was in the first instance the part of the brain concerned with -the distribution to the muscles of impulses generated in olfactory -organs. There is scarcely any indication in a fish’s brain of the -representation in the cerebral hemispheres of any other sense, even -that of vision.</p> - -<p>A bird’s brain presents a striking contrast to the brain of a fish. -With the exception of the apteryx and other ground-birds of New Zealand, -all birds are apparently destitute of the sense of smell. Vision is the -sense upon which their activity depends. It has invaded the cerebrum, -converting it into an organ in which sensations of sight are worked -up into “mind-stuff.” The optic lobe connection is restricted to the -production of reflex actions in which vision is immediately followed by -movement.</p> - -<p>All the senses are represented in the great brains of mammals. -The cerebrum, which owes its existence to its connection with the -favourably-situated sense-organ of the nose, and grew in importance -when vision invaded it, has now taken in the senses of hearing, taste, -and touch. Only what may be termed in general visceral sense, and the -sense of orientation, are excluded. -<span class="pagenum"><a name="Page_337" id="Page_337">[Pg 337]</a></span></p> - -<p>Looking back to the starting-point, we see a segmented animal; its -segments of equal value; its nervous reactions unisegmental, although -linked in functional sequence. If it starts to walk, owing to -stimulation of one of its sense-organs, the impulse to walk spreads -from segment to segment. Comparing the latest product of evolution -with the earliest, we find that nervous tissue has concentrated -at the anterior end of the body. The double chain of ganglia, now -condensed into the axis of the brain and the spinal cord, still -contain all the effector neurones by which muscles are called into -action. Sensory nerves still arborize in the axis, providing the -mechanism for actuating motor neurones. But the vast majority of -intermediate or intercalated neurones have been attracted to the two -huge brain-masses—the cerebellum and cerebrum. In the former all -sensations (not conscious) connected with tone, position, orientation -and equilibrium are worked into appropriate impulses for the regulation -of the muscular system. In the latter all sensations which convey -information regarding the relation of the environment, including the -body, to the ego—the <i>not-me</i> to the <i>me</i>—are transformed -into motor discharges which set a-going the movements (and the thoughts) by means -of which the purposes of life are fulfilled; for in the cortex of the -great brain alone is the passage of nerve-currents accompanied by -consciousness. Concentration of nerve-tissue allows of the combination -of sensations. It also facilitates the no less important effect -of mutual influence, interference. Sensations are suppressed, and -therefore the multitude of reactions to which they would give rise are -inhibited, in the interests of restricted and sustained movement or thought.</p> - -<p><b>The Cerebellum.</b>—Sharks and other swift-swimming fishes have -large, deeply fissured cerebella, for the cerebellum is the part of the -brain which has gathered into itself most of the grey matter associated -with balancing, attitude, posture. The cerebellum is in birds large -and deeply folded. Developed from the ganglia to which the auditory -nerve distributes impulses from the semicircular canals, it has -established connections with all the other nervous tissues concerned -with sensations of position, strain, or pressure, including the eyes, -which afford information regarding the position of our limbs relatively -<span class="pagenum"><a name="Page_338" id="Page_338">[Pg 338]</a></span> -to the trunk, and of the whole body relatively to external objects. -Morphologically it is a median growth. The adverb is one of those -qualifying terms, convenient in science, which direct thought without -confining it. As used above, it implies that anyone who passes before -his mind the cerebella of all animals from fishes to Man, and in all -stages of growth, from their earliest appearance in the embryo to -their condition in the adult, sees the organ as a median prominence -surmounting the medulla oblongata. The bulgings of its sides which, -in human anatomy, are termed hemispheres, do not disturb its central, -unpaired plan of structure. It has, it is true, a lateral appendage -on either side (the combined flocculus and paraflocculus of mammalian -anatomy), but this lobe, although of great historic interest, is -so small, as compared with the median growth, as not to affect our -general conception of the form of the organ. By transverse fissures the -cerebellum is divided into a series of lobes.</p> - -<p>In appearance the cerebellum varies greatly in the different classes -and orders of Vertebrata. Yet underlying this variety there is marked -unity of plan. A sagittal section of the organ of a shark, of a bird, -of a kangaroo, of a dog, of a whale, of Man, shows that it is divided, -from before backwards, into the same number of lobes in animals -occupying every position from the bottom to the top of the vertebrate -scale. A very little effort to grasp the significance of this mystic -number, nine, convinces one of the hopelessness of any attempt to -correlate the form of the cerebellum with the muscular development or -sensory endowments of vertebrates as a sub-kingdom. It is the same for -animals with limbs and animals without; animals with well-developed -noses or eyes, and animals destitute of one or other of these -sense-organs. This uniformity is extremely significant, when contrasted -with the wide differences exhibited by the cerebral hemispheres. -It shows that, unlike the great brain which mediates between the -several senses and the muscular system, the little brain is concerned -in bringing about adjustments to the environment which are equally -important to all animals, no matter how far they may depart from the -common type. The cerebellum is crossed by deep fissures, dividing it -into narrow convolutions or folia. The folia are grouped in nine lobes. -If the reader has secured as an illustration the brain of a sheep, he -<span class="pagenum"><a name="Page_339" id="Page_339">[Pg 339]</a></span> -will notice that the lateral regions of the cerebellum present a -complicated appearance owing to the contortion of the folia, which -results from the unequal development on its sides of the several lobes. -In its total size the cerebellum keeps step with the cerebrum, the -right side of one organ being associated with the left side of the other.</p> - -<div class="figcenter"> - <a id="I_339" name="I_339"> </a> - <img src="images/i_p339.jpg" alt="" width="600" height="488" /> - <p class="center"><span class="smcap">Fig. 23.</span>—<span class="smcap">Vertical - Section of the Cortex of the Cerebellum,<br /> cut Parallel with the Long - Axis of a Folium.</span></p> - <div class="blockquot2"> - <p class="neg-indent space-below2">A shows three cells of Purkinje, their espalier systems of - dendrites being seen in profile. A “mossy fibre” enters the granular layer from - the white matter. About a dozen of the granules are shown, each with - four or five dendrites and a single axon. The axon bifurcates in the - molecular layer, its two branches running for a considerable distance - to left and right along the folium. B shows the other nervous elements - which are found in the cortex: a cell of Golgi with a ramified axon, - a climbing fibre, a basket-cell, of which the axon divides into four - branches, and a small stellate cell.</p> -</div></div> - -<p>The grey matter which covers the surface of the cerebellum, its cortex, -is singularly regular in microscopic pattern (<a href="#I_339">Fig. 23</a>). -It is divided into three sheets: superficially, the molecular layer in which -the dendrites of the cells of Purkinje branch; beneath this, the thin layer -in which are situate the cell-bodies of these neurones; thirdly, the -layer of small cells, or granules. Cells of Purkinje and granules have -<span class="pagenum"><a name="Page_340" id="Page_340">[Pg 340]</a></span> -been already described (<a href="#Page_303">p. 303</a>). To these must be added -the stellate, bracketing cells of the molecular layer, the axons of which divide -to form baskets about a number of Purkinje-cells, and the cells of -Golgi of the granular layer. These last are comparatively large cells, -which have thornless dendrites, and axons which branch repeatedly -in the granular layer, without passing into the white matter which -underlies the cortex. Two kinds of nerve-fibre bring impulses to the -cortex: (1) “Mossy” fibres, which bear rosettes of filaments which -distribute impulses to the granules; and (2) “climbing” fibres or -“tendril” fibres, which, passing through the granular layer, cling like -ivy to the trunk and principal boughs of the dendritic processes of -Purkinje-cells. The axons of the cells of Purkinje undoubtedly carry -impulses away from the cortex, but their destination is not certainly known.</p> - -<p>The uniformity of structure of the cerebellum suggests that it “acts -as a whole.” Anatomy gives no warrant for the expectation that work of -different kinds is done by its several lobes. Its simplicity leads one -to hope that its mechanism may some day be understood; but at present -there are so many gaps in our knowledge that it is difficult, perhaps -hardly profitable, to attempt to string together the few anatomical -facts of which we are sure.</p> - -<p>By means of tracts of afferent fibres the cerebellum has a very -extensive connection with the grey matter of the cerebro-spinal axis -(including the optic thalamus) into which sensory impulses of all kinds -are poured. Experimental results indicate that the organ distributes -impulses to the whole length of the cerebro-spinal axis, from the level -of the neurones which govern the muscles which move the eyes to its far -hinder end. No nerve-roots enter it. Its afferent fibres are the axons -of cell-bodies which lie in the posterior horns of the grey matter of -the spinal cord and in the corresponding grey matter of the axis of the -brain, especially that part related to the nerve from the semicircular -canals. Another set of afferent fibres lies at the periphery of the -spinal cord, forming one of the best defined of the spinal tracts. It -is also one of the oldest, being found in the same situation in all -vertebrate animals. Its fibres, which are exceptionally large, are the -axons of cells which form a very definite column—the “vesicular column -<span class="pagenum"><a name="Page_341" id="Page_341">[Pg 341]</a></span> -of Clarke”—on the median side of the posterior horn. Further than this -we cannot go. We are ignorant of the nature of the sensory impressions -collected by the cells of Clarke. The cerebellum also receives through -its middle peduncle the axons of cells which lie in the pons Varolii on -the opposite side; which cells are discharged by impulses descending -from the cortex of the great brain. It is not improbable that it gives -to the great brain as many fibres as it receives from it.</p> - -<p>If we had no experimental evidence as to the part which the cerebellum -plays in the harmonious working of the whole nervous system, we -should infer from its structure and connections that it is somewhat -mechanical, a co-ordinator of the activities of other parts rather -than in itself a functionally independent organ. Pathological and -physiological observations very definitely justify this conclusion. -They show that the cerebellum is not essential to life. It may be -completely destroyed by disease or removed by operation without robbing -the individual of any single function or capacity. Disease of the -cerebellum does not diminish the patient’s sensitiveness to every kind -of stimulus, nor does it deprive him of the use of any single muscle; -but it reduces him to the condition of a person who in gait, but not -in mind, is habitually drunk. When he walks he staggers from side to -side; when he stretches out his hand it trembles. His movements are -jerky; his head shakes, his eyes oscillate; he suffers from a feeling -of giddiness; his speech comes haltingly. Cerebellar ataxia, which is a -rare disease, resembles in many respects the much commoner “locomotor -ataxia” produced by disease of the spinal ganglia and the parts of -the cord connected with posterior roots; but careful analysis of the -symptoms shows that they are due, not to want of the sensations which -guide movements, but to inability to regulate the force of muscular -contractions. A man suffering from locomotor ataxia falls when he -closes his eyes, because, not being able to feel with his feet, he is -dependent upon vision for information as to his attitude. When the -cerebellum is diseased, the patient is no less unsteady with his eyes -open than he is with them closed.</p> - -<p>The results of cerebellar disease or injury bring home to us the fact -that a nice adjustment of movements is needed to maintain equilibrium. -<span class="pagenum"><a name="Page_342" id="Page_342">[Pg 342]</a></span> -A dog from which the cerebellum has been removed retains all its -natural enterprise, all its instincts, all its emotions; but every -action which requires it to maintain its centre of gravity in an -unstable position gives it trouble. Placed in water so that its -body is supported, it swims almost as well as a normal dog. It is, -however, easy to lay too much stress upon the balancing function of the -cerebellum. The disturbance of this function attracts our attention; -yet it is probably but the indirect result of the suppression of -activities of a more widespread character. No animal ventures such -liberties with its centre of gravity as the biped Man accomplishes, -without thinking, every time that he descends a flight of stairs. Yet -the cerebellum of the limbless whale, that lives in a medium which -decentralizes its gravity, so to speak, bears the same proportional -relation to the rest of the nervous system as that of Man. Strangely -enough, it is the only cerebellum in the animal kingdom which so -closely resembles Man’s that it might be passed off as belonging to -a human giant; another reminder of the difficulty of deducing the -functions of the several parts of the organ from a study of their -relative development. What have a man and a whale in common which -determines the identity in form of their cerebella? How has it come -about that two cerebra as widely unlike as a man’s and a whale’s should -be associated with a common form of cerebellum?</p> - -<p>If we apply to grey matter the distinction between sensory and motor -nerve-tissue—having no exact terminology, it is difficult to avoid -these metaphorical expressions—the cerebellum is essentially a sensory -development. It grows from the very margin of the infolding groove, -which, when closed, becomes the central canal of the brain and spinal -cord, its elements being marshalled in intimate association with -sensory root-fibres. Its millions of loops formed by the axons of -granules and the collecting processes of Purkinje-cells, are by-paths -which tap the conductors of sensory impulses. From some—those, for -example, which originate in the muscles and tendons, and in the -semicircular canals—more of the impulse is diverted to the cerebellum, -from others less. The organ has no motor functions. It does not -discharge neurones which control skeletal muscles, or plain muscle, or -glands. Yet it influences the passage of impulses through sensori-motor -<span class="pagenum"><a name="Page_343" id="Page_343">[Pg 343]</a></span> -chains, and apparently its influence is universal. It regulates tone, -reflex action, voluntary action. There is no part of the nervous system -over which its control is not felt. By its action on the apparatus -which binds the infinity of receptors which the body contains to its -muscle-fibres and other effectors, it unifies the body. The cerebrum, -as we shall see, is the organ which unifies the personality. In the -progress of evolution two functions which were originally combined -have, for convenience of concentration, been divorced. The great brain -has been set free from the more mechanical part of the work. That it -can perform the functions of the cerebellum as well as its own is -proved in cases of congenital deficiency of that organ. In several -instances malformation, amounting to a very considerable reduction -in the size of the cerebellum, was not detected until after death, -there being no symptoms of a sufficiently pronounced character to call -attention to it during life.</p> - -<p><b>The Cerebrum.</b>—All observations made on the great brain prior -to 1870 showed it as absolutely inexcitable. Surgeons and physiologists -agreed that cutting, burning, passing electric currents through its -substance, neither yielded evidence of sensation nor movement of any -part of the body. Concerning its structure little was known beyond the -fact that whereas the grey matter, or cortex, which covers its surface -contains nerve-cells, only fibres are to be found in the white matter -which constitutes the greater part of its bulk. It seemed a hopeless -task to attempt to make anything out of a mass of tissue so uniform in -constitution and so irresponsive to experiment. Removing portions of it -appeared to cause a general dulling of the intellect without loss of -any particular mental quality. Physiologists, therefore, spoke of the -cerebrum as “functioning as a whole.” Phrenologists, having classified -the various phases of mental activity as “faculties,” discovered -“bumps” on the surface of the skull which they correlated with the -possession of the several faculties in a marked degree. They parcelled -out the brain in organs concerned with different kinds of thought; but -their localization of function was anatomically as baseless as their -classification of the various aspects of mind, viewed as a system of -philosophy, was absurd. In 1870 it was announced that electrical -<span class="pagenum"><a name="Page_344" id="Page_344">[Pg 344]</a></span> -stimulation of certain areas of the cortex of the cerebrum of an -animal under the influence of an anæsthetic, and therefore incapable -of voluntary action, induces definite movements. Although the surgical -applications of this discovery have proved immensely important, -its physiological value, as affording a method of investigating -the functions of the brain, is extremely small. Yet the discovery -gave an impetus to the further study of the cortex, which has been -rewarded with many exact results. By the discovery of its excitability -to electric currents it was proved that the whole cortex has not -exactly the same work to do, or—perhaps this is the safer form of -statement—does not do its work in exactly the same way. As soon as -it was known that it is divisible into areas differing in function, many -methods by which the delimitation of the areas might be attempted were -devised. The converging efforts made during the past forty years by -comparative anatomists, histologists, physiologists, pathologists, and -physicians, have resulted in the acquisition of an accurate, if very -restricted, understanding of the construction and mode of working of -the apparatus of thought. Of some of the new data the psychologist is -able to make use; but so far as the physiologist is concerned, it is -the vehicle of mind which is the subject of study, not its contents.</p> - -<p>A new subject has been created since 1870. There is therefore nothing -to be gained, so far as our present purpose is concerned, from the -consideration of views which were current before that date; and since, -as must always occur when a science is rapidly advancing, observations -which logically should have been the first to be made were not thought -of until it became necessary to devise methods of checking results -obtained in other ways, we will consider the various sources of our -information without regard to the chronological order in which they -were opened up.</p> - -<p>The cerebral hemisphere contains two large central masses of grey -matter, the nucleus caudatus and the nucleus lenticularis, often -described as a single structure under the name “corpus striatum.” Their -functions are unknown. The nerve-fibres which connect the cerebral -hemispheres with the rest of the central nervous system form two thick -limbs or crura on the under side of the brain. Each crus turns upwards -<span class="pagenum"><a name="Page_345" id="Page_345">[Pg 345]</a></span> -into its hemisphere, between the nucleus caudatus and optic thalamus -(the latter belongs to the “between-brain”) on the inner side, and -the nucleus lenticularis on the outer. In this passage the compact -crus, which is somewhat flattened, is termed the “internal capsule.” -Immediately above the three grey masses the internal capsule disperses -as a fountain of fibres which go to all parts of the cortex. Mingled -with these radiating fibres are vast numbers of others, proper to the -hemispheres, which run tangentially. Some, crossing the median plane, -as the corpus callosum, bind the two hemispheres together. Others form -tracts which can be followed from one end or pole of the hemisphere to -the other. Groups of fibres, dipping but little below the cortex, unite -nearly adjacent spots or neighbouring convolutions.</p> - -<p>The folding of the cortex beneath fissures is due to the necessity -of disposing of a certain bulk of grey matter without increasing its -thickness beyond the proper limit. Since the superficial area of a -sphere varies as the square of its radius, whereas its capacity varies -as the cube, it is possible for a fixed relation to be maintained -between the amount of cortex and the amount of white matter in the -brain, only by the folds increasing in depth as the size of the brain -increases. Fissuring is a response to a mechanical need. This does not -imply, however, that the lines along which it takes place are devoid of -morphological meaning. The similarity in pattern of the convolutions -and fissures in various animals, and the regular progress of their -development in each individual, prove the contrary. If they are not -absolutely trustworthy as boundaries of areas of separate function—and -further evidence will be needed before a decision can be pronounced -upon this disputed question—they are in the main satisfactory as -landmarks.</p> - -<p>As the nervous system grows, the axons of its neurones acquire their -fatty (myelin) sheaths in the order in which they come into functional -activity. The passage through them of impulses is the stimulus -which leads to the deposition of fat. The study of the progress of -myelination enabled the anatomist Flechsig to ascertain the situation -within the brain of the tracts of fibres related to the several senses, -and hence the traffic of the areas of the cortex to which they go. -Glistening white streaks appear successively in the pulpy yellowish-pink -<span class="pagenum"><a name="Page_346" id="Page_346">[Pg 346]</a></span> -substance of the interior of the brain. At the time of birth all the -fibres which enter or leave the cerebral hemispheres have acquired -their myelin sheaths. In the baby’s brain the sense-organs have -established all their connections with the cortex. No new fibres will -appear in the nerves of the eye, the ear, or the other sense-organs, -nor will their end-stations in the cortex be further multiplied. -(The use of the expression “end-stations” is legitimate so far as -sensations are concerned; notwithstanding that all sensory impulses -are retransmitted by neurones in the cerebro-spinal axis.) But the -cortex is very far from having finished its growth. It contains a large -amount of embryonic tissue, which gradually spreads outwards from the -developed areas into the surrounding unoccupied zones. The taking up of -new territory, and the consequent increase in the size of the brain, is -continued into adult life. The study of progressive myelination enabled -Flechsig to divide the cortex into “sensory centres,” and intervening -“association-zones”; although, doubtless, the difference in function -between the portions which receive sensations direct and the portions -in which the products of sensation are worked up is one of degree, and -not of kind.</p> - -<div class="figcenter"> - <a id="I_347" name="I_347"> </a> - <img src="images/i_p347.jpg" alt="" width="500" height="660" /> - <p class="center">Fig. 24.—<span class="smcap">Vertical Sections of the - Cortex of the Cerebrum—<br />A, of the Visual Sensory; B, of the Visual - Association Area.</span></p> - <div class="blockquot2"> - <p class="neg-indent space-below2">Between the two sections are shown the principal types of cell, - at the levels at which they are severally found: <i>a</i>, small pyramid; <i>b</i>, - medium-sized pyramid; <i>c</i>, large pyramid. The size of a pyramid is an - indication of the distance to which its axon extends before branching; - the longer its traject, the more widespread, it would seem, is its - terminal arborization. The axon of <i>c</i>, one of the very large pyramids - found in this association area, passes to the front of the cerebrum, - where it breaks up in an association area of the tactual sense of - the hand, or of sensations concerned with the regulation of gait, or - in a centre for movements of the eyeball. <i>d</i>, a tangential cell of - the surface; <i>e</i>, a Golgi cell with ramified axon; <i>f</i>, a polymorph - cell, with its axon directed towards the surface. In sensory areas, - tangential fibres and granules are more numerous; in association areas, - small and medium-sized pyramids.</p> -</div></div> - -<p><span class="pagenum"><a name="Page_347" id="Page_347">[Pg 347]</a></span> -The structure of the cortex is not quite the same in sensory and -association areas; but it is everywhere so far from showing the -diagrammatic simplicity which characterizes the cortex of the -cerebellum as to make it difficult to summarize the modifications which -distinguish its various regions. To a considerable extent its elements -shade one into the other, differing in size and in orientation rather -than in form. Commonly it is described as divisible into five layers: -(1) A thin superficial layer, containing cells of various forms and -fibres derived from the cells of the deeper strata. Some of the cells -are pluripolar, possessing several axons which run parallel with the -surface. Their destination is unknown. They do not appear to form -baskets like the cells of the molecular layer of the cerebellum. The -dendrites of pyramidal cells extend into this layer. (2) The layer -of small pyramids; cells with a branching apical process, root-like -dendrites from the basal angles of the pyramid, and an axon which sinks -into the white matter. (3) Granules. Carmine or other nuclear stains -show that small cells are present in very large numbers, especially in -<span class="pagenum"><a name="Page_348" id="Page_348">[Pg 348]</a></span> -the sensory areas; but since they are not, like the granules of the -cerebellum, coloured by the chrome-silver method, their form and the -disposition of their axons are unknown. (4) Large pyramids exactly -similar in form to the small ones. Their apical processes are very -thorny. Their axons give off several collaterals. Pyramids are the -most conspicuous elements in the cortex. Properly speaking, they do -not occur in layers, but are scattered throughout its whole thickness, -although their cell-bodies are not seen in either its most superficial -or its deepest strata. The largest are those of which the axons -either descend into the spinal cord or pass to a very distant region -of the cortex. They are found singly or in small clusters in the -deeper levels. (5) Polymorphous cells, some of them pyramids lying on -their sides, or even directing their axons towards the surface; some -fusiform or irregular cells; some Golgi-cells (<a href="#Page_340">p. 340</a>). -The axons of pyramids enter the white matter, and many fibres from the white -matter radiate towards the surface between the pyramids; but the way in which -afferent, sensory fibres are connected with the collecting processes, -dendrites, of the pyramids is not known. We have already referred to -thorns, and to the possible nerve-net (<a href="#Page_301">p. 301</a>). Sheets -of tangential fibres also occur in the cortex. A particularly distinct sheet -divides the granules in the visual cortex into two strata. In sections of this -region the sheet of fibres appears as a white line, distinctly visible -without a lens.</p> - -<p>The limits of the several areas can be determined by examining the -structure of the cortex; but the individual peculiarities of the -various regions are not so marked as to indicate that they have -different kinds of work to do; if by kinds of work we wish to imply -that one part is “sensory,” another “motor,” a third concerned with -“intellectual processes.” On the contrary, its relative uniformity -shows unmistakably that all parts are engaged in the same work. -Nevertheless, certain broad conclusions can be drawn with regard to -the form of the neurones more immediately concerned with sensation, -with motion—that is to say, with the discharge to the grey matter -of the cerebro-spinal axis of the impulses which call its neurones -into activity—and with the secondary processes, called collectively -“association,” which occur within the cortex. Granules, as everywhere -<span class="pagenum"><a name="Page_349" id="Page_349">[Pg 349]</a></span> -throughout the nervous system, are receivers and distributors of -sensory impulses; although a study of the cerebral cortex does -not justify the conclusion that they are necessary links in its -sensori-motor arcs. Large pyramids are occupied with the nutrition -of fibres which have a long traject through the system. Hence they -are “motor.” They constitute a marked feature of the area which -is susceptible to stimulation. They occur also in the visual area -and elsewhere. Small pyramids are associational; that is to say, -their axons do not leave the cerebral hemispheres. They distribute -impulses from sensory areas to association-zones, and from one part -of an association-zone to another. The layer of polymorphous cells -is relatively thicker in animals in which the cortex of the brain -exercises less control over action than in animals in which the cortex -is supreme—in a rabbit thicker than in a monkey; in a monkey thicker -than in Man. This layer is therefore said to be concerned with the -lower functions of the cortex, whatever this expression may mean. Since -the relative abundance of small pyramids is a test of the supremacy -of the cortex, we may speak of them vaguely as concerned with its -higher functions. But a surer test of the capacity of the cortex for -the elaboration of the raw materials of thought which sensory nerves -deliver to it is the relative abundance of the tissue which intervenes -between its cells. The number of cell-bodies to be counted in a square -millimetre of a section of a given thickness is smaller in Man than in -a monkey, in a monkey than in a dog, and in a dog than in a rabbit.</p> - -<p>A comparison of the brains of various mammals in which particular -sense-organs are either deficient or exceptionally well developed -affords the clearest proof of the localization of sensory areas. This, -if it were possible to make satisfactory measurements, would be by -far the best class of evidence as to the part played by the several -senses in an animal’s mental life. Unfortunately, measurement appears -to be out of the question; but a glance at a rabbit’s brain, placed by -the side of a mole’s, shows that vision is localized in the occipital -region. All marine mammals are destitute of the sense of smell; the -brain of a dog, compared with that of a porpoise or a whale, shows -that the sphenoidal region (<a href="#I_352"><i>cf.</i> Fig. 25</a>) is associated -with this sense. The brain of an otter exhibits very clearly the area into which -<span class="pagenum"><a name="Page_350" id="Page_350">[Pg 350]</a></span> -impulses arising in the nerve-endings of the sensory bristles of the -cheek are poured.</p> - -<p>“Nihil est in intellectu quod non prius in sensu fuerit.” The organ -of the intellect is the cortex of the great brain, a sheet of grey -matter which has developed in connection with the various sense-organs. -The cerebral hemisphere of an infant is merely an extension of the -nerve-tissue associated with its sense-organs. Such it remains in a -microcephalous idiot. In the lower animals its capacity of growth after -birth is very small. But in a normal child the inflow of impressions -through sense-organs, the experience acquired regarding itself and its -surroundings, education, whether accidental or directed, causes the -extension of nerve-tissue from the sensory areas into the expansible -intervening zones.</p> - -<p>There is still some uncertainty as to the nature of the sensations -received in the excitable area. They may be termed “kinæsthetic” -(sensations connected with movement) without more exact definition. -Some physiologists consider that tactile sensations, as well as the -obscure sensations, originated in the nerve-endings in muscles, around -tendons, or on joint-surfaces, are distributed to the areas, which, -when stimulated, are shown to represent fingers, hand, arm, and other -parts of the body. Others have sought, though with doubtful success, -for a tactile area, independent of the kinæsthetic centres. When first -discovered, these centres were termed “motor,” and still this term -may be retained, on the understanding that it does not imply that the -exchanges which occur in the kinæsthetic centres are of a different -nature to those which take place elsewhere. The region which they -occupy has become the motor area of the cortex because voluntary -movement is possible only under the guidance of sensations of movement. -A sound or a retinal image may prompt the movement; but the part of the -temporal region, or of the occipital region in which the sound-movement -exchange or sight-movement exchange occurs must act through the motor -area by opening kinæsthetic-movement arcs. Destruction of a part of -the kinæsthetic cortex causes in Man and the higher apes permanent -paralysis for the movements directed by the spot destroyed. In lower -animals the definition of the movement centres is vague, and their -<span class="pagenum"><a name="Page_351" id="Page_351">[Pg 351]</a></span> -removal produces only temporary results. Their mastery over the muscles -is less complete than in the higher apes and Man.</p> - -<p>Practically nothing is known with regard to localization of function in -the association-zones, with the exception of the localization of the -centres for words; but this exception is so remarkable as to suggest -that if there were any other faculties, interference with which caused -defects as distinct as those which characterize disorders of speech, -it would be found that the association-zones are made up of definite -centres. As the evidence stands with regard to the broadest continental -divisions, we can merely state that it points, although not very -clearly, to the connection of the frontal zone, the region in front of -the kinæsthetic area, with ideas of personality, of other zones with -ideas of environment. Injury to the frontal region has in certain cases -resulted in the victim’s losing his knowledge of himself, his name, and -his relation to his family. On the other hand, gunshot wounds and other -definite injuries have in a large number of cases destroyed portions -of the cortex behind the forehead without causing any recognizable -intellectual change. It is quite certain that this part of the brain -performs no functions which are of a different, or, as it is often -called, higher order than those of other association-zones. It has been -stated that disease of the zone which intervenes between the visual -and auditory areas is more likely to cause hallucinations, disease of -the frontal zone delusions. A patient fancies in the one case that -he sees things that are not there, or hears voices when no one is -speaking; in the other case he imagines himself a king; but evidence -connecting localized disease with mental derangement is very scanty. -The functional disturbance which causes lunacy is usually of a general -character; or, if local to begin with, it becomes general before the -death of the patient makes possible the examination of his brain. -<span class="pagenum"><a name="Page_352" id="Page_352">[Pg 352]</a></span></p> - -<div class="figcenter"> - <a id="I_352" name="I_352"> </a> - <img src="images/i_p352.jpg" alt="" width="600" height="499" /> - <p class="center"><span class="smcap">Fig. 25.</span>—<span class="smcap">The - Surface of the Left Cerebral Hemisphere, Cerebellum,<br /> and Medulla Oblongata.</span></p> - <div class="blockquot2"> - <p class="neg-indent space-below2">Sensory areas are enclosed by broken lines; certain centres - in the association-zones are marked by dots. The sensory area of smell is on - the inner aspect of the brain; so also is the area of vision which - borders the calcarine and retrocalcarine fissures, and only rarely - extends on to the external surface, as shown in the diagram. The - sensory area of hearing is largely hidden within the fossa of Sylvius, - the opening into which is indicated by the dark line above it. The - kinæsthetic-sensory areas for the various muscles of the body occupy - the territory between the dotted line in front and the bottom of the - fissure of Rolando behind. They do not extend on to the posterior wall - of this fissure. It is impossible at present to define the boundaries - of any of the centres in the association-zones.