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diff --git a/34211.txt b/34211.txt new file mode 100644 index 0000000..9078192 --- /dev/null +++ b/34211.txt @@ -0,0 +1,10280 @@ +The Project Gutenberg EBook of A Treatise on Physiology and Hygiene, by +Joseph Chrisman Hutchison + +This eBook is for the use of anyone anywhere at no cost and with +almost no restrictions whatsoever. You may copy it, give it away or +re-use it under the terms of the Project Gutenberg License included +with this eBook or online at www.gutenberg.org + + +Title: A Treatise on Physiology and Hygiene + For Educational Institutions and General Readers + +Author: Joseph Chrisman Hutchison + +Release Date: November 5, 2010 [EBook #34211] + +Language: English + +Character set encoding: ASCII + +*** START OF THIS PROJECT GUTENBERG EBOOK +A TREATISE ON PHYSIOLOGY AND HYGIENE *** + + + + +Produced by Bryan Ness, Keith Edkins and the Online +Distributed Proofreading Team at http://www.pgdp.net (This +file was produced from images generously made available +by The Internet Archive/American Libraries.) + + + + + +Transcriber's note: A few typographical errors have been corrected: they +are listed at the end of the text. + + * * * * * + + +[Illustration: THE VISCERA IN POSITION.] + + * * * * * + + +A + +TREATISE + +ON + +PHYSIOLOGY AND HYGIENE + +FOR + +EDUCATIONAL INSTITUTIONS AND GENERAL READERS. + +_FULLY ILLUSTRATED._ + +BY + +JOSEPH C. HUTCHISON, M. D., + +_President of the New York Pathological Society, Vice-President of the New +York +Academy of Medicine, Surgeon to the Brooklyn City Hospital, late +President +of the Medical Society of the State of New York, etc._ + + * * * * * + +NEW YORK: + +CLARK & MAYNARD, PUBLISHERS, + +5 BARCLAY STREET. + +1872. + + * * * * * + + +Entered according to Act of Congress, in the year 1870, +By CLARK & MAYNARD. +In the Office of the Librarian of Congress, at Washington. + + Stereotyped by LITTLE, RENNIE & CO. + 645 and 647 Broadway. + + * * * * * + + +TO MY WIFE, + +WHOSE SYMPATHY HAS, FOR MORE THAN TWENTY YEARS, LIGHTENED THE +CARES INCIDENT TO + +_AN ACTIVE PROFESSIONAL LIFE_, + +THIS HUMBLE VOLUME + +IS AFFECTIONATELY INSCRIBED. + + * * * * * + + +{3} + +PREFACE. + +------o------ + +This work is designed to present the leading facts and principles of human +Physiology and Hygiene in clear and concise language, so that pupils in +schools and colleges, and readers not familiar with the subjects, may +readily comprehend them. Anatomy, or a description of the structure of an +organ, is of course necessary to the understanding of its Physiology, or +its uses. Enough of the former study has, therefore, been introduced, to +enable the pupil to enter intelligently upon the latter. + +Familiar language, as far as practicable, has been employed, rather than +that of a technical character. With a view, however, to supply what might +seem to some a deficiency in this regard, a Pronouncing Glossary has been +added, which will enable the inquirer to understand the meaning of many +scientific terms not in common use. + +In the preparation of the work the writer has carefully examined all the +best material at his command, and freely used it; the special object being +to have it abreast of the present knowledge on the subjects treated, as far +as such is possible in a work so elementary as this. The discussion of +disputed points has been avoided, it being manifestly inappropriate in a +work of this kind. + +Instruction in the rudiments of Physiology in schools does not necessitate +the general practice of dissections, or of experiments upon animals. The +most important subjects may be illustrated by {4} drawings, such as are +contained in this work. Models, especially those constructed by AUZOUX of +Paris, dried preparations of the human body, and the organs of the lower +animals, may also be used with advantage. + +The writer desires to acknowledge his indebtedness to R. M. WYCKOFF, M.D., +for valuable aid in the preparation of the manuscript for the press; and to +R. CRESSON STILES, M.D., a skilful microscopist and physician, for the +chapter "On the Use of the Microscope in the Study of Physiology." Mr. AVON +C. BURNHAM, the well-known teacher of gymnastics, furnished the drawing of +the parlor gymnasium and the directions for its use. + +_Brooklyn, N. Y., 1870._ + + * * * * * + + +{5} + +CONTENTS. + + CHAPTER I. + + PAGE + + THE FRAMEWORK OF THE BODY 15 + + _The Bones--Their form and composition--The Properties of Bone--The + Skeleton--The Joints--The Spinal Column--The Growth of Bone--The + Repair of Bone._ + + CHAPTER II. + + THE MUSCLES 25 + + _The Muscles--Flexion and Extension--The Tendons--Contraction--Physical + Strength--Necessity for Exercise--Its Effects--Forms of + Exercise--Walking--Riding--Gymnastics--Open-air Exercise--Sleep-- + Recreation._ + + CHAPTER III. + + THE INTEGUMENT, OR SKIN 41 + + _The Integument--Its Structure--The Nails and Hair--The Complexion--The + Sebaceous Glands--The Perspiratory Glands--Perspiration + and its uses--Importance of Bathing--Different kinds of Baths--Manner + of Bathing--The Benefits of the Sun--Importance of + Warm Clothing--Poisonous Cosmetics._ + + CHAPTER IV. + + THE CHEMISTRY OF FOOD 53 + + _The Source of Food--Inorganic Substances--Water--Salt--Lime--Iron-- + Organic Substances--Albumen, Fibrin, and Casein--The Fats or + Oils--The Sugars, Starch, and Gum--Stimulating Substances--Necessity + of a Regulated Diet._ + + {6} + CHAPTER V. + + FOOD AND DRINK 64 + + _Necessity for Food--Waste and Repair--Hunger and Thirst--Amount + of Food--Renovation of the Body--Mixed Diet--Milk--Eggs--Meat--Cooking + --Vegetable Food--Bread--The Potato--Fruits--Purity + of Water--Action of Water upon Lead--Coffee, Tea, and Chocolate--Effects + of Alcohol._ + + CHAPTER VI. + + DIGESTION 80 + + _The Principal Processes of Nutrition--The General Plan of Digestion-- + Mastication--The Teeth--Preservation of the Teeth--Insalivation--The + Stomach and the Gastric Juice--The Movements of the + Stomach--Gastric Digestion--The Intestines--The Bile and Pancreatic + Juice--Intestinal Digestion--Absorption by means of Blood-vessels + and Lacteals--The Lymphatic or Absorbent System--The + Lymph--Conditions which affect Digestion--The Quality, Quantity, + and Temperature of the Food--The Influence of Exercise and + Sleep._ + + CHAPTER VII. + + THE CIRCULATION 101 + + _The Blood--Its Plasma and Corpuscles--Coagulation of the Blood--The + Uses of the Blood--Transfusion--Change of Color--The Organs of + the Circulation--The Heart, Arteries, and Veins--The Cavities + and Valves of the Heart--Its Vital Energy--Passage of the Blood + through the Heart--The Frequency and Activity of its Movements--The + Pulse--The Sphygmograph--The Capillary Blood-vessels--The + Rate of the Circulation--Assimilation--Injuries to the Blood-vessels._ + + CHAPTER VIII. + + RESPIRATION 123 + + _The Objects of Respiration--The Lungs--The Air-Passages--The Movements + of Respiration--Expiration and Inspiration--The Frequency + of Respiration--Capacity of the Lungs--The Air we Breathe--Changes + in the Air from Respiration--Changes in the Blood--Interchange + of Gases in the Lungs--Comparison between Arterial and + {7} + Venous Blood--Respiratory Labor--Impurities of the Air--Dust--Carbonic + Acid--Effects of Impure Air--Nature's Provision for + Purifying the Air--Ventilation--Animal Heat_. + + CHAPTER IX. + + THE NERVOUS SYSTEM 148 + + _Animal and Vegetative Functions--Sensation, Motion, and Volition--The + Structure of the Nervous System--The White and Gray Substances--The + Brain--Its Convolutions--Cerebellum--The Spinal + Cord and its System of Nerves--The Anterior and Posterior + Roots--The Sympathetic System of Nerves--The Properties of Nervous + Tissue--Excitability of Nervous Tissues--The Functions of the + Spinal Nerves and Cord--The Direction of the Fibres of the Cord--Reflex + Activity and its Uses--The Functions of the Medulla Oblongata + and the Cranial Ganglia--The Reflex Action of the Brain._ + + CHAPTER X. + + THE SPECIAL SENSES 177 + + _The Production of Sensations--Variety of Sensations--General + Sensibility--Pain + and its Function--Special Sensation, Touch, Taste, + Smell, Sight, and Hearing--The Hand, the Organ of Touch--The + Sense of Touch--Delicacy of Touch--Sensation of Temperature and + Weight--The Tongue the Organ of Taste--The Nerves of Taste--The + Sense of Taste, and its Relations with the other Senses--The + Influence of Education on the Taste--The Nasal Cavities, or the + Organs of Smell--The Olfactory Nerve--The Uses of the Sense of + Smell--The Sense of Sight--Light--The Optic Nerve--The + Eyeball and its Coverings--The Function of the Iris--The + Sclerotic, Choroid, and Retina--The Tears and their Function--The + Movements of the Eyeball--The Function of Accommodation--The + Sense of Hearing and Sound--The Ear, or + the organ of Hearing--The External, Middle, and Internal Ear._ + + CHAPTER XI. + + THE VOICE 227 + + _Voice and Speech--The Larynx, or the Organ of the Voice--The Vocal + Cords--The Laryngoscope--The Production of the Voice--The Use + of the Tongue--The different Varieties of Voice--The Change of + Voice--Its Compass--Purity of Tone--Ventriloquy._ + + {8} + CHAPTER XII. + + THE USE OF THE MICROSCOPE IN THE STUDY OF + PHYSIOLOGY 236 + + _The Law of Tissues--Necessity of the Microscope--Different kinds of + Microscopes--Additional Apparatus--Preliminary Studies--The + Study of Human Tissues--Tissues of the Inferior Animals--Incentives + to Study._ + + APPENDIX. + + POISONS AND THEIR ANTIDOTES 247 + + GLOSSARY 252 + +{9} + +LIST OF ILLUSTRATIONS + + FIG. PAGE + FRONTISPIECE, } + VISCERA IN POSITION,} + 1. Section of bone, 17 + 2. Structure of bone, magnified, 17 + 3. The skeleton, 18 + 4. Cells of cartilage, 20 + 5. Elbow-joint, 21 + 6. Spinal column, 22 + 7. The muscles, 24 + 8. Muscular tissue, magnified, 25 + 9. Biceps muscle of the arm, 26 + 10. View of knee-joint, 27 + 11. Appliance for strengthening the muscles, 35 + 12. Appliance for strengthening the muscles, 35 + 13. Parlor gymnasium, 36 + 14. Root and transverse section of hair, magnified, 43 + 15. Granules of potato starch, 61 + 16. Section of the trunk, 81 + 17. Section of a tooth, 82 + 18. Section of the jaws, 82 + 19. Section of the jaws--right side, 84 + 20. Structure of a salivary gland, 87 + 21. Head of a horse, showing salivary gland, etc. 87 + 22. Section of chest and abdomen, 90 + 23. Organs of digestion, 91 + 24. The lacteals, 97 + 25. Blood corpuscles, 102 + 26. Blood corpuscles of man and lower animals, 103 + 27. Circulation of the blood, 108 + {10} + 28. Heart and large vessels, 109 + 29. Section of the heart, 110 + 30. Form of the pulse, 116 + 31. Valves of the veins, 117 + 32. Web of frog's foot, magnified, 119 + 33. Circulation in a frog's foot, 119 + 34. Organs of the chest, 124 + 35. Larynx, trachea, and bronchial tubes, 125 + 36. Diagram of the structure of the air-cells, 125 + 37. Section of the lungs, 126 + 38. Section of mouth and throat, 127 + 39. Ciliated cells, 128 + 40. Cerebro-spinal system, 151 + 41. Upper surface of the cerebrum, 153 + 42. Vertical section of the brain, 154 + 43. Base of the brain, 155 + 44. Brain and spinal cord, 156 + 45. Sense of touch, 185 + 46. Section of nasal cavity, 193 + 47. Front view of the eye, 200 + 48. Vertical section of eye, 202 + 49. Diagram for blind point of eye, 