path: root/63089-0.txt

*** START OF THE PROJECT GUTENBERG EBOOK 63089 ***

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

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