</p> -</div></div> - -<p>Derangements of <b>speech</b> throw a flood of light upon the -organization and manner of working of the association-zones; and, owing -to the accident of the continuation of the line of the carotid artery -by the middle cerebral artery, which supplies the speech centre, there -is no other spot in the cortex so likely to be thrown out of gear. A -little plasma coagulates on one of the cardiac valves, or about an -atheromatous spot in the aorta. Detached by the blood-stream, it is -shot into one of the branches of the middle cerebral artery, which it -plugs, causing apoplexy. A larger or smaller number of muscles on the -opposite side of the body are paralysed. If the plugging occurs on the -left side of the brain, it is accompanied by aphasia; but only if it -occurs on the left side, owing to the fact—perhaps the most remarkable -in connection with the localization of speech—that only on the left -side is the cortex trained to utter words. In course of time the -patient may recover the power of speaking, but not until he has, with -almost as much labour as in childhood, educated the right side to do -<span class="pagenum"><a name="Page_353" id="Page_353">[Pg 353]</a></span> -the work. There are four speech-centres, quite distinct one from -the other. Near the visual area is the centre for seeing words, or -rather the centre for seeing the meaning of words. If this centre be -diseased, a written word is merely a crooked line. Behind the auditory -area is the centre for recognizing the meaning of words heard. If it -is interfered with, the most endearing or commanding phrases produce -no more impression on the hearer than a bird’s song. In front of the -hand-area—its localization is less certain than that of the other -three—is the centre for writing. In it are associated words heard -or seen, with the movements necessary for the making of letters. In -the centre first referred to, as being the one most often thrown out -of gear, which lies in front of the area for the mouth and throat, -words heard or seen are translated into movements of the parts which -give them sound. No other actions illustrate so clearly the “law of -neural habit.” In the infant’s brain sounds of words are distinguished -from other sounds. They are associated with the objects which they -name. Movements of the mouth and throat, made at first ineffectively, -blunderingly, succeed after a time in securing the thing of which -they sound the name to the child’s satisfaction. Thus, two centres -are gradually established in his mind. Sounds and ideas of things are -associated in the one; words and ideas of the movements necessary to -their pronunciation in the other. Either of the four speech-centres -may be placed out of action without the others suffering. A man may -be able to write without being able to read what he has written. He -may read aloud, although apparently deaf to speech. He may be unable -to write or unable to speak, although understanding what he reads or -hears. Aphasia, when partial, illustrates still further the law of -neural habit. The ability to remember nouns, especially proper names, -is most easily lost. Few are the people who, as age advances, do not -suffer from this failing. Even the names which are most familiar elude -the memory. From one point of view this is strange. Nouns-substantive -are the words first learned. Of all words they have the most definite -objective association. But it is just their definiteness which makes -them difficult of approach when the apparatus of mind is working badly. -There are so few paths by which they can be reached. Their mental -<span class="pagenum"><a name="Page_354" id="Page_354">[Pg 354]</a></span> -associations are limited. A patient who is recovering from the effects -of a lesion which has rendered him partially aphasic may be able to -recall adjectives when he cannot recall nouns. He may say, “Give me -the black,” when he wants ink, and “Give me the white,” when he needs -paper. Or he may retain control of verbs. “Where is the—— what -I put on—what I think with?” may be the circumlocution for hat.</p> - -<p>Psychologists explain the voluntary production of a movement as the -setting flowing of a sensori-motor current. Everyone agrees that it -is impossible to think of the impulses which produce movement as -originating without sensory antecedents. Hence psychologists picture -the nerve-current as originating on the sensory side. Kinæsthetic -images of the sensations which will result from the movement are -described as being called up in the mind by the agitation of the -part of the brain which, by association, is linked with the neurones -which discharge impulses to the appropriate spots in the grey -matter of the spinal cord. The idea of movement flows over to the -muscles. But this conception of the relation of mind to body assumes -too much. It postulates an existent mind in which the images of -movement-sensations—the memories, that is to say, of the sensations -which previously accompanied movement—are stored. The study of the -apparatus of mind does not warrant this assumption of an existent -mind. It finds nothing in the nervous system but apparatus. There is -no mind existent in the brain during sleep. It would appear to be -sufficient to describe the origination of a voluntary movement as -the opening of the channels which convey the afferent impulses which -are ceaselessly pouring into grey matter from nerve-endings in and -about muscles into efferent channels. Our conception of the number -of sensations which reach the realm of consciousness is ludicrously -restricted by our inability to pay attention to more than one sensation -at a time—a restriction, it is needless to remark, which is imperative -in the interests of consistency of behaviour. Two personalities paying -attention to different sequences of sensations would give incompatible -orders. One would command the muscles to cause the body to recline; -the other would direct them to make it stand up. From myriads of -sense-organs impulses are continuously rippling through the cortex -<span class="pagenum"><a name="Page_355" id="Page_355">[Pg 355]</a></span> -of the brain. The term “impulse” is too heavily weighted by its -association with the idea of currents which are strong enough to prove -effective without the intervention of consciousness; but no other is -available. They ring the bell of consciousness, however little may be -the attention which their summons secures. Attention cannot be directed -to two things simultaneously. It moves, as it were, on a succession of -points. On some it rests longer than on others. They make an impression -which can be recalled; the rest being passed by so rapidly that -they are not remembered, it is as if they had never been perceived. -They blend, as a succession of moving lights blend, in producing a -background to consciousness. Not recognizing their separateness, we -interpret them as fused. A good deal of misleading metaphor has been -used, as it seems to the writer, in accounting for the effect upon -the mind of impressions which make but a weak demand upon attention. -They are spoken of as “marginal” perceptions, from the analogy of -the ineffectiveness of impulses generated at the periphery of the -retina, as compared with those which give rise to direct vision. A -“subconscious,” or even “unconscious,” self is evoked. The self cannot -be less than conscious. Self is the passage of attention from sensation -to sensation. Its relation to the not-self is temporal, not spatial.</p> - -<p>Every sensation which is called up into consciousness, though it -occupy attention for the shortest possible time, tends to give rise to -movement—is, indeed, in its very nature an impulse flowing through -a sensori-motor arc. The circuit for the voluntary execution of a -movement is represented as flowing through kinæsthetic-movement arcs. -This may be necessary for volitional actions, but it is not essential -for reflex actions. A spinal frog will remove an irritant from its back -with its hind-leg, after the roots of all the afferent nerves of the -hind-leg have been cut. In this case the reflex is direct, from injured -skin to muscles of the leg. It is not double—muscular sensations from -the leg, liberated into efferent leg-muscle-nerves by skin-sensations -originated simultaneously in a part anterior to the segments in which -the roots have been cut.</p> - -<p>The unit of sensation to which attention can be directed has yet -to be defined. Like sensations—sensations which are correlated in -<span class="pagenum"><a name="Page_356" id="Page_356">[Pg 356]</a></span> -experience, that is to say—seem to fuse in consciousness. A sequence -of similar sensations appeals to attention. Unlike sensations interfere -one with another. The apparent fusion is not a composite neural effect -which consciousness views as a single unit. Not even identical images -simultaneously formed on the two retinæ produce a superimposed effect -upon a particular spot in the brain. Different brain-spots receive the -two separate images which the mind views as one. This raises a doubt -as to whether perceptions are, properly speaking, fused. It suggests -that they are separate points upon which attention rests in rapid -succession; but such a hypothesis does not preclude the conception -of the production of a composite sensation by impulses coming -simultaneously from the same sense-organ—<i>e.g.</i>, a unified -neural effect as the result of several musical tones.</p> - -<p>Every neural agitation which attracts attention has an effect upon -the growth of the nerve-strands in which it occurs. <b>Memory</b> is -not an existent. It is the repassage of the same strands. There is -no such <i>thing</i> as memory. It is the neural apparatus which responds -in a similar way to a similar agitation. It is difficult to speak of -association and neural habit, the phenomena upon which not only all -mental life, but all co-ordinated activities, are based, without using -such expressions as “the broadening of the path” or “the thickening of -the conductor” by the impulses which pass through it. Apparently these -analogies may with safety be pressed curiously far. Chaotic response to -stimulation is unknown. Thanks to the nervous system, action exhibits -an ordered relation to stimulation. This relation is determined by -education, giving the term a connotation wide enough to cover all -experience. Nerve-tissue adjusts itself to experience; and since the -nerve-matter which takes the pattern is not labile, the process of -organization is consecutive and the result permanent. One pattern is -not destroyed as another is impressed. Hence temporal associations -are formed. What has been thought once will be thought again, if -the circumstances in which it was thought recur. What has been done -once will be done again under the influence of a similar sequence of -stimuli. The conductors are widened every time that they are used. But, -so far as concerns the mind, a reversed influence comes into play. The -<span class="pagenum"><a name="Page_357" id="Page_357">[Pg 357]</a></span> -wider the conductor, the less appeal to attention is made by the -impulses which pass through it. It is as if currents which have to -overcome resistance in a narrow path acquire a higher potential than -those which find an open road. And since the making of the road depends -upon attention, the limit of broadening is reached when a volitional -act becomes a habit. The first time that a piece of music is played -consciousness is alert. Marks on the page and movements of the fingers -are felt intensely. With each repetition the need for attention subsides.</p> - -<p>A skilled movement is impossible in the absence of guiding sensations. -I decide to button my coat. Sensation-paths from the muscles of the -forearm are opened into motor paths extending from the large pyramids -in the arm-centres of the kinæsthetic cortex. But it is not sufficient -that the action be started: it must be guided by the sensations which -movement produces. If my fingers are numb with cold, I cannot button -the coat. The muscles which move the fingers are warm enough beneath -the sleeve, but my attempts to will them to move are as futile as they -would be if the muscles belonged to some other person. The will has -no power over the muscles. It is essential that the sensations which -accompany the act of buttoning the coat flow through the same paths as -hitherto in the cortex of the brain. Flowing through the same paths, -they produce the same effect in consciousness, the same perceptions. In -ordinary parlance, one cannot perform any act unless one can remember -what it felt like to perform it on a previous occasion. It is almost -as sound physiology to describe the voluntary action of fastening a -button as commencing in the skin of the fingers as to describe is as -commencing in the brain. The act is due to the direction of attention -to impulses which flow from muscle to muscle, and from skin to muscle.</p> - -<p>All skill in the use of muscles is acquired by the method of trial and -error. Familiar movements are tried, combined, modified with a view to -the production of a new result. A man accustomed to striking with the -right hand forwards endeavours to swing a golf-club with the left hand -backwards. For a long time the result is anything but a success. At -length the head of the club takes the right curve. It not only hits the -ball with its centre, but it carries it through in the right line. The -<span class="pagenum"><a name="Page_358" id="Page_358">[Pg 358]</a></span> -ball travels 120 yards or so towards the green. In golfing terminology, -a successful drive is always “an awful fluke”; but the fluke once -accomplished, nothing is easier for the golfer than to drive equally -well on all succeeding occasions. He need merely remember exactly what -it felt like to give the club a perfect swing, and exclude all other -sensations while he is passing these memories through his sensori-motor arcs!</p> - -<p>The fact that we can deliberately improve an action, fitting it to -the attainment of the object of desire, by suppressing wrong and -emphasizing right sensations, shows how large a part consciousness -plays in the affairs of the nervous system. This brings us to -the frontier of physiology. At this boundary the authority of -the physiologist ends. He cannot define consciousness; he cannot -investigate it. Yet he naturally asks whether the machine which he is -investigating is a machine and nothing more. When the possibilities of -reflex action were first recognized, thought tended to dethrone feeling -and Will in favour of automatism. If the actions of a spinal frog -exhibit so distinct a purposive character, why, it was asked, should -we assume that the frog with a brain is anything more than a reflex -machine? Light, heat, sound are playing upon its sense-organs; surely -these stimuli suffice to set going all the sensori-motor currents which -lead to the various movements which in their totality constitute the -frog’s behaviour! And why assign to a mammal a self-directing authority -which we deny to a frog? The increased complexity of its behaviour is -more than accounted for by the greater variety of its nervous arcs. -All animals, it was argued, including Man, are reflex machines. Their -thoughts and actions are the effects of the play upon their nervous -systems of forces from the outer world. Each inherits a nervous system -of a certain pattern. Its individual development is conditioned by -the sensations which pass through it. The sensations are impressed by -the environment. Therefore the individual is a puppet, his activities -the dance of circumstance. Consciousness is an “epiphenomenon.” Few -physiologists or students of animal behaviour take this material view -of life at the present day. The fact that it leads inevitably to the -conclusion that consciousness is an “epiphenomenon” (Huxley’s term) is -its <i>reductio ad absurdum</i>. It is not in harmony with the economy of -Nature that an animal should be endowed with the capacity of feeling -<span class="pagenum"><a name="Page_359" id="Page_359">[Pg 359]</a></span> -pain and pleasure, if such endowment is useless to it. It can be useful -only by directing activity towards the attainment of pleasure and -the avoidance of pain. This admitted, the mechanical theory falls to -the ground. There is an “It” which feels, selects feelings, chooses -those which have a pleasant tone, wills to perform the acts by which -they are attained. It follows that the value of consciousness lies in -the prerogative which it confers of adapting action, within certain -limits, to circumstance. An animal succeeds in life in proportion as -the nervous system which it inherits reacts satisfactorily to its -environment. A chick which, after being hatched in an incubator, has -been isolated for twelve hours without food, seizes a grain of corn the -instant that it sees it. Its brain contains ready-made sensori-motor -arcs connecting the spot in its cortex in which the visual impression -of the grain is perceived and the motor neurones which control the -pecking muscles. A sheep-dog is quickly broken to sheep, because its -ancestors have been selected by mankind from amongst dogs that readily -adapted themselves to this work. The breeder has selected a pattern of -brain with the same success with which, when appearance is the only -desideratum, he selects a pattern of coat. Beavers set to work at -constructing a dam at the only spot in a valley at which it is possible -to create an artificial lake, because for countless ages Nature has -ruled out the animals which constructed their dams in unsuitable -places. Man also inherits a brain-pattern; but, not being required to -shift for himself soon after birth, he goes through a long period of -infancy and tutelage, during which, by force of circumstance and his -own Will, the pattern is elaborated. His supreme success is due to his -capacity for adapting means to ends. He inherits very few instincts. -Except as regards organic functions, his spinal cord is subservient in -almost all respects to his brain. Most of the actions of an animal are -instinctive—a word which has been sadly misapplied. Its connotation -is negative rather than positive. Owing to the marked pattern of its -brain, an animal finds it difficult to avoid acting in a particular -way. As the nights grow longer and its hours for feeding are curtailed, -a swallow is impelled by its instinct to go South. It makes the same -use of its sensations during its migration, and is as completely -<span class="pagenum"><a name="Page_360" id="Page_360">[Pg 360]</a></span> -dependent upon them for its guidance as a man would be. The lower we -descend the scale, the more inevitable do an animal’s movements become; -but there can be no doubt but that consciousness is of value to an -animal, as to Man, in that it gives to its individuality the capacity, -within such limits as Nature has selected, of resisting or modifying -its ancestral instincts when they are not absolutely appropriate to the -occasion.</p> - -<p>Sentience implies personality. “No system of philosophy can extrude -the ego.” The difference between the performance of the animal machine -as a physiologist studies it, and its behaviour when under the control -of its own driver, is the difference between reflex action and choice. -The ego interacts with physical forces. It does not come within the -province of the physiologist to explain the source of the force which -interferes with force. He finds no trace of it on either credit or -debit side when making up the body’s accounts. He is unable to enter, -“Item, to the development of consciousness ... so much.” He can form -no conception of this immaterial manifestent which hovers over the -infinitely numerous sensori-motor exchanges which are always occurring -in the cortex of the brain, giving to a particular group of agitations, -now here, now there, a special quality; but the manifestent is needed -to account for the potency of the reinforced agitations which enables -them to take possession of the nerve-paths by which muscles are reached.</p> - -<p>It is for the psychologist to define the application of the terms -“consciousness,” “attention,” “will.” He cannot define the attributes -of the ego which these terms connote. The moralist must show the way -in which they determine, or should determine, conduct. Yet within the -plain limits of physiology, attention, using the word in its every-day -sense, modifies the responses of the nervous system in a degree which -cannot escape observation. It is astonishing to anyone accustomed to -hospital surgery (although even in this field singular exceptions are -met with) to see the grave operations which a veterinary surgeon may -perform, without the animal showing any evidence of pain, provided its -apprehension has not been aroused and its attention directed to what is -being done. A horse standing in front of a crib of oats, untied, will -hardly whisk its tail while the surgeon is making a great wound in its -<span class="pagenum"><a name="Page_361" id="Page_361">[Pg 361]</a></span> -flesh, and sawing off a bony excrescence. The knife does not come -within the experience of a horse. It has no anticipations, and its -skin, intensely sensitive to the tickling of a fly or the smart of a -whip, is relatively insensitive to a cut. An eminent surgeon of the -last generation (the writer, as a student, “dressed” for him in his -old age) was in the habit, having arranged that his patient could not -see what he was doing, of performing operations of a very painful -nature whilst assuring his patient, “I am merely making a thorough -examination, in order that I may be perfectly certain of the cuts that -I shall have to make to-morrow in the operating-theatre when you are -under chloroform.” We are not concerned with the ethics of his method; -but the assurance, “Now that’s all over; you will never need to have -that operation performed again,” saved many a sufferer from a night of -apprehension and a miserable “coming round.”</p> - -<p>It was stated, during the South African War, that at Ladysmith the -bearer of a critical despatch, who was struck in the palm of the hand -by a bullet which traversed the whole length of his forearm, did -not discover that he was wounded until he saw the dripping blood, -after his errand was successfully accomplished. To deliberately cut -oneself with a razor is most painful, yet shaving in the morning, with -thoughts concentrated on the doings of the day, it is often the sight -of blood which directs attention to the fact that the skin is severed. -Of all evidences of self, the power of paying attention is the most -noteworthy. We can direct attention to certain sensations, which then -become perceptions, and we can deliberately ignore others, within -certain restricted limits.</p> - -<p>The control of the nervous apparatus by the self is a truth which no -student of the physiology of human beings can ignore. Isolated from its -relation to all other scientific truths, it has been made the basis of -a nescience which, although positively merely foolish, is, negatively, -harmful—yet a form of folly which answers well to the needs of persons -of a certain category.</p> - -<p>It may be objected that the picture of the relation of mind to brain -which is here presented—the one, activity, motion, the other a -labyrinth of conducting paths—makes all mental phenomena entirely -dependent upon current sensations. No results could happen if the -<span class="pagenum"><a name="Page_362" id="Page_362">[Pg 362]</a></span> -sensations were not there. It affords no ground for the explanation of -<b>mental images</b>, <b>hallucinations</b>, <b>dreams</b>. A few lines -may be spared to show that this objection does not hold. We cannot -attempt to explain the conscious control of thought. It is a part of -the impenetrable mystery to which we have just referred. But, granted -that it obtains, the direction by the ego of afferent nerve-currents -through the same strands which formerly vibrated to sensations which -drew a picture, and hence the revival of its image, is no more -incomprehensible than the liberation of afferent impulses from muscles -into efferent channels. Brain-chains are composed of many links. Their -interconnection is illimitable. When I recall the appearance of the -house in which I lived as a child, I throw into the chain impulses -(from somewhere) which traverse the final links, where passage implies -consciousness. At the edge of the lace-work of linked threads the -impulses light up a pattern which childhood’s experience worked into -the apparatus of thought.</p> - -<p>If we were to admire the perfection of any special aspect of the -brain’s functioning, the rarity of hallucinations might give us cause -for wonder. That impulses so seldom leave their own paths is more -astonishing than that occasionally, when the brain is excited and its -nutritive conditions deranged, the impulses which the ego can direct -into channels where they revive an image should sometimes, and with -far greater force, make their own way down well-worn paths, lighting -up a picture which deceives the ego. Dreams, by contrast, throw up -in a strong light the part played by attention in intelligent life. -The capacity for alertness is due to the favouring of one set of -impulses by suppressing others. The favoured impulses hold the road. -Concentration of attention is keeping thought to one line by resisting -all temptation to wander into by-paths. The waking condition is the -state in which all nerve-ways are closed, with the exception of those -which consciousness is using. The more severe the closure, the more -vivid is consciousness. In sleep all paths are open. In none is the -potential acquired by impulses in the process of overcoming resistance -high enough to evoke consciousness. A burst of impulses ascends from -the stomach, set a-flowing by undigested fragments of salmon and -cucumber, or mounts from the arm on which the sleeper has been lying -<span class="pagenum"><a name="Page_363" id="Page_363">[Pg 363]</a></span> -until its circulation has been arrested. They reverberate through the -open corridors of the brain. If they are sufficiently noisy to awaken -the sleeper, he, detecting them in this path and in that, supposes -them to be on the same errands as the impulses which commonly pass -thus. If dreams are analysed, it will be found that, although the -combinations of impressions may be uncommon and extremely bizarre, the -impressions are selected from the most familiar. The images of which -the dream is compounded, which may have lost all normal relations and -may have assumed impossible proportions, are those which the mind -most frequently conjures up. In the large majority of instances some -happening of the day preceding can be recognized as the prompting -cause. A remembered dream is the photograph taken by consciousness -of the sensations which have bombarded it into activity. Especially -if due to impulses originated by visceral discomfort, the dream may -have an unpleasant tone. This may take various forms, but the emotion -most commonly aroused is fear. The objects visualized may have -preposterous dimensions, or they may be not sufficiently distinct for -recognition—elusive imps; but most commonly distress is caused by -the want of harmony of sensations, due to the absence of kinæsthetic -elements. A man is lying on the railway-line; a train is approaching -with increasing speed; he cannot get up. He is in the pulpit, but -cannot speak. Dreams thus confirm the view set forth above as to the -cause of volitional action. Ability to perform an act depends upon the -flow through the kinæsthetic centres of the brain of impulses generated -in the muscles by which the act is to be, or is being, performed. -Kinæsthetic sensations do not under any circumstances play the same -part in mental life as sensations from the skin, the eye, or the ear; -when the body is passive in bed they are not flowing into the cortex. -The dream-photograph shows elements demanding movement, but affords no -evidence that movement is in progress.</p> - -<hr class="chap" /> -<p><span class="pagenum"><a name="Page_364" id="Page_364">[Pg 364]</a></span></p> -<div class="chapter"> - <h2>CHAPTER XII<br /><span class="h_subtitle">SMELL AND TASTE</span></h2> -</div> - -<p>In Man the chief function of these senses is to guard the entrances -to the respiratory and digestive tracts. In this they are not -conspicuously efficient, since various poisonous gases, salts, and -powders, escape their vigilance. Merely a selection of the substances -which occur in air and in food are recognized as having odour or -flavour. Smell and taste are only partially distinguished in ordinary -parlance. No odorous substance is spoken of as tasteless when taken -into the mouth. Its volatile constituents, escaping to the chambers -of the nose, are said to afford a certain flavour. On the other -hand, it is recognized that substances which stimulate the tongue -alone—bitters, acids, sweets, and salts, unmixed with volatile -bodies—have no odour.</p> - -<p>Biassed as we necessarily are by the paltry rôle assigned to smell in -our mental life, it seems a little unworthy of the present functions of -the great brain that it should have developed in association with the -nose. Yet smell and taste are the oldest of the senses. Their origin -goes back to the days of chemiotaxis, when the organism, having no -specialized sense-organs, was attracted to its mate or to its food, and -repelled from conditions unsuitable for its well-being, by particles in -solution acting as chemical stimulants. An amœba is chemiotactically -drawn towards its food, one spore of an alga is attracted to another, -by the particles of matter which drift across the interval between them.</p> - -<p>In the life of many animals smell plays as important a part as that of -either of the other senses. One has but to watch a dog “looking” for -its master, already full in view, with its nose, to realize that smell -<span class="pagenum"><a name="Page_365" id="Page_365">[Pg 365]</a></span> -is the sense on which a dog chiefly relies. We describe it as looking, -because in ourselves the eye has so far outdistanced the other senses -as a channel of information that we speak of “looking” when we mean -seeking, and say that “we see” when we wish to imply that we understand.</p> - -<p>The difference between smell and taste is, in fishes, a difference in -the quality of the sensation, and not in its “modality” or kind; but -in terrestrial animals the olfactory membrane of the nose has become -specialized for the recognition of particles suspended in air, the -tongue for substances dissolved in water. The olfactory membrane, which -lines the upper two of the three chambers of the nose, is covered with -elongated cells of two kinds: (<i>a</i>) Columnar cells, fairly thick; and -(<i>b</i>) fusiform cells, each carrying at its free extremity a bunch of -exceedingly minute hairs. The fusiform cells are neuro-epithelial cells -of the most primitive type. Before nerve-cells, properly so called, -appeared, certain favourably-placed epithelial cells were connected -by protoplasmic bridges with muscle-fibres, to which they delivered -the impulses which were generated in them by external forces. Later -some of the neuro-epithelial cells sank beneath the surface, where, -as ganglion-cells, they served as intermediaries between groups of -sensory cells on the surface and the nerve-net which lay more deeply -in the tissues. The olfactory membrane perpetuates the earlier stage; -in so far as it consists of elements which are combinations of -sense-cells and nerve-fibres. Each of its fusiform cells sends inwards -a nerve-filament, which, traversing the submucous tissue of the nose -and the bone (cribriform plate) on the base of the skull, between the -orbits, enters the olfactory bulb. The olfactory bulb is a part of -the local nervous mechanism of smell. It is the ganglion of the nerve -of smell <i>plus</i> nerve-elements which in all segments behind the eye -have been withdrawn from the neighbourhood of the sense-organ into the -central nervous system (<a href="#Page_333"><i>cf.</i> p. 333</a>).</p> - -<p>The way in which odorous particles in air stimulate the fusiform cells -is unknown. The quantity which suffices as a stimulant is so small -as to put chemical stimulation out of the question. A few grains of -musk will scent a room for years. 0·00000004 milligramme of mercaptan -(sulphur-alcohol) is recognizable in a litre of air. This is a dilution -<span class="pagenum"><a name="Page_366" id="Page_366">[Pg 366]</a></span> -to 1 in 50,000,000,000. Probably even such figures as these would -be thrown into the shade if we could estimate the minimum amount of -human effluvium which will enable a dog to follow his master’s trail. -Explanations have been sought in alterations in the vibrations of -molecules of air caused by the presence amongst them of relatively -heavy molecules of volatile substances; but the difficulty of -accounting for the generation of nerve-impulses in the sensory cells -remains as great as ever. The hairs borne by olfactory cells are so -short that it is impossible that they should project beyond the film -of moisture on the surface of the membrane. This seems to preclude an -answering vibration. Yet an increase in the thickness of this layer -and in its density, due to the presence in it of mucus secreted during -a catarrh, renders the sense-cells incapable of responding to odorous -particles.</p> - -<p>Smell in an animal is not a test of the quality of the air it is -breathing, but a source of information as to the direction in which it -may seek its prey; or, although far more rarely, as to the direction -from which the advance of a foe is to be feared. Hunting animals depend -for the most part on the nose. Hunted animals rely chiefly on the eye.</p> - -<p>If we attempt to analyse our smell-sensations, we find that we can pick -out a number of varieties which appear so unlike as to have nothing -in common: Putrid meat, burning indiarubber, sulphuretted hydrogen, -ammonia, roses, onions, lemon verbena, methylated spirit. Everyone can -make for himself a list of typical odours which seem to have specific -qualities—odours so distinct that he never confuses one with another. -He can also class together scents about which he is often uncertain. -The type-odours he can distinguish when present in a mixture; whereas -odours which are less distinct reinforce or modify one another. It has -been found, by careful experiment, that certain type-odours even tend -to neutralize each other. Musk and bitter almonds, for example, if -present in small quantities and properly proportioned, produce a very -dim sensation, whether supplied as a mixture to both nostrils, or the -one assertive odour to one nostril and the other to the other. This -last observation is of great importance. It proves that their mutual -destruction does not occur on the olfactory membrane. It is not due to -<span class="pagenum"><a name="Page_367" id="Page_367">[Pg 367]</a></span> -physical interference. The sensation of musk is delivered to one side -of the brain, the sensation of bitter almonds to the other; but when -attention is directed to these two sensations there is found a quality -in the one which is irreconcilable with the quality of the other.</p> - -<p>In certain persons and under certain pathological conditions, -sensitiveness to particular odours, or groups of odours, is absent, -while for the rest the sense is normal. Methylated spirit, prussic acid -and mignonette, constitute a group which not infrequently drops out. -Instances have also been reported of persons unable to smell vanilla -(to which some are hyper-sensitive), and of others insensitive to -violets, although normally sensitive to the scents of other flowers. -The notes sounded in consciousness extend over a long gamut; but -there are reasons for thinking that the number of keys on the clavier -which odoriferous substances strike is limited. Eleven is the number -provisionally adopted. The effect in consciousness varies according as -one key or another is struck, or several at the same time with varying -degrees of force.</p> - -<p>Many attempts have been made to associate the sensation-qualities of -the various odours with the chemical or physical properties of their -odorants, with but little success as yet. To excite the sense of -smell, a gas must be at least a little heavier than air. No volatile -body, it is stated, is so heavy as to be odourless; on the contrary, -speaking generally, heavy molecules are more stimulating than light. -The quality of a smell-sensation would therefore appear to depend upon -the period of vibration of the molecules of the substance which evokes -it; but, as already stated, a consideration of the apparatus which -responds to stimulation by odoriferous particles does not help us to an -understanding of the way in which the particles act upon it. -<span class="pagenum"><a name="Page_368" id="Page_368">[Pg 368]</a></span></p> - -<div class="figcenter"> - <a id="I_368" name="I_368"> </a> - <img src="images/i_p368.jpg" alt="" width="400" height="568" /> - <p class="center"><span class="smcap">Fig. 26.</span>—<span class="smcap">Highly - Magnified Section through the Wall of a Circumvallate Papilla<br /> of the - Tongue, showing Two Taste-Bulbs.</span></p> - <div class="blockquot2"> - <p class="neg-indent space-below2">These sense-organs are groups of elongated epithelial cells, set - vertically to the surface. Their cells are of two kinds—the one - fusiform, slender, bearing each a bristle-like process which projects - through a minute pore left between the superficial cells of the general - epithelium; the other thicker and wedge-shaped. Nerve-fibres are - connected with the fusiform cells.</p> -</div></div> - -<p>Taste is far more limited in its range of sensations than smell. The -back of the tongue is sensitive to bitters, the tip to sweets and -salts, the sides to acids. Mixtures of these qualities are distinctly -analysable by the sense of taste. Our sensations of taste do not fuse. -Slight differences in the way in which the organs on the different -parts of the tongue react to stimulation enable us to recognize that a -sapid substance is a mixture. When, with a great flourish of trumpets, -saccharin was introduced as a safe sweetener for gouty people, an -attempt was made to provide them with saccharin-sweetened jam. The -effect of the jam upon the person who consumed it was truly humorous. -First a suspicion of tartness, then its adequate suppression, followed -by nauseating sweetness. The sense-organs which subserve the sense -of taste are clusters of fusiform epithelial cells, collected in -“taste-bulbs” (<a href="#I_368">Fig. 26</a>). Each gustatory cell bears a minute -bristle, which projects through the pore left by the cells of the surrounding -epithelium which constitute a globular case for the bulb. As in the -nose, eye, and ear, a second thicker variety of epithelial cell is -also present. The nerve-fibres of the taste-bulbs are not, as in the -olfactory membrane, processes of their cells, but branches of the fifth -nerve which ramify amongst them. On the back of the tongue taste-bulbs -<span class="pagenum"><a name="Page_369" id="Page_369">[Pg 369]</a></span> -are much more numerous than elsewhere. They are not as sensitive as the -cells of the olfactory membrane; nevertheless, they enable us to detect -1 part of quinine in 2,000,000 parts of water.