207 + 50. Retinal image, 210 + 51. Different shapes of the globe of the eye, 212 + 52. Function of accommodation, 214 + 53. Diagram of the ear, 218 + 54. Section of the ear, 221 + 55. Section of larynx and trachea, 229 + 56. View of vocal cords by the laryngoscope, 232 + 57. Different positions of vocal cords, 232 + 58. Simple microscope, 238 + 59. Compound microscope, 239 + 60. Household microscope, 240 + 61. Popular microscope, 241 + +{11} + +INTRODUCTION. + +------o------ + +The Human Body is the abode of an immortal spirit, and is the most complete +and perfect specimen of the Creator's handiwork. To examine its structure, +to ascertain the uses and modes of action of its various parts, how to +protect it from injury, and maintain it in a healthy condition, is the +design of this work. + +The departments of knowledge which are concerned in these investigations, +are the science of Human Physiology and the art of Hygiene. + +PHYSIOLOGY treats of the vital actions and uses of the various parts of +living bodies, whether vegetable or animal. Every living thing, therefore, +has a Physiology. We have a _Vegetable_ Physiology, which relates to +plants; and an _Animal_ Physiology, relating to the animal kingdom. The +latter is also divided into _Comparative_ Physiology, which treats of the +inferior races of animals, and _Human_ Physiology, which teaches the uses +of the various parts of the human body. + +HYGIENE, or the art of preserving health, is the practical use of +Physiology. It teaches us how to cultivate our bodily and mental powers, so +as to increase our strength and to fit us for a higher enjoyment of life. +It also shows us how to prevent some of the accidents which may befall the +body, and to avoid disease. It is proper that we should {12} understand the +construction and powers of our bodies; but it is our duty, as rational +beings, to know the laws by which health and strength may be maintained and +disease warded off. + +There are various means by which we gain important information respecting +the Physiology of man. Plants aid us in understanding the minute structure +of the human body, its circulation, and absorption. From inferior animals +we learn much in respect to the workings of the different _organs_, as we +call those parts of the system which have a particular duty to perform. In +one of them, as in the foot of the frog, we can study the circulation of +the blood; in another, we can study the action of the brain. + +By _vivisection_, or the laying bare of some organ of a living animal, we +are able to investigate certain vital processes which are too deeply hidden +in the human body to be studied directly. This is not necessarily a cruel +procedure, as we can, by the use of anaesthetics, so blunt the sensibility +of the animal under operation, that he need not suffer while the experiment +is being performed. There are other means by which we gather our +information. There are occasionally men, who, from some accident, present +certain parts, naturally out of view, in exposed positions. In these cases, +our knowledge is of much greater value than when obtained from creatures +lower in the scale of being than man. + +We are greatly aided, also, by the use of various instruments of modern +invention. Chief among these is the microscope, which is, as we shall learn +hereafter, an arrangement and combination of lenses in such a way as +greatly to magnify the objects we wish to examine. {13} + +We have much to say of Life, or vital activity, in the course of our study +of Physiology; but the most that we know of it is seen in its results. What +Life is, or where its precise position is, we are not able to determine. We +discover one thing, however, that all the parts of the body are united +together with wonderful sympathy, so that one part cannot be injured and +other parts not suffer damage. It is further evident that all organs are +not equally important in carrying on the work of Life; for some may +temporarily suspend their action, without serious results to the system, +while others must never cease from acting. Yet there is nothing superfluous +or without aim in our frames, and no part or organ can suffer harm without +actual loss to the general bodily health. On this point Science and Holy +Writ strictly agree. + + * * * * * + + +{14} + +{15} + +PHYSIOLOGY, + +AND + +HYGIENE. + +CHAPTER I. + +THE FRAMEWORK OF THE BODY. + +_The Bones--Their Form and Composition--The Properties of Bone--The +Skeleton--The Joints--The Spinal Column--The Growth of Bone--The Repair of +Bone._ + +[Sidenote: 1. The framework of the body? The superstructure? Softness and +delicacy of the organs? How protected?] + +1. THE BONES.--The framework which sustains the human body is composed of +the _Bones_. The superstructure consists of the various organs on which the +processes of life depend. These organs are soft and delicately formed, and, +if unprotected, would, in most cases, rapidly be destroyed when subjected +to violence, however slight. The bones, having great strength and power of +resistance, afford the protection required. + +[Sidenote: 2. The more delicate the organ? Example in relation to the +brain? The eye? The lungs? The services performed by the bones?] + +2. The more delicate the organ, the more completely does Nature shield it. +For example: the brain, which is soft in structure, is enclosed on all +sides by a complete box of bone; the eye, though it must be near the +surface of the body to command an extensive view, is sheltered from injury +within a deep recess of bone; the lungs, requiring freedom of motion as +well as protection, are surrounded by a large case of bone and muscle. The +bones serve other useful purposes. They give permanence of form to the +body, by {16} holding the softer parts in their proper places. They assist +in movement, by affording points of attachment to those organs which have +power of motion--the muscles. + +[Sidenote: 3. Their shape and size? Of what composed? Possibility of being +separated? Effect of fire? Of dilute acid?] + +3. THE FORM AND COMPOSITION OF THE BONES.--Their shape and size vary +greatly in different parts of the body, but generally they are arranged in +pairs, one bone for each side of the body. They are composed of both +mineral and animal substances, united in the proportion of two parts of the +former to one of the latter; and we may separate each of these substances +from the other for examination. First, if we expose a bone to the action of +fire, the animal substance is driven off, or "burned out." We now find +that, though the shape of the bone is perfectly retained, what is left is +no longer tough, and does not sustain weight as before. Again, we may +remove the mineral portion, which is a form of lime, by placing a bone into +a dilute acid. The lime will be dissolved out, and the shape of the bone +remain as before; but now its firmness has disappeared, and it may be bent +without breaking. + +[Sidenote: 4. Effect of deficiency of ingredient? Usefulness of the lime? +Of the animal substance? Effect of their union? Condition, in youth? Old +age?] + +4. If, for any reason, either of these ingredients is disproportionate in +the bone during life, the body is in danger. The lime is useful in giving +rigidity of form, while the animal substance insures toughness and +elasticity. By their union, we are able to withstand greater shocks and +heavier falls than would be possible with either alone. In youth, the +period of greatest activity, the animal constituent is in excess: a bone +then does not break so readily, but, when broken, unites with great +rapidity and strength. On the other hand, the bones of old persons are more +easily broken, and in some cases fail to unite. The mineral matter being +then in excess, indicates that the period of active exertion is drawing to +a close. + +{17} + +[Illustration: FIG. 1.--SECTION OF BONE.] + +[Illustration: FIG. 2. + +Structure of bone enlarged.] + +[Sidenote: 5. In what respect admirably fashioned? Its formation? +Microscopic examination? The inference? "Line of beauty?"] + +5. THE STRUCTURE OF THE BONES.--If we examine one of the long bones, which +has been sawn through lengthwise, we observe that it is admirably fashioned +for affording lightness as well as strength (Fig. 1). Its exterior is hard +and resisting, but it is porous at the broad extremities, while through the +central portion there is a cavity or canal which contains an oily +substance, called _marrow_. Let us now take a thin section of bone, and +examine it under the microscope; we discover that it is pierced by numerous +fine tubes (Fig. 2), about which layers of bone-substance are arranged. +Accordingly, though a bone be as hard as stone externally, it is by no +means as heavy as stone, by reason of its light interior texture. Another +element of power is found in the curved outline of the bones. The curved +line is said to be "the line of beauty," as it certainly is the line of +strength, and is uniformly employed in the bones whose position exposes +them to accident. + +{18} + +[Illustration: FIG. 3.--THE SKELETON] + +{19} + +[Sidenote: 6. Number of bones? Skeleton? The skull? Chest? The trunk? The +trunk and skull, how maintained? What of the arms? Legs?] + +6. THE SKELETON.--The number of bones in the human body exceeds two +hundred. When these are joined together in the proper places, they form +what is termed the _Skeleton_ (Fig. 3). It embraces three important +cavities. The first, surmounting the frame, is a box of bone, called the +_skull;_ below this, is a bony case, or "chest;" and lower down is a bony +basin, called the _pelvis_. The two latter compose the trunk. The trunk and +skull are maintained in their proper relations by the "spinal column." +Branching from the trunk are two sets of limbs: the arms, which are +attached to the chest by means of the "collar-bone" and "shoulder-blade;" +and the legs, directly joined to the lower part of the trunk. + +[Sidenote: 7. Design of the cavities? Give the examples.] + +7. The cavities of which we have spoken, are designed for the lodgment and +protection of the more delicate and perishable parts of the system. Thus, +the skull, together with the bones of the face, shelters the brain and the +organs of four senses--sight, hearing, smell, and taste. The chest contains +the heart, lungs, and great blood-vessels, while the lower part of the +trunk sustains the liver, stomach, and other organs. + +[Sidenote: 8. Joint or articulation? Movable joints, how compacted? The +ligaments of the movable joints? What is a sprain? Consequence of a serious +sprain?] + +8. THE JOINTS.--The point of union of two or more bones forms a joint or +_articulation_, the connection being made in various ways according to the +kind and amount of motion desired. The movable joints are compacted +together by certain strong fibrous bands, called ligaments. These ligaments +are of a shining, silvery whiteness, and very unyielding; so much so, that +when sudden violence is brought to bear in the vicinity of a joint, the +bone to which a ligament is attached may be broken, while the ligament +itself remains uninjured. When this connecting material of the joints is +strained or lacerated by an {20} accident, a "sprain" is the consequence. +An injury of this sort may be, and frequently is, quite as serious as the +breaking of a bone. + +[Illustration: FIG. 4.