</p> - -<p>Sensations of taste and smell endure for a long time after stimulation, -because the odorous or sapid substance remains in contact with the -sense-organs. This accounts for the confusion into which a man is -thrown if he sip alternately port and sherry. After a short time he -cannot tell the one from the other. The organs are quickly fatigued, -using the term loosely. How intolerable patchouli would be to the -ladies who use it were it otherwise! If for some time one sniffs the -odour of mignonette, it ceases to be recognizable; whereas, turning to -a rose, the olfactory membrane is found to be as sensitive as usual. -When the sense is fatigued for a particular smell, it is dull for -others of the same group, thus affording an opportunity of classifying -smell-sensations according to their qualities; but the method is -difficult to apply. Taste-organs are greatly affected by temperature. -Quinine is not tasted just after drinking ice-cold water. Alcohol, -ether, or chloroform paralyses the organs much in the same way. -Castor-oil slips down the throat unnoticed if the mouth, just before -swallowing it, has been rinsed with brandy or with a strong solution -of tincture of chloroform.</p> - -<p>Englishmen make but little use of their sense of smell. It might -teach them much regarding the various emanations from putrid matter -which are produced by bacterial action; but, dreading drains, they -decline to cultivate proficiency in the exercise of this sense. The -nose is valued for the warning it gives of “nasty smells,” but is not -allowed to analyse them. Burnt milk, soap-boilings, rancid oils, are -taboo, because they are associated with bungling in the kitchen. With -moderated ardour, we allow our sense of smell to distinguish foods and -beverages, but we are not a race of epicures. The perfumes of flowers -are classed as “nice smells.” The idea of greediness is not associated -with their enjoyment; besides, they remind us of gardens, sunshine, -pretty forms and colours. When bottled, musk, orange-blossom, violets, -lavender, are valued not so much for their own sweetness, as for their -singular efficiency in obscuring nasty smells. Few persons practise the -<span class="pagenum"><a name="Page_370" id="Page_370">[Pg 370]</a></span> -recognition and distinction of even pleasant odours. Very few, on first -coming across a scented herb or shrub, pay sufficient attention to its -perfume to impress it on their memories. They note the shape of its -leaves and the colour of its flowers, but they are unable to identify -it by its odour when they meet with it again. It is not much to be -wondered at, therefore, that this slighted sense tends to leave us -after middle life. It has been asserted—and probably the statement is -justified—that rarely is the olfactory bulb of a man over forty free -from signs of atrophy. We have no statistics concerning the brains of -Japanese, who regard the sense of smell as one of the chief avenues -of pleasure; but it may be that in this respect their brains present -a contrast to our own. Yet the deadening of the sense is scarcely -noticed, since its results are of little consequence as compared with -those which follow loss of sight or loss of hearing. Many a man, as he -grows older, declares that the cook of his club has lost his cunning, -or frankly asserts that he “no longer cares for kickshaws. Cold beef, -beer, and pickles, are good enough for him.” He little suspects that -his palate has lost its power of distinguishing the flavours of -dainty meats and wines. Others continue to be exacting, because their -imaginations still endow food with the qualities which they remember, -just as people eat preserved asparagus or tinned peas because they -look—however little they taste—like the gifts of Spring.</p> - -<p>Taste accompanies the reception of food in the mouth. We have no -knowledge of the situation of our own olfactory membranes, and -therefore we suppose that a flavour, whether it be due to stimulation -of taste-bulbs or olfactory membrane, is in the mouth. The odour of -a flower we mentally project to a distance, because we associate the -sight of a flower with its perfume. A dog, able to judge the freshness -or staleness of a scent, must project its sensations of smell in the -same way in which we project our sensations of sight. It forms an -estimate, of a sort, of the time that it will take in reaching the -source of the scent. Its excitement increases as the trail grows fresher.</p> - -<p>Taste and smell are heavily laden with affective tone. When -disagreeable, the feeling which they evoke is near akin to pain. It -may gather head until, like hunger, it causes the discharge of motor -<span class="pagenum"><a name="Page_371" id="Page_371">[Pg 371]</a></span> -neurones; but under its influence food is ejected, instead of -preparation being made for its reception.</p> - -<p>Taste and smell are senses which afford us no information with -regard to time or space. They give rise to massive sensations. Such -sensations, devoid of detail, produce a frame of mind rather than -thought. The smell of tobacco does not distract attention. On the -contrary, the steady flow of impulses to which it gives rise helps -to inhibit, to subdue, the yapping of more exigent sensations. And -since sensations of smell have no features of their own, they form -a background to sensations of other kinds, entering with them into -memory. No two scenes are exactly alike. One cannot recall another. But -the scent of syringa is always the same. Wherever smelled, it opens the -pathways in the brain in which were first associated a June evening and -syringa, with a scene and a situation upon which memory loves to dwell.</p> - -<hr class="chap" /> -<p><span class="pagenum"><a name="Page_372" id="Page_372">[Pg 372]</a></span></p> -<div class="chapter"> - <h2>CHAPTER XIII<br /><span class="h_subtitle">VISION</span></h2> -</div> - -<p>The eye is enclosed in a globe of fibrous tissue, of which the front -part, or cornea, being transparent, admits light. The epithelial -layer which covers the cornea, conjunctiva, is also transparent. No -bloodvessels enter these colourless tissues, unless as the result of -inflammation due to infection or to exposure to sunshine or dust. For -nutrition they are dependent upon the plasma which, exuding from, and -returning to, the vessels which surround them, circulates in their -tissue-spaces. In advancing years, when the circulation is less brisk, -a ring of opaque tissue, arcus senilis, encroaches on the cornea. In -the interior of the globe, just behind the cornea, is a projecting -shelf, formed of a ring of tissue supported by buttresses, ciliary -processes. It is continued inwards as the iris, a muscular curtain. The -“hyaloid membrane” lines the back portion of the globe. Continued on -the inner side of the ciliary processes, it splits into several layers, -which pass, one in front of the lens, others to its edge, to which -they are attached, and still another, very thin, behind it. Since it -holds the lens in place, the anterior portion of the hyaloid membrane -is known as its “suspensory ligament.” Thus the eyeball is divided -into three chambers. The anterior is filled with watery lymph, aqueous -humour. In it, resting on the anterior surface of the suspensory -ligament of the lens, is the iris. The middle chamber contains the -lens. The posterior chamber is filled with a liquid jelly, vitreous -humour.</p> - -<p>By the contraction of the circular fibres of the iris, the aperture of -the pupil is diminished, limiting the light which enters the globe. -This adjustment occurs when the illumination is bright. It is also -<span class="pagenum"><a name="Page_373" id="Page_373">[Pg 373]</a></span> -brought into action for the purpose of cutting out divergent rays, -which would not be clearly focussed when objects near at hand are -looked at. The posterior surface of the iris and the inner surfaces of -the ciliary processes are covered with dense black pigment. It is this -pigment, showing through the uncoloured connective tissue and plain -muscle-fibres of which the iris is composed, that gives their colour to -grey and blue eyes. In many eyes the iris contains a brown pigment in -its substance.</p> - -<div class="figcenter"> - <a id="I_373" name="I_373"> </a> - <img src="images/i_p373.jpg" alt="" width="500" height="473" /> - <p class="center"><span class="smcap">Fig. 27.</span>—<span class="smcap"> - Horizontal Section through the Right Eye.</span></p> - <div class="blockquot2"> - <p class="neg-indent space-below2">The slight depression in the retina in the axis of the - globe is the fovea centralis, or yellow spot; the optic nerve pierces the ball - to its inner or nasal side. The lens, with its suspensory ligament, - separates the aqueous from the vitreous humour. On the front of the - lens rests the iris, covered on its posterior surface with black - pigment. On either side of the lens is seen a ciliary process, with - the circular fibres of the ciliary muscle cut transversely, and its - radiating fibres disposed as a fan.</p> -</div></div> - -<p>The back portion of the globe of the eye is covered with a curtain, the -retina, formed by the spreading out of the fibres of the optic nerve -in front of various layers of nerve-cells and the sensory cells of the -organ of vision, rods and cones. The retina lies between the hyaloid -membrane, which encloses the vitreous humour, and a layer of pigment -which “backs” it, as a photographer backs a plate when he proposes to -use it towards a source of light—to take a photograph of a window from -within a room. The serrated margin of the retina is somewhat anterior -to the equator of the eyeball. The pigment which backs the retina is -<span class="pagenum"><a name="Page_374" id="Page_374">[Pg 374]</a></span> -contained in a sheet of cells which belongs to the pouch of brain that -extended outwards towards the eye-pit (<a href="#Page_334">p. 334</a>). Properly -speaking, therefore, it is a layer of the retina.</p> - -<div class="figcenter"> - <a id="I_374" name="I_374"> </a> - <img src="images/i_p374.jpg" alt="" width="600" height="342" /> - <p class="center"><span class="smcap">Fig. 28.</span>—<span class="smcap">Diagrams - showing the Mode of Formation of the Crystalline Lens.</span></p> - <div class="blockquot2"> - <p class="neg-indent space-below2">A, A pit in the epithelium on the surface of the head has closed - into a hollow sphere. B, The cells of the posterior wall of this sphere are - growing forward, as the fibres of the lens which traverse its whole - thickness, with the exception of the cubical epithelium on its front.</p> -</div></div> - -<p>Three sets of tissues take part in the development of the eyeball. (1) -The epithelium covering the surface of the head is depressed as a pit, -which gradually closes into a hollow sphere. This sphere, when its -cavity is filled up, owing to the great elongation of the cells of its -posterior half, becomes the lens. It breaks away from the rest of the -epithelium of the surface, which clears to transparency as that part -of the conjunctiva termed the “corneal epithelium.” (2) The retina, -as already stated, is a hollow outgrowth from the interbrain. As this -pouch approaches the lens, its anterior half is pushed back into the -posterior half, forming a cup with a double wall. The anterior, or -inner, sheet of the bowl of the cup develops into the nervous layers -of the retina, the posterior sheet into its pigmented epithelium. (3) -Connective tissues are transformed into the other constituents of the -globe—cornea, iris, vitreous humour, etc. The globe is complete, -except at a spot on the nasal side of its posterior pole where the -optic nerve pierces it.</p> - -<p>The bloodvessels of the retina, entering with the optic nerve, ramify -on its anterior surface. Under ordinary circumstances we ignore the -shadows which they cast, as we ignore the blind spot which coincides -<span class="pagenum"><a name="Page_375" id="Page_375">[Pg 375]</a></span> -with the disc of insensitive tissue presented by the end of the optic -nerve, and many other imperfections; but it was shown by Purkinje many -years ago that by a very simple manœuvre they may be forced upon our notice.</p> - -<div class="figcenter"> - <a id="I_375" name="I_375"> </a> - <img src="images/i_p375.jpg" alt="" width="400" height="454" /> - <p class="center"><span class="smcap">Fig. 29.</span>—<span class="smcap">Purkinje’s Shadows.</span></p> - <div class="blockquot2"> - <p class="neg-indent space-below2">A beam of light traversing the eyeball in the direction A - throws a shadow of the vessel <i>v</i>, lying on the front of the retina, upon the - sensitive layer at its back. When the light is moved from A to B the - shadow moves from <i>a</i> to <i>b</i>. The mind, supposing the shadow to be a - dark mark on the nearest wall or screen, infers that this mark moves - from A′ to B′.</p> -</div></div> - -<p>By making use of <i>Purkinje’s figures</i>, it can be proved that the level -in the retina at which undulations of light give rise to the impulses -which evoke visual sensations coincides with the back of its anterior -sheet—<i>i.e.</i>, with the layer of rods and cones. A person stares -fixedly at a white sheet in a dimly lighted room while an assistant, by -the help of a lens, focuses a strong light on the front of his eyeball, -to the outer side of the cornea. The rays, traversing the white of the -eye, throw shadows of the retinal vessels on the layers behind them; -but this not being the way in which light normally enters the eyeball, -the person experimented upon supposes that he sees the shadows in front -of him. He mentally projects them on to the white sheet. The pattern -of his retinal vessels appears on the sheet in grey streaks. When the -spot of light is moved, the shadow-pattern shifts, and in the same -<span class="pagenum"><a name="Page_376" id="Page_376">[Pg 376]</a></span> -direction; since, as the retinal image is reversed, a movement from -right to left is interpreted by consciousness as a movement from -left to right. Given the angle through which the light is moved, and -the apparent displacement of the shadows, it is a simple matter to -calculate the distance behind the bloodvessels of the sensitive layer -of the eye. So definite are Purkinje’s figures that the shadows of -individual blood-corpuscles can be followed, and the rate at which they -are moving in the capillaries of the retina calculated.</p> - -<p>The retina is the organ of vision. Cornea, iris, lens, vitreous humour, -are parts of the camera in which this sensitive screen is exposed; and -of the retina, the sensitive layer is the layer of rods and cones. -Interest therefore centres in these structures. They are disposed with -the utmost regularity on the posterior surface of a thin, reticulated -membrane—the outer limiting membrane. But rods and cones are only -the outer halves of sensory cells, the inner portions of which, reduced -to a minimum in thickness, except where they contain their nuclei, -lie in the outer nuclear layer. Rods are the larger elements. Each -consists of an outer segment, or limb, of relatively firm substance -transversely striated, and liable to break into discs; and an inner -limb of much softer substance, again divisible into two parts, the -outer longitudinally striated, the inner granular. Cones are almost -identical in structure with rods, save that their outer limbs are much -smaller, their inner limbs rather fuller. In frogs and various other -animals, but not in Man, each cone contains at the junction of its two -limbs a highly refracting globule of oil, often brightly coloured, red, -yellow, or green. -<span class="pagenum"><a name="Page_377" id="Page_377">[Pg 377]</a></span></p> - -<div class="figcenter"> - <a id="I_377" name="I_377"> </a> - <img src="images/i_p377.jpg" alt="" width="400" height="705" /> - <p class="center"><span class="smcap">Fig. 30.</span>—<span class="smcap">The - Retina in Vertical Section—<br />A, after Exposure to Bright Light;<br /> B, - After Resting in the Dark.</span></p> - <div class="blockquot2"> - <p class="neg-indent space-below2">The arrow shows the direction in which light traverses the - retina. C, Retinal epithelium, with its pigmented fringe. 1, Layer of rods and - cones, separated by the external limiting membrane from 2, the layer of - the nuclei of the rods and cones. 3, The ganglion-cells of the retina, - which are homologous with the cells of the afferent root of a spinal - nerve. Their peripheral axons ramify beneath the sensory epithelium - (rods and cones and their nucleus-bearing segments), their central - axons in 4, the inner molecular layer. D, Collecting cells on the front - of the retina; <i>a a a</i>, their axons which conduct impulses to the - brain; <i>b</i>, an efferent fibre from the brain.</p> -</div></div> - -<p><span class="pagenum"><a name="Page_378" id="Page_378">[Pg 378]</a></span> -The layers in front of the rods and cones contain nervous elements -accessory to them. In the “inner nuclear layer” are the ganglion-cells -of the retina, homologous with the cells of the ganglia on the -posterior roots of spinal nerves; but, in the retina, bipolar and -extremely minute. On either side of the rather thick layer occupied by -the nuclei of these ganglion-cells (and of cells of other types which, -for the sake of clearness, we omit) is a felt-work of nerve-filaments -in which their two extremities arborize. The most internal, or -anterior, layer consists of a single sheet of rather large collecting -cells and of their axons, which stream towards the optic nerve. Each -cone has its proper ganglion-cell, collecting cell, and efferent -fibre. Rods are served in groups by ganglion-cells and collecting -cells. From this it may be inferred that a cone is a sensory unit, an -inference confirmed, as we shall show presently, by direct evidence. -The connections of the rods show that they also are sensory elements, -although it may be doubted whether they are sensory units. The optic -nerve contains a very large number of fibres—about a million—all -small, but some distinctly larger than the rest. The largest very -probably belong to the collecting cells of rods. But the retina -certainly does not contain a million collecting cells. A considerable -residue of fibres is therefore unaccounted for. It is supposed that -they are afferent to the retina, but we have no knowledge regarding the -nature of the impulses which descend from the brain.</p> - -<p>The retinal pigment is not merely a backing for the sensitive screen. -It undoubtedly plays an important part in vision. That it is not -essential is evident from the fact that albinos, whose eyes appear pink -owing to the absence of pigment, and the consequent showing through -of the blood in the exceedingly vascular membrane which lies behind -the retina, can see; although their visual sense cannot be described -as normal. They are exceptionally sensitive to an excess of light. We -shall return to this subject after describing the differences in manner -of functioning which distinguish rods from cones, differences so marked -as to justify us in speaking of two kinds of vision.</p> - -<p>During twilight warm tones gradually fade out of the landscape; cold -blues and greys predominate. A time arrives when scarlet poppies -look black, although yellow and blue flowers and green leaves can -still be dimly distinguished. In full daylight colours are seen at -their brightest in the high lights; where the light is dim they -tend to appear in different shades of grey. At night, if the sky is -star-lit, all colours give place to a slightly bluish grey in the high -lights, black in the shade. But a not very uncommon abnormality is -night-blindness—inability to see at all when the light is not bright -enough for the recognition of colours. In persons so affected the rods -do not function; for it is with the rods that we see in weak light. -<span class="pagenum"><a name="Page_379" id="Page_379">[Pg 379]</a></span> -They record differences in intensity between the lower limit of their -sensitiveness and the higher degree of brightness, at which they -are superseded by cones; but they afford no information regarding -colour. Their monochrome is interpreted by the mind as a bluish grey, -apparently because, since they are insensitive to red rays, the -sensations of which they are the source are associated with the blue -end of the spectrum. When the cones are stimulated very slightly, -the reinforcing grey of the rods enables us to distinguish all other -colours, save red, which appears black. In bright light the rods are -in a permanent state of exhaustion; they do not contribute to vision. -Rods respond to stimulation more slowly than cones. This fact enables -us, by a very pretty experiment, to distinguish the two kinds of -vision. A disc of green paper about the size of a threepenny-bit is -pasted on a red surface. Held at arm’s length in a room lighted by a -single candle, the disc looks dull green when the gaze is directed at -it; but if the gaze be directed 2 or 3 inches to one side of it, it -appears brighter than before, but less distinct and almost grey. The -explanation of this is to be found in the fact that at the posterior -pole of the eye there is a shallow cup—fovea centralis—which carries -cones only, without rods. This small depression is the area of direct -vision, the only spot at which we see things quite distinctly. At the -fovea the nuclei and nerve-cells of the retina are withdrawn from in -front of the cones to the margin of the cup, in order that they may -not interfere with the passage of light. The pit and the ring round it -contain some yellow pigment. Hence it is usually termed the “yellow -spot.” When we are looking straight at the green disc, it is focussed -on the yellow spot. It then excites a sensation of greenness; but since -this is not reinforced by any rod-sensations, the green is dull. When -it is focussed outside the yellow spot, it stimulates rods and the -sparse cones which lie amongst them; and the rods being more sensitive -than cones to light of low intensity, the disc looks brighter. If, -while the observer is still gazing fixedly at a spot to the side of the -disc, the red paper be waved rapidly, but gently, to right and left, a -brightish grey cover seems at each movement to slip off the dark green -disc, and to regain its position a moment later, with a jump. The grey -rod-sensation, developing more slowly than the green cone-sensation, -<span class="pagenum"><a name="Page_380" id="Page_380">[Pg 380]</a></span> -is, as it were, left behind. The two are separated at the moment when -the paper starts to right or to left.</p> - -<p>Astronomers have long recognized that one of the smaller stars which -catches the attention when they are not looking directly at it may -be invisible when the gaze is directed to the spot where it ought -to be. It was visible when focussed on rods, but it is not visible -when focussed on cones. In most birds the retina shows cones alone. -To anyone who for the first time enters a dovecote at night the -experience is very curious. A candle is for him a sufficiently strong -illuminant, but it does not give light enough to enable the pigeons -to see. Although evidently alarmed by the noise made by the intruder, -they allow themselves to be taken down from their perches without -making any attempt to escape. If, startled by the touch of a hand, they -take to flight, they fly against the wall. Pigeons are night-blind. -The retina of an owl bears chiefly rods, the outer limbs of which are -exceptionally long.</p> - -<p>The outer limbs of the rods are coloured reddish-purple. This colour -is quickly bleached by light. If a frog which has been kept for a -short time in the dark be decapitated, its head fixed for ten minutes -in a situation in which a window is in front of it, then carried to a -photographic dark-room, where an eye is taken out by red light, opened, -and the retina removed, a print of the window will be seen upon it. -Such an optogram may be fixed by dipping the retina in alum.</p> - -<p>The retina is easily detached from its pigment-layer. If it has been -bleached by exposure to light, it regains its “visual purple” when -again placed in contact with its pigment. Evidently the visual purple -is renewed from the pigment which lies behind (and around) the rods.</p> - -<p>From the cells of the pigment-layer a fringe of streaming processes -depends amongst the outer limbs of the rods and cones (<a href="#I_377">Fig. 30</a>). -In a dull light the processes hang but a short way down; in a bright light -they react almost to the outer limiting membrane. They supply pigment -to the rods, but their relation to cones is not understood. It is -clear, however, that the cones, although they are not coloured, are -dependent upon the pigment-fringe, since they always remain in contact -with it. Their inner limbs elongate in the dark, lifting them to the -<span class="pagenum"><a name="Page_381" id="Page_381">[Pg 381]</a></span> -pigment, and shorten in bright light. These movements may merely -indicate that the cones require a backing of pigment, but it would seem -more probable that, like the rods, they absorb a substance which is -sensitive to light, although we cannot recognize it by its colour.</p> - -<p>The responsiveness of the rods to light is due to visual purple. -As every lady is aware, colours, especially mauves and lilacs, are -bleached by light. The chemical change affected by light in the colour -of the outer limbs of the rods is the stimulant which originates -impulses in the nerve-fibres connected with them, and it is generally -believed that cones—the more highly specialized sensory cells—are -stimulated in the same way. Visual purple is particularly abundant in -all animals that range at night, with the exception of the bat. But -its absence in the bat does not militate against the theory that it -is the cause of night-vision, for it has been shown that a blind bat -flies with almost as much freedom, and avoids obstacles—even threads -stretched across the room—with as much skill as one that can see. It -is guided by the bristles of its cheek. So, too, is the cat, which has -the reputation of being able to see in the dark. Undoubtedly a cat’s -eye is an exceptionally efficient organ in dim light, just as it is -exceptionally sensitive to sunshine—it is provided with an iris which -contracts the pupil almost to a pinhole—but the cat trusts to the -bristles of its cheek for information regarding the things which block -its path.</p> - -<p>Most of the peculiarities which distinguish the reactions of the eye -from those of other sense-organs can be explained by its mode of -stimulation—the initiation of a nerve-current by a chemical change. -No stimulus, if sufficiently strong, can be too brief. The retina reacts -to an electric spark in the same way as a photographic plate; but, -unlike the plate, the retina is restored to its previous condition of -sensitiveness in about one-tenth of a second. A visual sensation lasts -about one-tenth of a second. This prolongation of the sensation is, -however, a mental, not a retinal, effect. The mind continues to see an -object which has been illuminated by a flash until the retina is again -in a condition to send brainwards a second impulse. Were our sensations -coincident in duration with the stimulation of our sense-organs, we -<span class="pagenum"><a name="Page_382" id="Page_382">[Pg 382]</a></span> -should live in a flickering cinematograph. When one is watching a -moving point of light—the glowing end of a match, for example—the -prolongation of sensation has its disadvantages; the moving point -is interpreted as a streak of light. If the illumination be very -brilliant, the object seen may give rise to a prolonged after-image. A -glance at the sun leaves in the mind for seconds, or even for minutes, -the image of a glowing disc. Sensations due to stimulation of the -yellow spot last longer than those which originate in the peripheral -retina. If, in a train, one is being carried at a certain pace, past a -fence composed of upright palings, one sees the separate slats until -the eyes are directed towards them, when they fuse into a continuous screen.</p> - -<p>The phenomena of negative or complementary images are of retinal -origin. The bright image of the sun, if the stimulus has not been too -violent, gives place to a black disc. If one closes the eyes after -staring at a window, a black surface crossed by bright lines is seen -in place of a white surface with dark frames to the panes. If, after -staring at a red surface, one looks at the ceiling, a green patch is -seen; after yellow, blue. Every colour has its complement, which may be -determined in this way. There is much uncertainty as to the exact terms -in which this phenomenon is to be accounted for, but little doubt as to -its being due to the peculiar mode of reaction of the retina to light. -Chemical substances which have been used up have to be restored, and -during the period in which they are coming back to what may be termed -a neutral condition the retina delivers to the brain impulses of the -opposite sign.</p> - -<p>Contrasts which are experienced simultaneously are more difficult to -understand than those which appear successively. In <a href="#I_383">Fig. 31</a> -the half of the grey cross which is surrounded by black appears brighter than -the half which lies on white paper. A grey cross on a red background -looks green; on a green background, red; on yellow, blue; on blue, -yellow. If green is on red, it looks greener than if it is on white or -black. These simultaneous contrasts are seen best when the strength of -the colours is reduced by covering them with tissue-paper. It is as if -activity of any one part of the retina is accompanied by activity of -the opposite sign in the remainder. But it is unsafe, in explaining our -various sensations, to lay too much stress on the mode of stimulation. -<span class="pagenum"><a name="Page_383" id="Page_383">[Pg 383]</a></span> -The mind judges sensations in the light of previous experience. In -anatomical language, the effect of sensations upon the personality -depends upon the paths which impulses follow in the brain, and the -associations which have been established by previous impulses which -have followed the same paths. The retina enables us to distinguish -tone and colour. By the variations in tone, the juxtapositions of -light and shade, we recognize form. All streams of impulses which do -not present tone-variations—do not, that is to say, reproduce the -details of a scene—are interpreted in terms of colour. Every child -discovers that the tedium of the intervals during which it is proper -that his eyes should be closed may be relieved by pressing his knuckles -against the lids. Although the world is shut out, a phosphene offers -itself for his consideration—a yellow or white disc of irregular form -with a red margin, changing into lilac bordered with green, and then -into yellowish-green with a blue edge. Such, if my recollection can -be trusted, were the pictures which I used to see as a boy; but no -adjustment of pressure calls them forth with anything like the same -vividness now.</p> - -<div class="figcenter"> - <a id="I_383" name="I_383"> </a> - <img src="images/i_p383.jpg" alt="" width="400" height="399" /> - <p class="center"><span class="smcap">Fig. 31.</span>—<span class="smcap"> - Simultaneous Contrast.</span></p> - <div class="blockquot2"> - <p class="neg-indent space-below2">The shading of the two V’s is exactly similar; - but the figure in half-tone on black appears brighter than the figure - in half-tone on a white ground.</p> -</div></div> - -<p>All the senses show a tendency to rebound after activity, exhibiting -contrast-phenomena; but the contrasts of vision are more marked and -varied than those of the other senses, as everyone who is curious in -the observation of his own sensations is aware. Negative after-images -are generally referred to the retina; but various other kinds of -after-image and contrast-phenomena must be attributed to the judgments -<span class="pagenum"><a name="Page_384" id="Page_384">[Pg 384]</a></span> -passed by the mind upon the sensations which it receives; and not to -physical changes in sense-organs. Positive after-images are well-marked -appearances, although less common, perhaps, than the phenomena of -reversal of sensation of which we have just written. On waking in the -morning, one looks at the window; shifting the gaze to the ceiling, -an after-image of the window appears, just as one saw it, with bright -panes and dark frame. The “dark adapted eye,” being exceptionally -sensitive, yields the same persistent positive after-image as the eye -in its usual condition yields, after being directed towards the sun -at mid-day. Movement-after-images can be explained only by referring -them to misdirection of judgment. If the gaze is fixed on a rock close -beside a waterfall, then shifted to a bank covered with grass or -bushes, the part of the bank which occupies the lateral part of the -field of vision appears to rush upwards, reversing the movement of the -water. When the gaze has been fixed upon falling water—a narrow stream -sparkling in sunlight—a central strip of the field moves upwards, the -margins remaining stationary. If one stares at the spot on the surface -of a basin of water on which drops are falling from a tap, and then -looks at the floor, it is seen to contract towards the spot looked at, -reversing the movement of the ripples in the basin. These observations -reveal a fact of great importance in the physiology of vision. It is, -probably, impossible truly to fix the gaze. The muscles of the eyeball -keep the retinal field in constant movement—larger movements with -minute oscillations superposed. When, as in watching a waterfall, -movement has for a time taken a definite direction, its cessation is -judged to mean reversal.</p> - -<p>The anatomical unit of sensation is a cone. The fovea centralis, the -only part of the retina capable of receiving sensations sufficiently -discrete for reading, contains cones alone. If the gaze be directed -but a very few millimetres on to the white margin of the page, letters -lose their form. In the fovea the centre of one cone is 3·6 µ distant -from the centre of the next. Two stars are visible as separate stars if -they subtend an angle of at least 60 seconds with the eye. Their images -on the retina are then 4 µ apart. Parallel white lines ruled on black -paper, held at such a distance as causes them to subtend angles of 60 -<span class="pagenum"><a name="Page_385" id="Page_385">[Pg 385]</a></span> -seconds with the eye, appear not straight but wavy, showing that -their images are taken up, not by a continuous substance, but by the -mosaic of cones. So far the explanation of the visual unit is strictly -anatomical; but it must be added that trained observers can recognize -the separateness of objects which subtend angles of much less than 60 -seconds—not more than 5 or 6 seconds. This can be accounted for only -on the hypothesis that images far closer together than the width of a -cone produce a specific effect in passing across the anatomical unit.</p> - -<p>In 1807 Thomas Young, the physicist, formulated a theory to account -for <b>colour-vision</b>. He supposed that the retina contains three -kinds of apparatus—<i>a</i>, <i>b</i>, and <i>c</i>—each especially -responsive to a particular kind of light, all three slightly stimulated by rays -of all colours. (Young imagined three kinds of nerve, but modern supporters -of his theory suppose three different substances chemically changed -by light.) A prism spreads out the rays which are combined in white -light into a band in the order of their wave-lengths—those which have -the longest wave-length (0·8 µ) and the slowest rate of vibration (381 -billions to the second) at one end, those which have the shortest -wave-length (0·4 µ) and the most rapid vibration (764 billions to the -second) at the other: between these two extremes every intermediate -grade of length and rapidity. These are a mere fraction—a small -group—of the waves which the æther transmits, but they are all that -we can see. The long, slow vibrations give rise to sensations which we -describe as red; the short, rapid vibrations we describe as violet. -Our names for the tints which intervene are singularly old-fashioned -and unsatisfactory, but all persons agree that they recognize in the -spectrum a certain number of definite colours. Some normal-sighted -persons say twelve, others eighteen. It is largely a question of terminology.</p> - -<p>Many considerations show that it is quite unnecessary to imagine -that the retina is affected in a different kind of way by every kind -of light, or by each of several groups of waves. If the red of the -spectrum is mixed with yellow, we receive an impression of orange, -which is identical with the impression produced by waves of the mean -length of red and yellow; orange and green give yellow; yellow and -blue, green. Any two complementary colours yield white. By taking three -<span class="pagenum"><a name="Page_386" id="Page_386">[Pg 386]</a></span> -colours—say, red, green, and violet—we obtain, when they are duly -mixed, not white light only, but light of any other tint, although not -of spectral purity, since it is mixed with white. Young considered that -all the conditions of colour-vision would be satisfied, all our various -sensations provided for, if the retina contain three kinds of apparatus -which light, according to its quality, affects in varying degrees; -and with this theory of three kinds of apparatus—<i>a</i>, <i>b</i>, and -<i>c</i>—the theory of three elementary or fundamental colour-sensations is -indissolubly linked. The colour <i>x</i> produces its intensest effect when -<i>a</i> is stimulated, with the least possible stimulation of <i>b</i> and <i>c</i>; -<i>y</i> is the reaction of <i>b</i>, <i>z</i> of <i>c</i>. Recent studies of the curves -of intensity give us the tints of <i>x</i>, <i>y</i>, and <i>z</i> as carmine-red, -apple-green, and ultramarine blue.</p> - -<p>The blending of sensations is illustrated with the well-known -colour-top. But perhaps the most striking proof that three elementary -colour-sensations are adequate to produce our visual world is afforded -by photographs taken with the three-colour method. Three plates are -exposed—(<i>a</i>) behind a red screen, (<i>b</i>) behind a greenish-yellow -screen, (<i>c</i>) behind a blue screen. They are fixed in such a way that -the portions acted upon by light are rendered insoluble, whereas the -rest of the film can be dissolved away; <i>a</i> is then stained red, <i>b</i> -greenish yellow, <i>c</i> blue. The three are superposed, and the result -appears to the eye as an exact reproduction of the subject of the -photograph in all its hues. It shows every shade of orange and green -and violet. It is as bright—that is to say, as full of white light—as -the original.</p> - -<p>Various objections may, however, be brought against Young’s theory. -Of these, the most weighty are: (1) The retina does not contain three -kinds of apparatus, as Young supposed; nor can we find three kinds of -photochemical substances, as required by the theory in its modern form. -If we could find them, a fresh difficulty would arise; for we have no -reasons for supposing that one and the same nerve-ending can receive -stimuli of three different kinds. (2) The theory offers no explanation -of negative after-images—the complementary colours experienced -when the eye is closed after staring at a brightly coloured object. -(3) It does not adequately account for the various deficiencies of -colour-blindness. -<span class="pagenum"><a name="Page_387" id="Page_387">[Pg 387]</a></span></p> - -<p>It is well recognized that there are various degrees of -colour-blindness, and that the colour-vision of persons considered -normal presents different grades of refinement. Nevertheless, the -abnormalities of colour-blind persons are so marked that cases fall -into definite classes. Those whose cones do not function—which means -that their yellow spots are either undeveloped or diseased—see all -things grey. They are totally colour-blind. Excluding these, the -colour-blind may be grouped in one or other of two divisions—(<i>a</i>) -those who confuse red and green, (<i>b</i>) those who confuse yellow and -blue. One person out of every thirty-five is red-green blind. The -proportion is even higher if males only are considered, showing -how very unfortunate is our choice of warning signals. A man who -is red-green blind cannot tell the port from the starboard light. -Blue-yellow blindness is, on the other hand, extremely rare. According -to Young’s theory, colour-blindness is due to the absence of one of the -three sets of visual apparatus. But cases do not altogether conform to -this hypothesis. We knew an amateur water-colourist, since deceased, -who derived intense pleasure from the beauties of Nature, and showed -no mean skill in reproducing them with his brush, notwithstanding the -fact that he was red-green blind. Each night his sister arranged his -paint-box for him, and only rarely did he use vermilion to fill in a -foreground of lush green grass. But this mistake, when he made it, did -not destroy his own satisfaction in the picture. It was clear that -red had a value for him, although he confused it with green. It is -impossible for a normal person to see through the eye of one who is -colour-blind, and there is no other means of comparing his sensations -with our own. The mistakes which the colour-blind make in sorting -coloured objects and in naming mixtures of light selected from various -parts of the spectrum show the range of their deficiency, but give us -no information regarding the qualities of the sensations which they retain.</p> - -<p>The test of colour-sensitiveness usually employed is the grading of a -large number of wools of different tint. The order in which the colours -should be arranged is not a matter of opinion. They must be placed -in the order in which they occur in the spectrum—<i>i.e.</i>, arranged -according to their wave-lengths. In the cases of colour-blindness which -<span class="pagenum"><a name="Page_388" id="Page_388">[Pg 388]</a></span> -are most frequently met with the defect may be described as due to -an absence of the sense of redness, or as an absence of the sense of -greenness. The two conditions can be distinguished. But since the eye -is not dark for red (although in certain cases vision is very weak for -the red end of the spectrum) or dark for green, the abnormality cannot -be adequately accounted for on structural grounds. It is not explicable -on the hypothesis that one of three sets of responsive sense-organs -(or nerve-fibres) or photochemical substances is absent from the eye. -Again, it is generally agreed that the sensations of white, yellow, -and blue of the red-green colour-blind are similar to those of normal -persons. This is not in harmony with the theory of the omission from -their eyes of one of three pieces of colour-apparatus.</p> - -<p>Professor Hering, of Leipsic, adopting the generally accepted view that -light effects chemical changes in substances contained in the retina, -to which changes stimulation of nerve-endings is due, formulated a -theory of colour-vision which many physiologists prefer to Young’s. He -imagines that the retina contains three kinds of pigment, each of which -is, as he believes all living substance to be, in a constant state of -change. It is at the same time being built up and destroyed. Using -the terms which connote the opposite directions of metabolism, the -pigment is simultaneously undergoing anabolism and katabolism; the two -processes, when the retina is at rest, maintaining equilibrium. When -light acts upon either of the substances, it hastens, according to its -quality, either the one process or the other; and the chemical change, -whether it be constructive or destructive, stimulates the endings of -optic nerves. Hering assumes, therefore, that there are six elementary -qualities of visual sensation—red, green, yellow, blue, white, black. -Red, yellow, white are due to anabolism of the visual substances; -green, blue, black are due to their katabolism. The installation of -yellow amongst the unanalysable colours is a relief to many minds. It -is almost impossible to think of yellow as a compounded colour. White -also, we feel, is not a compounded colour, despite our knowledge that a -prism scatters from it all the hues of the rainbow. Black, many persons -assert, gives them a definite sensation, and not merely a sense of -<span class="pagenum"><a name="Page_389" id="Page_389">[Pg 389]</a></span> -rest. (Parenthetically, it may be observed that the <i>feeling</i> that a -colour is pure or mixed is not to be trusted. It may be based upon -the chromatic aberration of the eye, or it may be reminiscent of the -paint-box. We know that we cannot make yellow by mixing red and green -pigments, hence we feel that it is pure. Of green we are not by any -means sure; gamboge and Prussian blue come into our minds.) Except when -the light which falls upon the retina is giving rise to one of the -four pure colour-sensations, all three substances are simultaneously -affected, although one may be undergoing katabolism while the other two -are being built up, or <i>vice versa</i>. Hering accounts for simultaneous -contrast by assuming that the activity of any one part of the retina -induces an opposite kind of change in the remainder, and especially -in the vicinity of the primarily active part. When a certain patch -is developing a sensation of red, the rest of the retina develops a -sensation of green.</p> - -<p>The great merit of the theory is, however, to be found in its offering -an explanation of complementary after-images. The green patch seen -with closed eyes after one has stared at a red object is due to -the rebound of metabolism. In returning to a condition of chemical -equilibrium the retinal substance acts as a stimulant which evokes -the antagonistic colour. But it is a theory which makes very large -assumptions. It assumes, for example, the possibility of the existence -of a substance which is built up by light from one end of the spectrum, -and decomposed by light from its centre. Not that Hering regards the -existence of three retinal substances as essential to his theory. He -is prepared to transfer to the brain the seat of the substances, or -the substance, which, by their, or its, anabolism and katabolism, -produces antagonistic colour-perceptions; but in this he is abandoning -physiology for metaphysics. We have no warrant for imagining that there -exists in the brain any substance which, by undergoing physical changes -of various kinds, produces various psychical effects. The problem to -be solved is physiological. Rays of light of different wave-lengths -excite the retina to discharge impulses which are variously distributed -in the brain. The effects which they produce in consciousness depend -upon their distribution. The impulses to which the longest rays give -rise evoke sensations of red, those due to the shortest, sensations of -<span class="pagenum"><a name="Page_390" id="Page_390">[Pg 390]</a></span> -violet. And what is true of the retina as a whole is true, apparently, -of each individual cone. In what way does light act upon a cone? It -is one of the most fascinating problems in physiology. Round it our -thoughts revolve whenever we are trying to form conceptions of the -nature of stimulation, sensation, and perception. Each of the two -theories which we have expounded above helps to group together certain -of the more striking phenomena of colour-vision, but neither gives a -satisfying explanation of their causation.</p> - -<p>The sensitiveness of the retina is in a remarkable degree adjusted to -the intensity of the light. When a dark room is entered, the pupil -dilates; but one’s power of distinguishing objects continues to -increase after the pupil has reached its maximum size. At the end of -ten minutes the eye may be twenty-five times as sensitive as it was -when the room was entered. This <i>adaptation to darkness</i> is due in -large degree to the substitution of rods for cones as the organs on -which vision chiefly depends. But it cannot be wholly due to this, -since it occurs when one is working with a red light. Probably the red -used in a “dark-room” is not sufficiently near the end of the spectrum -to be completely without influence upon visual purple, but it is a -colour to which rods are comparatively insensitive. Other evidence also -points to an adaptation of cones as well as of rods. -<span class="pagenum"><a name="Page_391" id="Page_391">[Pg 391]</a></span></p> - -<div class="figcenter"> - <a id="I_391" name="I_391"> </a> - <img src="images/i_p391.jpg" alt="" width="600" height="208" /> - <p class="center"><span class="smcap">Fig. 32.</span>—<span class="smcap">The - Formation of an Image by the Refracting Media of the Eye.</span></p> - <div class="blockquot2"> - <p class="neg-indent space-below2"><i>x</i>, The common centre of curvature (nodal point of the - several media). Rays which pass through this point are not deflected. <i>y</i>, The - principal focus of the system. All rays which are parallel to the optic - axis converge to this point. The image of the point A is formed at <i>a</i>, - the spot at which a ray parallel with the optic axis meets an unbent - ray—the image of B at <i>b</i>.</p> -</div></div> - -<p><i>Accommodation of the eye for distance</i> is brought about by a mechanism -which allows the lens to change in shape. It becomes more convex when -a near object is looked at than it was when adjusted for an unlimited -distance, which is its condition when the eye is at rest. Adjustment -for near objects involves muscular action, and is accompanied by a -sense of effort, however slight. Whilst the eye is at rest the lens is -mechanically compressed against the anterior layer of its suspensory -ligament. Accommodation for near vision is effected by the ciliary -muscle, which is placed in the shelf of tissue which projects into -the interior of the eyeball. This muscle is made up of a ring of -circular fibres, and to the outer side of this, of fibres which radiate -backwards and outwards. The longitudinal, or radiating, fibres obtain -their purchase by attachment to the firm wall of the globe just beyond -the cornea. They spread into the front of the loose chorioid membrane -which lines the eye behind the retina. By the joint action of these two -sets of plain muscle-fibres the suspensory ligament is slackened, and -the extremely elastic lens, previously compressed, bulges forwards. -The radius of curvature of its anterior surface changes from 10·3 -millimetres for distance to 6 millimetres for vision at the “near -point.” It was stated, in connection with the development of the lens -(<a href="#Page_374">p. 374</a>), that the cells of the posterior half of the hollow -sphere out of which it is formed grow forwards into extremely long fibres, which -traverse its whole thickness. These fibres are bent like the segments -of a carriage-spring. Their anterior ends rest against the flattened -ligament of the lens; the vitreous humour, which is always under -tension, compresses their posterior ends. When removed from the eye, -the lens becomes rounder than it is <i>in situ</i>, even when accommodated -for near objects. But in later life it grows stiff. It ceases to bulge -forwards when its ligament is slackened. Hence it becomes necessary -to aid the presbyopic eye with convex glasses when it is used for -near objects, although for distant vision it remains as effective as -ever. If the ciliary muscle is constantly and completely relieved of -the labour of accommodation, it grows lazy, or rather wastes from -want of use. A person who relies on spectacles loses his power of -accommodation; but ophthalmologists agree that self-focussing, if it -give rise to a sensation of strain, is bad for the eyes. In myopic -persons the eyeball is too deep; objects are focussed in front of the -retina. In hypermetropia (“long sight”) the eyeball is too shallow; -<span class="pagenum"><a name="Page_392" id="Page_392">[Pg 392]</a></span> -objects are focussed behind the retina. Concave glasses correct -the one condition, convex glasses correct the other. Glasses are -also very commonly called for to neutralize another defect—regular -astigmatism—which may be present by itself, or may accompany -insufficient length or too great length of the optic axis. It is due -to unequal curvature of the cornea. Usually the curvature is sharper -in the vertical than in the horizontal meridian (<a href="#Page_269"><i>cf.</i> p. 269</a>); -as a consequence, points in a vertical line are focussed in front of points -<span class="pagenum"><a name="Page_393" id="Page_393">[Pg 393]</a></span> -in a horizontal line. Cylindrical glasses, not lenses, are required to -correct this defect. And here it may be well to call attention to the -fact that rays of light are more sharply refracted by the surface of -the cornea than they are by the crystalline lens. The lens has a high -index of refraction (1·45), but it does not lie in air (the index of -refraction of which is 1), but between two humours which have about the -same index as water—namely, 1·336. The bending by the combined action -of the cornea and the lens of rays of light which come from a source so -distant that they may be considered as parallel brings them to a focus -on the retina, when the lens is at its flattest. When the lens is at -its roundest, rays which diverge from a point only 5 inches in front of -the eye are focussed on the retina. The lens is therefore essential for -accommodation, but, after its removal for cataract, vision, even for -near objects, is rendered possible by the use of convex glasses.</p> - -<div class="figcenter"> - <a id="I_392" name="I_392"> </a> - <img src="images/i_p392.jpg" alt="" width="500" height="590" /> - <p class="center"><span class="smcap">Fig. 33.</span></p> - <div class="blockquot2"> - <p class="neg-indent space-below2">A, The normal eyeball, in which, when the ciliary muscle is - relaxed, parallel rays are brought to a focus on the retina. B, A hypermetropic - eyeball. Its depth being less than normal, parallel rays are not - brought to a focus on the retina when the eye is adjusted for distant - vision without the aid of a convex glass. C, A myopic eyeball. Its - depth being more than normal, a concave lens is needed to diminish the - convergence of parallel rays.</p> -</div></div> - -<p>A star or a distant gas-lamp is seen as a point of light with rays. -Usually this figure, which has given origin to the expression -“star-shaped,” shows three greater rays alternating with three lesser -rays. Such an image is not produced by a point of light near to the -eye, since it is due to the puckering of the lens when flattened -against its ligament. It brings into evidence the three axes on the -front of the lens and the three axes which alternate with them on the -back, with regard to which the lens-fibres are disposed.</p> - -<p>As an adaptation of living tissues to optical purposes the eye is above -admiration, yet it presents many <b>defects</b>, which an optician -corrects in the instruments which he manufactures. A remarkable fact in -the physiology of vision is our unconsciousness of the imperfections -of its organ. An unusual experiment is needed to bring them to our -notice. If we look through a common glass lens uncorrected for unequal -refraction of rays of different wave-lengths, we recognize that a -bright object is shown with a colour-fringe, yet we take no cognizance -of the colour-fringes which surround the images of all bright objects -focussed upon our retinæ. If we think about the matter, we recognize -a feeling that blue in a window of stained glass appears farther away -than red; but this might well be due to association. Blue glass is -chiefly used for the sky. If we look at a bright object through purple -<span class="pagenum"><a name="Page_394" id="Page_394">[Pg 394]</a></span> -glass, we her red with a blue fringe or blue with a red fringe, -according as the eye is focussed for red or for blue. The purple glass -having absorbed all intermediate rays, we become aware that we cannot -focus the two extreme ends of the spectrum at the same place. Since a -greater effort of accommodation is needed to focus red, we judge that -the bright object is nearer to us when it appears red than when it -appears blue.</p> - -<p>Spherical aberration is another fault of the lens. The rays which -enter its margin are brought to a focus sooner than those which pass -through its centre. This is due to the fact that its surfaces are -regularly curved, whereas a glass lens is corrected by grinding it -flatter towards the margin. This defect is partly corrected by the -cornea, which has an ellipsoidal surface, and partly by the greater -density of the centre of the lens. Yet it is still necessary for the -eye to be “stopped down” by the iris when a near object is looked at, -although less light is entering the eye than when it is directed to -the horizon—a condition which would lead a photographer to open -his iris-diaphragm.</p> - -<div class="figcenter"> - <a id="I_394" name="I_394"> </a> - <img src="images/i_p394.jpg" alt="" width="600" height="173" /> -</div> - -<p>Of all the imperfections of the eye which the mind ignores, the most -remarkable is the gap in the field of vision, due to the gap in the -sensitive layers of the retina, which occurs where the optic nerve -enters it—the blind spot. Hold this page of the book 10 inches from -the face, keeping the lines of print horizontal. Close the left eye and -look at X with the right eye. The black disc disappears, because its -image is focussed on the blind spot. Since the picture on the retina -is reversed, it is clear that the optic nerve enters the globe to its -inner side, and slightly above its horizontal meridian. But, unless we -employ an unusual test, we are quite unconscious of the fact that a -definite hole is punched in the picture. The mind fills it in, and the -<span class="pagenum"><a name="Page_395" id="Page_395">[Pg 395]</a></span> -way in which it does so is extremely suggestive. It lies about -it—in a downright ingenuous fashion if it is confident of -credence, in a more subtle way if a simple falsehood is likely to -be challenged. In place of the black disc make nine conspicuous crosses:</p> - -<div class="figcenter"> - <a id="I_395" name="I_395"> </a> - <img src="images/i_p395a.jpg" alt="" width="600" height="116" /> -</div> - -<p>Hold the paper in such a position that <i>X</i> falls upon the blind -spot. It ought to disappear, but the mind assures you that -there is a cross at that spot. The mind completes the field. -In place of the crosses use noughts and crosses, thus:</p> - -<div class="figcenter"> - <img src="images/i_p395b.jpg" alt="" width="600" height="133" /> -</div> - -<p>Now let <i>X</i> fall on the blind spot, and allow the eye to go just -a little out of focus. The four marginal crosses draw inwards:</p> - -<div class="figcenter"> - <img src="images/i_p395c.jpg" alt="" width="600" height="177" /> -</div> - -<p>The mind contracts the field. Still denying the gap, but not having -sufficient data from which to invent an object, the fraudulent -nature of which would not be found out the instant that the gaze is -shifted, the mind lies regarding the position on the paper occupied by -surrounding objects.</p> - -<p>Is it quite fair to the mind to say that it lies about the blind spot? -The mind judges sensations in the light of experience. An association -of previous sensations teaches me that the wall of the room is not -pierced by a round hole a foot in diameter opening into outer darkness. -<span class="pagenum"><a name="Page_396" id="Page_396">[Pg 396]</a></span> -Many sensations to me the fact that the designs on a wall-paper succeed -one another with unbroken regularity. Fixing my gaze on one of them, I -cannot by any effort of attention efface the pattern which happens to -be focussed on the blind spot. I know that I shall see it the instant -that I move the eye. If I let my eye roam until the face of my wife -falls on the blind spot, its image disappears. I know its lineaments -far better than I know the pattern on the wall-paper, but I cannot -fill it into the picture. Her hands are visible, and the work which -is resting in her lap, but in a mysterious way the background draws -together where the face should be. My mind refuses to pass a false -judgment; but it also refuses to see that there is a gap.</p> - -<p>This exceedingly instructive observation teaches the relativity of -sensations. It shows that a sensation has no objective value until -judgment has been passed upon it by the mind. The meaning of this -we express in figurative language, none other being available. We -speak of a new sensation as being compared with sensations previously -received—taken into the picture-gallery of the mind, and placed in -its due position amongst the infinitely numerous records which are stored -there. If we try to make a nearer approach to correlating physical with -psychical activity, we say that sensation has no value save that which -it acquires from its temporal relation in the sequence of sensations -to which attention is directed, and that this value depends upon the -relation which similar sensations have possessed in former sequences. -There is no gap in binocular vision. An object focussed on the inner -(nasal) side of the right eye, where the blind spot is situate, is -focussed on the outer (temporal) side of the left eye. The left eye -sees the object to which the right eye is blind. Since we have almost -invariably used two eyes in the past, experience teaches that there is -no gap in the field of vision. Hence the new group of sensations which -alleges that there is a gap must be corrected. The field must be filled -up in the way which experience shows to be most likely. The retina is -a sheet of rods and cones, each of which has a nervous connection with -the brain proper to itself. The retinal field is associated with the -brain-field. But this does not imply that we may think of the mind as -<span class="pagenum"><a name="Page_397" id="Page_397">[Pg 397]</a></span> -having a spatial distribution on A or button B in the retina causes -bell A´ or bell B´ to ring in the brain, but it does not follow that -perception A´´ or perception B´´ will be heard in the mind. It will be -heard if this is the association established by custom, since mind is -the product of experience. But the new sensation is creating precedent -as well as being judged by it.</p> - -<div class="figcenter"> - <a id="I_398" name="I_398"> </a> - <img src="images/i_p398.jpg" alt="" width="400" height="335" /> -</div> - -<p>Point A in the right retina is associated by experience with point <i>a</i> -in the left, and point B with <i>b</i>. These are termed <i>corresponding -points</i>, because they are similarly stimulated in binocular vision. -The mind, therefore, judges that it receives the same information from -each pair of corresponding points. The position of corresponding points -will be understood if the right retina is imagined as put inside the -left, precautions being taken to make the yellow spots coincide, and -to avoid twisting the retinal cups in taking them out of the eyeballs. -Great care is taken to maintain the points in correspondence during the -various movements of the two eyeballs. In addition to the four recti -muscles which move the eyeball upwards, downwards, to right and left, -two oblique muscles give it the requisite amount of rotation. We have -learned to give the same value to the impulses from two corresponding -points. But under changed conditions the correspondence changes. When -a squint develops in childhood, it follows one of two courses; either -the obliquity of one of the eyeballs increases until it looks towards -the nose, and its images cease to interfere with the images in the -dominant eye—they are ignored by the mind—or a fresh correspondence -is established between points in the oblique eye and points in the eye -which looks straight forward. If we are severely critical, we find, -from a study of the form of the eyeball, that it is impossible that the -same rods and cones should occupy corresponding points in different -positions of focus and with different degrees of convergence of the -eyeballs. To permit of this the retinal cups would need to change in -shape. But again mechanical correspondence is of little consequence. In -the light of experience the mind judges that points correspond. When -we are gazing at a flat surface, the mind judges that corresponding -points are giving it similar information. It does not see a flower on a -<span class="pagenum"><a name="Page_398" id="Page_398">[Pg 398]</a></span> -wall-paper twice as bright or twice as red with two eyes as with one. -If the eyes are normal, the impression received through the two is -precisely the same as the impression received through either singly. -But when we are looking at solid objects, the image on one retina is -not the same as the image on the other. One eye sees farther round -the object on the one side, the other on the other; and it is just -this disparity in the pictures, aided by the feeling that the eyes -are converging, that gives the impression of solidity. Correspondence -of points, on the other hand, is not necessarily sufficient by itself -to convince the mind that the pictures presented by the two eyes are -identical. When a flat triangle such as this is regarded with the two -eyes, its black lines fall on corresponding points; but the figure is -associated in the mind with other sensations—sensations of movement -and touch. Notwithstanding the identity of the retinal images, the mind -tries to see them as disparate. The figure troubles the eyes. At one -moment the meeting-point of the three central lines projects forwards, -at the next it recedes. That similarity of retinal images counts for -something is shown by closing one eye. The uncertainty of shape of the -figure is rendered more troublesome. It changes still more rapidly from -convex to concave. When the point seems to be in front of the page, the -accommodation of the eyes is adjusted for nearness; when behind the -page, for greater distance. But the illusion that the object occupies -three dimensions is not dependent upon the sense of contraction of the -ciliary muscle. When the paper is moved towards the eye, its centre -recedes; it is left behind until the ciliary muscle has had time to -contract. When it is moved away from the eye, it projects until the -ciliary muscle has had time to relax. Accommodation follows judgment, -<span class="pagenum"><a name="Page_399" id="Page_399">[Pg 399]</a></span> -not judgment accommodation. The mind is extremely suspicious of the -veracity of its newsagents. Disparateness of images, convergence of the -eyeballs, shifting of accommodation for the various levels of an object -in space, should be indisputable evidence of solidity or of hollowness. -Conversely, the absence of either factor should be conclusive proof of -flatness. But the mind does not trust to isolated sensations; it looks -for associations of sensations. When the finger hints, “I could touch -that sharp point,” it is useless for the eye to aver that there is no -point to be touched.</p> - -<p>If two exactly similar photographs are placed in a stereoscope, the -fact that the eyes are not converged gives to the common picture an -appearance of depth, notwithstanding the fact that corresponding points -on the two retinæ are stimulated. If the two photographs have been -taken, as they should be taken for this purpose, with a double camera, -the disparity of the retinal images immensely enhances the impression -of solidity.</p> - -<p>It is impossible to exaggerate the dependence of sensation on -<b>judgment</b>. At birth a child commences the long process of -education which enables it to associate the sensations derived from -its retinal images with the movements which place it in contact with -things. It discovers that, when it is necessary to make the eyes -converge, the object is near at hand. It also associates the voluntary -action of contracting its ciliary muscle with nearness. Unconverged and -unaccommodated eyes come to mean distance. So, too, do indistinctness -due to absorption by the atmosphere, blueness due to the same cause, a -small image on the retina. But there are obvious limits to its power -of ascertaining the distance of an object, and therefore, conversely, -of its power of estimating size. We have no idea of the size of the -retinal image of the sun. Very few people would be prepared to believe -that the angle which the sun subtends with the eye barely exceeds half -a degree. (The first finger, viewed in profile, at arm’s length, covers -one degree of arc.) A disc of paper of the right size, placed at the -right distance, looks far too small to represent the sun. The most -brilliant of orbs bulks larger than this in our minds. Everyone who for -the first time looks at the sun through well-smoked glass, or, better, -<span class="pagenum"><a name="Page_400" id="Page_400">[Pg 400]</a></span> -through a flat-sided vessel filled with ink and water, is astonished -that it looks so small. Nor are we prepared to accept the evidence of -a camera that the sun at the zenith does not produce a smaller image -on the retina than the sun when rising above the horizon. Yet if a -photographic plate is exposed to the rising sun, and again, without -changing its focus, to the sun at the zenith, the two images are -practically equal. There is a slight difference due to the greater -refraction of rays passing tangentially through the atmosphere, but -it is so slight as to bear no relation to the difference between our -two judgments of size. When the sun is rising behind trees and houses, -we compare it with objects which we know to be large and distant; -yet it looks almost as large when rising out of the sea. One of the -causes of the illusion is our conviction that the sky is flattened; -and this, again, is due partly to its paler tint—its less substantial -blueness—near the horizon, and partly to our impression that it is -spread out over a flat earth. When the sun is in what we deem to be the -more distant part of the vault of heaven, we judge it to be farther -from us, and therefore larger than when it is above us. Yet the last -word has not been said in explanation of a phenomenon which has been -studied by mankind since the dawn of science. Helmholtz attributed the -apparent greater distance, and consequent greater size, of the sun -and moon when near the horizon to the indistinctness of their discs. -When its image is so reflected from the zenith as to cause the moon -to appear to rest upon the horizon, it does not, he said, increase in -size. In answer to Helmholtz’s explanation, it may be objected that, -when at midnight he brought the full moon down from the zenith, he did -not bring with her the conditions of light and colour by which she is -customarily surrounded when floating on the horizon. If, when watching -the moon which has just risen, vast in diameter, out of the sea, one -interposes between it and the eye a sheet of paper in which a small -hole has been made, and looks at the moon with one eye through the -hole, it instantly shrinks to the size which it appears to have at the -zenith. It is not even necessary to blot out the whole of its trail of -light on the sea. At the same time, it appears to retreat to a great -distance. This shows how complicated are the associations upon which -judgments of size and distance are based, and to how small an extent -<span class="pagenum"><a name="Page_401" id="Page_401">[Pg 401]</a></span> -they are determined by the size of the image on the retina. This -observation is most surprising if made one or two nights after full -moon, when twilight is already dim at moon-rise.</p> - -<p>Our estimate of the distance away from us of an object on the horizon -is based upon the time and effort which experience tells us we should -need to spend in reaching it. The untried appears shorter than the -tried. Anyone who compares his feeling of the number of yards he would -have to climb up a pole reaching to the zenith with his feeling of -the number of steps he would need to take to reach the horizon will -recognize that the horizon appears to him to be the farther away.</p> - -<div class="figcenter"> - <a id="I_401" name="I_401"> </a> - <img src="images/i_p401.jpg" alt="" width="400" height="413" /> - <p class="center space-below2"><span class="smcap">Fig. 36.</span>—<span class="smcap">A - Symmetrical Arch, divided by a Vertical Line, A,<br /> which passes through - its Apex.</span></p> -</div> - -<p>In representing a solid object an artist conveys theidea that light -is falling obliquely upon it. One side of the object, therefore, is -more strongly illuminated than the other. By depth and gradation of -shade he indicates the extent to which the thing projects forwards, if -solid, or falls back, if hollow. He makes the margin of a ball hazy, -in the expectation that the spectator will look at the spot nearest -to him—an artifice which he may easily press too far, since the eyes -wander restlessly over a flat surface. In representing distance he -is dependent upon giving to the various objects in his picture sizes -equivalent to the sizes of their images on the retina, making them -brighter or paler and more or less distinct. Yet he cannot hope to -simulate the convincing evidence of distance which is afforded by our -<span class="pagenum"><a name="Page_402" id="Page_402">[Pg 402]</a></span> -sense of the degree of convergence of our eyes. Hence, as Francis Bacon -pointed out, a picture appears more real when one eye is closed than -when both are open. Its middle distance at once falls back.</p> - -<div class="figcenter"> - <a id="I_402" name="I_402"> </a> - <img src="images/i_p402a.jpg" alt="" width="500" height="250" /> - <p class="center space-below2"><span class="smcap">Fig. 37.</span>—<span class="smcap">Two Horizontal Lines of Equal -Length—the One with Diverging,<br /> the Other with Converging, Terminal Lines.</span></p> -</div> - -<p>Innumerable are the illustrations which may be given of errors -of sensory judgment, but none are more striking than the various -figures which may be drawn with converging or diverging lines. The -mind under-estimates acute and over-estimates obtuse angles. It is -impossible to convince oneself that in <a href="#I_401">Fig. 36</a> the line -A bisects a symmetrical arch. Equally difficult is it to believe that in -<a href="#I_402">Fig. 37</a> the line with diverging terminal segments and the line -with converging terminal segments are of exactly equal length. In the Ruskin Museum -at Sheffield there is a sketch by the master of the façade of a church -which shows a vertical tower to one side of a triangular pediment, or, -rather, this is what the sketch was meant to show, and does show, when -measured on an architect’s table. In effect the tower appears to be -leaning towards the pediment. Errors of judgment of this type have been -attributed to the curvature of the lines of a rectilinear image on the -retina, the mind judging the distance between two points by the length -of the chord, and not the length of the arc which joins them. This is -very simply illustrated by the example of the apparently greater length -of a filled space than of a vacant one.</p> - -<div class="figcenter"> - <img src="images/i_p402b.jpg" alt="" width="400" height="100" /> -</div> - -<p>A B looks longer than B C. If A B C be represented as a curved line, -the arc A B will, of course, be longer than the chord B C. But it is -not safe to suppose that the mind compares the length of an arc with -the length of a chord. Judgment is based upon experience, and probably -the illusion is due to more subtle causes than the curvature of the -<span class="pagenum"><a name="Page_403" id="Page_403">[Pg 403]</a></span> -retina. The mind does not look at the retina. If it did, it would -find the reversal of the picture the least of the inaccuracies which -it had to correct. It would find it very difficult, for example, to -superpose in its stereoscope the photographs of a vertical tower -taken simultaneously by the right eye and the left. The curved images -on the retina of the vertical lines which define the angles of the -tower, as seen with one eye, could not be made to correspond with -the images focussed by the other eye. The Greeks felt this when they -settled the form of a column. The canon of the swelling entasis and -increasing taper above it did not destroy the appearance of uniform -thickness which the shaft presented. It gave to the eye just the slight -help which it needs to enable it to picture the shaft as of the same -thickness from base to capital.</p> - -<hr class="chap" /> -<p><span class="pagenum"><a name="Page_404" id="Page_404">[Pg 404]</a></span></p> -<div class="chapter"> - <h2>CHAPTER XIV<br /><span class="h_subtitle">HEARING</span></h2> -</div> - -<p>The ear, like the eye, records amplitude of vibration; loudness. It -also records rapidity of vibration, musical pitch, which corresponds -with colour. But it seems to have a more difficult task than the eye, -since it has to analyse, or at any rate has to transmit information -regarding the form of compound vibrations. The meanings of these -distinctions may be illustrated by reference to a tracing on the -cylinder of a phonograph. A needle attached to the posterior surface of -the thin metal plate against which one speaks scratches the surface of -a rotating cylinder of hardened wax. Examined with a lens, the record -is seen to be an irregularly changing line. The depth of the marks is a -measure of loudness. Their varying number in a given time indicates the -changing pitch of the voice which produced them. Their form is a record -of the quality of its tone. The work of the ear, so far as it consists -in the estimation of the amplitude and rapidity of pulsations of sound, -is easy to describe, but the acoustics of form are complicated.