--CELLS OF CARTILAGE.] + +[Sidenote: 9. Office of the ligament? What must it have? How accomplished? +Describe it. Synovia?] + +9. The ligament, then, secures firmness to the joint; it must also have +flexibility and smoothness of motion. This is accomplished by a beautiful +mechanism the perfection of which is only feebly imitated by the most +ingenious contrivance of man. The ends of the bones are covered by a thin +layer of _cartilage_, which being smooth and elastic, renders all the +movements of the joint very easy. In addition to this, there is an +arrangement introduced for "lubricating" the joint, by means of a delicate +sac containing fluid. This fluid is constantly supplied in small +quantities, but only so fast as it is used up in exercise. In appearance, +it is not unlike the white of an egg, and hence its name _synovia_, or +egg-like. + +[Sidenote: 10. What do we observe as regards the composition of a joint? +The ligament and cartilage? What varies? Example of the skull? Other +examples? The ball-and-socket joint?] + +10. Thus, we observe, that two very different substances enter into the +composition of a joint. The ligament, very unyielding, affords strength, +while the cartilage, elastic and moist, gives ease and smoothness of +motion. The amount of motion provided for varies greatly in different +joints. In some there is none at all, as in the skull, where one bone is +dove-tailed into another by what are termed _sutures_. Others have a +hinge-like motion, such as those of the elbow, wrist, ankle, and knee; the +most complete of these being the elbow-joint (Fig. 5). Belonging to another +class, the {21} ball-and-socket joint, is that at the shoulder, possessing +a freedom of motion greater than any other in the body. + +[Illustration: FIG. 5.--ELBOW JOINT. A, Bone of the arm; B, C, Bones of the +fore-arm.] + +[Sidenote: 11. What is the spinal column? What does it connect and form? +Joints of the vertebrae? Amount of motion? Result?] + +11. THE SPINAL COLUMN.--The spinal column is often spoken of as the +"back-bone," as if it were a single bone, while, in reality, it is composed +of a chain of twenty-six small bones, called _vertebrae_. The spinal column +is a wonderful piece of mechanism. It not only connects the important +cavities of the body, as has already been shown, but, also, itself forms a +canal, which contains the spinal cord. The joints of the vertebrae are +remarkable for the thick layers of cartilage which separate the adjacent +surfaces of bone. The amount of motion between any two of these bones is +not great; but these little movements, taken together, admit of very +considerable flexibility, in several directions, without endangering the +supporting power of the column. + +{22} + +[Sidenote: 12. Elasticity of the frame? Protection of the brain from +shocks? Tallness of persons? Effects of reclining?] + +[Illustration: FIG. 6--THE SPINAL COLUMN.] + +12. The abundant supply of intervertebral cartilage has another important +use, namely, it adds greatly to the elasticity of the frame. It is due, in +part, to this elastic material, and in part to the frequent curves of the +spine, that the brain and other delicate organs are protected from the +shock of sudden falls or jars. During the day, the constant pressure upon +these joints, while the body is erect, diminishes the thickness of the +cartilages; so that a person is not so tall in the evening as in the +morning. The effects of this compression pass away when the body reclines +in a horizontal position. + +[Sidenote: 13. Change in bone? Example--animal and madder. Rapidity of +change in color? Waste and repair?] + +13. THE GROWTH OF BONE.--Bone, like all the other tissues of the body, is +constantly undergoing change, old material being withdrawn, and new +particles taking their place. This has been shown conclusively by +experiments. If an animal be fed with madder--a red coloring matter--for a +day or two, the bones soon become tinged; then, if the madder be +discontinued for a few days, the original color returns. If, however, this +material be alternately given and withheld, at short intervals, the bone +will be marked by a succession of red and white rings. In very young +animals, all the bones become colored in a single day; in older ones, a +longer time is required. The process of waste and repair, therefore, is +constantly taking place in this hard substance, and with astonishing +rapidity. {23} + +14. THE REPAIR OF BONE.--Nature's provision for uniting broken bones is +very complete. At first, blood is poured out around the ends of the bone, +as a result of the injury. This is gradually absorbed, and gives place to a +watery fluid, which, thickening from day to day, acquires, at the end of +two weeks, the consistency of jelly. This begins to harden, by a deposit of +new bone-substance, until, at the expiration of five or six weeks, the +broken bone may be said to be united. It is, however, still fragile, and +must be used carefully a few weeks longer. The process of hardening +continues, and months must pass before the union can be said to be +complete. + +QUESTIONS FOR TOPICAL REVIEW. + + PAGE + 1. What useful purposes do the bones serve? 15, 16 + 2. State what you can of the composition of the bones. 16 + 3. Of the usefulness of lime in the bones. 16 + 4. Of the usefulness of animal substance in the bones. 16 + 5. State what you can of the structure of the bones. 17 + 6. Of the strength belonging to the bones. 15, 16, 17 + 7. What is meant by the human skeleton? 19 + 8. Give a description of its construction. 19 + 9. What is meant by a joint in the human frame? 19 + 10. State what you can of the movable joints. 19, 20 + 11. What office is performed by the ligaments of the joints? 19, 20 + 12. What by the cartilage at the joints? 20 + 13. What movable joints are there? 20, 21 + 14. Describe the construction of the spinal column. 21 + 15. What properties and powers does the spinal column possess? 21, 22 + 16. When is a person taller than at other times? 22 + 17. Give the reason for this. 22 + 18. What can you state of the growth of bone? 22 + 19. Describe the process by which a broken bone is repaired. 23 + + * * * * * {24} + + +[Illustration: FIG. 7.--THE MUSCLES.] + +{25} + +CHAPTER II. + +THE MUSCLES. + +_The Muscles--Flexion and Extension--The Tendons--Contraction--Physical +Strength--Necessity for Exercise--Its Effects--Forms of +Exercise--Walking--Riding--Gymnastics--Open-air +Exercise--Sleep--Recreation._ + +[Sidenote: 1. What are the muscles? Their number? The design of most of +them? Of a few?] + +1. THE MUSCLES.-- The great mass of the body external to the skeleton, is +composed of the flesh, or _Muscles_, which largely determines its outline +and weight. The muscles are the organs of motion. Their number is about +four hundred, and to each of them is assigned a separate and distinct +office. They have all been studied, one by one, and a name given to each, +by the anatomist. Each is attached to bones which it is designed to move. A +few are circular in form, and enclose cavities, the size of which they +diminish by contraction. + +[Illustration: FIG. 8.--MUSCULAR TISSUE. + +_a_, _b_, Striped muscular fibres: _c_, The same more highly magnified.] + +[Sidenote: 2. The structure of flesh? Its color, etc.? The composition of +the fibres? How marked?] + +2. If we examine a piece of flesh, we observe that it is soft, and of a +deep red color. Its structure appears to be composed of layers and bundles +of small fibres. Let us further examine these fibres under the microscope. +We now discover that they are, in turn, made up of still finer fibres, of +_fibrillae_: these are seen in Fig. 8. The fibres are beautifully {26} +marked by parallel wavy lines, about ten thousand to an inch, which give +the fibre its name of the _striped_ muscular fibre. All of the voluntary +muscles present this appearance. + +[Sidenote: 3. Arrangement of the muscles? Their action? Flexion and +extension? Action of the muscles when we stand erect?] + +3. FLEXION AND EXTENSION.--The muscles are, for the most part, so arranged +in pairs, or corresponding sets, that when motion is produced in one +direction by one set, there is, opposite to it, another muscle, or group of +muscles, which brings the limb back to its place. When they act +alternately, a to-and-fro movement results. When a joint is bent, the +motion is called _flexion_; and when it is made straight again, it is +called _extension_. When both sets act equally, and at the same moment, no +motion is produced, but the body or limb is maintained in a fixed position: +this occurs when we stand erect. The muscles which produce extension are +more powerful than those opposite to them. + +[Illustration: FIG. 9.--A, Biceps muscle of the arm: B, C, Its tendons.] + +[Sidenote: 4. Kinds of muscles? The voluntary? Involuntary? The heart? Give +the example. The hand? Arm?] + +4. The muscles are also distinguished, on the other hand, as the voluntary +and involuntary muscles, according as they are, or are not, under the +control of the will. The heart is an example of the involuntary variety. We +cannot change its action in the least by an effort of the will. When we +sleep, and the will ceases to act, the heart continues to beat without +cessation. The voluntary muscles, on the other hand, are such as are used +only when we wish or _will_ to use them--as the muscles of the hand or arm +(Fig. 9). + +{27} + +[Sidenote: 5. What are the tendons or sinews? Their strength? Color? +Location? Tendon of Achilles? The fable? Muscles of the leg?] + +5. THE TENDONS.--Tendons, or sinews, are the extremities of muscles, and +are compactly fastened upon bone. They are very strong, and of a silvery +whiteness. They may be felt just beneath the skin, in certain parts of the +body, when the muscles are being used, as at the bend of the elbow or knee. +The largest tendon of the body is that which is inserted into the heel, +called the tendon of Achilles, after the hero of the Grecian poet, the +fable relating that it was at this point that he received his death-wound, +no other part of his body being vulnerable. The muscles which extend into +the leg unite to form a single and very powerful tendon, and enclose a +small bone called the knee-pan, which, acting like a pulley, greatly +increases their power, and at the same time protects the front of the +knee-joint (Fig. 10). + +[Illustration: FIG. 10.--VIEW OF KNEE-JOINT. A, Thigh bone: B, Knee-pan: C, +D, Leg bones.] + +[Sidenote: 6. Contraction of the muscles? Bending of the arm or finger? +Other agencies? Automatic movements? In cold-blooded animals?] + +6. MUSCULAR CONTRACTION.