</p> - -<p>Light is transmitted as vibrations of æther. They are transverse to -the direction in which the light is travelling. Sound cannot travel -through a vacuum, since it is dependent upon displacements of material -particles. The particles move forwards and backwards in the direction -in which sound is progressing. Sound is a sequence of pulsations, -alternate condensations and rarefactions of the media which conduct -it. Their particles are first pressed together, and then rebound to -positions farther apart. A sequence of to-and-fro movements, each -smoothly continuous throughout the whole duration of a pulsation, would -produce a pure musical tone. Tuning-forks carefully bowed settle down -after a few seconds into unbroken oscillations, which convey to the air -the to-and-fro movements of pure tones. Such tones vary in nothing but -loudness and pitch. If their pulsations are slow, we speak of the pitch -<span class="pagenum"><a name="Page_405" id="Page_405">[Pg 405]</a></span> -as “low”; if they are rapid, we say that their pitch is high. But if -the sound produced by tuning-forks (and low-toned stopped organ-pipes) -be omitted from the list, no pure tones reach our ears. The notes -of flutes, fiddles, trumpets, pianos, have each a certain “quality” -characteristic of the instrument. Even in a violin the G string has -not the same timbre as the D string. Owing to the elasticity of the -substances which originate and of the substances which transmit sound, -its pulsations are not simple to-and-fro movements, uninterrupted from -beginning to end. Each pulsation is partially broken at intervals; -and the quality of the sound depends upon the number and relative -accentuation of these partial interruptions. Sound travels through -air at the rate of 1,100 feet per second. This figure, divided by the -number of vibrations per second of a tone, gives the wave-length in -air of a tone of that particular pitch. For example, the middle C has -a vibratory rate of 256. Its wave-length is, therefore, somewhat over -4 feet. The lowest tone of an organ has a wave-length of 37 feet; its -highest of 3½ inches. These figures give no information, however, -regarding the movement of the particles which pass on the sound. When -air is transmitting a note—say the middle C—its separate molecules -do not move through a distance of 4 feet. Each molecule moves but -a short distance, varying with the loudness of the tone; but the -“wave” of crowding runs straight forward from the piano-string to the -ear, the molecules at the end of each stage of 4 feet taking on a -backward movement, so that the crowding, so far as the molecules of -that particular section are concerned, returns to its starting-point. -Between the piano-string and the ear there is a crowding and forward -movement at 0, 4, 8, 12 ... feet; a spreading and backward movement at -2, 6, 10, 14 ... feet. Most illustrations which are intended to aid -the mind in forming a definite picture of the transmission of sound -are liable to be misinterpreted, because they translate rectilinear -movements into waves. They represent the movements of the string, and -not the movements of the molecules of air between the string and the -ear; but with the aid of the imagination one may picture the positions -of the particles in this path. The pulse, we will suppose, has just -reached the limit of 12 feet. Half-way from its 8-foot halting place -the molecules are again crowded, although not so densely. One-third of -the distance from the same point there again appears a tendency to -<span class="pagenum"><a name="Page_406" id="Page_406">[Pg 406]</a></span> -crowd. This latter point marks an interval of one-third of this wave -<i>plus</i> the wave which led up to it. At the end of the ninth foot there -is a crowding, though less marked—this wave <i>plus</i> the two preceding -waves, divided into fourths. Within these intervals are other points -at which the molecules have closed together, the distances from a -nodal point depending upon the number of waves involved, and, speaking -generally, growing less marked as the number increases. Such are the -very complex pulsatile movements which reach the ear.</p> - -<p>Every musical sound produced by a piano, a violin, or other instrument, -is compounded of a fundamental or prime tone, and overtones, partial -tones, or harmonics. The following table shows the more important -partial tones which accompany the prime tone when the middle C on a -pianoforte is struck:</p> - -<table border="0" cellspacing="0" summary="Overtones for Middle C" cellpadding="0" > - <thead><tr> - <th class="tdc bb">Note. </th> - <th class="tdc bb" colspan="2">Number of<br /> Vibrations. </th> - <th class="tdc bb">Interval.</th> - <th class="tdc bb"> Ratio. </th> - <th class="tdc bb">Number of<br />Overtone.</th> - </tr> - </thead> - <tbody><tr> - <td class="tdl">C‴</td> - <td class="tdr">2,048</td> - <td class="tdc" rowspan="3"><img src="images/cbr-3.jpg" alt="" width="16" height="57" /></td> - <td class="tdl_ws1"> </td> - <td class="tdc"> </td> - <td class="tdc">7th</td> - </tr><tr> - <td class="tdl"> </td> - <td class="tdr"> </td> - <td class="tdl_ws1">Super-Second</td> - <td class="tdc">8/7</td> - <td class="tdc"> </td> - </tr><tr> - <td class="tdl"> </td> - <td class="tdr"> </td> - <td class="tdl_ws1"> </td> - <td class="tdc"> </td> - <td class="tdc"> </td> - </tr><tr> - <td class="tdl">B″♭</td> - <td class="tdr">1,792</td> - <td class="tdc" rowspan="3"><img src="images/cbr-3.jpg" alt="" width="16" height="57" /></td> - <td class="tdl_ws1"> </td> - <td class="tdc"> </td> - <td class="tdc">6th</td> - </tr><tr> - <td class="tdl"> </td> - <td class="tdr"> </td> - <td class="tdl_ws1">Sub-minor third</td> - <td class="tdc">7/6</td> - <td class="tdc"> </td> - </tr><tr> - <td class="tdl"> </td> - <td class="tdr"> </td> - <td class="tdl_ws1"> </td> - <td class="tdc"> </td> - <td class="tdc"> </td> - </tr><tr> - <td class="tdl">G″</td> - <td class="tdr">1,536</td> - <td class="tdc" rowspan="3"><img src="images/cbr-3.jpg" alt="" width="16" height="57" /></td> - <td class="tdl_ws1"> </td> - <td class="tdc"> </td> - <td class="tdc">5th</td> - </tr><tr> - <td class="tdl"> </td> - <td class="tdr"> </td> - <td class="tdl_ws1">Minor third</td> - <td class="tdc">6/5</td> - <td class="tdc"> </td> - </tr><tr> - <td class="tdl"> </td> - <td class="tdr"> </td> - <td class="tdl_ws1"> </td> - <td class="tdc"> </td> - <td class="tdc"> </td> - </tr><tr> - <td class="tdl">E″</td> - <td class="tdr">1,280</td> - <td class="tdc" rowspan="3"><img src="images/cbr-3.jpg" alt="" width="16" height="57" /></td> - <td class="tdl_ws1"> </td> - <td class="tdc"> </td> - <td class="tdc">4th</td> - </tr><tr> - <td class="tdl"> </td> - <td class="tdr"> </td> - <td class="tdl_ws1">Major third</td> - <td class="tdc">5/4</td> - <td class="tdc"> </td> - </tr><tr> - <td class="tdl"> </td> - <td class="tdr"> </td> - <td class="tdl_ws1"> </td> - <td class="tdc"> </td> - <td class="tdc"> </td> - </tr><tr> - <td class="tdl">C″</td> - <td class="tdr">1,024</td> - <td class="tdc" rowspan="3"><img src="images/cbr-3.jpg" alt="" width="16" height="57" /></td> - <td class="tdl_ws1"> </td> - <td class="tdc"> </td> - <td class="tdc">3rd</td> - </tr><tr> - <td class="tdl"> </td> - <td class="tdr"> </td> - <td class="tdl_ws1">Fourth</td> - <td class="tdc">4/3</td> - <td class="tdc"> </td> - </tr><tr> - <td class="tdl"> </td> - <td class="tdr"> </td> - <td class="tdl_ws1"> </td> - <td class="tdc"> </td> - <td class="tdc"> </td> - </tr><tr> - <td class="tdl">G′</td> - <td class="tdr">768</td> - <td class="tdc" rowspan="3"><img src="images/cbr-3.jpg" alt="" width="16" height="57" /></td> - <td class="tdl_ws1"> </td> - <td class="tdc"> </td> - <td class="tdc">2nd</td> - </tr><tr> - <td class="tdl"> </td> - <td class="tdr"> </td> - <td class="tdl_ws1">Fifth</td> - <td class="tdc">3/2</td> - <td class="tdc"> </td> - </tr><tr> - <td class="tdl"> </td> - <td class="tdr"> </td> - <td class="tdl_ws1"> </td> - <td class="tdc"> </td> - <td class="tdc"> </td> - </tr><tr> - <td class="tdl">C′</td> - <td class="tdr">512</td> - <td class="tdc" rowspan="3"><img src="images/cbr-3.jpg" alt="" width="16" height="57" /></td> - <td class="tdl_ws1"> </td> - <td class="tdc"> </td> - <td class="tdc">1st</td> - </tr><tr> - <td class="tdl"> </td> - <td class="tdr"> </td> - - <td class="tdl_ws1">Octave</td> - <td class="tdc">2/1</td> - <td class="tdc"> </td> - </tr><tr> - <td class="tdl">C</td> - <td class="tdr">256</td> - - <td class="tdl_ws1"> </td> - <td class="tdc"> </td> - <td class="tdc"> <b>Fundamental</b> </td> - </tr><tr> - <td class="tdc bt" colspan="6"> </td> - </tr> - </tbody> -</table> - -<p class="space-above1"><span class="pagenum"><a name="Page_407" id="Page_407">[Pg 407]</a></span> -The quality of a musical note depends upon the number and relative -loudness of its overtones. When several notes are sounded -simultaneously, they blend into a chord or harmony, provided the -intervals which separate them are equal to the intervals which separate -the simpler overtones. Each of the notes yields overtones. The tones -blend into a concord. Their partials are in unison. The variations in -air-pressure of the compound tone are strictly periodic. If the ratios -of the frequencies of its constituent notes are simple the product is a -rich, full sound, such as a common chord.</p> - -<p>At least one other character of the pulsations of sound must be taken -into consideration if we wish to picture the nature of the force to -which the ear responds. Tones which reach it from several instruments -simultaneously are not necessarily in unison, or even in harmony. The -overtones of a single note sounded on a piano or violin—the statement -does not hold good for bells, nor is it strictly true of flutes or -horns—must necessarily bear a simple proportional relation to their -prime tone. They divide the grand pulsation into fractions “without a -remainder.” But the vibrations of two tuning-forks which are slightly -out of unison interfere one with the other at regular intervals. They -produce “beats.” Everyone is familiar with the curious effect which -is produced upon the eye when one row of railings is seen through -another, or one expanse of wire-netting behind another. Sets of lines -which occupy nearly the same positions in the line of sight combine -to make a large pattern, which overlies the smaller pattern of the -rails or netting. The same thing happens with sounds which coincide at -considerable intervals, although in the case of sounds interference -is as marked as reinforcement. If whilst a tuning-fork yielding 101 -vibrations per second is singing another of 100 vibrations is brought -into play, the vibrations of the second fork are superposed on those -of the first. At a certain moment the forward movement of molecules of -air induced by the first fork is reinforced by a forward push from the -second. But half a second after this coincidence of phase an opposite -result is produced—50½ vibrations of No. 1 have passed, but only -50 of No. 2. No. 2 is going backwards (inwards), whilst No. 1 is moving -forwards (outwards). The same molecules are impelled backwards by No. 2 -<span class="pagenum"><a name="Page_408" id="Page_408">[Pg 408]</a></span> -and forwards by No. 1. The result is a pause. The compound sound -produced by the two forks reaches the ear in throbs. If the forks -were vibrating at the rates of 101 and 99, there would be two pauses -and two beats in every second; if at the rate of 202 and 198, four. -The number of beats per second equals the difference in frequency of -vibration of the tones. A pianoforte tuner does his work best if he has -a musical ear, yet he may discharge his duties with competence without -one. Having struck a note, he sounds its octave, holding both keys -down, and listens for the beat. If the first note gave no beat with -his tuning-fork, the second is in time when it likewise gives no beat -with the first. We have met a tuner who did his work in this way; but -it must be admitted that his tempering of the intervals of the octave -with which he commenced, and consequently of the other octaves above -and below it, left something to be desired. The result might have been -satisfactory had he been provided with twelve tuning-forks.</p> - -<p>The question as to whether beats, when sufficiently rapid, blend into a -tone has been much discussed, without a decision. Probably they do not. -The complementary question as to the cause of dissonance is also not -completely closed. Two notes harmonize, as we have seen, when the ratio -of their frequencies is a simple fraction. Musicians are not quite -agreed as to the level of numerical complexity at which a compound -tone first produces a feeling of discomfort. A good deal depends upon -its position in the scale and the instruments which are combining -to produce it. A minor third (⁶/₅) is on the safe side. This is the -first chord in our list of intervals in which a beat can be detected. -Slow beats, however, do not distress us. It is the rapid beats of -conflicting overtones which give a harsh, rough character to a compound -note. The level at which a line is drawn between harmony and dissonance -seems to depend to a considerable extent upon musical education, using -the term in its widest sense. In primitive music—Hungarian, Scotch, -Welsh—intricate minor chords predominate. The minute subdivision of -the octave in Indian music is quite incomprehensible to a European -ear. Musical cultivation tends to eliminate complex fractions. It is, -however, to be noted that the history of Western music also shows the -<span class="pagenum"><a name="Page_409" id="Page_409">[Pg 409]</a></span> -influence of an opposite tendency. Later generations have admitted as -harmonies combinations which earlier generations could not tolerate.</p> - -<p>Pitch, quality, harmony, and dissonance are distinguished by the -human ear. These are the attributes of musical or periodic sounds. -In a separate class must be included noises of all kinds, termed in -acoustics “aperiodic,” because the vibrations which cause them are -not rhythmic. The teeth of a policeman’s rattle may click a hundred -times a second, but it does not make music. Even with a rapidity of -interruption greater than this (at least 500 times per second) a -succession of noises fails to blend into a smooth, continuous sound. -The ear recognizes the loudness, duration, and even to a very high -frequency the repetition of unmusical sounds.</p> - -<p>The ear as a sense-organ can be followed down the zoological scale to -jelly-fish. In its primitive form it is a chamber lined with epithelial -cells bearing hairs, containing an otolith, or ear-stone. Otoliths are -rounded calcareous masses which play an important part in the ears -of all animals up to fishes. Even in man they are found in the more -subdivided form of otoconia. Contact of the otoliths with the sensory -hairs originates impulses in the nerves with which primitive ears -are abundantly provided. Advisedly we use the word “ear” in place of -“auditory organ.” In all animals this organ affords information of -a double nature-movement of the external medium in which the animal -lives, and movements of the animal in the medium. When the animal -moves, its sensory hairs are displaced with regard to the otolith; when -the water in which it is swimming pulsates, its otoliths are shaken -against the sensory hairs. Displacements of the animal and agitations -of the water produce similar effects. The ear in this stage is an -organ of touch. It might well be questioned whether an animal fitted -with a piece of sensory apparatus of this kind is endowed with a sense -which we may properly, after reflecting upon our own sensations, term -“hearing.” It is, however, stated that certain transparent crustaceans, -in which the functioning of the ear-organs may be watched through a -lens, show in these organs hairs of varying length which vibrate to -tones of different frequency. This observation apart, it might be -doubted whether fishes hear, if we mean by the word “hearing” the -<span class="pagenum"><a name="Page_410" id="Page_410">[Pg 410]</a></span> -recognition and discrimination of tones of high frequency—musical -tones. Their ears serve equally to inform them of the changes in -position of their heads and of the tremblings of the sea. The shocks -transmitted through the sea are near akin to the slower vibrations of -sound, if the fishermen of the Mediterranean are justified in their -practice of beating a wooden clapper which rests upon the seat of the -boat as they row backwards and forwards in front of a curved net. They -believe that the fish are frightened by the noise; but it matters -little whether we describe the fish as hearing a noise, or as feeling -the percussions of the clapper conducted through the water. To the more -rapid vibrations of the clapper, the fish are probably insensitive. -The cochlea, which we have every reason for regarding as the organ by -which sound is analysed, is not possessed by fishes. It makes its first -appearance in reptiles. Birds, it is evident, are able to distinguish -musical tones. Their cochleæ are very short, and are destitute of -“rods of Corti.” For a moment this appears surprising, but it must be -remembered that the range of tones which any bird discriminates is -very short, however nicely it may value the notes within its range. In -mammals the ear is clearly divided into three parts, to which the three -functions which have grown out of the specialization of the sense of -touch are allocated. (1) The semicircular canals are concerned with -the sense of orientation. (2) The utricle and saccule reverberate to -noise—the rumbling of trains, the boom of guns, the beats of dissonant -musical tones. We do not know how to classify the agitations of the -atmosphere which surrounds us and of the earth on which we stand, nor -can we point with any certainty to the groups of stimuli which for -us have taken the place of the grinding of stones on the beach and -slapping of rocks by waves. (3) The organ of Corti in the cochlea -discriminates and analyses musical sounds. To these three sense-organs, -which are situate in the inner ear, certain structures are accessory.</p> - -<p>The concha, which enables a horse or a cat to collect sound and to localize -its source, is in ourselves merely an ornament to the side of the head. -<span class="pagenum"><a name="Page_411" id="Page_411">[Pg 411]</a></span></p> - -<div class="figcenter"> - <a id="I_411" name="I_411"> </a> - <img src="images/i_p411.jpg" alt="" width="600" height="495" /> - <p class="center"><span class="smcap">Fig. 38.</span>—<span class="smcap">The - External, Middle, and Internal Ear of the Left Side.</span></p> - <div class="blockquot2"> - <p class="neg-indent space-below2">From right to left, the figure shows the concha and - lobule of the ear in profile; the external meatus (abbreviated); the drum, divided - vertically, its posterior half visible; the hammer-bone, with the tip - of its long arm attached to the drum, an arrow indicating the point of - attachment and line of action of the tensor tympani muscle; the anvil - attached by a ligament to the bony wall of the middle ear; the stirrup, - with its foot-plate almost filling the oval window; the labyrinth, with - the three semicircular canals above, and the scala vestibuli below. The - curled black line shows the situation of the scala media, or ductus - cochleæ (which contains the organ of Corti). Pulsations of sound which - move the membrana tympani are transmitted by the three bones to the - oval window. They shake the perilymph, producing waves which travel - along the scala vestibuli to the apex of the cochlea, whence they - return by the scala tympani to the round window (if they do not take a - shorter course through the ductus cochleæ). The Eustachian tube opens - out of the lower part of the middle ear.</p> -</div></div> - -<p>The external meatus is a curved tube, about an inch long. Frequently -a tuft of hairs guards its entrance. The wax secreted by its wall -serves to attach particles of dust, and to deter insects from entering -the tube. The air at the end of it is at a uniform temperature. It -is closed by the membrana tympani, or drum. This membrane receives -the vibrations of sound; and, in order that it may collect them with -absolute impartiality, it is in every respect the opposite in shape and -structure to the top of a drum. The stretched parchment which covers a -<span class="pagenum"><a name="Page_412" id="Page_412">[Pg 412]</a></span> -drum is flat. Its tension is uniform in all its parts. Movements have -the greatest amplitude at the centre. Every precaution is taken to -insure its emitting, with as little confusion as may be, the particular -note to which it is tuned. The drum of the ear is shaped like the -mouth of a trumpet, depressed to a point, but convex from this point -outwards. Its elastic fibres, which are partly radial, partly circular, -are at many different tensions. Its deepest part, to which the long arm -of the hammer-bone is attached, is not its centre.</p> - -<p>The “middle ear” is an irregular cavity communicating with the -pharynx by the Eustachian tube. It is filled with air at the same -pressure as the atmosphere. Except during the act of swallowing, when -it is at first shut tightly and then opened, the pharyngeal end of -the Eustachian tube is gently closed. When one is dropped in a lift -rapidly down the shaft of a mine, the difference in pressure between -the external air and the air in the middle ear stretches the drum to -such an extent that deafness to low tones is produced. Conversation -becomes inaudible. The deafness is remedied by swallowing saliva, -and thus opening the end of the Eustachian tube. The commonest cause -of permanent deafness is inflammation followed by thickening of the -mucous membrane of the lower end of the Eustachian tube, with its -consequent closure, due to frequent sore throats. The air in the middle -ear is slowly absorbed. It needs to be constantly renewed through the -Eustachian tube.</p> - -<p>On the inner wall of the middle ear are two small apertures—the -oval window and the round window. Both are closed with membrane. -Into the oval window is fitted the sole-plate of the stirrup-bone. -Three bones—hammer, anvil, and stirrup—combine in transferring -the movements of the membrana tympani to the oval window. They constitute a -jointed lever, which swings about an axis passing through the ligament -of the anvil (<a href="#I_411">Fig. 38</a>), the excursions of the long arm of -the hammer being reduced in amplitude by one-third at the stirrup-plate. As -the oval window has only one-twentieth of the area of the drum, the -movements of the latter are transmitted with concentrated force. Two -points in the mechanism of these bones may be specially noticed: (1) -The head of the hammer is free to rotate in the cavity of the anvil, -checked by a cog. Every inward movement of the drum is faithfully -<span class="pagenum"><a name="Page_413" id="Page_413">[Pg 413]</a></span> -transmitted to the oval window; but when the drum moves outwards, -the hammer does not necessarily carry the anvil with it. (2) A -muscle—tensor tympani—is inserted near the elbow of the long -arm of the hammer. When high notes are listened to its contraction tightens -the drum, rendering it more responsive to rapid vibrations. It has a -tonic action, but it does not make any special contraction for low notes.</p> - -<p>Behind the two windows, within the solid bone, is the inner ear, -which our ancestors very aptly termed a “labyrinth.” It is filled -with fluid—perilymph—which is shaken by every movement of the -stirrup-plate. Since water is incompressible, no waves could be raised -in the perilymph were there no second aperture. Every vibration -conveyed by the stirrup-plate after passing through the labyrinth ends -as a vibration of the membrane which closes the round window.</p> - -<p>Nowhere does perilymph come in contact with auditory cells. All the -endings of the nerve of hearing are contained within a membranous -labyrinth which lies within the bony cavities. The way in which the -waves of the perilymph are dispersed over the surface of this closed -sac can be inferred from the diagram (<a href="#I_411">Fig. 38</a>). They sweep -round the utricle and saccule, are lost in the narrow spaces which surround the -semicircular canals, run up the scala vestibuli of the cochlea. The -course of the waves which traverse the cochlea is of especial interest -in connection with the physiology of hearing.</p> - -<p>The cochlea—snail-shell—is a spiral tunnel of three turns, -in hard bone, about an inch in length. A shelf of bone—lamina -spiralis—projects into the tunnel on its convex side. From the free -margin of this spiral lamina two membranes extend to the outer wall -of the tunnel—one firm, containing straight, stiff, and probably -elastic fibres which radiate outwards (the basilar membrane); the -other an extremely delicate film of connective tissue. The tunnel is -thus divided into three compartments, known as the scala vestibuli, -scala media, scala tympani. The scala media belongs to the membranous -labyrinth. Waves transmitted through perilymph pass, as we have already -explained, up the scala vestibuli. At the apex of the cochlea the -two scalæ are in communication; but the aperture is small, and it is -unlikely that waves reach the lower passage from the upper through this -<span class="pagenum"><a name="Page_414" id="Page_414">[Pg 414]</a></span> -opening. They pass through the thin membrane which roofs the scala -media, shake its endolymph, and reach the lower passage through the -basilar membrane. It is noteworthy that, since the round window at -the lower end of the scala tympani is, with the exception of the oval -window, the only opening of the bony labyrinth, all waves transmitted -through the oval window must travel part of the way or all the way up -and down the cochlea.</p> - -<div class="figcenter"> - <a id="I_414" name="I_414"> </a> - <img src="images/i_p414.jpg" alt="" width="600" height="360" /> - <p class="center"><span class="smcap">Fig. 39.</span>—<span class="smcap">A Section - through the Axis of the Column of the Cochlea.</span></p> - <div class="blockquot2"> - <p class="neg-indent space-below2">The spiral sheet of nerve-fibres which supplies the - organ of Corti is cut in eight places. If the bundle to the lowest coil of the - shell (on the left side of the diagram) is followed, it will be seen to bear - ganglion-cells where it enters the bony spiral lamina. This lamina - divides the tube into two canals—scala vestibuli above, scala - tympani below. From the edge of the lamina the membrane of Corti - stretches to the outer wall. Above the organ of Corti is the membrana - tectoria, and above this a very thin membrane which cuts off the ductus - cochleæ from the scala vestibuli.</p> -</div></div> - -<p>The organ of Corti is spread out on the basilar membrane. It is an -epithelial structure of extreme regularity and uniformity. Near to the -edge by which the basilar membrane is attached to the spiral lamina -rests a double row of rods of Corti, stiff pillars which lean one -towards the other, over the tunnel of Corti, the convex head of the -outer rod fitting into a concavity in the head of the inner one; in -some places one outer rod fits against two inner rods, as the latter -are rather the more numerous. On the inner side of the inner rod is -seen, in transverse sections a single plump cell filled with cloudy -protoplasm, and bearing on its free surface a tuft of very short hairs. -<span class="pagenum"><a name="Page_415" id="Page_415">[Pg 415]</a></span> -On the outer side of the outer rod are three or four hair-cells, -each with a cloudy outer segment containing the nucleus, a granular -middle segment, and a stiffish stalk, which attaches it to the basilar -membrane. Between the hair-cells are supporting cells, thicker below, -tapering above, containing in their substance a firm fibre. Still -farther to the outer side are epithelial cells, of no special interest. -The purpose of the rods of Corti and the supporting cells is to give -attachment and support to a reticulated membrane of exquisite delicacy, -through the oblong apertures of which the hairs of the hair-cells -project into the endolymph. The spiral lamina is traversed by a vast -number of fibres of the auditory nerve, which, losing their medullary -sheaths, pass across the tunnel of Corti as naked axons, to end amongst -the hair-cells. Above the organ of Corti, attached by its edge to the -spiral lamina, is a thick, gelatinous, fibrillated structure—membrana -tectoria—which rests as a coverlet on the surface of the organ. It has -been supposed that it serves to damp the vibrations of the hairs after -they have been set in motion by the waves passing across the scala media; -but it not impossibly plays a more active part in hearing than this.</p> - -<div class="figcenter"> - <a id="I_415" name="I_415"> </a> - <img src="images/i_p415.jpg" alt="" width="600" height="239" /> - <p class="center"><span class="smcap">Fig. 40.</span>—<span class="smcap">Organ - of Corti.</span></p> - <div class="blockquot2"> - <p class="neg-indent space-below2">The spiral lamina, on the left of the drawing, gives - attachment to the membrane of Corti, which stretches to the opposite wall. - Below the membrane is a bloodvessel which runs its whole length beneath the - tunnel of Corti. The tunnel is formed by pillars—the inner on the - left, the outer on the right—which meet above it. On the left of - the inner pillar is a hair-cell; to the left of this a nerve-cell with - two nuclei. To the right of the outer pillar is a space; to the right - of this four hair-cells alternating with four supporting cells, which - hold up the reticulated membrane through apertures in which the tufts - of hairs project. Three nerve-fibres are seen in the spiral lamina; - they cross the tunnel to ramify between the rows of outer hair-cells. - The lamina tectoria rests upon the tufts of hairs.</p> -</div></div> - -<p>The organ of Corti is, beyond doubt, the apparatus which analyses -<span class="pagenum"><a name="Page_416" id="Page_416">[Pg 416]</a></span> -sounds; but the problem of the way in which it responds to tones -of different pitch, or analyses compound tones, is not as yet even -approximately solved. To escape the acoustic difficulties which have -to be faced by anyone who endeavours to expound the theory of the -cochlea as a piece of analytical apparatus, various suggestions as -to the possibility of an action <i>en masse</i> have been advanced. For -example, the basilar membrane has been compared to a telephone-plate -which takes up vibrations and transmits them through the auditory nerve -to the brain. But if the organ of Corti be the transmitter, there is -no ear in the brain to analyse the vibrations given out by a receiving -telephone-plate; and without a receiving plate and a listening ear a -telephone is purposeless. According to this hypothesis, the basilar -membrane vibrates as a whole, moving the hair-cells in various -“patterns”; the pressure of the hairs against the tectorial membrane -causing irritation of the cells which bear them, and hence producing -stimulation of various groups of nerves. Other pattern theories -are somewhat similar. But it is obvious that all hypotheses of the -vibration of the whole of the basilar membrane, or of large parts of -it, simultaneously, leave to the mind the responsibility of reading -the pattern which the impulses generated in the organ of Corti make in -the brain. It is conceivable that every fraction of a semitone which a -musician can discriminate, and every combination of tones which he can -analyse, is transmitted to the brain by a large number of co-operating -nerve-impulses; but such a theory involves a complexity of mental -associations difficult to contemplate.</p> - -<p>According to the general principles enunciated in this book, analysis -of stimuli is the function of sense-organs. It cannot in all cases -be compared with the analysis effected in a physical laboratory; nor -is this necessary; but it must be carried so far that nerve-impulses -which have no specific qualities apart from their source shall give -rise to effects in consciousness which have no basis other than the -topographical distribution of the said impulses in the brain. There -may be sensory impulses of different orders; there may be in the brain -psycho-physical substances which react to impulses of various orders in -various ways; but until we have some hint of the existence of specific -impulses and specific psycho-physical substances, we are not justified -<span class="pagenum"><a name="Page_417" id="Page_417">[Pg 417]</a></span> -in postulating their existence simply in order that we may escape from -physiological embarrassments.</p> - -<p>The organ of Corti has in the highest degree the appearance of a piece -of apparatus for the analysis of sound. If the basilar membrane, with -the cells which rest upon it, be cut out and laid flat, the suggestion -of some kind of instrument is very strong. It is a long narrow ribbon, -narrowest at the bottom of the spiral, increasing to about twice the -width at the apex. It is crossed by radiating fibres, presumably -elastic. The cells which rest upon it carry vibrating hairs, and -are supplied with nerves. The rods of Corti hold up the reticulated -membrane, which keeps the hair-cells in place. It is not to be wondered -at that when its structure was first discovered it was thought that the -problem of the analysis of musical tones was solved. If two pianos in -perfect tune are in the same room, when one is played the corresponding -wires of the other twang. Anyone who sings into a piano, whilst the -loud pedal raises the dampers, feels an increased fulness in his voice. -This is the familiar phenomenon of resonance. Why should not the fibres -of the basilar membrane resonate to the tones conveyed to the ear—the -shorter ones at the base of the cochlea to high tones, the longer ones -at the apex to low tones? This is the order in which we should expect -the pulsations of sound which ascend the scala vestibuli to be taken -up—the more rapid, near its commencement, the less rapid farther up -it. But an explanation of the physics of the selection of vibrations -of different frequencies by different sets of the elements which make -up the organ of Corti, if such selection occurs, is still to seek. In -the first place, the fibres of the basilar membrane are so exceedingly -short. What could a fibre less than 0·5 millimetre in length make of -the vibrations of a 36-foot organ-pipe? Even if this objection be -waived, as certain eminent physicists hold that it may be, there is not -a sufficient difference in length between the longest and the shortest -fibres to account for the great range of tones which we are able to -discriminate; nor is there any evidence that some fibres are more -tightly stretched than others.</p> - -<p>A further consideration which tempts physiologists to look upon -the organ of Corti (including the basilar membrane) as a series of -<span class="pagenum"><a name="Page_418" id="Page_418">[Pg 418]</a></span> -resonators is the somewhat remarkable agreement between the number of -separate pieces of apparatus of which it appears to be composed and -the number of different musical sounds which, if it were a series of -resonators, it might be called upon to discriminate.</p> - -<p>The squeak given by a bat at each turn in its flight has a pitch of -about 11,000 vibrations to the second—the sixth E above the middle -C (Tyndall). In a group of persons listening for the squeak there -are usually some who cannot hear it. Above this the range of hearing -is very variable. The suddenness of transition from perfect hearing -to total want of perception makes experiments with small pipes or -with a siren somewhat amusing, when a number of persons are tested -at the same time. One complains that the note is intolerably loud -and shrill, whilst others assert that there is perfect silence. -Thirty-three thousand vibrations is usually regarded as the upper limit -for the human ear, but certain physiologists place it at 40,000, or -even higher. The upper limit is of little consequence, since there -is very little power of discriminating rapidities above the highest -note used in music—the piccolo stop of the organ, with a pitch of -4,096. It is possible that a sound with a lower frequency than 27 -(the contra-bassoon) may be heard as a tone—16 according to certain -writers; but again our power of discriminating very low notes is small. -Over a certain range a skilled musician can tell that a note is out of -tune when it is one sixty-fourth of a semitone higher or lower than -it ought to be. If we assume that by allowing equal sensitiveness for -a range of seven octaves, the excess of the allowance over the actual -sensitiveness towards either end of this stretch would compensate for -the comparatively few distinctions which the ear can make either below -or above it—64 × 12 × 7 = 5,376. A much higher estimate, based upon -observations which seem to show that the ear can distinguish sounds -less than one sixty-fourth of a semitone apart, places the total number -at 11,000.</p> - -<p>On the assumption that one piece of apparatus is tuned to resonate -for every distinguishable sound, between 5,000 and 11,000 pieces of -apparatus would be required. Taking one of Corti’s arches as the -centre-piece of the resonator, although the rods are certainly not -vibratile structures, we find the number to be 3,848 (the number of the -<span class="pagenum"><a name="Page_419" id="Page_419">[Pg 419]</a></span> -outer rods); if either rod with a hair-cell, or hair-cells, is the -analytical element, 9,438. Counting gives 3,487 inner, 11,700 outer, -hair-cells. The fibres of the basilar membrane are estimated at 24,000; -the fibres of the cochlear nerve at 14,000. It will be understood that -the counting of structures as minute as these yields results which -cannot be more than approximately accurate. Helmholtz, assuming that -each arc of Corti indicates an analytical element, accounted for the -apparent deficiency in their number by assuming that a tone of which -the pitch fell between two arches set both in sympathetic vibration, -the arch which was nearest in pitch to the tone vibrating the more -strongly. In this way he anticipated an objection which has often been -brought against his theory of a long series of resonators.</p> - -<p>In opposition to Helmholtz’s theory it is pointed out that when a -violinist runs his finger up a bowed string, the pitch rises with -perfect smoothness; it does not bump along from resonator to resonator. -Especially in the case of very high tones given out by a siren, it is -urged that at the rare intervals at which a resonator in the ear is -tuned for the tone which the siren is emitting it should sound much -louder than when the tone falls midway between two resonators. But the -whole question of the nature of the response of the analytical elements -is too obscure at present for the discussion of points so nice as this.</p> - -<p>Many who think that Helmholtz’s theory of resonators is based upon -principles of physics and of physiology which must be regarded as -the starting-points of any explanation of the analysis of sounds by -the ear and the mind, hold that it goes too far in searching for a -separate resonator for every distinguishable tone. The cochlea, as we -have already said, does not offer anything like so extensive a choice -as this, if regard be had to the tension or length of its elements, -and not to their numbers. Those who accept it as an axiom that the -cochlea contains a series of responding instruments—but a series far -more limited in range than the gamut of our sound-perceptions—seek -to discover in musical tones qualities which unite them in groups. -Just as in the case of colour-sensations they recognize four (or six) -elementary qualities which excite four (or six) pieces of responding -apparatus, so also in the case of hearing they seek for a limited -<span class="pagenum"><a name="Page_420" id="Page_420">[Pg 420]</a></span> -number of tone-qualities and a correspondingly limited number of -elementary sensations. The ideal of those who take this view is an -octave of qualities and of elementary sensations sounded in the middle -of the scale when <i>x</i> nerve-endings are stimulated, as the octave -above when 2<i>x</i> nerves respond, the octave below with <i>x</i>/2. Such a -conception seems to guide thought round insurmountable barriers. There -is, however, a risk of making too much of the periodic intervals, -because they take so important a place in music. At one side of the gap -which sound bridges between the individual and his environment is an -elastic body shaking at any possible rate within the range of hearing. -At the other side of the gap is the ear. If, having arranged several -thousands of stones along the side of the road in order of size, I were -to state, picking up No. 512, “This is the fundamental of which No. -1,024 is the octave,” answer would be made to me: “It may be that the -larger could be broken into halves, each as heavy as the smaller stone; -but I recognize no difference between the stones in shape, colour, or -hardness.” A vibrating string divides into equal segments, each of -which vibrates within the vibrations of the whole string, sounding -the octave. We recognize a similarity in quality between tones and -their octaves because we are accustomed to hear the octave, the most -prominent of overtones, in all musical sounds. Hence, from association, -it has become more difficult to distinguish a note from its octave -than it is to distinguish it from its fifth; but it does not follow -that the effect of 1,024 vibrations upon the sensory cells more nearly -resembles the effect of 512 than does that of 768. But at this point -we are compelled to construct some hypothesis as to the way in which -the vibrations affect the sensory cells. The protoplasm of the cells -is not directly sensitive to them. We can account for the generation -of impulses in the nerve connected with a particular cell, or group -of cells, only on the supposition that a resonating mechanism which -responds to vibrations of a certain frequency shakes the cell. Even -then it seems necessary to suppose that there is an accessory mechanism -which disturbs the cell-protoplasm sufficiently to render the shake -effective, probably the hairs rubbing against the tectorial membrane. -<span class="pagenum"><a name="Page_421" id="Page_421">[Pg 421]</a></span> -Anatomical study gives us no confidence in the theory of the existence -of several thousands of resonators tuned to as many notes of different -pitch. It remains for the physicists to say whether or not we may -picture one of these minute resonators as responding to a given note -in 10 separate octaves, another in 9 ... another in only 1. The -physicists, on their part, may very properly ask the anatomists to -point out the resonators, and even to reproduce them in models of -dimensions which allow of experimental investigation.</p> - -<p>It is generally agreed that the sensation of a chord is compounded of -the sensations to which each of its constituent tones gives rise, and -that our power of analysing the compound is a question of attention. A -musician can direct his attention to either sensation at will. It is -not equally certain that a person who has no knowledge of music can -do the same. Familiarity with musical instruments gives us so exact -a knowledge of the way in which compound tones are produced that it -becomes a difficult matter to decide whether, when we say that we can -pick out the E or the G of the common chord, it means that we can hear -it as distinct from <b>C</b> and C′, or whether it means that, knowing -the constitution of the chord, we think about the E or the G when we -hear the compound tone, to the exclusion of its other constituents. -Then, again, the several strings which we try to strike simultaneously -do not actually “toe the line.” Their vibrations are not in the same -phase, even though the strings be in absolute tune. Discrepancy of -phase may favour the singling out of the several constituents of the -chord. There we touch upon a problem which we passed over in silence -when attempting to give an idea of the nature of the pulsations which -reach the ear. We then (<a href="#Page_405">p. 405</a>) described the partial pulsations -which are superimposed upon the main pulsation as if they necessarily started -simultaneously with it. We assumed that the phase difference of the -partials was zero. But it is clear that differences of phase of its -constituent tones may produce an almost infinite number of variations -in the form of a compound “wave” of sound. Is the ear variously -affected by different forms of wave? Does difference of phase result in -difference of sensation? In broad terms, the answer to this question -must be in the negative; although it can be shown that in certain cases -<span class="pagenum"><a name="Page_422" id="Page_422">[Pg 422]</a></span> -a change in phase of the several constituents of a compound tone, -without any alteration in their number or their loudness, makes a -change in its acoustic quality. Any attempt to correlate physical -changes—the movements of air in the outer ear—with the effects -which they may be supposed to have upon the organ of Corti must take into -account this wide range of variation of wave-form. We have called -attention to the difficulties which it introduces; but have no hope of -indicating the way in which they may be overcome.</p> - -<p>Nothing connected with the physiology of the sense of hearing is -more remarkable than its capacity for education. The cochlea of one -human being is as extensive and as elaborate in structure as that of -another, yet some men can make an infinitely more refined use of it -as an analytical apparatus than can others. A native of the Torres -Straits cannot distinguish as two separate notes sounds which are less -than a semitone apart. Sir Michael Costa could distinguish sounds into -the sixty-fourth parts of semitones. The cochlea of a cat is not less -elaborate than that of a man, yet Man’s mental life is based upon the -analysis of auditory sensations. His supreme advance in the animal -scale has depended upon the invention of language, by means of which -he communicates and receives information, thus rendering experience -eternal, notwithstanding the transience of the individuals who acquire -and transmit it. An animal is born, finds out, dies. A man starts with -the wisdom of the race beneath his feet.</p> - -<p>Hearing has a nebulous origin in sensations of movement or -displacement. The connection between the two special senses—the sense -of orientation and the sense of hearing, properly so-called—remains -always intimate. David danced before the Ark of the Lord. All people, -savage and civilized, associate music with movement. High in the -animal scale appears the sense-organ which enables its possessor to -discriminate musical tones. By its use Man has developed with great -rapidity—as secular time is reckoned—an intelligence which removes -him from all other animals a planet’s space. The sounding of his organ -of Corti by pure tones and combinations of pure tones gives him extreme -pleasure, although it in no way ministers to his intelligence. Yet -there is in the enjoyment of music a quality of pleasure which makes -it near akin to the satisfaction which we experience in exercising the -intellect.</p> - -<hr class="chap" /> -<p><span class="pagenum"><a name="Page_423" id="Page_423">[Pg 423]</a></span></p> -<div class="chapter"> - <h2>CHAPTER XV<br /><span class="h_subtitle">SKIN-SENSATIONS</span></h2> -</div> - -<p>The senses, according to a time-honoured classification, are five -in number—smell, sight, taste, hearing, and common sensation, or -touch; but such a classification of our sensations and of the organs -which originate them is too crude for modern needs. Already we have -shown that, whereas the nose and the tongue afford the same kind of -information, the ear affords information of two, perhaps of three, -different kinds. Within the realm of common sensation we pick out three -special senses served by specialized sense-organs—touch, cold and -heat—and, possibly, a fourth, served by non-specialized nerves, to -which alone the epithet “common” properly applies.</p> - -<p>The skin is supplied with nerves—naked fibrils—in the richest -abundance. They are most easily demonstrated in the layer which covers -the cornea, thanks to its transparency; in this, as shown in <a href="#I_424">Fig. 41</a>, -having branched on the front of the fibrous tissue of which the cornea -is composed, the nerves pass towards the surface, forming connections -with every one of its cells, or, at any rate, with every cell of the -more superficial of the three or four layers of which the epithelium -is made up. Ramified nerve-twigs of this type do not, under ordinary -conditions, convey any sensations to consciousness. So long as the -skin-cells with which they are connected are healthy, the nerve-twigs -establish for them connections with the central nervous system by which -their nutrition is regulated; but they carry no impulses to which we -can direct attention. The movement of blinking is accompanied by no -sensation until the edges of the eyelids come in contact. A pencil -pressed against the lid evokes touch-sensations from the skin, but none -<span class="pagenum"><a name="Page_424" id="Page_424">[Pg 424]</a></span> -from the cornea which underlies it. When a tiny beetle injures the -surface of the cornea by scratching the epithelial cells with its horny -wings and legs, the ruptured nerve-filaments convey to consciousness -impulses, or, as we prefer to express it, an influence which is felt -as pain. But even the pain caused by injury to the cornea is trifling -as compared with that which originates in the under-sides of the lids, -where not only is the epithelium supplied with branching nerve-twigs, -but specialized organs of touch are present to localize the seat of -injury. Everywhere the epithelium covering the surface of the body is -so abundantly supplied that a successful staining of nerve-filaments -induces one to think that every epithelial cell has its nervous -affiliation. These are the nerves of common sensation, if we retain the -term; but sensation so common, so obscure, so little differentiated -that we know no more about it than we know about the air which envelops -our hands and faces on a warm, windless day. Yet the air, when it -moves, gives rise to a dim, broad, generalized sensation, which may be -focussed into definiteness by a sensitive nerve.</p> - -<div class="figcenter"> - <a id="I_424" name="I_424"> </a> - <img src="images/i_p424.jpg" alt="" width="600" height="292" /> - <p class="center"><span class="smcap">Fig. 41.</span>—<span class="smcap">Vertical - Section of the Epithelium which covers the Surface of the Cornea,<br /> and - of a Small Portion of the Corneal Substance, highly Magnified.</span></p> - <div class="blockquot2"> - <p class="neg-indent space-below2">The black lines are naked nerve-fibres (stained with - chloride of gold), which are distributed amongst the cells of the more superficial - strata of the epithelium in very great abundance. The corneal substance is - composed of sheets of transparent fibres with intervening cells. As the - fibres of the several sheets cross one another at various angles, they - are cut, some transversely, others in the direction of their length.</p> -</div></div> - -<p>An observer who has devoted himself for many years to the investigation -of skin-sensations, and especially of the “referred pains” which are -due to diseases of the viscera, recently caused the large cutaneous -<span class="pagenum"><a name="Page_425" id="Page_425">[Pg 425]</a></span> -nerve which supplies the thumb side of the forearm and hand to be cut -in his own arm, in order that he might study carefully the revival -of sensations. He found that he never lost his ability to recognize -displacements of the tissues beneath the skin. Pacinian bodies and -other end-organs of deep-lying nerves recorded pressure and tension -caused by pushing or rubbing with a blunt instrument. Seven weeks after -the injury he began to recognize stimuli that do harm—hot things, -cold things, pricking with a pin—although his power of localizing -the spot injured was extremely vague. In seven weeks, that is to say, -the protopathic nerves, which do not follow the same definite lines -as the nerves of the special senses, but form open networks with many -alternative paths, had re-established their skin connections. Only -gradually and very slowly did critical sensations return—the ability -to distinguish degrees of warmth, to recognize as separate two points -of a pair of compasses, to feel a touch with cotton-wool.</p> - -<p>According to a theory set forth in this book (<a href="#Page_312">p. 312</a>), -pain is not a set of sensations, but a condition of the central nervous system -which renders it unduly excitable, or excitable in a particular manner, to -impulses which have the same local origin as the nerve-current which -sets up the condition of pain. When a nerve of the skin has been cut, -the epithelial ramifications are renewed before any specialized tactile -or other sense-organs have regained their nervous connections. When -the area which has regained its surface ramifications, but has not -regained its sense-organs, is injured, no localization of pain results. -Indeed, the obscure sensations which are then experienced if the skin -be injured can hardly be described as painful. The ramified nerves -pour their agitation into the grey matter of the spinal cord; but it -is not the agitation <i>per se</i> which causes pain. It is the passage of -impulses through the agitated area that gives to them, when they reach -consciousness, not only a topographical meaning, but also a distressful -feeling. Until the specialized organs of the skin have been restored to -working order, there are no impulses to pass through the agitated grey -matter, and therefore no feelings of pain. According to this view there -are two systems of afferent nerves, the protopathic and the specialized -or critical. The former is very widely and very abundantly distributed -<span class="pagenum"><a name="Page_426" id="Page_426">[Pg 426]</a></span> -to the surface of the body, the lungs, the alimentary canal, and other -viscera. It has no end-organs, no defined tracts in the central nervous -system, no definite connections with the cortex of the great brain. -The currents which it conducts, if they originate in the visceral part -of this system, have no direct effect in consciousness; but if they -originate on the surface of the body, or in the alimentary canal at -the lower end of the œsophagus, or in certain other situations, they -co-operate with stimuli of heat, cold, or traction. The critical system -works in a more definite way. Its impulses originate in sense-organs. -Starting with a certain potential, they are transmitted by the -discharge of a succession of linked neurones. When they reach the -cortex their potential is sufficiently high to evoke consciousness. -Their distribution in the cortex is as definite as their origin.</p> - -<p>Specialized sense-organs are necessary for the origin of all -sensations. Within the epithelium are certain cells which look as -if they were specialized for sensory purposes. The deeper sheet, or -derma, of the skin is abundantly provided with structures in which -nerves end in the most elaborate and complicated ways (<a href="#I_427">Fig. 42</a>). -They are found especially in the papillæ of connective tissue, which, set -in rows, form the ridges that one can see at the finger-tips and in -various other situations. All of these organs are made up of groups -of epithelial cells which, displaced from the epidermis, have sunk -into the derma, with the nerves connected with them. In their further -development the nervous part of the apparatus is complicated by -branching, the branches being thickened and usually flattened into -ribbons, which lie on the external surfaces of the cells or between -them. A more or less marked capsule is provided for the organ by -condensation of connective tissue.</p> - -<p>Anyone can convince himself that the skin is not uniformly sensitive. -He may test it first for the minimal stimulus which excites a sensation -of touch. With a hair of the head—it must not be a very fine one—cut -across with scissors, and held between finger and thumb at the right -distance from the cut end, the skin of the palm of the hand is prodded. -Every here and there a spot is found which is insensitive to so slight -a pressure. These spots are neither large nor very close together. If -<span class="pagenum"><a name="Page_427" id="Page_427">[Pg 427]</a></span> -the hairless skin of the arm between the elbow and the armpit be -investigated in the same way, much larger blank areas are met -with—oval patches more than ¼ inch in diameter. When a hairy surface -is tested, it is found that contact with a hair can always be felt; and -when the hairs are shaved, the touch-spots are found to extend around -or from the points at which hairs pierce the epidermis. Touchless areas -lie between them. Hair-follicles receive tufts of nerve-filaments, and -it appears that they are the chief organs of touch. “Touch-corpuscles,” -which are found in great numbers in the papillæ of the skin of the -fingers and elsewhere, may probably be regarded as, genetically, -hair-follicles which have not developed hairs.</p> - -<div class="figcontainer"> - <a id="I_427" name="I_427"> </a> - <div class="figsub"> - <img src="images/i_p427a.jpg" alt="" width="250" height="299" /> - </div> - <div class="figsub"> - <img src="images/i_p427b.jpg" alt="" width="210" height="296" /> - </div> - <p class="center"><span class="smcap">Fig. 42.</span>—<span class="smcap"> - Sense-Organs susceptible to Pressure.</span></p> - <div class="blockquot2"> - <p class="neg-indent space-below2">All are formed on essentially the same plan; - a fibrous capsule invests a group of epithelial cells amongst which a - nerve ramifies. The simplest form is known as a Grandry’s corpuscle-a - nerve ending in one or two plates between two or three epithelial cells. - These organs are found in great numbers in the bills of aquatic birds. - If a duck is watched whilst it is gobbling mud at the margin of a pond, - it will be seen to have a remarkable capacity for discriminating between - the shells of small snails, which it can crush, and stones, which it needs - to drop from its bill. Its bill is also provided with small Pacinian - corpuscles (<a href="#I_428">Fig. 43</a>). Touch-corpuscles, more elaborate - in form than the one figured, are found in the papillæ of the skin of the - fingers and elsewhere. They appear to be modified hair-follicles. End-bulbs - occur in the conjunctiva and elsewhere, and especially in the peritoneum. - Together with Pacinian corpuscles, they are accountable for sensations - connected with the distension of the stomach and intestines.</p> -</div></div> - -<p>If sensitiveness to pain is investigated by tapping very gently with a -needle—or, better, by using a stiff horsehair fixed in a cleft stick, -from which it projects about ¼ inch—it will be found that every here -and there are spots which are exceedingly sensitive, whilst adjoining -them are areas which are moderately sensitive, and between these areas -<span class="pagenum"><a name="Page_428" id="Page_428">[Pg 428]</a></span> -small spots or stretches of skin which do not give the smarting -sensation even though the horsehair be pushed until it doubles up.</p> - -<div class="figcenter"> - <a id="I_428" name="I_428"> </a> - <img src="images/i_p428.jpg" alt="" width="350" height="567" /> - <p class="center"><span class="smcap">Fig. 43.</span>—<span class="smcap"> - Pacinian Corpuscle.</span></p> - <div class="blockquot2"> - <p class="neg-indent space-below2">These organs are especially numerous in the - neighbourhood of tendons and ligaments. They are also present beneath - the skin of the hands and feet. Their capsules are formed of a great number - of concentric lamellæ of connective tissue, enclosing lymph-spaces. Within the - capsule is a core of finely granular substance, which also shows a - tendency to a lamellar disposition. The structure of these relatively - large sense-organs is highly suggestive of sensitiveness to pressure, - traction, or rubbing.</p> -</div></div> - -<p>Testing now for sensitiveness to cold with a cold blunt metal point, -“cold-spots” can be mapped on the skin. If the metal is warmed to about -50° C., “heat-spots” are found. The different kinds of spot are very -irregularly distributed. They may coincide, or overlap, or leave blank -spaces. Their relative abundance varies. In some regions touch-spots, -<span class="pagenum"><a name="Page_429" id="Page_429">[Pg 429]</a></span> -in others cold-spots, in others heat-spots, are more closely grouped. -The tongue and the hand, and especially the tips of the fingers, are -most sensitive to touch; but whereas the tongue is also exceedingly -sensitive to warmth, the hands are relatively insensitive. Yet, -speaking generally, parts especially sensitive to touch are little -sensitive to temperature, and <i>vice versa</i>. Sensitiveness to cold is -much more widespread than sensitiveness to heat. It is concentrated in -the skin covering the abdominal viscera. A cold douche directed between -the shoulders is doubtfully felt as cold. There is no doubt whatever -about it when it strikes the skin over the stomach.</p> - -<p>From these observations it appears that the skin contains three -sets of organs sensitive respectively to touch, cold, and heat. -Certain investigators hold that it also contains specific organs, or -nerve-endings, sensitive to painful stimulants; but in this case there -is the obvious difficulty of distinguishing between pain and touch. At -no spot can pure pain be evoked free from any consciousness of touch.</p> - -<p>To a certain extent the combinations of epithelial cells and -nerve-endings in the skin fulfil the negative requirement of -sense-organs; each kind, whilst specially sensitive to its own specific -stimulant, is insensitive to stimulants of other kinds. But mutual -exclusion is not absolute in the case of cold and warmth. If a warmed -metal point be applied to a cold spot, it produces a sensation of cold. -Our feelings of warmth and cold are to a large degree comparative. -Luke-warm water feels cold to hands just taken out of hot water; -moderately cold water appears luke-warm to hands that have been in -contact with ice. The sensory apparatus for cold and heat soon adapts -itself, or, in physiological language, it is soon fatigued. If after a -prolonged bath at the body temperature a foot be plunged into very hot -water and withdrawn quickly, the feeling which first ensues is one of -cold. It is indistinguishable from the feeling provoked by dipping the -foot into cold water. The sensation of cold subsequently gives place -to one of painful warmth. This does not indicate that the heat-spots -have been waked out of their lethargy by excessive stimulation. On the -contrary, it is the cold-spots which, when they were first stimulated -by the very hot water, answered “Cold,” that now cry out “Hot”; for -<span class="pagenum"><a name="Page_430" id="Page_430">[Pg 430]</a></span> -both cold-spots and heat-spots, when strongly stimulated, yield the -same sensation. Indeed, it appears that the mind relies upon the -simultaneous stimulation of adjacent heat-spots and cold-spots for the -assurance that the thing with which the skin is in contact is really -hot. If two metal points, one kept warm and the other cold, are applied -simultaneously to two closely adjacent spots of skin, the resulting -sensation is “hot.” When the cold point is withdrawn, or replaced by a -second warm point, the sensation sinks to “warm.”</p> - -<hr class="chap" /> -<p><span class="pagenum"><a name="Page_431" id="Page_431">[Pg 431]</a></span></p> -<div class="chapter"> - <h2>CHAPTER XVI<br /><span class="h_subtitle">VOICE AND SPEECH</span></h2> -</div> - -<p>A cut carried horizontally backwards across the cartilage which -projects forwards as Adam’s apple, a quarter of an inch below its -notch, would show that it is <b>V</b>-shaped, the point of the <b>V</b> -in front. Each limb of the <b>V</b> is a broad plate. In the mid-line -is a gap, the rima glottidis, through which the windpipe communicates -with the pharynx (<a href="#I_433">Fig. 45</a>). It is overhung by the stiff -leaf-shaped epiglottis, the edge of which can be felt with the finger behind -the tongue. (γλωττίς, the mouthpiece of a reed-pipe, is the term -commonly used, for short, for the rima glottidis.) When air is being -drawn into the lungs, the glottis is widely open. In speaking or -singing it is almost closed. It is tightly shut whilst food is passing -down the gullet.</p> - -<p>The glottis is bounded, as to its anterior two-thirds, by two -membranous folds, the vocal cords. In its posterior third it has a -triangular cartilage, the arytenoid, on either side. A distinction is -sometimes drawn between the anterior part, bounded by the vocal cords, -and the whole glottis, the former being termed “rima vocalis”; but it -is scarcely justified, for, although it is true that the anterior part -is essentially the organ of voice, and its margins alone vibrate when -high notes are sung, the anterior ends of the arytenoid cartilages -also vibrate during the production of low notes. (The substance of -these processes is not, properly speaking, cartilage; it resembles the -epiglottis in containing a great abundance of elastic fibres.) And -here we must warn the reader not to picture to himself a vocal “cord” -as a kind of fiddle-string. It bears no resemblance to a cord, as we -ordinarily understand the word; it is but a fold of mucous membrane, -such as one might pinch up between finger and thumb from the inner side -<span class="pagenum"><a name="Page_432" id="Page_432">[Pg 432]</a></span> -of the cheek. Its capacity for vibration depends upon the tenseness -which is given to it by the pressure of the lymph with which it is -distended, and vast numbers of exceedingly slender elastic fibres which -traverse it.</p> - -<div class="figcenter"> - <a id="I_432" name="I_432"> </a> - <img src="images/i_p432.jpg" alt="" width="600" height="597" /> - <p class="center"><span class="smcap">Fig. 44.</span>—<span class="smcap">The - Anterior Half of the Larynx seen from Behind.</span></p> - <div class="blockquot2"> - <p class="neg-indent space-below2">The drawing shows the folds of mucous membrane, - the vocal cords, which stretch from the tips of the arytenoid cartilages to - the recess behind the median portion of the thyroid cartilage. To the outer - side of each vocal cord is seen the thyro-arytenoid muscle (cut across), - consisting of a broad outer portion, chiefly concerned in closing the glottis - during the act of swallowing, and a smaller internal portion, which - regulates the length and the thickness of the segment of the cord - allowed to vibrate.</p> -</div></div> -<hr class="r25" /> - -<div class="figcenter"> - <a id="I_433" name="I_433"> </a> - <img src="images/i_p433.jpg" alt="" width="600" height="396" /> - <p class="center"><span class="smcap">Fig. 45.</span>—<span class="smcap">The - Aperture of the Glottis seen from Above.</span></p> - <div class="blockquot2"> - <p class="neg-indent space-below2">The leaf-like structure in front of it is the epiglottis; the two - triangular structures at the back, the arytenoid cartilages; the - white bands on either side, the vocal cords. A, The glottis is widely - open during inspiration. Arrows show the lines of action of the - muscles which rotate, and approximate, the cartilages. Attached to - their outer angles, and pulling these angles forwards, the lateral - crico-arytenoid muscles; pulling them backwards and inwards, the - posterior crico-arytenoid muscles. Drawing the cartilages together, - the arytenoid muscles. B, The glottis during speaking in a deep - chest-voice, or when a low note of the lower register is being sung. C, - During the production of a high note of the lower register. D, During - the production of a note of the head-register. E, During the act of - swallowing; the arytenoid cartilages are drawn towards the epiglottis - the aperture is folded into a <b>T</b>; the pharynx (the tube behind - the glottis) is distended.</p> -</div></div> - -<p>The first cartilage below the thyroid—it may be felt with the -finger—is termed “cricoid” (κρίκος, a ring), from its resemblance to -a signet-ring. Narrow in front, its large signet projects upwards, -within the <b>V</b> of the thyroid, behind, and on the top of the -signet rest the two arytenoids. Each arytenoid is a triangular pyramid, -its anterior, external, and upper angles prolonged into processes. -It is united with the cricoid by a swivel joint, which allows its -anterior process to swing inwards or outwards under the influence of -two antagonistic muscles attached to its outer angle—the lateral -and posterior crico-arytenoids. Another muscle attached only to the -<span class="pagenum"><a name="Page_433" id="Page_433">[Pg 433]</a></span> -arytenoids draws them together. Still another muscle—or two muscles, -for it is in two separate bands—unites the anterior process of the -arytenoid with the back surface of the thyroid just on the outer side -of the attachment into that cartilage of the vocal cord. The internal -thyro-arytenoid muscle is a comparatively narrow band; the external -thyro-arytenoid muscle is thick and broad.<a name="FNanchor_3_3" id="FNanchor_3_3"></a><a href="#Footnote_3_3" class="fnanchor">[3]</a> -By the simultaneous contraction of the encircling muscles the larynx -is closely squeezed together, the anterior portion of the slit forming -a <b>T</b>, with the transverse limb in front. This occurs only in -swallowing. Under the co-operating contractions of the several muscles, -the glottis assumes a variety of shapes. The external crico-arytenoids -<span class="pagenum"><a name="Page_434" id="Page_434">[Pg 434]</a></span> -rotate the anterior angles of the arytenoid cartilages inwards (<a href="#I_433">Fig. 45, A</a>). -If at the same time the arytenoid muscle draws the cartilages -together, the glottis is reduced to a slit (<a href="#I_433">Fig. 45, C</a>). -The posterior crico-arytenoid muscles rotate the cartilages outwards. If -the arytenoid muscle is at the same time relaxed, the glottis gapes to -its fullest extent (<a href="#I_433">Fig. 45, A</a>). The freer the opening, the -less is the resistance to the blast of air, the gentler the vibrations of the -cords, the lower the voice. The closer the slit, the greater is the -resistance which the air in the windpipe has to overcome in passing -through it, and consequently the more ample the vibrations into which -it throws the vocal cords.</p> - -<p>The vocal cords are the tongues of a reed-pipe, which, commencing -in the chest at the point where the great bronchi join to form the -windpipe, comprises the larynx, and, above the larynx, the complicated -chambers of the throat, mouth, and nasal cavities, including the spaces -within the bones of the head which open out of them. The pitch of the -voice depends upon (1) the length of the vocal cords, and (2) their -tension. The first factor is fixed for every individual. The voice is -base, baritone, tenor, in a man; contralto, mezzo-soprano, soprano, -in a woman—in proportion as the cords are long, of medium length, or -short. A man’s vocal cords measure, on the average, 15 millimetres, a -woman’s 11 millimetres. When a boy is from twelve to fifteen years of -age his vocal cords double in length, and the “breaking” of the voice -occurs as he gives up trying to get high notes out of his longer cords, -and allows them to produce manly tones of an octave lower.</p> - -<p>The lower posterior angles of the thyroid cartilages articulate with -the cricoid. If the four cartilages are freed from all soft tissues -without disturbing the thyro-cricoid, or crico-arytenoid joints, and -if, while the thyroid is held in one hand, a finger of the other is -placed on the front of the cricoid, it will be found that as this is -depressed the arytenoid cartilages which rest upon its signet are -tilted upwards and forwards within the thyroid; as it is raised, -they are tilted away from it. In life this movement is effected by -a muscle—the crico-thyroid (<a href="#I_435">Fig. 46</a>)—attached to -the front of the cricoid cartilage and to the under border of the lateral plate -of the thyroid. This is the muscle of supreme importance in the production of -the voice. The thyroid cartilage is slung in a fixed position by the -<span class="pagenum"><a name="Page_435" id="Page_435">[Pg 435]</a></span> -hyoid bone (to be felt in the neck above it). The crico-thyroid muscle, -being unable to depress the thyroid, raises the front of the cricoid -cartilage, tilts back the arytenoids, tightens the vocal cords. As the -voice ascends the scale, the tension of the cords is progressively -increased, and their vibrations rendered proportionately more rapid. -The range of the human voice is about three and a half octaves; of -individual voices about two octaves; if the shrill cry of a baby, -which may reach the third G above the middle C, or even higher (E⁗ or F⁗), -be excluded. Exceptional voices have a range far greater than two -octaves. Falsetto voice is produced by throwing half of the vocal cord -out of vibration (the way in which this is accomplished is not clear), -and at the same time raising the back of the tongue to the wall of the -throat in such a manner as to cut off all the lower part of the upper -resonating chamber, leaving it only the mouth and the cavities of the nose.</p> - -<div class="figcenter"> - <a id="I_435" name="I_435"> </a> - <img src="images/i_p435.jpg" alt="" width="350" height="546" /> - <p class="center"><span class="smcap">Fig. 46.</span>—<span class="smcap">The - Larynx from the Right Side.</span></p> - <div class="blockquot2"> - <p class="neg-indent space-below2">From above downwards: the hyoid bone, thyro-hyoid membrane, thyroid - cartilage, cricoid cartilage, trachea. The upper and posterior angle - of the wing of the thyroid cartilage is suspended from the hyoid bone; - its lower and posterior angle articulated with the cricoid cartilage. - On the summit of the cricoid cartilage it articulates the arytenoid. - Dotted lines indicate the position of the vocal cord. The crico-thyroid - muscle, which raises the front of the cricoid, tilting the arytenoid - cartilage backwards and tightening the vocal cord, extends, fan-like, - from the front of the cricoid to the lower border of the wing of the thyroid.</p> -</div></div> - -<p>So far the mechanism of voice is easily understood. As the scale -is ascended, the vocal cords are progressively tightened by the -<span class="pagenum"><a name="Page_436" id="Page_436">[Pg 436]</a></span> -contraction of the crico-thyroid muscles. But an analysis of the -feelings experienced during singing (and of the quality of the sounds -produced) shows that by themselves these muscles are not able to make -changes in the tension of the cords sufficient to account for the full -range of the voice. Or, put in another way, the tension of the vocal -cords is not altered to the extent which would be necessary if upon it -alone depended a range of from two to three octaves. It is obvious that -by some means the length or thickness, or both, of the portions of the -cords vibrating is changed as the scale is ascended. If commencement be -made on a low note, a point is reached, after a certain number of notes -have been sung, at which a sudden change occurs. There is an alteration -in the quality of sound, the more marked, the less well trained the -singer. The singer experiences a feeling of relief. If a finger be -placed on his crico-thyroid muscle, a relaxation of its anterior fibres -can be detected. As he proceeds up the scale, these fibres again -tighten. At a certain point there is again a change in the quality of -voice, and in the feelings which accompany its production. The two -points at which change occurs are said to divide the voice into three -“registers”—the lower, or chest-register, the middle, and the upper, -or head-register. A great effort is needed to hold either register -above its natural range.