--The muscles, when acted upon by the appropriate +stimulus, contract, or so change {28} their shape, that their extremities +are brought nearer together. The bending of the arm, or of a finger, is +effected in this manner, by the will; but the will is not the only means of +producing this effect. Electricity, a sharp blow over a muscle, and other +stimuli, also cause it. Contraction does not always cease with life. In +man, after death from cholera, automatic movements of hands and feet have +been observed, lasting not less than an hour. In certain cold-blooded +animals, as the turtle, contraction has been known to take place for +several days after the head has been cut off. + +[Sidenote: 7. Contractility? Give the illustration. What was supposed? What +is the case?] + +7. The property which, in muscle, enables these movements to take place is +called _contractility_. If we grasp a muscle while in exercise (for +example, the large muscle in the front of the arm), we notice the alternate +swelling and decrease of the muscle, as we move the forearm to and fro. It +was at one time supposed that the muscle actually increased in volume +during contraction. This, however, is not the case; for the muscle, while +gaining in thickness, loses in length in the same proportion; and thus, the +volume remains the same in action and at rest. + +[Sidenote: 8. What further in relation to contraction? Weariness of a +muscle? Beating of the heart? Standing and walking?] + +8. Contraction is not the permanent, or normal, state of a muscle. It +cannot long remain contracted, but after a shorter or longer time, it +wearies and is obliged to relax. After a short rest, it can then again +contract. It is for this reason that the heart can beat all through life, +night and day, by having, as we shall hereafter see, a brief interval of +rest between successive pulsations. For the same reason, it is more +fatiguing to stand for any great length of time in one position, than to be +walking the same period. + +[Sidenote: 9. Muscular power of animals? How tested? Man's power? Horse's? +The comparison?] + +9. RELATIVE STRENGTH OF ANIMALS.--The amount of muscular power which +different animals can exert, has {29} been tested by experiment. By +determining the number of pounds which an animal can drag upon a level +surface, and afterward comparing that with its own weight, we can judge of +its muscular force. It is found that man is able to drag a little less than +his own weight. A draught-horse can exert a force equal to about two-thirds +of his weight. The horse, therefore, though vastly heavier than man, is +relatively not so powerful. + +[Sidenote: 10. Power of insects? Beetles? Give the conclusion.] + +10. Insects are remarkable for their power of carrying objects larger and +heavier than themselves. Many of them can drag ten, and even twenty times +their weight. Some of the beetles have been known to move bodies more than +forty times their own weight. So far, therefore, from it being a fact that +animals have strength in proportion to their weight and bulk, the reverse +of that statement seems to be the law. + +[Sidenote: 11. Difference in strength of individuals? How caused?] + +11. PHYSICAL STRENGTH.--The difference in strength, as seen in different +individuals, is not due to any original difference in their muscles. Nature +gives essentially the same kind and amount of muscles to each person, and +the power of one, or the weakness of another, arises, in great part, from +the manner in which these organs are used or disused. + +[Sidenote: 12. Complaint in relation to degeneracy? How true? How +determined by armor? The fair supposition?] + +12. Many authors complain of the physical degeneracy of men at the present +day, as compared with past generations. There is room for doubt as to the +correctness of this statement. Certain experiments have recently been made +with the metallic armor worn seven hundred years ago, by which it is found +that any man, of ordinary height and muscular development, can carry the +armor and wield the weapons of an age supposed to be greatly our superior +in strength. When we consider that in those days, only very strong men +could endure the hardships of soldier-life, {30} it is fair to suppose that +our age has not so greatly degenerated in respect to physical strength. + +[Sidenote: 13 Action? Use of organs? Training of the mind? The child's +brain? Education of the body?] + +13. IMPORTANCE OF EXERCISE.--Action is the law of the living body. Every +organ demands use to preserve it in full vigor, and to obtain from it its +best services. The value of that training of the mind, which we call +education, is everywhere recognized. The child is early put to school, and +for many years continues to study, in order that his brain, which is the +great centre of mental power, may act healthfully and with force. It is +important that the body, also, should receive its education by exercise. +This is especially true of persons belonging to certain classes of society, +whose occupation confines them within doors, and requires chiefly +brain-work. + +[Sidenote: 14 Work in the open air? A perfect business? The consequence of +universal perfect business? Occupation of children?] + +14. Persons who are engaged in manual labor in the open air obtain all the +exercise necessary for bodily health in their regular business: their need +is more likely to be a discipline or exercise of the mind. A perfect +business of life, therefore, would be one which would combine both physical +and mental labor in their proper proportions. If such a business were +possible for all the human race, life would thereby be vastly prolonged. +Such is, in fact, to a large extent, the occupation pertaining to one +period of life--childhood. A part of the time is spent by the child in +improving his mind by study, and another part of the time he has physical +exercise in his games and sports. + +[Sidenote: 15 In what does exercise consist? Effects of it?] + +15. THE EFFECTS OF EXERCISE.--Exercise consists in a well-regulated use of +the voluntary muscular system. The effects, however, are not limited to the +parts used. Other organs, which are not under the control of the will, are +indirectly influenced by it. For instance, the heart beats more rapidly, +the skin acts more freely, and {31} becomes hotter, as well as the parts +beneath it, and the appetite and power of digestion are increased. An +increased exhalation from the lungs and skin purifies the current of the +circulation, and the body as a whole thrives under its influence. + +[Sidenote: 16. General effect upon the muscles? Special effect? Effects of +inaction? Of excessive exercise?] + +16. The immediate effect of exercise, however, is upon the muscles +themselves; for by use they become firm and large, and increase in power. +If we examine a muscle thus improved by exercise, we find that its fibres +have become larger and more closely blended together, that its color is of +a darker red, and that the supply of blood-vessels has increased. Without +exercise the muscle appears thin, flabby, and pale. On the other hand, +excessive exercise, without sufficient relaxation, produces in the muscle a +condition not very different from that which follows disuse. The muscle is +worn out faster than nature builds it up, and it becomes flabby, pale, and +weak. + +[Sidenote: 17. Of violent and spasmodic efforts? Strength, how attained? +Give the particulars.] + +17. Violent exercise is not beneficial; and spasmodic efforts to increase +the muscular strength are not calculated to secure such a result. Strength +is the result of a gradual growth, and is most surely acquired if the +exercise be carried to a point short of fatigue, and after an adequate +interval of rest. To gain the most beneficial results, the exercise should +be at regular hours, and during a regular period. The activity of the +exercise, and the time devoted to it must vary, of course, with the +strength of the individual, and should be carefully measured by it. + +[Sidenote: 18. What may walking be called? What further is said of +walking?] + +18. DIFFERENT MODES OF EXERCISE.--There are very few who have not the power +to walk. There is required for it no expensive apparatus, nor does it +demand a period of preliminary training. _Walking may be called the +universal exercise._ With certain foreign nations, the English {32} +especially, it is a very popular exercise, and is practised habitually by +almost every class of society; by the wealthy as well as by those who have +no carriages; by women as well as by men. + +[Sidenote: 19. What is said of running, and other like movements? What, as +related to childhood? What instances are alluded to? Example?] + +19. Running, leaping, and certain other more rapid and violent movements, +are the forms of exercise that are most enjoyed in childhood. For the +child, they are not too severe, but they may be so prolonged as to become +injurious. Instances have been recorded where sudden death has resulted +after violent playing, from overtaxing the heart: for example, we have the +case of a young girl who, while skipping the rope, and endeavoring to excel +her playmates by jumping the greatest number of times, fell dead from +rupture of the heart. + +[Sidenote: 20. Carriage-riding? Horseback-riding?] + +20. Carriage-riding, as a means of passive exercise, is particularly well +suited to invalids, and persons advanced in life. Horseback exercise brings +into use a greater number of muscles than any other one exercise, and with +it there is an exhilaration of feeling which refreshes the mind at the same +time. It is one of the manliest of exercises, but not less suitable for +women than for men. To be skilful in riding, it is best to begin its +practice in youth; but there are very few kinds of exercise of which the +same is not equally true. + +[Sidenote: 21. Boating, swimming, and skating?] + +21. For those who live near streams or bodies of water, there are the +delightful recreations of boating, swimming, and skating. Certain of these +exercises have a practical importance aside from and above their use in +increasing the physical vigor. This is especially true of boating and +swimming, since they are often the means of saving life. Practice in these +exercises also teaches self-reliance, courage, and presence of mind. +Persons who have become proficient in these vigorous exercises are +generally the ones, {33} who, in times of danger, are the quickest to act +and the most certain to do so with judgment. + +[Sidenote: 22. What kind of exercise yields the best results? What advice +is given?] + +22. PHYSICAL CULTURE.--That form of exercise which interests and excites +the mind, will yield the best results; but to some persons no kind of +exertion whatever is, at first, agreeable. They should, nevertheless, make +a trial of some exercise, in the expectation that, as they become +proficient in it, it will become more pleasant. In exercise, as many sets +of muscles should be employed as possible, open-air exercise being the +best. Parlor gymnastics, and the discipline of the gymnasium are desirable, +but they should not be the sole reliance for physical culture. No in-door +exercise, however excellent in itself, can fill the place of hearty and +vigorous activity in the open air. + +[Sidenote: 23. Physical culture among the ancients? In Greece? In schools +and colleges at the present time? Result to the body and mind?] + +23. GYMNASTIC EXERCISES FOR SCHOOLS AND COLLEGES.