</p> - -<p>The physiology of the registers is a subject far too thorny for -handling in this book. The larynx can be watched with the laryngoscope -during the production of notes of different pitch, but observers are -not in accord regarding the appearances which it presents, or their -interpretation. The possibilities of changing the reed which vibrates, -the vocal cord, otherwise than by increasing the direct pull upon it -exerted by the crico-thyroid muscle, appear to be as follows: (1) -During the production of the lowest notes the elastic portion of the -arytenoid cartilage may be included with the cord. It may be thrown out -of vibration by its rotation inwards (under the action of the lateral -crico-arytenoid muscle) until it is pressed against its fellow. (2) -Certain portions of the cord may be damped by partial contractions of -the internal thyro-arytenoid muscle. It has been frequently stated, -although the statement is not accepted by all anatomists, that some of -the fibres which take origin from the arytenoid cartilage end in the -<span class="pagenum"><a name="Page_437" id="Page_437">[Pg 437]</a></span> -cord, instead of passing right through to the thyroid. It is supposed -that by their contraction they throw the posterior portion of the -cord—even, it is asserted, as much as its posterior two-thirds in the -higher head-notes—out of vibration. (3) It appears that the width -(thickness) of the cord vibrating is also regulated by the contraction -of the thyro-arytenoid muscle. Those who regard the diminution in -the thickness and width of the vibrating fold of mucous membrane and -underlying elastic tissue as the chief factor in the adaptation of -the larynx for the middle register lay great stress upon the sense of -relief from muscular effort which accompanies the transition. Less -force is needed to tighten the thinner cord. They also call attention -to the loss in volume of the voice when the lower register is left, and -to its greater softness. The lower is spoken of as the thick register, -the middle as thin, and the upper (on the hypothesis that part only of -the cord vibrates) as the small register.</p> - -<p>Singing reveals the possibilities of the larynx as a musical -instrument. In <b>speech</b> the larynx plays a part, but the form -of the syllabic sounds and the relative prominence of overtones in -the vowels is of more importance than pitch. Flexibility of voice -is dependent upon ability to increase or diminish at will the size -of the resonating chambers of the throat, mouth, and nose, or the -freedom of access to them. Conversation is carried on in the lower or -chest-register. When a practised speaker mounts a platform, he spends -the first few minutes in ascertaining the pitch of the hall—that is -to say, the pitch of his voice to which the room resonates most freely. -Having found the proper tone, he endeavours to maintain a uniform -tension of his vocal cords, and therefore a uniform pitch. He relieves -the monotony of speech by suitable variations of its overtones. -Nothing is more uncomfortable to listen to than an oration delivered -in cadences. The speaking voice should be full, round, and musical, -and free from affectation—as guiltless of the intoning or preaching -quality as it is of harshness or of vulgar flatness. A flexible voice -is capable of producing, as occasion calls for them, tones of any and -every quality. With the throat and mouth set for the syllable “haw,” it -is impossible to do justice to such words as “king ” and “queen.” The -<span class="pagenum"><a name="Page_438" id="Page_438">[Pg 438]</a></span> -voice-tones of a superior person are as distasteful to the hearer as -those of a vulgarian. Unpleasant also is a nasal twang, illogically so -called, since it is due, not to the opening of the resonating chambers -of the nose, but to the restriction of the entry of air into them. In -this it is somewhat similar to the effect produced by a severe cold. -Resonance in the nasal chambers produces a clear, ringing voice.</p> - -<p>A little consideration of the varying qualities of different -voices suffices to show how largely they depend on resonance. When -vowel-sounds are analysed, it is found that the distinctive character -of each of them is dependent upon the overtones which it contains. For -every vowel the overtones are fixed, or very nearly so, no matter what -may be the pitch of the note to which the vowel is sounded.</p> - -<p>It is much to be regretted that the alphabet was settled before the -physiology of speech was understood. Were it based upon reasonable -principles, children would be spared the bewilderment which overtakes -them when they endeavour to establish in their minds some kind of -relation between the names of consonants and their effects upon the -blast of air as it passes through throat and mouth, and between tongue -and palate, teeth and lips. The vowels, had physiologists defined them, -would have been real pure vowel-tones—<i>ōō</i>, <i>o</i>, <i>ah</i>, <i>ēē</i>—sounds -which can be sustained for an indefinite time, and allowed to die -away without deterioration in their quality. <i>A</i> (<i>é</i> as pronounced -in France) is doubtfully pure—it has a tendency to tail off in -<i>ēē</i>; <i>ī</i> is frankly a diphthong, <i>ai</i> (<i>ah-ēē</i>). Try to hold a long -final note on the syllable “nigh ”! An international standard of -vowel-sounds would have been fixed, by giving the vibrating periods of -the tuning-forks for which in each several case the resonating chambers -are shaped, and defining the relative accentuation of each overtone. -Greatest boon of all, the irruption of the Essex dialect would have -been dammed. It would not have been allowed to inundate London, or to -submerge Australia, debasing our English tongue. In Cockney speech -vowels degenerate down the line of greatest indolence. <i>Aw</i> becomes -<i>or</i>, or <i>ar</i>; <i>a</i> becomes <i>i</i>. It requires a greater effort to -pronounce a full <i>a</i> than a flat <i>a</i>, a definite flat <i>a</i> than <i>i</i>. And -worse than a Cockney’s unwillingness to take the trouble necessary for<span class="pagenum"><a name="Page_439" id="Page_439">[Pg 439]</a></span> -the production of dignifiedvowel-tones is his reluctance to make the -effort required for the holding of any tone. In his mouth virile, -self-reliant vowels are replaced by emasculated diphthongs, which -collapse as they present themselves to the ear. It costs trouble to fix -the mouth-chamber before a vowel is sounded and to hold it steady until -it is finished. <i>Ah</i> slides down through <i>ai</i> to <i>ēē</i>; <i>i</i> slips into -<i>ēē</i>. “Cow” becomes <i>kyow</i>; “you,” <i>ye-u-ow</i>; “cart,” <i>kyart</i>. And just -as the effort needed for the filling of the vowels is shirked, so also -is grudged the expenditure of an accessory blast for their aspiration.</p> - -<p>When a vowel is whispered, although the vocal cords do not vibrate, the -blast passing through the resonating chambers produces the overtones -characteristic of the vowel. Anyone who feels his own larynx while he -sings, to the same note, the various vowels between <i>ōō</i> and <i>ēē</i>—he -may please himself as to the number of <i>ai</i>, <i>eu</i>, and <i>ŭ</i> vowels he -interposes between these two extremes—will recognize that it is pulled -farther and farther upwards by the muscles which surround it. The -cavity of the mouth is at the same time made shorter and broader for -each succeeding vowel. Singing the several vowels before a piano, and -at the same time striking various keys, it is felt in the mouth that -the resonance of that chamber is reinforced by certain selected notes. -Certain tuning-forks, when sounded in front of the mouth shaped for -a vowel, ring out more loudly, because the mouth-cavity resonates to -their prime tones. The overtones of the vowels can be analysed in this -way. Conversely, by sounding simultaneously an appropriate selection -of tuning-forks, each with the right degree of force, the overtones of -a vowel can be synthesised. Thus if whilst one tuning-fork is sounding -B₁♭ (B♭ above middle C), two others be added giving B₂♭ (loud) and F₃ -(soft), the composite sound resembles the vowel <i>o</i>. If to these same -three forks, with F₃ sounding more strongly, B₃♭ and a loud D₄ be -added, the sound changes to <i>ah</i>.</p> - -<p>The organ of voice is a combination of a reed-pipe with resonating -chambers, the shape of which can be changed at will. The quality -characteristic of a vowel is given to it by adding to the note produced -in the larynx sounds due to the resonance of the throat and mouth. -On the assumption (not allowed by all authorities) that, since the -<span class="pagenum"><a name="Page_440" id="Page_440">[Pg 440]</a></span> -resonating chambers are not sound-producers, they can only add to the -larynx-tone, as “formants” of a vowel, its own harmonics—sounds which -they have picked out of it—it follows that, if, when the prime is -changed, the resonators were not adapted to the new note, they would -be dumb. If this attitude in regard to the question be justified, -there must be a certain amount of variation in the quality of a vowel -as the scale is ascended. But a vowel is not a musical tone; it is -a conventional sound. Its whole value depends upon its retaining, -as nearly as may be, the same quality, whatever be the pitch of its -prime tone. By adjusting the form of the throat and mouth, we can not -only prevent one vowel from passing into another, but we can keep it -so nearly true to itself as to convince the ear that its quality is -unchanged: <i>ōō</i> remains <i>ōō</i>, and <i>ah</i> <i>ah</i>, -although the form of the sound as produced on C♯ is different to its -form when sung to C.</p> - -<p>Apart from the general distinction that low notes are taken more -easily with vowels requiring a large mouth-cavity, and high notes with -those providing a small one, there are certain very distinct relations -between vowel-sounds and musical tones which need to be borne in mind -in setting words to music. A singer changes a word when he feels that -its vowel-tone does not allow him to give to the note to which it is -set the fullest expression of which he is capable.</p> - -<p>An account of the physiology of the production of consonants is to be -found in most text-books of grammar.</p> - -<hr class="chap" /> -<p><span class="pagenum"><a name="Page_441" id="Page_441">[Pg 441]</a></span></p> -<div class="chapter"> - <h2>INDEX</h2> -</div> - -<ul class="index"> -<li class="isub1">Absorption from alimentary canal, <a href="#Page_129">129</a></li> -<li class="isub1">Accelerator nerves of heart, <a href="#Page_237">237</a></li> -<li class="isub1">Accommodation of the eye for distance, <a href="#Page_391">391</a></li> -<li class="isub3">for light, <a href="#Page_390">390</a></li> -<li class="isub1">Acromegaly, <a href="#Page_93">93</a></li> -<li class="isub1">Addison’s disease, <a href="#Page_91">91</a></li> -<li class="isub1">Adrenalin, action on the kidney, <a href="#Page_209">209</a></li> -<li class="isub3">formed in suprarenal capsule, <a href="#Page_92">92</a></li> -<li class="isub1">Air, quantity inspired, <a href="#Page_173">173</a></li> -<li class="isub3">quantity needed by individual, <a href="#Page_191">191</a></li> -<li class="isub1">Air-cells of lungs, <a href="#Page_168">168</a></li> -<li class="isub1">Albumin made by plants, <a href="#Page_12">12</a></li> -<li class="isub1">Alcohol, effect on nerve conduction, <a href="#Page_301">301</a></li> -<li class="isub1">Alimentary canal, morphology of, <a href="#Page_98">98</a></li> -<li class="isub3">nerves of, <a href="#Page_104">104</a></li> -<li class="isub1">Altitude, highest, attained by climbers, <a href="#Page_187">187</a></li> -<li class="isub1">Alveoli of lungs, their number, <a href="#Page_169">169</a></li> -<li class="isub1">Amides produced from proteins, <a href="#Page_119">119</a></li> -<li class="isub1">Amœba, irritability of its protoplasm, <a href="#Page_10">10</a></li> -<li class="isub1">Amyl nitrite, effect on vascular system, <a href="#Page_237">237</a></li> -<li class="isub1">Anæmia, treatment with iron, <a href="#Page_67">67</a></li> -<li class="isub1">Anæsthetics, influence on protoplasm, <a href="#Page_11">11</a></li> -<li class="isub1">Analysis by animals, <a href="#Page_12">12</a></li> -<li class="isub1">Angina pectoris, <a href="#Page_237">237</a></li> -<li class="isub1">Angler fish, its nerve-cells, <a href="#Page_31">31</a></li> -<li class="isub1">Animal machine and its driver, <a href="#Page_354">354</a>, <a href="#Page_358">358</a></li> -<li class="isub1">Animals, hunting <i>versus</i> hunted, <a href="#Page_366">366</a></li> -<li class="isub3">not reflex machines, <a href="#Page_358">358</a></li> -<li class="isub3">relative insensibility to the knife, <a href="#Page_361">361</a></li> -<li class="isub1">Antitoxins, formation by protoplasm, <a href="#Page_20">20</a></li> -<li class="isub1">Aorta, diameter of, <a href="#Page_232">232</a></li> -<li class="isub1">Aphasia, <a href="#Page_352">352</a></li> -<li class="isub1">Apnœa, condition of arrested respiration, <a href="#Page_181">181</a></li> -<li class="isub1">Appendicitis, increased frequency of, <a href="#Page_101">101</a></li> -<li class="isub1">Appetite, a safe guide, <a href="#Page_114">114</a></li> -<li class="isub1">Arteries, blood-pressure in, <a href="#Page_234">234</a>, <a href="#Page_239">239</a></li> -<li class="isub3">structure of wall of, <a href="#Page_233">233</a></li> -<li class="isub1">Artificial respiration, <a href="#Page_183">183</a></li> -<li class="isub1">Asphyxia, <a href="#Page_182">182</a></li> -<li class="isub1">Association-zones in the cortex of the great brain, <a href="#Page_348">348</a></li> -<li class="isub1">Asthma, due to reflex contraction of small bronchi, <a href="#Page_167">167</a></li> -<li class="isub1">Astigmatism, correction by glasses, <a href="#Page_393">393</a></li> -<li class="isub3">due to modern print, <a href="#Page_269">269</a></li> -<li class="isub1">Attention, effect of, in heightening pain, <a href="#Page_361">361</a></li> - -<li class="isub1 ifrst">Bacteria, diminution of number in intestine on milk diet, <a href="#Page_138">138</a></li> -<li class="isub3">of alimentary canal, <a href="#Page_135">135</a></li> -<li class="isub3">of Bulgarian sour milk, <a href="#Page_138">138</a></li> -<li class="isub3">of the River Ganges, <a href="#Page_141">141</a></li> -<li class="isub3">in an infant’s intestine, <a href="#Page_136">136</a></li> -<li class="isub3">their rôle in nature, <a href="#Page_20">20</a></li> -<li class="isub1">Balance-sheet of body, how drawn up, <a href="#Page_149">149</a></li> -<li class="isub1">Balloon, highest altitude attained in, <a href="#Page_187">187</a></li> -<li class="isub1">Basket-cells in nervous system, <a href="#Page_324">324</a>, <a href="#Page_340">340</a></li> -<li class="isub1">Bat’s squeak, number of vibrations, <a href="#Page_418">418</a></li> -<li class="isub1">Bats, flight not dependent on vision, <a href="#Page_381">381</a></li> -<li class="isub1">Beats in music, explanation of, <a href="#Page_407">407</a></li> -<li class="isub1">Beetle, muscle of, <a href="#Page_261">261</a></li> -<li class="isub1">Belladonna, physiological action, <a href="#Page_109">109</a></li> -<li class="isub1">Bile, composition, <a href="#Page_117">117</a></li> -<li class="isub3">function in regard to absorption of fat, <a href="#Page_133">133</a></li> -<li class="isub3">relation to digestion, <a href="#Page_117">117</a></li> -<li class="isub1">Bile-pigment, origin from hæmoglobin, <a href="#Page_69">69</a>, <a href="#Page_82">82</a>, <a href="#Page_118">118</a></li> -<li class="isub1">Bioplasm, the essential substance of a living cell, <a href="#Page_148">148</a></li> -<li class="isub1">Birds, sense of hearing of, <a href="#Page_410">410</a></li> -<li class="isub1">Blind spot, how filled in, <a href="#Page_395">395</a></li> -<li class="isub1">Blisters, <a href="#Page_41">41</a></li> -<li class="isub1">Blood, amount ejected by heart, <a href="#Page_219">219</a></li> -<li class="isub3">circulation-time, <a href="#Page_219">219</a></li> -<li class="isub3">composition of, <a href="#Page_59">59</a></li> -<li class="isub3">gases of, amount, <a href="#Page_190">190</a></li> -<li class="isub3">tension, <a href="#Page_61">61</a></li> -<li class="isub3">lodged in abdominal veins, <a href="#Page_234">234</a>, <a href="#Page_236">236</a></li> -<li class="isub1">Blood-corpuscles, cellular nature, <a href="#Page_28">28</a></li> -<li class="isub3">life-story, <a href="#Page_62">62</a></li> -<li class="isub3">number, <a href="#Page_61">61</a></li> -<li class="isub3">origin, <a href="#Page_63">63</a>, <a href="#Page_64">64</a></li> -<li class="isub3">structure, <a href="#Page_60">60</a></li> -<li class="isub1">Blood-platelets, <a href="#Page_74">74</a></li> -<li class="isub1">Blood-poisoning, <a href="#Page_57">57</a></li> -<li class="isub1">Blushing, <a href="#Page_243">243</a></li> -<li class="isub1">Bowman’s description of kidney, <a href="#Page_200">200</a></li> -<li class="isub3">discs in muscle, <a href="#Page_259">259</a></li> -<li class="isub1">Brain. <i>Cf.</i> Cerebellum, Cortex of cerebrum</li> -<li class="isub3">blood-supply of, <a href="#Page_352">352</a></li> -<li class="isub1">Bread, digestion of, <a href="#Page_120">120</a></li> -<li class="isub1">Breathing, mechanism of, <a href="#Page_171">171</a></li> -<li class="isub1">Bruises, explanation of play of colours, <a href="#Page_69">69</a> -<span class="pagenum"><a name="Page_442" id="Page_442">[Pg 442]</a></span></li> -<li class="isub1">Bulgarian milk-germ, <a href="#Page_138">138</a></li> - -<li class="isub1 ifrst">Capillary vessels, circulation of blood in, <a href="#Page_232">232</a></li> -<li class="isub3">migration of leucocytes from, <a href="#Page_232">232</a></li> -<li class="isub3">structure of their walls, <a href="#Page_38">38</a></li> -<li class="isub1">Carbohydrate foods, chemical composition, <a href="#Page_147">147</a></li> -<li class="isub1">Carbonic acid, carried by blood, <a href="#Page_60">60</a></li> -<li class="isub3">liberation in lungs, <a href="#Page_61">61</a>, <a href="#Page_189">189</a></li> -<li class="isub1">Carbonic oxide, compound with hæmoglobin, <a href="#Page_187">187</a></li> -<li class="isub1">Carnivora, absorption of fat from alimentary canal of, <a href="#Page_133">133</a></li> -<li class="isub1">Cartilage, growth, <a href="#Page_28">28</a></li> -<li class="isub1">Catalysis, <a href="#Page_17">17</a></li> -<li class="isub1">Cell theory, <a href="#Page_26">26</a></li> -<li class="isub1">Cells, constituent parts, <a href="#Page_26">26</a>, <a href="#Page_28">28</a></li> -<li class="isub3">size, <a href="#Page_30">30</a></li> -<li class="isub3">specialization of function in, <a href="#Page_35">35</a></li> -<li class="isub1">Cells of Purkinje in the cerebellum, <a href="#Page_303">303</a>, <a href="#Page_340">340</a></li> -<li class="isub1">Cellulose, digestion of, <a href="#Page_137">137</a></li> -<li class="isub1">Cerebellum, cases of deficiency of, <a href="#Page_341">341</a></li> -<li class="isub3">connections with cerebro-spinal axis, <a href="#Page_340">340</a></li> -<li class="isub3">development of granules of, <a href="#Page_299">299</a>, <a href="#Page_303">303</a></li> -<li class="isub3">lobes, <a href="#Page_338">338</a></li> -<li class="isub3">minute anatomy, <a href="#Page_339">339</a></li> -<li class="isub3">phylogeny, <a href="#Page_338">338</a></li> -<li class="isub3">relation to tone of muscles, <a href="#Page_342">342</a></li> -<li class="isub1">Cerebral hemisphere, an outgrowth towards olfactory pit, <a href="#Page_334">334</a></li> -<li class="isub3">in animals with various sensory endowments, <a href="#Page_349">349</a></li> -<li class="isub1">Cerebro-spinal fluid, <a href="#Page_50">50</a></li> -<li class="isub1">Chemical activity of protoplasm, <a href="#Page_12">12</a></li> -<li class="isub3">messengers, <a href="#Page_89">89</a>, <a href="#Page_123">123</a></li> -<li class="isub3">processes in plants, <a href="#Page_15">15</a></li> -<li class="isub1">Chemiotaxis of leucocytes, <a href="#Page_56">56</a>, <a href="#Page_364">364</a></li> -<li class="isub1">Children, brain in, <a href="#Page_346">346</a></li> -<li class="isub3">development of astigmatism in eyes of, <a href="#Page_269">269</a></li> -<li class="isub1">Chill, catching a, <a href="#Page_242">242</a></li> -<li class="isub1">Chloroform. <i>Cf.</i> Anæsthetics</li> -<li class="isub1">Cholesterin, <a href="#Page_118">118</a></li> -<li class="isub1">Chromatolysis in nerve-cells, <a href="#Page_320">320</a></li> -<li class="isub1">Chrome-silver method of colouring nerve-tissue, <a href="#Page_293">293</a></li> -<li class="isub1">Chyme, food converted into, <a href="#Page_126">126</a></li> -<li class="isub1">Circulation of the blood, <a href="#Page_218">218</a></li> -<li class="isub1">Circulation-time, <a href="#Page_219">219</a></li> -<li class="isub1">Cirrhosis of liver, <a href="#Page_42">42</a></li> -<li class="isub1">Coagulation of blood, <a href="#Page_69">69</a></li> -<li class="isub1">Cochlea, anatomy, <a href="#Page_413">413</a></li> -<li class="isub1">Cockney dialect, the degradation of vowel-sounds, <a href="#Page_439">439</a></li> -<li class="isub1">Coke-fire, poisonous fumes from, <a href="#Page_186">186</a></li> -<li class="isub1">Cold-spots in skin, <a href="#Page_429">429</a></li> -<li class="isub1">Collaterals of nerves, <a href="#Page_297">297</a></li> -<li class="isub1">Colon, length and disposition of, <a href="#Page_101">101</a></li> -<li class="isub1">Colour-blindness, <a href="#Page_385">385</a></li> -<li class="isub1">Colour-vision, <a href="#Page_385">385</a></li> -<li class="isub1">Colours, reason for apparent fading in twilight, <a href="#Page_378">378</a></li> -<li class="isub1">Conductivity of protoplasm, <a href="#Page_248">248</a></li> -<li class="isub1">Consciousness, does not come within physiological investigation, <a href="#Page_360">360</a></li> -<li class="isub3">its part in animal life, <a href="#Page_359">359</a></li> -<li class="isub1">Control experiments, their value, <a href="#Page_72">72</a></li> -<li class="isub1">Convolutions of brain, <a href="#Page_345">345</a></li> -<li class="isub1">Cooking, effect upon digestibility of meat, <a href="#Page_120">120</a></li> -<li class="isub1">Corneal epithelium, sensitiveness of, <a href="#Page_424">424</a></li> -<li class="isub1">Corpus striatum of brain, <a href="#Page_344">344</a></li> -<li class="isub1">Cortex of cerebrum, discovery of excitability of, <a href="#Page_344">344</a></li> -<li class="isub3">fissures and convolutions, <a href="#Page_345">345</a></li> -<li class="isub3">functional areas, <a href="#Page_352">352</a></li> -<li class="isub3">myelination of its fibres, <a href="#Page_345">345</a></li> -<li class="isub3">sensory and association areas, <a href="#Page_346">346</a></li> -<li class="isub3">structure of, <a href="#Page_347">347</a></li> -<li class="isub3">variations in different animals, <a href="#Page_349">349</a></li> -<li class="isub1">Corti, organ of, its structure, <a href="#Page_414">414</a></li> -<li class="isub3">theories of function of, <a href="#Page_416">416</a></li> -<li class="isub1">Coughing, mechanism of, <a href="#Page_180">180</a></li> -<li class="isub1">Crayfish, tone of claw-muscle of, <a href="#Page_273">273</a></li> -<li class="isub1">Cretinism, <a href="#Page_85">85</a>, <a href="#Page_90">90</a></li> -<li class="isub1">Cricket, chirp of, <a href="#Page_261">261</a></li> -<li class="isub1">Crypts of Lieberkühn, <a href="#Page_103">103</a></li> -<li class="isub1">Curdling of milk, <a href="#Page_75">75</a></li> - -<li class="isub1 ifrst">Dancing, association of sound with movement, <a href="#Page_422">422</a></li> -<li class="isub1">Day’s work, food required for, <a href="#Page_151">151</a></li> -<li class="isub1">Deafness due to sore throat, <a href="#Page_412">412</a></li> -<li class="isub1">Degeneration of nerves after section, <a href="#Page_326">326</a></li> -<li class="isub1">Depressor nerve of the heart, <a href="#Page_237">237</a></li> -<li class="isub1">Diabetes, excretion of more carbohydrate than contained in food, <a href="#Page_143">143</a></li> -<li class="isub1">Dialysis, explanation of the process, <a href="#Page_40">40</a>, <a href="#Page_128">128</a></li> -<li class="isub1">Diaphragm, function in respiration, <a href="#Page_171">171</a></li> -<li class="isub1">Diastases, destructive ferments, <a href="#Page_18">18</a></li> -<li class="isub1">Diet, limits of possible variations in, <a href="#Page_153">153</a></li> -<li class="isub3">of labouring classes, <a href="#Page_152">152</a></li> -<li class="isub1">Digestibility of bread, meat, fish, etc., <a href="#Page_120">120</a>, <a href="#Page_125">125</a></li> -<li class="isub1">Digestion, mechanism of, <a href="#Page_96">96</a></li> -<li class="isub3">vascular changes during, <a href="#Page_235">235</a></li> -<li class="isub3">waits on appetite, <a href="#Page_114">114</a></li> -<li class="isub1">Digitalis, action on heart and kidney, <a href="#Page_209">209</a></li> -<li class="isub1">Diphtheria, antitoxin of, <a href="#Page_20">20</a></li> -<li class="isub1">Diuretics, <a href="#Page_209">209</a></li> -<li class="isub1">Dog’s sense of smell, <a href="#Page_370">370</a></li> -<li class="isub1">Dreams, theory of, <a href="#Page_362">362</a> -<span class="pagenum"><a name="Page_443" id="Page_443">[Pg 443]</a></span></li> -<li class="isub1">Dropsy, <a href="#Page_42">42</a></li> -<li class="isub1">Drowning, resuscitation from, <a href="#Page_183">183</a></li> -<li class="isub1">Drugs, physiology of, <a href="#Page_95">95</a></li> -<li class="isub1">Ductless glands, <a href="#Page_94">94</a></li> -<li class="isub1">Dyspnœa, difficult respiration, <a href="#Page_181">181</a></li> - -<li class="isub1 ifrst">Ear, anatomy, <a href="#Page_411">411</a></li> -<li class="isub3">bones of, <a href="#Page_412">412</a></li> -<li class="isub3">differentiation into separate sense-organs, <a href="#Page_410">410</a></li> -<li class="isub3">in fishes, <a href="#Page_410">410</a></li> -<li class="isub3">phylogeny, <a href="#Page_409">409</a></li> -<li class="isub1">Eel’s blood injected into mammal, <a href="#Page_20">20</a></li> -<li class="isub1">Effector, an organ which exhibits change in response to stimulation, <a href="#Page_253">253</a></li> -<li class="isub1">Egg-albumin destroyed by blood, <a href="#Page_19">19</a></li> -<li class="isub1">Electric organs, <a href="#Page_288">288</a></li> -<li class="isub3">phenomena of muscles, <a href="#Page_279">279</a></li> -<li class="isub1">Emotions, their relation to vaso-motor changes, <a href="#Page_242">242</a></li> -<li class="isub1">Energy, expended by body, <a href="#Page_151">151</a></li> -<li class="isub3">source of the body’s, <a href="#Page_152">152</a></li> -<li class="isub3">of stimulus compared with energy of muscular response, <a href="#Page_254">254</a></li> -<li class="isub1">Engines, body compared with, <a href="#Page_152">152</a>, <a href="#Page_256">256</a></li> -<li class="isub1">Epiglottis during swallowing, <a href="#Page_433">433</a></li> -<li class="isub1">Equilibrium, maintenance of, in walking, <a href="#Page_342">342</a></li> -<li class="isub1">Erepsin, ferment of intestinal juice, <a href="#Page_119">119</a></li> -<li class="isub1">Errors of sensory judgment, <a href="#Page_402">402</a></li> -<li class="isub1">Excretion, <a href="#Page_195">195</a></li> -<li class="isub1">Eye, accommodation for distance, <a href="#Page_391">391</a></li> -<li class="isub3">adaptation for darkness, <a href="#Page_390">390</a></li> -<li class="isub3">blind spot, <a href="#Page_394">394</a></li> -<li class="isub3">optical defects of, <a href="#Page_393">393</a></li> -<li class="isub3">phylogeny, <a href="#Page_334">334</a></li> -<li class="isub3">refractive media, formation of image by, <a href="#Page_391">391</a></li> -<li class="isub1">Eyeball, abnormalities in shape of, <a href="#Page_392">392</a></li> -<li class="isub3">anatomy, <a href="#Page_373">373</a></li> -<li class="isub3">development, <a href="#Page_374">374</a></li> -<li class="isub3">muscles of, indefatigable, <a href="#Page_269">269</a></li> - -<li class="isub1 ifrst">Fat, absorption of, <a href="#Page_131">131</a>, <a href="#Page_132">132</a></li> -<li class="isub3">accumulation of, relation to foods consumed, <a href="#Page_144">144</a></li> -<li class="isub3">chemistry, <a href="#Page_132">132</a></li> -<li class="isub3">digestion, <a href="#Page_133">133</a></li> -<li class="isub3">laid down in connective tissues, <a href="#Page_145">145</a></li> -<li class="isub3">stored in liver, <a href="#Page_145">145</a></li> -<li class="isub1">Fatigue, causes of, <a href="#Page_45">45</a>, <a href="#Page_268">268</a></li> -<li class="isub1">Fermentation, <a href="#Page_16">16</a></li> -<li class="isub1">Ferments, chemical nature, <a href="#Page_18">18</a></li> -<li class="isub3">classification, <a href="#Page_16">16</a>, <a href="#Page_18">18</a></li> -<li class="isub3">physiological importance, <a href="#Page_18">18</a></li> -<li class="isub1">Fibrin of blood, its antecedents, <a href="#Page_75">75</a></li> -<li class="isub1">Fireflies, source of their light, <a href="#Page_291">291</a></li> -<li class="isub1">Fish, sense of smell of, <a href="#Page_365">365</a></li> -<li class="isub3">supposed to be frightened by noise, <a href="#Page_410">410</a></li> -<li class="isub1">Flatulence, cause of, <a href="#Page_114">114</a>, <a href="#Page_125">125</a>, <a href="#Page_136">136</a></li> -<li class="isub1">Foods, classification, <a href="#Page_142">142</a></li> -<li class="isub3">history of, after absorption, <a href="#Page_142">142</a></li> -<li class="isub3">relative value, <a href="#Page_147">147</a>, <a href="#Page_151">151</a>, <a href="#Page_153">153</a>, <a href="#Page_157">157</a></li> -<li class="isub3">residue after digestion and absorption, <a href="#Page_194">194</a></li> -<li class="isub1">Foramen ovale of heart, sometimes perforate, <a href="#Page_218">218</a></li> -<li class="isub1">Frigate-bird, turbinate bones of, <a href="#Page_166">166</a></li> -<li class="isub1">Frog, supposed to be found entombed in rock, <a href="#Page_164">164</a></li> -<li class="isub1">Functional interdependence of organs, <a href="#Page_94">94</a></li> -<li class="isub1">Functions transferred to other organs, <a href="#Page_87">87</a></li> - -<li class="isub1 ifrst">Gall-stones, cause of formation of, <a href="#Page_118">118</a></li> -<li class="isub1">Galvani’s observation of contraction of a frog’s muscles, <a href="#Page_277">277</a></li> -<li class="isub1">Ganges, purifying water of, <a href="#Page_138">138</a></li> -<li class="isub1">Ganglia of sympathetic chain, function, <a href="#Page_325">325</a></li> -<li class="isub1">Ganglion-cells of retina, <a href="#Page_376">376</a></li> -<li class="isub3">spinal, <a href="#Page_299">299</a>, <a href="#Page_333">333</a></li> -<li class="isub1">Gaseous tension, meaning of expression, <a href="#Page_188">188</a></li> -<li class="isub1">Gases of blood, their exchange in the lungs, <a href="#Page_184">184</a></li> -<li class="isub1">Gastric glands, structure, <a href="#Page_123">123</a></li> -<li class="isub3">juice, amount secreted, <a href="#Page_114">114</a></li> -<li class="isub3">composition, <a href="#Page_114">114</a></li> -<li class="isub3">digestive action, <a href="#Page_115">115</a></li> -<li class="isub1">Gelatin as article of diet, <a href="#Page_158">158</a></li> -<li class="isub1">Giant cells, <a href="#Page_65">65</a></li> -<li class="isub1">Glands, vaso-motor nerves of, <a href="#Page_109">109</a>, <a href="#Page_241">241</a></li> -<li class="isub1">Glycogen, formula, <a href="#Page_147">147</a></li> -<li class="isub3">as muscle food, <a href="#Page_148">148</a></li> -<li class="isub3">stored in liver, <a href="#Page_147">147</a></li> -<li class="isub1">Goitre, cause of, <a href="#Page_84">84</a></li> -<li class="isub1">Granules, appearance of, in glands, <a href="#Page_110">110</a></li> -<li class="isub3">of cerebellum, development of, <a href="#Page_299">299</a>, <a href="#Page_303">303</a></li> -<li class="isub1">Grey matter, formation of paths in, <a href="#Page_356">356</a></li> -<li class="isub1">Growth, a function of protoplasm, <a href="#Page_24">24</a></li> -<li class="isub3">a reaction to work, <a href="#Page_47">47</a></li> - -<li class="isub1 ifrst">Hæmatin, <a href="#Page_68">68</a></li> -<li class="isub1">Hæmatoidin, <a href="#Page_68">68</a></li> -<li class="isub1">Hæmochromogen, <a href="#Page_68">68</a></li> -<li class="isub1">Hæmoglobin, crystalline form, <a href="#Page_66">66</a>, <a href="#Page_186">186</a></li> -<li class="isub3">formula, <a href="#Page_66">66</a></li> -<li class="isub3">as oxygen carrier, <a href="#Page_66">66</a>, <a href="#Page_186">186</a></li> -<li class="isub3">spectrum, <a href="#Page_68">68</a>, <a href="#Page_185">185</a></li> -<li class="isub1">Hæmophilia, non-coagulability of blood, <a href="#Page_76">76</a></li> -<li class="isub1">Hallucinations, <a href="#Page_362">362</a></li> -<li class="isub1">Headache, a pain in the scalp, <a href="#Page_106">106</a>, <a href="#Page_319">319</a></li> -<li class="isub3">the brain’s warning of fatigue, <a href="#Page_269">269</a></li> -<li class="isub3">from strain of eye-muscles, <a href="#Page_268">268</a></li> -<li class="isub1">Hearing, analysis of compound vibrations, <a href="#Page_405">405</a> -<span class="pagenum"><a name="Page_444" id="Page_444">[Pg 444]</a></span></li> -<li class="isub3">capacity dependent upon education, <a href="#Page_422">422</a></li> -<li class="isub1">Hearing, Helmholtz’s theory of analysis of sounds, <a href="#Page_419">419</a></li> -<li class="isub3">range of sensations, <a href="#Page_418">418</a></li> -<li class="isub3">sense of, <a href="#Page_404">404</a></li> -<li class="isub3">upper limit, <a href="#Page_418">418</a></li> -<li class="isub1">Heart, anatomy, <a href="#Page_217">217</a></li> -<li class="isub3">automatism of, <a href="#Page_238">238</a></li> -<li class="isub3">development, <a href="#Page_218">218</a></li> -<li class="isub3">murmurs, <a href="#Page_229">229</a></li> -<li class="isub3">muscular tissue, minute structure, <a href="#Page_261">261</a></li> -<li class="isub3">nerves regulating beat, <a href="#Page_237">237</a>, <a href="#Page_239">239</a></li> -<li class="isub3">sounds of, <a href="#Page_228">228</a></li> -<li class="isub3">valves, their mechanism, <a href="#Page_226">226</a></li> -<li class="isub3">work done by, <a href="#Page_219">219</a>, <a href="#Page_223">223</a></li> -<li class="isub1">Heat, production of, by muscles, <a href="#Page_254">254</a>, <a href="#Page_256">256</a></li> -<li class="isub1">Heat-spots in skin, <a href="#Page_429">429</a></li> -<li class="isub1">Helmholtz’s theory of organ of Corti, <a href="#Page_419">419</a></li> -<li class="isub1">Hering’s theory of colour-vision, <a href="#Page_388">388</a></li> -<li class="isub1">Hormones, meaning of term, <a href="#Page_89">89</a>, <a href="#Page_124">124</a></li> -<li class="isub3">of pancreas and liver, <a href="#Page_127">127</a></li> -<li class="isub3">of stomach, <a href="#Page_123">123</a></li> -<li class="isub1">Humours in ancient medical theory, <a href="#Page_79">79</a></li> -<li class="isub1">Hunter, experiment of grafting cock’s spur in its comb, <a href="#Page_47">47</a></li> -<li class="isub1">Hydrochloric acid, part taken in digestion, <a href="#Page_114">114</a></li> -<li class="isub1">Hydrophobia, protective inoculation, <a href="#Page_78">78</a></li> -<li class="isub1">Hyperpnœa, excessive respiratory efforts, <a href="#Page_182">182</a></li> -<li class="isub1">Hypoblast, a layer of the embryo, <a href="#Page_97">97</a></li> - -<li class="isub1 ifrst">Illusions of movement, <a href="#Page_335">335</a>, <a href="#Page_384">384</a></li> -<li class="isub3">of size and distance, <a href="#Page_400">400</a></li> -<li class="isub1">Immunity, acquisition of, <a href="#Page_20">20</a></li> -<li class="isub1">Impulse of the heart, <a href="#Page_225">225</a></li> -<li class="isub3">rate of passage in muscle, <a href="#Page_280">280</a></li> -<li class="isub3">in nerve, <a href="#Page_278">278</a>, <a href="#Page_280">280</a></li> -<li class="isub3">theory of nerve conduction, <a href="#Page_282">282</a></li> -<li class="isub1">Inhibition, explanation of term, <a href="#Page_311">311</a></li> -<li class="isub3">of reflex actions, <a href="#Page_311">311</a></li> -<li class="isub1">Insects, efficiency of their muscles, <a href="#Page_261">261</a></li> -<li class="isub1">Instinct, due to brain-pattern, <a href="#Page_359">359</a></li> -<li class="isub1">Intelligence of animals, <a href="#Page_359">359</a></li> -<li class="isub1">Internal secretions, <a href="#Page_83">83</a></li> -<li class="isub1">Intestinal juice, digestive action, <a href="#Page_119">119</a></li> -<li class="isub1">Intestine, large, sacculation of its walls, <a href="#Page_101">101</a></li> -<li class="isub3">small, folds and glands of mucous membrane, <a href="#Page_102">102</a></li> -<li class="isub1">Intestines, movements of, <a href="#Page_103">103</a></li> -<li class="isub3">nerves of, <a href="#Page_105">105</a></li> -<li class="isub3">size and situation, <a href="#Page_100">100</a></li> -<li class="isub1">Iodine, importance of, to economy, <a href="#Page_89">89</a></li> -<li class="isub1">Iodothyrin, goitre due to deficiency of, <a href="#Page_90">90</a></li> -<li class="isub1">Iris, its function in regulating admission of light to eye, <a href="#Page_394">394</a></li> -<li class="isub1">Iron in food, <a href="#Page_67">67</a></li> -<li class="isub3">in hæmoglobin, <a href="#Page_67">67</a></li> -<li class="isub3">use of, in treatment of anæmia, <a href="#Page_67">67</a></li> -<li class="isub1">Irritability, a function of protoplasm, <a href="#Page_10">10</a></li> - -<li class="isub1 ifrst">Japanese, cultivation of sense of smell by, <a href="#Page_370">370</a></li> -<li class="isub1">Judgment of angles, <a href="#Page_402">402</a></li> -<li class="isub3">of distance and size, <a href="#Page_401">401</a></li> -<li class="isub3">of meaning of sensations, <a href="#Page_396">396</a>, <a href="#Page_399">399</a></li> - -<li class="isub1 ifrst">Kidney, ancestral history, <a href="#Page_195">195</a></li> -<li class="isub3">elimination of indigo by, <a href="#Page_207">207</a></li> -<li class="isub3">of birds and reptiles, <a href="#Page_200">200</a>, <a href="#Page_207">207</a></li> -<li class="isub3">hydrostatic mechanism, <a href="#Page_189">189</a></li> -<li class="isub3">minute anatomy, <a href="#Page_196">196</a></li> -<li class="isub1">Kinæsthetic sensations, absence from dreams, <a href="#Page_363">363</a></li> -<li class="isub3">part played by, in voluntary actions, <a href="#Page_354">354</a></li> -<li class="isub3">representation in cortex of brain, <a href="#Page_350">350</a>, <a href="#Page_352">352</a></li> -<li class="isub1">Knee-jerk, <a href="#Page_274">274</a></li> - -<li class="isub1 ifrst">Labyrinth of ear, <a href="#Page_413">413</a></li> -<li class="isub1">Lactate of ammonia, relation to urea, <a href="#Page_13">13</a></li> -<li class="isub1">Lacteals, lymphatic vessels of alimentary canal, <a href="#Page_43">43</a>, <a href="#Page_131">131</a></li> -<li class="isub1">Lactic acid produced in muscle, <a href="#Page_46">46</a>, <a href="#Page_146">146</a></li> -<li class="isub1">Larynx, closure during swallowing, <a href="#Page_433">433</a></li> -<li class="isub3">structure of, <a href="#Page_430">430</a></li> -<li class="isub1">Latent period of muscle after nervous impulse reaches it, <a href="#Page_278">278</a></li> -<li class="isub1">Laughter, respiratory mechanism of, <a href="#Page_180">180</a></li> -<li class="isub1">Lecithin produced by metabolism of nerve-tissue, <a href="#Page_118">118</a></li> -<li class="isub1">Leech, ganglion-cells of, <a href="#Page_298">298</a></li> -<li class="isub1">Leucocytes as protective agents, <a href="#Page_52">52</a></li> -<li class="isub3">death of, <a href="#Page_54">54</a>, <a href="#Page_57">57</a></li> -<li class="isub3">migration of, <a href="#Page_49">49</a></li> -<li class="isub3">number in lymph and in blood, <a href="#Page_49">49</a>, <a href="#Page_61">61</a></li> -<li class="isub3">origin of, <a href="#Page_33">33</a>, <a href="#Page_51">51</a></li> -<li class="isub3">source of fibrin-ferment, <a href="#Page_74">74</a></li> -<li class="isub1">Leucocythæmia, excess of leucocytes in the blood, <a href="#Page_215">215</a></li> -<li class="isub1">Levers to which muscles are attached, <a href="#Page_286">286</a></li> -<li class="isub1">Light, emission of, by animals, <a href="#Page_291">291</a></li> -<li class="isub1">Lime, influence upon coagulation of blood, <a href="#Page_75">75</a></li> -<li class="isub3">curdling of milk, <a href="#Page_75">75</a></li> -<li class="isub1">Lithates, or urates, constituents of calculi, <a href="#Page_213">213</a></li> -<li class="isub1">Liver, destruction of red blood-corpuscles in, <a href="#Page_83">83</a></li> -<li class="isub3">form and structure of, <a href="#Page_160">160</a></li> -<li class="isub3">former theories of its functions, <a href="#Page_129">129</a>, <a href="#Page_163">163</a></li> -<li class="isub3">manufactures urea and uric acid, <a href="#Page_146">146</a>, <a href="#Page_162">162</a></li> -<li class="isub3">of well fed sheep, <a href="#Page_147">147</a> -<span class="pagenum"><a name="Page_445" id="Page_445">[Pg 445]</a></span></li> -<li class="isub3">origin of, in vertebrate phylogeny, <a href="#Page_34">34</a></li> -<li class="isub1">Liver stores food, especially glycogen, <a href="#Page_46">46</a>, <a href="#Page_145">145</a>, <a href="#Page_147">147</a>, <a href="#Page_161">161</a></li> -<li class="isub1">Locomotor ataxy, <a href="#Page_341">341</a></li> -<li class="isub1">Ludwig’s view of mechanism of kidney, <a href="#Page_200">200</a></li> -<li class="isub1">Luminous glands, <a href="#Page_291">291</a></li> -<li class="isub1">Lung, exchange of gases in, <a href="#Page_173">173</a>, <a href="#Page_184">184</a>, <a href="#Page_190">190</a></li> -<li class="isub3">nerve-supply, <a href="#Page_178">178</a></li> -<li class="isub3">structure, <a href="#Page_168">168</a></li> -<li class="isub1">Lymph, amount of, in body, <a href="#Page_37">37</a></li> -<li class="isub3">composition, <a href="#Page_49">49</a></li> -<li class="isub3">relation to blood, <a href="#Page_51">51</a></li> -<li class="isub1">Lymph-spaces, <a href="#Page_37">37</a>, <a href="#Page_43">43</a>, <a href="#Page_49">49</a></li> -<li class="isub1">Lymphatic glands, structure of, <a href="#Page_54">54</a></li> -<li class="isub1">Lymphatic vessels, <a href="#Page_43">43</a></li> - -<li class="isub1 ifrst">Malapterurus, electric organs, <a href="#Page_288">288</a></li> -<li class="isub3">immense neurones of, <a href="#Page_295">295</a></li> -<li class="isub1">Manometer for measuring blood-pressure, description of, <a href="#Page_238">238</a></li> -<li class="isub1">Man’s ancestry, <a href="#Page_153">153</a></li> -<li class="isub1">Massage of abdominal viscera, <a href="#Page_101">101</a></li> -<li class="isub3">of muscles, <a href="#Page_48">48</a></li> -<li class="isub1">Meal, the story of a, <a href="#Page_120">120</a></li> -<li class="isub1">Meat, diet consisting solely of, <a href="#Page_157">157</a></li> -<li class="isub3">digestion of, <a href="#Page_121">121</a></li> -<li class="isub3">extracts of, as articles of diet, <a href="#Page_159">159</a></li> -<li class="isub1">Megacaryocytes, <a href="#Page_65">65</a></li> -<li class="isub1">Memory, physiological explanation, <a href="#Page_356">356</a></li> -<li class="isub1">Metabolism, chemical change in living tissue, <a href="#Page_12">12</a>, <a href="#Page_273">273</a></li> -<li class="isub1">Methæmoglobin, <a href="#Page_69">69</a></li> -<li class="isub1">Microscope, its discovery, <a href="#Page_26">26</a></li> -<li class="isub1">Migration of birds, <a href="#Page_359">359</a></li> -<li class="isub1">Milk, call for secretion of, by a hormone, <a href="#Page_94">94</a></li> -<li class="isub3">chemical and physical constitution, <a href="#Page_132">132</a></li> -<li class="isub3">digestion of, <a href="#Page_127">127</a></li> -<li class="isub1">Milk diet, reduction of bacteria in alimentary canal on, <a href="#Page_138">138</a></li> -<li class="isub1">Mind, physiology of, <a href="#Page_354">354</a></li> -<li class="isub1">Mosquitoes, production of sound by, <a href="#Page_261">261</a></li> -<li class="isub1">Motile cells, <a href="#Page_32">32</a></li> -<li class="isub1">Mountain sickness, <a href="#Page_187">187</a></li> -<li class="isub1">Mountains, highest climbed, <a href="#Page_187">187</a></li> -<li class="isub1">Mucous membrane, use of term, <a href="#Page_97">97</a></li> -<li class="isub1">Murmurs, in chest, in diseases of lungs, <a href="#Page_169">169</a></li> -<li class="isub3">of heart, <a href="#Page_229">229</a></li> -<li class="isub1">Muscle, change in appearance under microscope during contraction, <a href="#Page_263">263</a></li> -<li class="isub3">chemistry of contraction, <a href="#Page_266">266</a></li> -<li class="isub3">contraction a phenomenon of osmosis, <a href="#Page_258">258</a></li> -<li class="isub3">electric phenomena of, <a href="#Page_278">278</a></li> -<li class="isub3">means of promoting growth of, <a href="#Page_271">271</a></li> -<li class="isub3">measurement of its power, <a href="#Page_285">285</a></li> -<li class="isub3">nature of impulse which leads to contraction of, <a href="#Page_282">282</a></li> -<li class="isub1">Muscle of heart, its minute structure, <a href="#Page_224">224</a></li> -<li class="isub3">of insects, its efficiency, <a href="#Page_261">261</a></li> -<li class="isub3">plain, its minute structure, <a href="#Page_258">258</a></li> -<li class="isub3">plasma, its coagulation, <a href="#Page_266">266</a></li> -<li class="isub3">rhythm of voluntary contraction, <a href="#Page_279">279</a></li> -<li class="isub3">theory of its structure as a mechanism liberating energy, <a href="#Page_234">234</a>, <a href="#Page_255">255</a></li> -<li class="isub3">tone of, <a href="#Page_272">272</a></li> -<li class="isub3">tracings taken of contracting, <a href="#Page_278">278</a></li> -<li class="isub3">voluntary, its minute structure, <a href="#Page_259">259</a></li> -<li class="isub3">wastes when its nerve is severed, <a href="#Page_274">274</a></li> -<li class="isub3">work done by, proportional to load, <a href="#Page_286">286</a></li> -<li class="isub1">Muscles, arrangement in regard to the bones which they move, <a href="#Page_286">286</a></li> -<li class="isub3">co-operation in lifting a weight, <a href="#Page_287">287</a></li> -<li class="isub1">Muscular energy, source of, <a href="#Page_235">235</a></li> -<li class="isub1">Muscularis mucosæ of alimentary canal, <a href="#Page_103">103</a></li> -<li class="isub1">Musculi papillares of heart, <a href="#Page_227">227</a></li> -<li class="isub1">Music, chords admissible in, <a href="#Page_408">408</a></li> -<li class="isub3">Indian, division of octave, <a href="#Page_408">408</a></li> -<li class="isub3">primitive, prevalence of minor chords, <a href="#Page_408">408</a></li> -<li class="isub1">Musical tones and overtones, <a href="#Page_406">406</a></li> -<li class="isub1">Myelination of nerves, order of, <a href="#Page_345">345</a></li> -<li class="isub1">Myxœdema, dependent on disease of thyroid gland, <a href="#Page_85">85</a></li> -<li class="isub1">Myxomycetes, fusion of cell-bodies of, <a href="#Page_27">27</a></li> - -<li class="isub1 ifrst">Nasal chambers, air warmed in, <a href="#Page_166">166</a></li> -<li class="isub1">Negroes, their long heels, <a href="#Page_285">285</a></li> -<li class="isub1">Nerve, conduction in, theory of, <a href="#Page_282">282</a></li> -<li class="isub3">degeneration, <a href="#Page_326">326</a></li> -<li class="isub3">electrical phenomena, <a href="#Page_279">279</a></li> -<li class="isub3">indefatigable, <a href="#Page_282">282</a></li> -<li class="isub3">regeneration, <a href="#Page_326">326</a></li> -<li class="isub3">structure, <a href="#Page_296">296</a></li> -<li class="isub1">Nerves, depressor, <a href="#Page_237">237</a></li> -<li class="isub3">experiment of crossing, <a href="#Page_327">327</a></li> -<li class="isub3">fifth, <a href="#Page_316">316</a></li> -<li class="isub3">of heart, <a href="#Page_239">239</a></li> -<li class="isub3">of intestines, <a href="#Page_426">426</a></li> -<li class="isub3">protopathic and critical systems of, <a href="#Page_425">425</a></li> -<li class="isub3">secretory, of the salivary glands, <a href="#Page_109">109</a></li> -<li class="isub3">splanchnic, <a href="#Page_236">236</a></li> -<li class="isub3">superior laryngeal, <a href="#Page_178">178</a></li> -<li class="isub3">vagus, <a href="#Page_104">104</a></li> -<li class="isub3">vaso-motor, <a href="#Page_239">239</a></li> -<li class="isub1">Nerve-cells last throughout life, <a href="#Page_148">148</a></li> -<li class="isub3">limitations of their functions, <a href="#Page_321">321</a></li> -<li class="isub3">store of energy in, <a href="#Page_320">320</a></li> -<li class="isub3">transfer of impulses from cell to cell, <a href="#Page_177">177</a>, <a href="#Page_300">300</a></li> -<li class="isub3">their relation to muscle-fibres, <a href="#Page_274">274</a></li> -<li class="isub3">varying size of, <a href="#Page_295">295</a>, <a href="#Page_322">322</a> -<span class="pagenum"><a name="Page_446" id="Page_446">[Pg 446]</a></span></li> -<li class="isub1">Nerve-centres, <a href="#Page_176">176</a></li> -<li class="isub1">Nerve-force, improper use of expression, <a href="#Page_281">281</a></li> -<li class="isub1">Nerve-impulses, distribution in grey matter, <a href="#Page_305">305</a></li> -<li class="isub3">reinforcement of, <a href="#Page_320">320</a></li> -<li class="isub3">resistance to, at synapses, <a href="#Page_306">306</a></li> -<li class="isub1">Nerve-nets, pericellular, <a href="#Page_301">301</a>, <a href="#Page_319">319</a></li> -<li class="isub1">Nervous system, neuronic and extra-neuronic conduction, <a href="#Page_310">310</a></li> -<li class="isub3">phylogeny of, <a href="#Page_332">332</a></li> -<li class="isub1">Neuro-fibrillæ, <a href="#Page_298">298</a></li> -<li class="isub1">Neurone, origin of term, <a href="#Page_293">293</a></li> -<li class="isub3">transmission of current by, <a href="#Page_328">328</a></li> -<li class="isub3">various types of, <a href="#Page_296">296</a>, <a href="#Page_323">323</a></li> -<li class="isub1">Night-blindness, <a href="#Page_378">378</a></li> -<li class="isub1">Nissl’s bodies, source of nervous energy, <a href="#Page_320">320</a></li> -<li class="isub1">Nitric oxide, combination with hæmoglobin, <a href="#Page_186">186</a></li> -<li class="isub1">Nitrogenous equilibrium, <a href="#Page_150">150</a></li> -<li class="isub3">food, stimulating effect of, <a href="#Page_157">157</a></li> -<li class="isub3">waste, <a href="#Page_210">210</a></li> -<li class="isub1"><i>Nœud vital</i> of Flourens, <a href="#Page_176">176</a></li> -<li class="isub1">Normal diet, <a href="#Page_151">151</a></li> -<li class="isub1">Normal salt-solution, <a href="#Page_82">82</a></li> -<li class="isub1">Nucleo-proteins, source of uric acid, <a href="#Page_215">215</a></li> - -<li class="isub1 ifrst">Odours, classification of, <a href="#Page_366">366</a></li> -<li class="isub1">Œdema, or dropsy, <a href="#Page_42">42</a></li> -<li class="isub1">Olfactory membrane, structure, <a href="#Page_366">366</a></li> -<li class="isub1">Optic nerve, number of fibres, <a href="#Page_378">378</a></li> -<li class="isub1">Organ of Corti, structure, <a href="#Page_415">415</a></li> -<li class="isub3">theory of function, <a href="#Page_417">417</a></li> -<li class="isub1">Organs that have lost their prime functions, <a href="#Page_87">87</a></li> -<li class="isub1">Orientation, sense of, <a href="#Page_335">335</a></li> -<li class="isub1">Osmosis, <a href="#Page_40">40</a>, <a href="#Page_128">128</a>, <a href="#Page_201">201</a></li> -<li class="isub3">cause of muscular contraction, <a href="#Page_235">235</a></li> -<li class="isub1">Osteoblasts, bone-forming cells, <a href="#Page_32">32</a></li> -<li class="isub1">Osteoclasts, bone-eating cells, <a href="#Page_65">65</a></li> -<li class="isub1">Oxygen, amount required per diem, <a href="#Page_166">166</a></li> -<li class="isub3">carried by red blood-corpuscles, <a href="#Page_66">66</a></li> - -<li class="isub1 ifrst">Pain, influence of, upon action, <a href="#Page_359">359</a></li> -<li class="isub3">referred from viscera to surface of body, <a href="#Page_316">316</a></li> -<li class="isub3">relation to sensation, <a href="#Page_313">313</a>, <a href="#Page_425">425</a></li> -<li class="isub3">theory of, <a href="#Page_312">312</a>, <a href="#Page_425">425</a></li> -<li class="isub1">Pancreas, structure, <a href="#Page_116">116</a></li> -<li class="isub1">Pancreatic juice, constitution, <a href="#Page_116">116</a></li> -<li class="isub3">fat-splitting ferment of, <a href="#Page_133">133</a></li> -<li class="isub1">Papillæ of the tongue, various forms of, <a href="#Page_97">97</a></li> -<li class="isub1">Parathyroids, <a href="#Page_86">86</a></li> -<li class="isub1">Pepsin, digestive action, <a href="#Page_115">115</a></li> -<li class="isub1">Peptone prevents coagulation of blood, <a href="#Page_77">77</a></li> -<li class="isub1">Pericellular nerve-nets, <a href="#Page_301">301</a></li> -<li class="isub1">Perspiration, cools the surface of the body, <a href="#Page_236">236</a></li> -<li class="isub3">repressed during fever, <a href="#Page_257">257</a></li> -<li class="isub1">Peyer’s patches of lymph-follicles in intestine, <a href="#Page_53">53</a></li> -<li class="isub1">Phagocytes, germ-eating leucocytes, <a href="#Page_60">60</a></li> -<li class="isub3">consumption of red blood-corpuscles by, <a href="#Page_82">82</a></li> -<li class="isub1">Phosphenes, developed by pressure on eyeball, <a href="#Page_383">383</a></li> -<li class="isub1">Phosphorescence, cause of, <a href="#Page_291">291</a></li> -<li class="isub1">Phrenology, <a href="#Page_343">343</a></li> -<li class="isub1">Pictures, suggestion of solidity in, <a href="#Page_401">401</a></li> -<li class="isub1">Pineal body, phylogeny, <a href="#Page_334">334</a></li> -<li class="isub1">Pituitary body, <a href="#Page_93">93</a></li> -<li class="isub1">Plants, anæsthetized by ether, <a href="#Page_12">12</a>, <a href="#Page_24">24</a></li> -<li class="isub3">their metabolism, <a href="#Page_15">15</a></li> -<li class="isub3">their respiration, <a href="#Page_24">24</a></li> -<li class="isub1">Pleura, lining membrane of chest, <a href="#Page_172">172</a></li> -<li class="isub1">Pleurisy, pain of, <a href="#Page_313">313</a></li> -<li class="isub1">Pleuritic fluid, absorption of, <a href="#Page_223">223</a></li> -<li class="isub1">Pneumonia, changes in lung during, <a href="#Page_169">169</a></li> -<li class="isub1">Portal system of bloodvessels, <a href="#Page_80">80</a></li> -<li class="isub3">regulator of vascular tone, <a href="#Page_236">236</a></li> -<li class="isub1">Power of muscles, <a href="#Page_285">285</a></li> -<li class="isub1">Precipitins formed in blood, <a href="#Page_19">19</a></li> -<li class="isub1">Proteins, absorption by alimentary canal, <a href="#Page_145">145</a></li> -<li class="isub3">chemical constitution, <a href="#Page_6">6</a></li> -<li class="isub3">dietetic value, <a href="#Page_157">157</a></li> -<li class="isub3">fate after absorption, <a href="#Page_212">212</a></li> -<li class="isub1">Protopathic nerves, <a href="#Page_425">425</a></li> -<li class="isub1">Protoplasm, arrangement in cells, <a href="#Page_30">30</a></li> -<li class="isub3">constitution, <a href="#Page_7">7</a></li> -<li class="isub3">Huxley’s definition, <a href="#Page_6">6</a></li> -<li class="isub1">Pulse, cause of, <a href="#Page_244">244</a></li> -<li class="isub3">records of, <a href="#Page_245">245</a></li> -<li class="isub3">variations, <a href="#Page_247">247</a></li> -<li class="isub1">Purgatives, theory of action, <a href="#Page_128">128</a></li> -<li class="isub1">Purkinje-cells of cerebellum, <a href="#Page_303">303</a>, <a href="#Page_340">340</a></li> -<li class="isub3">shadows of retinal vessels, <a href="#Page_375">375</a></li> -<li class="isub1">Pus, origin of, from leucocytes, <a href="#Page_57">57</a></li> -<li class="isub1">Pyramids of cortex of great brain, <a href="#Page_346">346</a></li> - -<li class="isub1 ifrst">Rabbit’s ear, vaso-motor changes in, <a href="#Page_235">235</a></li> -<li class="isub1">Receptor, an organ specially sensitive to stimulation, <a href="#Page_253">253</a></li> -<li class="isub1">Referred pains from viscera, <a href="#Page_316">316</a></li> -<li class="isub1">Reflex action, inhibition of, <a href="#Page_311">311</a></li> -<li class="isub3">of scratching, <a href="#Page_330">330</a></li> -<li class="isub3">vinegar experiment with frog, <a href="#Page_307">307</a></li> -<li class="isub1">Regeneration of nerves, <a href="#Page_326">326</a></li> -<li class="isub1">Renal-portal circulation, <a href="#Page_199">199</a></li> -<li class="isub1">Renewal of tissues, <a href="#Page_148">148</a></li> -<li class="isub1">Rennin, ferment of milk, <a href="#Page_16">16</a></li> -<li class="isub1">Resistance in nervous system, laws of, <a href="#Page_177">177</a>, <a href="#Page_307">307</a></li> -<li class="isub1">Respiration, artificial, <a href="#Page_179">179</a></li> -<li class="isub3">effect on circulation, <a href="#Page_221">221</a></li> -<li class="isub3">a function of protoplasm, <a href="#Page_23">23</a>, <a href="#Page_164">164</a></li> -<li class="isub3">movements of, <a href="#Page_171">171</a></li> -<li class="isub3">nervous mechanism, <a href="#Page_175">175</a>, <a href="#Page_179">179</a> -<span class="pagenum"><a name="Page_447" id="Page_447">[Pg 447]</a></span></li> -<li class="isub3">in tissues, <a href="#Page_165">165</a>, <a href="#Page_193">193</a></li> -<li class="isub1">Respiratory centre in medulla oblongata, <a href="#Page_176">176</a>, <a href="#Page_178">178</a>, <a href="#Page_182">182</a></li> -<li class="isub1">Respiratory quotient, <a href="#Page_174">174</a></li> -<li class="isub1">Retina, structure, <a href="#Page_374">374</a></li> -<li class="isub1">Retinal pigment, relation to vision, <a href="#Page_381">381</a></li> -<li class="isub1">Rice ordeal, arrest of secretion of saliva, <a href="#Page_112">112</a></li> -<li class="isub1">Rigor mortis, <a href="#Page_266">266</a></li> -<li class="isub1">Rods and cones, respective functions in vision, <a href="#Page_378">378</a></li> -<li class="isub1">Rowing, value of, as exercise, <a href="#Page_287">287</a></li> - -<li class="isub1 ifrst">Saccharin, taste of, <a href="#Page_367">367</a></li> -<li class="isub1">Saline frog, respiration in, <a href="#Page_193">193</a></li> -<li class="isub1">Saliva, chemical constitution, <a href="#Page_107">107</a></li> -<li class="isub3">function of, <a href="#Page_96">96</a>, <a href="#Page_107">107</a></li> -<li class="isub1">Salivary glands, mechanism of secretion, <a href="#Page_108">108</a></li> -<li class="isub3">nerves of, <a href="#Page_109">109</a>, <a href="#Page_236">236</a></li> -<li class="isub1">Salts, absorption of, in alimentary canal, <a href="#Page_128">128</a></li> -<li class="isub1">Scientific method, definition of, <a href="#Page_71">71</a></li> -<li class="isub1">Scratch reflex, in dog, <a href="#Page_330">330</a></li> -<li class="isub1">Sea-sickness, <a href="#Page_106">106</a></li> -<li class="isub1">Secretin, hormone of pancreas and liver, <a href="#Page_127">127</a></li> -<li class="isub1">Secretion, accumulation of granules in cells, and their discharge, <a href="#Page_110">110</a></li> -<li class="isub3">a response to stimulation, <a href="#Page_111">111</a></li> -<li class="isub3">not a process of filtration, <a href="#Page_110">110</a></li> -<li class="isub1">Semicircular canals, their functions, <a href="#Page_410">410</a></li> -<li class="isub3">their positions in space, <a href="#Page_335">335</a></li> -<li class="isub1">Sensations, their apparent fusion, <a href="#Page_356">356</a></li> -<li class="isub3">many which escape attention, <a href="#Page_318">318</a>, <a href="#Page_355">355</a></li> -<li class="isub3">neutralization of one by another, <a href="#Page_356">356</a></li> -<li class="isub1">Sense-organs, origin in vertebrata, <a href="#Page_336">336</a></li> -<li class="isub1">Sensory areas in cortex of the great brain, <a href="#Page_348">348</a></li> -<li class="isub1">Sensory nerves, their connection with cerebro-spinal axis, <a href="#Page_304">304</a></li> -<li class="isub1">Shell-fish, poisonous extract of, <a href="#Page_41">41</a></li> -<li class="isub1">Shivering due to loss of heat from skin, <a href="#Page_257">257</a></li> -<li class="isub1">Sight. <i>Cf.</i> Vision</li> -<li class="isub1">Skate, electric organs of, <a href="#Page_289">289</a></li> -<li class="isub1">Skilled movements, dependent upon kinæsthetic sensations, <a href="#Page_357">357</a></li> -<li class="isub1">Skin, experiment of cutting nerve, <a href="#Page_424">424</a></li> -<li class="isub3">variety of sensations from, <a href="#Page_423">423</a></li> -<li class="isub1">Sleep, condition of neurones in, <a href="#Page_362">362</a></li> -<li class="isub1">Sleeping sickness, <a href="#Page_33">33</a></li> -<li class="isub1">Smallpox, protection against, <a href="#Page_78">78</a></li> -<li class="isub1">Smell, disappearance of sense of, in later life, <a href="#Page_370">370</a></li> -<li class="isub3">dog’s dependence upon sense of, <a href="#Page_366">366</a></li> -<li class="isub3">reason for mental associations with sensations of, <a href="#Page_371">371</a></li> -<li class="isub3">sensitiveness to mercaptan, <a href="#Page_365">365</a></li> -<li class="isub1">Smells, nice and nasty, <a href="#Page_369">369</a></li> -<li class="isub1">Smoking, mental effect of, <a href="#Page_371">371</a></li> -<li class="isub1">Sneezing on looking at bright light, <a href="#Page_317">317</a></li> -<li class="isub1">Sore throat, cause of deafness, <a href="#Page_412">412</a></li> -<li class="isub1">Soul, Aristotle’s definition, <a href="#Page_32">32</a></li> -<li class="isub1">Sound, mode of conduction, <a href="#Page_404">404</a></li> -<li class="isub3">rapidity of vibrations of, <a href="#Page_406">406</a>, <a href="#Page_418">418</a></li> -<li class="isub1">Sounds of the heart, <a href="#Page_228">228</a></li> -<li class="isub3">periodic and aperiodic, <a href="#Page_409">409</a></li> -<li class="isub1">Spectacles, defects of eyeball which call for, <a href="#Page_392">392</a></li> -<li class="isub1">Speech, derangements of, due to disease of the brain, <a href="#Page_353">353</a></li> -<li class="isub3">mechanism of, <a href="#Page_437">437</a></li> -<li class="isub1">Sphygmographs for recording pulse, <a href="#Page_245">245</a></li> -<li class="isub1">Spinal dog, reflex action in, <a href="#Page_330">330</a></li> -<li class="isub3">frog, reflex action in, <a href="#Page_307">307</a></li> -<li class="isub3">ganglia, development of cells, <a href="#Page_299">299</a></li> -<li class="isub1">Splanchnic nerves, regulation of blood-pressure by, <a href="#Page_236">236</a></li> -<li class="isub1">Spleen, destruction of blood-corpuscles in, <a href="#Page_80">80</a></li> -<li class="isub3">structure, <a href="#Page_79">79</a></li> -<li class="isub1">Squint, correction of double vision in, <a href="#Page_397">397</a></li> -<li class="isub1">Starch, formula, <a href="#Page_15">15</a></li> -<li class="isub1">Star-shapes due to puckering of crystalline lens, <a href="#Page_393">393</a></li> -<li class="isub1">Starvation, statistics of, <a href="#Page_156">156</a></li> -<li class="isub1">Stiffness of muscles, cause of, <a href="#Page_45">45</a>, <a href="#Page_271">271</a></li> -<li class="isub1">Stimuli to muscles and nerves, <a href="#Page_248">248</a></li> -<li class="isub1">Stokes, discovery of spectrum of blood, <a href="#Page_68">68</a></li> -<li class="isub1">Stomach, digestion in, <a href="#Page_120">120</a></li> -<li class="isub3">glands of, <a href="#Page_123">123</a></li> -<li class="isub3">referred pains from, <a href="#Page_316">316</a></li> -<li class="isub3">shape and size, <a href="#Page_99">99</a></li> -<li class="isub1">Stone in the bladder, its cause, <a href="#Page_213">213</a></li> -<li class="isub1">Subconscious self, <a href="#Page_355">355</a></li> -<li class="isub1">Sugars, digestion of, <a href="#Page_120">120</a>, <a href="#Page_136">136</a></li> -<li class="isub3">formulæ, <a href="#Page_15">15</a></li> -<li class="isub1">Sun, apparent size near horizon, <a href="#Page_399">399</a></li> -<li class="isub1">Suprarenal capsules, their structure and function, <a href="#Page_91">91</a></li> -<li class="isub1">Sweetbread as article of diet, <a href="#Page_215">215</a></li> -<li class="isub1">Sympathetic system of nerves, <a href="#Page_243">243</a>, <a href="#Page_325">325</a></li> -<li class="isub3">diameter of fibres, <a href="#Page_325">325</a></li> -<li class="isub1">Synapses of nerve-cells, resistance interposed at, <a href="#Page_306">306</a></li> -<li class="isub1">Synaptases, constructive ferments, <a href="#Page_18">18</a></li> -<li class="isub1">Synthesis by plants, <a href="#Page_15">15</a></li> - -<li class="isub1 ifrst">Tapeworms, resist digestion in the intestines, <a href="#Page_21">21</a></li> -<li class="isub1">Taste, confusion with sense of smell, <a href="#Page_364">364</a></li> -<li class="isub3">localization on tongue, <a href="#Page_367">367</a></li> -<li class="isub3">sense of, in fishes, <a href="#Page_365">365</a></li> -<li class="isub3">sensitiveness to quinine, <a href="#Page_369">369</a></li> -<li class="isub1">Taste-bulbs, their structure, <a href="#Page_368">368</a></li> -<li class="isub1">Tattooing, removal of pigment by leucocytes, <a href="#Page_55">55</a> -<span class="pagenum"><a name="Page_448" id="Page_448">[Pg 448]</a></span></li> -<li class="isub1">Tea, its dietetic value, <a href="#Page_122">122</a></li> -<li class="isub1">Teeth, <a href="#Page_96">96</a></li> -<li class="isub1">Tendon, the growth of, from cells, <a href="#Page_28">28</a></li> -<li class="isub1">Tension of gases in the lungs, <a href="#Page_190">190</a></li> -<li class="isub1">Tetanus, the vibratile contraction of muscle, <a href="#Page_279">279</a></li> -<li class="isub1">Thoracic duct, discharges lymph into veins, <a href="#Page_43">43</a>, <a href="#Page_131">131</a></li> -<li class="isub1">Thorax, negative pressure in, <a href="#Page_222">222</a></li> -<li class="isub1">Thorns on dendrites of nerve-cells, <a href="#Page_300">300</a></li> -<li class="isub1">Thyroid body or gland, forms an internal secretion, <a href="#Page_86">86</a></li> -<li class="isub3">relation to goitre, <a href="#Page_85">85</a></li> -<li class="isub3">structure of, <a href="#Page_85">85</a></li> -<li class="isub1">Tight-lacing, deformation of organs which it causes, <a href="#Page_220">220</a></li> -<li class="isub1">Tigroids, in nerve-cells, stores of energy, <a href="#Page_320">320</a></li> -<li class="isub1">Tissues, respiration in, <a href="#Page_165">165</a>, <a href="#Page_193">193</a></li> -<li class="isub1">Tone of muscles, <a href="#Page_272">272</a></li> -<li class="isub1">Tongue, as organ of taste, <a href="#Page_367">367</a></li> -<li class="isub1">Tonsils, function as guardians of the fauces, <a href="#Page_53">53</a></li> -<li class="isub3">structure, <a href="#Page_52">52</a></li> -<li class="isub1">Torpedo, electric organs of, <a href="#Page_290">290</a></li> -<li class="isub1">Touch, sensations of, <a href="#Page_426">426</a></li> -<li class="isub1">Toxins produced by microbes, <a href="#Page_20">20</a></li> - -<li class="isub1 ifrst">Urea, amount relatively to proteins consumed, <a href="#Page_155">155</a></li> -<li class="isub3">antecedents of, <a href="#Page_146">146</a>, <a href="#Page_212">212</a></li> -<li class="isub3">chemical formula, <a href="#Page_211">211</a></li> -<li class="isub3">secreted during period of starvation, <a href="#Page_156">156</a></li> -<li class="isub1">Uric acid, amount secreted daily, <a href="#Page_213">213</a></li> -<li class="isub3">artificial production of, <a href="#Page_13">13</a></li> -<li class="isub3">chemical formula, <a href="#Page_13">13</a>, <a href="#Page_214">214</a></li> -<li class="isub3">diathesis, its relation to diet, <a href="#Page_140">140</a></li> -<li class="isub3">due to metabolism of leucocytes, <a href="#Page_53">53</a>, <a href="#Page_216">216</a></li> -<li class="isub3">form in which excreted, <a href="#Page_207">207</a></li> -<li class="isub3">made in the liver of birds, <a href="#Page_13">13</a></li> -<li class="isub1">Urticaria due to abnormal composition of lymph, <a href="#Page_41">41</a></li> - -<li class="isub1 ifrst">Vaccination, protective value of, <a href="#Page_22">22</a></li> -<li class="isub1">Valves of heart, their mechanism, <a href="#Page_226">226</a></li> -<li class="isub1">Vascular system, tone of, <a href="#Page_236">236</a>, <a href="#Page_240">240</a></li> -<li class="isub1">Vaso-constrictor nerves, <a href="#Page_236">236</a></li> -<li class="isub1">Vaso-dilator nerves, <a href="#Page_236">236</a></li> -<li class="isub1">Vegetables, dietetic value of, <a href="#Page_139">139</a></li> -<li class="isub3">digestion of, <a href="#Page_125">125</a>, <a href="#Page_137">137</a></li> -<li class="isub1">Vermiform appendix, <a href="#Page_88">88</a></li> -<li class="isub1">Villi of intestine, absorption of food by, <a href="#Page_130">130</a></li> -<li class="isub3">fat seen in, during active digestion, <a href="#Page_134">134</a></li> -<li class="isub1">Viscera, their insensitiveness to injury, <a href="#Page_316">316</a>, <a href="#Page_426">426</a></li> -<li class="isub1">Vision, colour contrasts, <a href="#Page_382">382</a></li> -<li class="isub3">duration of images, <a href="#Page_382">382</a></li> -<li class="isub3">judgment of distance and size, <a href="#Page_411">411</a></li> -<li class="isub3">solidity, <a href="#Page_401">401</a></li> -<li class="isub3">stereoscopic, doctrine of corresponding points, <a href="#Page_397">397</a></li> -<li class="isub1">Visual purple, <a href="#Page_381">381</a></li> -<li class="isub1">Vital action, definition of expression, <a href="#Page_205">205</a></li> -<li class="isub1">Vivisection, <a href="#Page_4">4</a></li> -<li class="isub1">Vocal cords, structure, <a href="#Page_431">431</a></li> -<li class="isub3">how modified in singing, <a href="#Page_435">435</a></li> -<li class="isub1">Voice, breaking of, in boys, <a href="#Page_434">434</a></li> -<li class="isub3">falsetto, how produced, <a href="#Page_435">435</a></li> -<li class="isub3">range of human, <a href="#Page_435">435</a></li> -<li class="isub3">registers, <a href="#Page_436">436</a></li> -<li class="isub1">Vomiting, <a href="#Page_105">105</a></li> -<li class="isub1">Vowels, synthesis by tuning-forks, <a href="#Page_439">439</a></li> - -<li class="isub1 ifrst">Wandering cells, <a href="#Page_33">33</a></li> -<li class="isub1">Warmth, appreciation of, by skin, <a href="#Page_429">429</a></li> -<li class="isub1">Waste substances, classification, <a href="#Page_194">194</a></li> -<li class="isub3">how eliminated from body, <a href="#Page_59">59</a></li> -<li class="isub1">Waterfall, negative after-image of, <a href="#Page_384">384</a></li> -<li class="isub1">Water-weed, experiment proving that it respires, <a href="#Page_24">24</a></li> -<li class="isub1">Wear and tear of bioplasm, <a href="#Page_145">145</a></li> -<li class="isub1">Wisdom-tooth, tending to disappear, <a href="#Page_96">96</a></li> - -<li class="isub1 ifrst">Yawning, beneficial effect on circulation, <a href="#Page_222">222</a></li> -<li class="isub3">nervous mechanism of, <a href="#Page_180">180</a></li> -<li class="isub1">Young’s theory of colour-vision, <a href="#Page_385">385</a></li> - -<li class="isub1 ifrst">Zymogen, <a href="#Page_110">110</a></li> -</ul> - -<hr class="chap" /> -<p class="f120">THE END</p> - -<p class="center">BILLING AND SONS, LTD., PRINTERS, GUILDFORD</p> -<hr class="chap" /> - -<div class="footnotes"> -<p class="f150"><b>FOOTNOTES:</b></p> - -<div class="footnote"><p class="no-indent"> -<a name="Footnote_1_1" id="Footnote_1_1"></a><a href="#FNanchor_1_1"><span class="label">[1]</span></a> -Proteïn, <i>subs.</i>, proteïd, <i>adj.</i>, general terms for complex -nitrogenous substances, such as albumin (white of egg), the less -soluble globulins, fibrin of blood, casein of milk, etc.</p></div> - -<div class="footnote"><p class="no-indent"> -<a name="Footnote_2_2" id="Footnote_2_2"></a><a href="#FNanchor_2_2"><span class="label">[2]</span></a> -Notice the distinction between filtration and dialysis. If water -containing soluble and insoluble substances is placed in a porous -jar, the water and the soluble substances pass through the pores of -the jar. The rate of flow varies as the pressure. If water containing -soluble substances is placed in a bladder, and the bladder is -suspended in a vessel of water, some of the substances which it may -contain—white of egg, for example—are non-diffusible; -others will pass from the water inside the bladder to the water which -surrounds it. But every diffusible substance has its own osmotic -value. Some pass through the membrane rapidly, soon establishing a -condition of equilibrium in the two fluids; others take a long time. -Further, if the water on one side of the membrane contains a certain -percentage weight of a salt, the molecules of which are large—say -sulphate of magnesia—and the water on the other side the same -percentage weight of a salt of smaller molecule—say chloride -of sodium—water containing the salt of smaller molecule will -pass into the water containing the salt of heavier molecule with a -certain force. If, to start with, the two solutions are at the same -level, the level of the solution containing the less diffusible salt, -sulphate of magnesia, will at the commencement of the experiment rise. -It is therefore said to exert a greater osmotic pressure than the more -diffusible salt—chloride of sodium. Equilibrium will not be -established until the fluid on one side of the membrane contains the -same <i>number</i> of molecules per unit volume as the fluid on the other -side. If the molecules of magnesic sulphate are pictured as oranges, -and the molecules of sodic chloride as nuts, it will be understood -that equilibrium is not established until the oranges and nuts to the -pint on one side equal in number the oranges and nuts to the pint on -the other. When these principles are applied to the passage of water -containing products of digestion through the wall of the alimentary -canal, it is evident that, if we understand all the conditions, the -process cannot be explained as merely an exhibition of osmosis. Take -the simplest illustration. When blood-serum is placed in the intestine -it is absorbed. If it were in a dialyser, there would be equilibrium -between the serum inside the intestine and the lymph on the outside. -There would be no osmosis. Or, again, supposing water containing 2% -of common salt is placed in the intestine, we find that both salt and -water pass through into the lymph. In a dialyser water would pass from -the lymph (which contains salts equal to about 0·9% of sodic chloride) -through the membrane into the stronger solution. A salt-solution needs -to be very concentrated to cause water to take the reverse course -through the intestinal wall, and so to act as a purgative. When we -study absorption from the alimentary canal, we find that its wall, -if it wants a salt or any other substance, sets the laws of osmosis -at defiance. If the salt is not wanted, the ordinary phenomena of -osmosis are exhibited. Sulphate of magnesia (Epsom salt) would be -deleterious if absorbed. The intestinal wall behaves towards it like -a dead membrane. The salt retains the water in which it is dissolved: -possibly water passes out of the lymph into the solution of the salt. -The contents of the intestines are in consequence unduly liquid. The -salt acts as a purge.</p></div> - -<div class="footnote"><p class="no-indent"> -<a name="Footnote_3_3" id="Footnote_3_3"></a><a href="#FNanchor_3_3"><span class="label">[3]</span></a> -A bullock’s larynx is an admirable object of study. In almost all -points of form and structure it is practically identical with the human -larynx, and its large size makes it easy to dissect.</p></div> -</div> -<hr class="chap" /> - -<div class="transnote bbox"> -<p class="f120 space-above1">Transcriber’s Notes:</p> -<hr class="r5" /> -<p class="indent">The illustrations have been moved so that they do not break up - paragraphs and so that they are next to the text they illustrate.</p> -<p class="indent">Old or antiquated spellings have been preserved.</p> -<p class="indent">Typographical errors have been silently corrected.</p> -</div> - - - - - - - - -<pre> - - - - - -End of the Project Gutenberg EBook of The Body at Work, by Alexander Hill - -*** END OF THIS PROJECT GUTENBERG EBOOK THE BODY AT WORK *** - -***** This file should be named 63089-h.htm or 63089-h.zip ***** -This and all associated files of various formats will be found in: - http://www.gutenberg.org/6/3/0/8/63089/ - -Produced by Paul Marshall, Turgut Dincer and the Online -Distributed Proofreading Team at https://www.pgdp.net (This -file was produced from images generously made available -by The Internet Archive) - -Updated editions will replace the previous one--the old editions will -be renamed. - -Creating the works from print editions not protected by U.S. copyright -law means that no one owns a United States copyright in these works, -so the Foundation (and you!) can copy and distribute it in the United -States without permission and without paying copyright -royalties. 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