--In the system of +education among the ancients, physical culture predominated. In ancient +Greece, physical exercises in schools were prescribed and regulated by law, +and hence these schools were called _gymnasia_. At the present time, on the +contrary, this culture is almost wholly unknown, as a part of the course of +education, in our schools and colleges. In a few of our institutions of +learning, however, physical exercises have been introduced, with manifest +advantage to the students, and they form a part of the regular curriculum +of exercises,--as much so as the recitations in geography, grammar, or +Greek. The good effect of the experiments, as shown in improved scholarship +as well as increased bodily vigor, in the institutions where the plan has +been tried, will, it is hoped, lead to its universal adoption. We should +then hear less frequently of parents being obliged to withdraw their +children from school, because they become exhausted {34} or, perchance, +have lost their health from intense and protracted mental application. + +[Sidenote: 24. The result of gymnastics in our colleges and other +institutions of learning?] + +24. Were gymnastics more common in our educational institutions we should +not so often witness the sad spectacle of young men and women leaving our +colleges and seminaries, with finished educations it may be, but with +constitutions so impaired, that the life which should be devoted to the +accomplishment of noble purposes must be spent in search of health. Spinal +curvatures, which, according to the experience of physicians, are now +extremely frequent, especially among ladies, would give place to the steady +gait and erect carriage which God designed his human creatures should +maintain. + +25. All the exercises necessary for the proper development of the body may +be obtained from the use of a few simple contrivances that every one can +have at home, at little cost--less by far than is spent for useless toys. +Many of these may be made available in the parlor or chamber, though all +exercises are far more useful in the open air. A small portion of the day +thus spent will afford agreeable recreation as well as useful exercise. The +Indian club, the wand, the ring, and the dumb-bells answer ordinary +purposes very well. Illustrations are here introduced of a few simple +contrivances that may be useful for general exercises, and are specially +suitable for persons with _weak spines_, or with spines that are the +subject of lateral curvature. + +26. One of the simplest appliances for strengthening the muscles of the +spine, designed chiefly to exercise the muscles on either side of the +spine, consists of two wooden handles attached to india-rubber cords, one +of which is attached to a hook made fast in the ceiling, or in the top of +the door-case; and the other to another hook fastened in the wall, +door-post, or window-casing, about the height {35} of the shoulder. When +traction is made with the left hand, it exercises the muscles on the left +side of the spine, while those on the opposite side are left almost at +rest, owing to the oblique direction given to the shoulders when the right +hand grasps the horizontal cord. (This appliance will be understood by +referring to Fig. 13.) + +[Illustration: FIG. 11.] + +[Illustration: FIG. 12.] + +{36} + +27. Fig. 11 shows an appliance consisting of two strong elastic cords, with +handles, secured to a hook in the floor, so arranged that the patient has +to stoop forward to reach them. On raising the body the spinal muscles are +powerfully exercised. Fig. 12 shows other modes of using the elastic cords +for strengthening the spine and chest. + +[Illustration: FIG. 13.] + +28. These various appliances have been combined so as to form a system of +gymnastics suitable for parlor use; other appliances have been added by +which the muscles of {37} the legs may be called into action as well as +those of the spine and upper part of the body (Fig. 13). Combinations of +cords suitable for particular cases may also be made, and by using one or +several cords on the same hook, their power may be adapted to the strength +of the most robust as well as to that of the invalid, or of the most +delicate child. The entire apparatus is quite simple in its construction +and inexpensive, requiring but little space, and at the same time affording +a great variety of exercises. + +EXERCISES THAT MAY BE PRACTISED ON THIS APPARATUS. + + EXERCISE I. (Fig. 13).--Stand erect under the cords and place the heels + together. Grasp the handles firmly, keeping the knees and elbows stiff, + and pull downward and forward until the fingers nearly touch the toes. + Return slowly to the erect position. Repeat. + + EXERCISE II. (Fig. 13).--Stand erect, and having grasped the handles + overhead firmly, separate them and bring them down slowly until they + touch the sides: then return them slowly to the original position. + Repeat. + + EXERCISE III. (Fig. 13).--Stand erect, heels together, grasp the + handles overhead, and charge forward with the right foot. Return to + first position, and then charge with the left. Repeat, using the right + and left foot alternately. + + EXERCISE IV. (Fig. 13).--Stand erect, heels together. Grasp the handle + overhead, and charge forward with the right foot, knee bent. Remain in + this position and bring the arms down to the sides so that the arm and + fore-arm may form a right angle. Still holding the handles, thrust + forward, first the right hand and then the left, until the arm is + straight. Repeat. Return to first position, then charge forward with + the left foot, performing the same movements as before. + + EXERCISE V. (Fig. 13).--In this exercise we change to the pulleys + leading from the side posts, which can be used in several different + ways. 1st. Stand erect, heels together, facing one of the posts, grasp + the handle with the right hand, the arm being extended, then flex the + fore-arm on the arm. Repeat. Perform the same movements with the left + hand. 2d. Stand with back to the post; grasp the pulley behind with the + right hand, then gradually bring the hand forward until it is extended + in a straight line in front. Repeat. Perform the same exercise with the + left hand. + + {38} EXERCISE VI. (Fig. 13).--This exercise is especially adapted to + the legs. Stirrups are so arranged that they can be attached to the + pulleys overhead, and can hang down to within three or four feet of the + floor. Place the foot in the stirrup, and then press down until it + touches the floor. Repeat. Exercise the left foot in the same way. + + EXERCISE VII. (Fig. 13.)--This exercise requires a little attention in + the adjustment of the apparatus. Under the pulleys in the floor are + passed ropes which can be attached to the snap-hooks that hold the + handles overhead. Stoop forward with the knees stiff, and take hold of + the handles, and then raise the body to the erect position. Repeat. + + EXERCISE VIII. (Fig. 13).--Sit on the floor or on a seat three or four + inches high; bend forward, take hold of the handles, and perform the + same movements that you would in rowing a boat. + + EXERCISE IX. (Fig. 13).--The trapeze can now be let down; take hold of + it with both hands, sustaining the weight of the body with the arms, + then rotate the body first from right to left, then from left to right + alternately. This exercise is especially suitable for females. + + EXERCISE X. (Fig. 13).--Grasp the trapeze as before, bearing all the + weight with the arms: then draw the body up slowly until you can place + the chin over the bars. This requires strength of muscle, and might + strain if done too violently; if slowly performed there is no danger. + + These are but a few of the exercises that can be practised with this + apparatus. As these become familiar they can easily be modified, and + new ones can be arranged to meet the requirements of particular cases. + Most of the exercises described can be practised with one hand so as to + strengthen the muscles on one side. + +[Sidenote: 29. Need of repose? How do we obtain rest? Alfred the Great? The +eight-hour division of time?] + +29. REST.--We cannot always be active: repose must succeed labor. We obtain +this rest partly by suspending all exertion, as in sleep, and partly by a +change of employment. It is said that Alfred the Great recommended that +each day should be divided in the following manner: "Eight hours for work, +eight hours for recreation, and eight hours for sleep." This division of +time is as good as any that could now be made, if it be borne in mind that, +when the work is physical, the time of recreation should {39} be devoted to +the improvement of the mind; and when mental, we should then recreate by +means of physical exercise. + +[Sidenote: 30. Cessation of voluntary activity? Temperature of the body? +Consequence? Body and mind during sleep? Nutrition? Describe it. +Consequences of insufficient sleep?] + +30. During sleep, all voluntary activity ceases, the rapidity of the +circulation and breathing diminishes, and the temperature of the body falls +one or two degrees. In consequence, the body needs warmer coverings than +during the hours of wakefulness. During sleep, the body seems wholly at +rest, and the mind is also inactive, if we except those involuntary mental +wanderings which we call dreams. Nevertheless a very active and important +physical process is going on. Nutrition, or the nourishing of the tissues, +now takes place. While the body is in action, the process of pulling down +predominates, but in sleep, that of building up takes place more actively. +In this way we are refreshed each night, and prepared for the work and +pleasures of another day. If sleep is insufficient, the effects are seen in +the lassitude and weakness which follow. Wakefulness is very frequently the +forerunner of insanity, especially among those who perform excessive mental +labor. + +[Sidenote: 31. Amount of sleep in different persons? Cases? Frederick the +Great? Bonaparte? Instances of long deprivation of sleep?] + +31. All persons do not require the same amount of sleep, but the average of +men need from seven to nine hours. There are well-authenticated cases where +individuals have remained without sleep for many days without apparent +injury. Frederick the Great required only five hours of sleep daily. +Bonaparte could pass days with only a few hours of rest. But this long +continued absence of sleep is attended with danger. After loss of sleep for +a long period, in some instances, stupor has come on so profoundly, that +there has been no awaking. + +[Sidenote: 32. Instances of sailors? French soldiers? During torture?] + +32. There are instances related of sailors falling asleep {40} on the +gun-deck of their ships while in action. On the retreat from Moscow, the +French soldiers would fall asleep on the march, and could only be aroused +by the cry, "The Cossacks are coming!" Tortured persons are said to have +slept upon the rack in the intervals of their torture. In early life, while +engaged in a laborious country practice, the writer not unfrequently slept +soundly on horseback. These instances, and others, show the imperative +demand which nature makes for rest in sleep. + +QUESTIONS FOR TOPICAL REVIEW. + + PAGE + 1. What can you state of the number and division of the muscles? 25, 26 + 2. Describe the structure of the muscles. 25, 26 + 3. Their arrangement in pairs and consequent action. 26 + 4. What is the difference between the motion called flexion and that + called extension? 26 + 5. Describe their action, and state which are the more powerful. 26 + 6. What is the difference between voluntary and involuntary muscles? 26 + 7. Illustrate the difference between the two. 26 + 8. State all you can of the tendons or sinews. 27 + 9. What is meant by contraction of the muscles? 27, 28 + 10. In how many and what ways may contraction be effected? 28 + 11. What is stated of after-death contraction? 28 + 12. Why cannot a muscle in life continue contracted a long time? 28 + 13. How then can the constant beating of the heart be explained? 28 + 14. How does the strength of a man compare with that of a horse? 29 + 15. What can you state in relation to the relative strength + of animals? 28, 29 + 16. What, in relation to physical strength? 29 + 17. What, in relation to physical degeneracy? 29, 30 + 18. What, in relation to the importance of exercise? 30 + 19. What is the effect of exercise upon the heart, skin, + and appetite? 30, 31 + 20. How does exercise affect the current of the body's circulation? 31 + 21. How does judicious exercise affect the muscles? 31 + 22. What is stated of violent and spasmodic exercise? 31 + 23. Of the exercise of walking? 31, 32, 33 + 24. Of running, leaping, and other modes of exercise? 32 + 25. Of physical culture, in connection with out-door exercises? 33 + 26. Of the importance of gymnastics in our schools and colleges? 33, 34 + 27. Of the importance of rest from labor or exercise? 38, 39 + 28. What processes take place during sleep? 39 + 29. What effects follow insufficient sleep? 39 + + * * * * * + + +{41} + +CHAPTER III. + +THE INTEGUMENT, OR SKIN. + +_The Integument--Its Structure--The Nails and Hair--The Complexion--The +Sebaceous Glands--The Perspiratory Glands--Perspiration and its +Uses--Importance of Bathing--Different kinds of Baths--Manner of +Bathing--The Benefits of the Sun--Importance of Warm Clothing--Poisonous +Cosmetics._ + +[Sidenote: 1. What is the skin? Parts directly beneath? What is shown?] + +1. THE INTEGUMENT.--The skin is the outer covering of the body. The parts +directly beneath it are very sensitive, and require protection. This is +shown whenever by accident the skin is broken, pierced, or torn off, the +bared surface being very tender, and painful to the touch. Nature has +provided the body with a garment that is soft, pliable, close-fitting, and +very thin, and yet sufficiently strong to enable us to come in contact with +the objects that surround us, without inconvenience or suffering. + +[Sidenote: 2. Microscopic examination? What is the cutis? The cuticle? +Their union? How separated? What further is said of the cuticle?] + +2. THE STRUCTURE OF THE SKIN.--When examined with the aid of the +microscope, the skin is found to be made up of two layers--the outer and +the inner. The inner one is called the _cutis_, or true skin; the outer one +is the _epidermis_, or scarf-skin. The latter is also known as the +_cuticle_. These two layers are closely united, but they may be separated +from each other. This separation takes place whenever, from a burn, or +other cause, a blister is formed; a watery fluid is poured out between the +two layers, and lifts the epidermis from the true skin. + +Of the two layers, the cuticle is the thinner in most parts of the body, +and has the appearance of a whitish membrane. It is tough and elastic, is +without feeling, and does {42} not bleed, when cut. Examine it more +closely, and we observe that it is composed of minute flat cells, closely +compacted, and arranged layer upon layer. + +[Sidenote: 3. Wearing out of the cuticle? What then? Variety in thickness +of cuticle? How accounted for?] + +3. The outer layer is constantly being worn out, and falls from the body in +the form of very fine scales. It is, also, continually forming anew on the +surface of the inner layer. Its thickness varies in different parts of the +body. Where exposed to use, it is thick, hard, and horn-like, as may be +seen on the soles of the feet, or on the palms of the hands, especially of +those who are accustomed to perform much manual labor. This is an admirable +provision for the increased protection of the sensitive parts below the +skin against all extraordinary exposure. Even the _liabilities_ of these +parts to injury, are thus kindly provided for by "the Hand that made us." + +[Sidenote: 4. Location and office of the cutis? What further is said of it? +Papillae? Touch?] + +4. The cutis, or true skin, lies beneath the epidermis, and is its origin +and support. It is firm, dense, elastic, very sensitive, and is freely +supplied with blood-vessels. It is closely connected with the tissues below +it, but may be separated by means of a sharp instrument. The surface of the +cutis is not smooth, but is covered here and there with minute elevations, +called _papillae_. These are arranged in rows, along fine lines, or ridges, +such as those which mark the palm and fingers; their number is about 80 to +the square line (a line being one-twelfth of an inch). These _papillae_ +contain the blood-vessels which carry the supply of blood needed by the +ever-wasting skin. They contain nerves also, and are largely concerned in +the sense of touch; hence they are particularly abundant where the touch is +most delicate, as at the ends of the fingers. + +[Sidenote: 5. What are the nails and hair? The growth of the nail? The +rapidity of its growth? Accident to the nail?] + +{43} [Illustration: Fig. 14. + +_a, b._ THE ROOT OF A HAIR. + +1, 2, 3. The skin forming the hair sac. 4. Sebaceous glands. 5. The hair +sac. + +_c._ TRANSVERSE SECTION OF A HAIR.] + +5. THE NAILS AND HAIR.--These are appendages of the skin, and although very +unlike the cuticle as it appears on the surface of the body, they are, in +reality, modified forms of that layer of the skin. The nail grows from a +fold of the cuticle at the root, and from the under surface. As fast as it +is formed, it is constantly being pushed outward. The rapidity of its +growth can be ascertained by filing a slight groove on its surface, and +noticing how the space between it and the root of the nail increases, in +the course of a few weeks. When the nail is removed by any accident, it +will be replaced by a new one, if the root be not injured. + +[Sidenote: 6. How are the hairs produced? Difference in their length?] + +6. The hairs are produced in a similar manner; the skin forming +depressions, or hair sacs, from the bottom of which they grow and are +nourished (Fig. 14). They are found, of greater or less length, on almost +all parts of the surface, except the palms of the hands and soles of the +feet. On certain parts of the body, they grow to great length; on other +parts they are so short, that they do not rise beyond the hair sac in which +they originate. + +[Sidenote: 7. Root of the hair? Shaft? Firmness and softness of the hair?] + +7. The bulb, or root, from which the hair arises, is lodged in a small +pouch, or depression in the skin. The shaft is the part which grows out +beyond the level of the skin. Its growth is altogether in one direction, in +length alone. The outer part of the hair is quite firm, while its {44} +interior is softer, and probably conveys the fluids by which it is +nourished. The hair is more glossy in health than at other times. + +[Sidenote: 8. Office of the nail? Of the hair? Give the illustrations.] + +8. The nail serves as a protection to the end of the finger, and also +enables us to grasp more firmly, and to pick up small objects. The hair, +too, is a protection to the parts it covers. On the head, it shields the +brain from extremes of heat and cold, and moderates the force of blows upon +the scalp. On the body, it is useful in affording a more extensive surface +for carrying off the perspiration. + +[Sidenote: 9. On what does the complexion depend? Light and dark races? +Freckles?] + +9. COMPLEXION.--In the deeper cells of the cuticle lies a pigment, or +coloring matter, consisting of minute colored grains. On this pigment +_complexion_ depends; and, according as it is present in less or greater +amount, occasions the difference of hue, that exists between the light and +dark races of men, and between the blonde and brunette of the white races. +Freckles are due to an irregular increase of coloring matter. + +[Sidenote: 10. Influence of the sun? How illustrated? Jews?] + +10. The sun has a powerful influence over the development of this pigment, +as is shown by the swarthy hue of those of the white race who have +colonized in tropical climates. It is also well illustrated by the fact, +that among the Jews who have settled in northern Europe, there are many who +are fair complexioned, while those residing in India, are as dark as the +Hindoos around them. + +[Sidenote: 11. What is an Albino? Where are Albinos found?] + +11. An Albino is a person who may be said to have no complexion; that is, +there is an entire absence of coloring matter from the skin, hair, and +_iris_ of the eye. This condition more frequently occurs among the dark +races, and in hot climates, although it has been observed in almost every +race and clime. + +[Sidenote: 12. What are sebaceous glands? How do they act? Sebaceous glands +of the face? How do they act?] + +12. SEBACEOUS GLANDS.--There are in the skin certain {45} small glands, +which produce an oily substance, called _sebaceous_ matter. These glands +are little rounded sacs, usually connected with the hair-bulbs; and upon +these bulbs, they empty their product of oil, which acts as a natural and +adequate dressing for the hair (4, Fig. 14). A portion of the sebaceous +matter passes out upon the surface, and prevents the cuticle from becoming +dry and hard. The glands situated upon the face and forehead, open directly +upon the skin. In these, the sebaceous matter is liable to collect, and +become too hard to flow off naturally. + +[Sidenote: 13. Black points, called worms? Animalcules? Service performed +by sebaceous matter?] + +13. These glands on the face and forehead frequently appear, on the faces +of the young, as small black points, which are incorrectly called "worms." +It is true, that occasionally living animalcules are found in this +thickened sebaceous matter, but they can only be detected by the aid of the +microscope. This sebaceous matter acts not only to keep the skin flexible, +and furnish for the hair an oily dressing, but it especially serves to +protect the skin and hair, from the acridity arising from the perspiration. + +[Sidenote: 14. Perspiration? Sweat glands? Of what do they consist? +Dimension of the tubes?] + +14. THE PERSPIRATORY GLANDS.--The chief product of the skin's action is the +perspiration. For the formation of this, there are furnished countless +numbers of little sweat-glands in the true skin. They consist of fine +tubes, with globe-like coils at their deeper extremity. Their mouths or +openings may be seen with an ordinary magnifying glass, upon the fine +ridges which mark the fingers. These tubes, if uncoiled, measure about +one-tenth of an inch in length. In diameter, they are about one +three-hundredth of an inch, and upon certain parts of the body there are +not far from three thousand of these glands to the square inch. Their whole +number in the body is, therefore, very great; and, in fact, it is computed +if they were all united, end to end, their combined measurement would +exceed three miles. {46} + +[Sidenote: 15. What is sensible perspiration? Insensible perspiration?] + +15. THE SENSIBLE AND INSENSIBLE PERSPIRATION.--The pores of the skin are +constantly exhaling a watery fluid; but, under ordinary circumstances, +there is no moisture apparent upon the surface, for it evaporates as +rapidly as it is formed. This is called insensible perspiration. Under the +influence of heat or exercise, however, this fluid is excreted more +abundantly, and appears on the surface in the form of minute, colorless +drops. It is then termed sensible perspiration. + +[Sidenote: 16. Components of perspiration? Upon what does perspiration +depend? Amount of perspiration daily?] + +16. Water is the chief component of this fluid, there being about +ninety-eight parts of water to two parts of solid matter. The quantity +escaping from the body varies greatly, according to the temperature of the +air, the occupation of the individual, and other circumstances. The average +daily amount of this excretion, in the adult, is not far from thirty +ounces, nearly two pints, or more than nine grains each minute. + +[Sidenote: 17. What does perspiration set free from the blood? What other +service does perspiration perform? Explain the process.] + +17. THE USES OF THE PERSPIRATION.--Besides liberating from the blood this +large amount of water, with the effete matter it contains, the perspiration +serves to regulate the temperature of the body. That is to say, as +evaporation always diminishes temperature, so the perspiration, as it +passes off in the form of fine vapor, cools the surface. Accordingly, in +hot weather this function is much more active, and the cooling influence +increases in proportion. When the air is already charged with moisture, and +does not readily receive this vapor of the body, the heat of the atmosphere +apparently increases, and the discomfort therefrom is relatively greater. + +[Sidenote: 18. Effect of interruption of excretion? What experiments are +mentioned?] + +18. The importance of this excretion is shown by the effects that often +follow its temporary interruption, namely, headache, fever, and the other +symptoms that accompany {47} "taking cold." When the perspiration is +completely checked, the consequences are very serious. Experiments have +been performed upon certain smaller animals, as rabbits, to ascertain the +results of closing the perspiratory tubes. When they are covered by a +coating of varnish impervious to water and gases, death ensues in from six +to twelve hours; the attendant symptoms resembling those of suffocation. + +[Sidenote: 19. Give the story in relation to the boy covered with gold +foil.] + +19. It is related that, at the coronation of one of the Popes about three +hundred years ago, a little boy was chosen to act the part of an angel; and +in order that his appearance might be as gorgeous as possible, he was +covered from head to foot with a coating of gold foil. He was soon taken +sick, and although every known means were employed for his recovery, except +the removal of his fatal golden covering, he died in a few hours. + +[Sidenote: 20. Give the quotation. Perspiration?] + +20. THE IMPORTANCE OF BATHING.--From these considerations, it is evident +that health must greatly depend upon the free action of the skin. "He who +keeps the skin ruddy and soft, shuts many gates against disease." When the +watery portion of the perspiration evaporates, the solid matter is left +behind on the surface. There, also, remain the scales of the worn-out +cuticle, and the excess of sebaceous matter. In order to secure the natural +action of the skin, these impurities require to be removed by the frequent +application of water. + +[Sidenote: 21. Ablution in warm climates? What advice is given?] + +21. In warm climates, and during hot weather, ablution should be more +frequently practised. For a person in good health, a daily cold bath is +advisable. To this should be added occasionally a tepid bath, with soap, +water alone not being sufficient to remove impurities of a greasy nature. +Soap facilitates this, by forming with such substances a chemical mixture, +which is readily soluble in water, and is by it removed from the body. {48} + +[Sidenote: 22. Liebig's maxim? What further is added?] + +22. There is a maxim by the chemist Liebig, to the effect, that the +civilization of a nation is high, in proportion to the amount of soap that +it consumes; and that it is low, in proportion to its use of perfumes. In +some degree, we may apply the same test to the refinement of an individual. +The soap removes impurity; the perfume covers, while retaining it. + +[Sidenote: 23. What is said about cold bathing?] + +23. THE DIFFERENT KINDS OF BATHS.--All persons are not alike able to use +the cold bath. When the health is vigorous, and the system does not feel a +shock after such a bath, a prompt reaction and glow upon the surface will +show that it is beneficial. Where this pleasurable feeling is not +experienced, but rather a chill and sense of depression ensues, we are +warned that the system will not, with impunity, endure cold bathing. + +[Sidenote: 24. What is said about warm bathing?] + +24. It should also be borne in mind, that the warm or hot bath cannot be +continued so long, or repeated so frequently as the cold, on account of the +enervating effect of unusual heat so applied to the body. For persons who +are not in robust health, one warm bath each week is sufficient; this class +should be careful to avoid every extreme in reference to bathing, clothing, +and whatever greatly affects the action of the skin. + +[Sidenote: 25. What is said about sea-bathing?] + +25. Sea-bathing is even more invigorating than fresh-water bathing. Those +who cannot endure the fresh water, are often benefited by the salt-water +baths. This may be accounted for, in part, by the stimulant action upon the +surface, of the saline particles of the sea-water; but the exciting scenes +and circumstances of sea-bathing also exert an important influence. The +open-air exercise, the rolling surf, the genial weather, and usually the +cheerful company, add to its intrinsic benefits. {49} + +[Sidenote: 26. What is said as to the time and manner of bathing?] + +26. TIME AND MANNER OF BATHING.--A person in sound health may take a bath +at almost any time, except directly after a full meal. The most appropriate +time is about three hours after a meal, the noon-hour being probably the +best. For the cold bath, taken rapidly, no time is better than immediately +after rising. Those beginning the use of cold baths should first try them +at 70deg Fahr., and gradually use those of a lower temperature. From five +to twenty minutes may be considered the proper limit of time to remain in a +bath; but a sensation of chilliness is a signal to withdraw instantly, +whether at home, or at the sea-side. Two sea-baths may be taken daily; one +of any other kind is sufficient. + +[Sidenote: 27. Condition of the body when bathing? Direction, after +bathing?] + +27. The body should be warm, rather than cold, when stepping into the bath; +and after it, the skin should be thoroughly dried with a coarse towel. It +is best to continue friction until there is a sensation of warmth or "glow" +throughout the entire surface. This reaction is the test of the good +effects of the bath. If reaction is still incomplete, a short walk may be +taken, especially in the sunshine. It is very congenial, however, both to +health and comfort, to rest for a short time directly after bathing, or to +take some light refreshment. This is better than severe exercise or a full +meal. + +[Sidenote: 28. Bathing among the ancients? Baths of Rome?] + +28. BATHING AMONG THE ANCIENTS.--The Romans and other nations of antiquity +made great use of the vapor-bath as a means of preserving the health, but +more particularly as a luxury. Their method was not unlike that employed in +northern Europe at the present day. The public baths of Rome and other +cities are among the grandest and most interesting monuments of ancient +luxury and splendor; and from their ruins have been recovered some of the +most beautiful works of art. {50} + +[Sidenote: 29. After the bath? Swimming among the ancients?] + +29. The Thermae, as the baths of Rome were called, were of great extent, +built very substantially, and ornamented at vast expense. They were +practically free to all, the cost of a bath having been less than a cent. +It is related that some persons bathed seven times a day. After the bath +their bodies were anointed with perfumed oil. If the weather was fine, they +passed directly from the Thermae into the gymnasium, and engaged in some +gentle exercise previous to taking the midday meal. Between two and three +in the afternoon was the favorite hour for this ancient luxury. Swimming +was a favorite exercise, and a knowledge of it was regarded as necessary to +every educated man. Their common expression, when speaking of an ignorant +person, was, "He can neither read nor swim." + +[Sidenote: 30. The Sun-bath? The story of Pliny?] + +30. THE SUN-BATH.--Some also were accustomed daily to anoint themselves, +and lie or walk in apartments arranged for the purpose, with naked bodies +exposed to the direct rays of the sun. There is an interesting allusion to +this practice, in a letter of the younger Pliny to the historian Tacitus, +describing the destruction of Pompeii by an eruption of Vesuvius. "My +uncle," (Pliny the elder,) "was at that time in command of the fleet at +Misenum. On the 24th of August, about one in the afternoon, my mother +desired him to notice a cloud which seemed of unusual shape and dimensions. +He had just returned from _taking the benefit of the sun_, and after a cold +bath, and a slight repast, had retired to his study." Then follows a +description of the destruction of Pompeii, and the death of the elder +Pliny. + +[Sidenote: 31. Benefit of the sun? Effect upon plants? Skin?] + +31. We may judge somewhat of "the benefits of the sun," by observing the +unnatural and undeveloped condition of plants and animals which are +deprived of light. Plants become blanched and tender; the fish of {51} +subterranean lakes, where no light enters, are undersized, and have no +eyes; tadpoles kept in the dark do not develop into frogs; men growing up +in mines are sallow, pale, and deformed. Besides the well-known effect of +solar light in tanning the skin, it also makes it thicker and better able +to resist exposure; though the complexion may be thereby injured, the +health gains more than compensates for the loss of beauty. "To make good +the loss of the lily, where the sun has cast his ray, he seldom fails to +plant the rose." + +[Sidenote: 32. Direction about clothing? Exposing limbs of children? +Clothing, night and day?] + +32. CLOTHING.--In reference to clothing, we are far more apt, in our +changeful climate, to use too little than too much. An aphorism of +Boerhaave, worth remembering, if not of adopting, is, "We should put off +our winter clothing on midsummer's day, and put it on again the day after." +He also says, "Only fools and beggars suffer from the cold; the latter not +being able to get sufficient clothes, the others not having the sense to +wear them." The practice of exposing the limbs and necks of young children, +for the alleged purpose of "hardening" them, is quite hazardous. It is not +to be denied that some seem to be made tough by the process; but it is so +only with the rugged children, the delicate ones will invariably suffer +under this fanciful treatment. As has been stated before, the skin is +constantly acting, by night as well as by day. It is therefore conducive +both to cleanliness and comfort to change entirely the clothing on retiring +for the night. The day-clothing should be aired during the night, and the +bedding should be aired in the morning, for the same reason. + +[Sidenote: 33. Cosmetics? Painters' colic?] + +33. POISONOUS COSMETICS.--The extensive use of _cosmetics_ for the +complexion is a fertile source of disease. The majority of these +preparations contain certain poisonous mineral substances, chiefly lead. +Now, the skin rapidly absorbs the fine particles of lead, and the system +{52} experiences the same evil effects that are observed among the +operatives in lead works and painters, namely, "painters' colic," and +paralysis of the hands, called "wrist-drop." + +34. Certain hair-dyes also contain lead, together with other noxious and +filthy ingredients. These do not work as great harm as the cosmetics, since +they are purposely kept away from the skin, but they rob the hair of its +vitality. Eye-washes, too, are made from solutions of lead, and many an eye +has been ruined by their use. They deposit a white metallic scale on the +surface of the eye, which becomes a permanent obstruction to the vision. + +QUESTIONS FOR TOPICAL REVIEW. + + PAGE + + 1. What are the characteristics of the skin, and what office + does it perform? 41 + 2. What can you state of the structure of the skin? 41 + 3. Describe the cuticle and tell its use. 41, 42 + 4. Describe the cutis or true skin and tell its use. 42 + 5. What can you state of the nature and growth of the nail? 42, 43 + 6. Of the nature and growth of the hair? 42, 43, 44 + 7. Of the offices performed by the nails and hair? 44 + 8. How is the difference in complexion in different persons + accounted for? 44 + 9. How is the presence of freckles accounted for? 44 + 10. How does Nature provide a dressing for the hair? 44, 45 + 11. What other service do the sebaceous glands perform? 45 + 12. State what you can of the perspiratory glands. 45 + 13. What is the difference between sensible and insensible + perspiration? 46 + 14. State the uses and importance of perspiration. 46, 47 + 15. What impurities gather naturally on the skin? 47 + 16. Repeat what is said of the importance of bathing. 47, 48 + 17. When should we indulge in cold, warm, and sea bathing? 48, 49 + 18. What is the effect in each case? 48 + 19. What directions are given as to the time and manner for + bathing? 49 + 20. What is related of bathing among the ancients? 49, 50 + 21. What is related to show the antiquity of sun-bathing? 50 + 22. What are the effects of sun-bathing? 50, 51 + 23. What directions are given in relation to clothing the body? 51 + 24. What can you state of poisonous cosmetics? 51, 52 + 25. Of hair-dyes and eye-washes? 52 + + * * * * * + + +{53} + +CHAPTER IV. + +THE CHEMISTRY OF FOOD. + +_The Source of Food--Inorganic Substances--Water--Salt--Lime--Iron--Organic +Substances--Albumen, Fibrin, and Casein--The Fats or Oils--The Sugars, +Starch, and Gum--Stimulating Substances--Necessity of a Regulated Diet._ + +[Sidenote: 1. The term food? Source of food? Need of preparing food?] + +1. THE SOURCE OF FOOD.--The term _food_ includes all those substances, +whether liquid or solid, which are necessary for the nourishment of the +body. The original source of all food is the earth, which the poet has +fitly styled the "Mother of all living." In her bosom, and in the +atmosphere about her, are contained all the elements on which life depends. +But man is unable to obtain nourishment directly from such crude chemical +forms as he finds in the inorganic world. They must, with a few exceptions, +be prepared for his use, by being transformed into new and higher +combinations, more closely resembling the tissues of his own body. + +[Sidenote: 2. Usefulness and hurtfulness of plants? What then must man do? +Parts of the same plant or tree?] + +2. This transformation is effected, first, by the vegetable world. But all +plants are not alike useful to man; while some are absolutely hurtful. +Accordingly, he must learn to discriminate between that which is poisonous +and that which is life-supporting. Again, all parts of the same plant or +tree are not alike beneficial: in some, the fruit, in others, the leaves, +and in others, the seeds only are sufficiently refined for his use. These +he must learn to select; he must also learn the proper modes of preparing +each kind for his table, whether by cooking or other processes. {54} + +[Sidenote: 3. Certain forms of vegetable creation? Example of the bee? +Cattle? The inference?] + +3. Again, certain forms of the vegetable creation which are unfit, in their +crude state, for man's food, and which he rejects, are chosen as food by +some of the lower animals, and are, by them, made ready for his use. Thus +the bee takes the clover, that man cannot eat, and from it collects honey. +The cattle eat the husks of corn and the dried grass, that are by far too +coarse for man, and in their own flesh convert them into tissues closely +resembling his muscular tissue. In this way, by the aid of the transforming +processes of the vegetable and animal creations, the simple chemical +elements of the mineral kingdom are elaborated into our choice articles of +food. + +[Sidenote: 4. What classification? Define organic substances. Inorganic. +Organic, how spoken of? The inorganic? Water and salt?] + +4. INORGANIC SUBSTANCES.--The substances we use as food are classified as +_organic_ and _inorganic_. By organic substances are meant those derived +from living forms, such as vegetables and animals. Inorganic substances are +those simpler inanimate forms which belong to the mineral kingdom. The +former alone are commonly spoken of as food, but the latter enter very +largely into the constitution of the body, and must therefore be present in +our food. With the exception of two articles, water and common salt, these +substances only enter the system when blended with organic substances. + +[Sidenote: 5. Water in physiology? Where found? Computation? Water in the +teeth? Muscle, tendons, and ligaments? How ascertained? Water in the fluids +of the body? What is the advantage?] + +5. WATER.--Water, from a physiological point of view, is the most important +of all the articles of food. It is everywhere found in the body, even in +the bones and the teeth. It has been computed that as large a proportion as +two-thirds of the body is water. The teeth, the densest of the solids in +the human system, contain ten per cent. of water. The muscles, tendons, and +ligaments are more than half water; for it is found that they lose more +than half their {55} weight when dried with moderate heat. But it is in the +_fluids_ of the body that water is found most abundantly. It gives to them +the power of holding a great variety of substances in solution, and is the +great highway by which new supplies are conveyed to the point where they +are required, and by which old particles of matter, that have served their +uses, are brought to the outlets of the body to be thus removed from the +system. + +[Sidenote: 6. Length of time man can do without food or water? Give the +comparison? Bulk of drinks? Constituent of meats, etc.? Fruits?] + +6. Man can remain a longer time without solid food than without water. He +may be deprived of the former for ten to twelve hours without great +suffering, but deprivation of water for the same length of time will +produce both severe pain and great weakness. The food should contain not +less than two parts of water to one of solid nutriment. Water constitutes +the great bulk of all our drinks, and is also a large constituent of the +meats, vegetables, and fruits which come upon the table. Fruits, +especially, contain it in great abundance, and, in their proper season, +furnish most agreeable and refreshing supplies of the needed fluid. + +[Sidenote: 7. Salt, how obtained? Where found? In the human body? +Importance of salt? What else can you state of the value of salt?] + +7. COMMON SALT.--Salt, or sodium chloride, as an article of food, is +obtained chiefly from the mineral kingdom; although plants contain it in +small quantities, and it is also found in the tissues of nearly all animals +used as food. In the human body, it is an ingredient of all the solids and +fluids. The importance of salt to animal life in general, is shown by the +great appetite for it manifested by domestic animals, and also by the +habitual resort of herds of wild beasts to the "salt-licks" or springs. In +those parts of the world where salt is obtained with difficulty, man places +a very high price upon it. + +[Sidenote: 8. Experiments upon animals?] + +8. Experiments upon domestic animals show that the withdrawal of salt from +their food, not only makes their {56} hides rough and causes the hair to +fall out, but also interferes with the proper digestion of food. If it be +withheld persistently, they become entirely unable to appropriate +nourishment, and die of starvation. + +[Sidenote: 9. Salt, how taken into the system? Its use in cooking? +Consumption?] + +9. Salt is usually taken into the system in sufficient quantities in our +food. Even the water we drink often has traces of it. The habitual use of +much salt in cooking, or as a seasoning at the table, is not wise; and +while it may not lead to consumption, as some writers declare, it is a bad +habit in itself, and leads to the desire for other and more injurious +condiments. + +[Sidenote: 10. Lime in the bones? What does it impart? Chief ingredient of +the bones and teeth? Where else found?] + +10. LIME.--This is the mineral substance which we have spoken of before as +entering very largely into the composition of the bones. It is the +important element which gives solidity and permanence to the framework upon +which the body is built. Calcium tri-phosphate, or "bone-earth," is the +chief ingredient of the bones and teeth, but is found in the cartilages and +other parts of the body in smaller quantities. + +[Sidenote: 11. How does lime find its way into the body? Early life? Effect +of its derivation?] + +11. How does this substance find its way into the body? Meat, milk, and +other articles obtained from the animal kingdom contain it, and it is +abundantly stored away also in the grains from which our bread is made, in +wheat, rye, and Indian corn. In early life, while the body is growing, the +supplies of this substance should be carefully provided. The evil effects +of the deprivation of it are too often and painfully evident in the +softening of the bones, and in the predisposition to curvature of the +spine--deformities which are most deplorable and which continue through +life. + +[Sidenote: 12. Iron, its abundance and diffusion? Where found? What part of +the blood is it? How supplied to the system? In case of loss of blood or +wasting disease?] + +12. IRON.--This substance is probably the most abundant and widely diffused +of the metals. It is found in {57} most of the vegetables, and is a very +important component of animal tissues. It enters into the composition of +human blood in about one part per thousand. Ordinarily, the food conveys to +the system enough iron for its use, but it must sometimes be introduced +separately as a remedy, especially after great loss of blood, or after some +wasting disease. Under its influence the blood seems to be rapidly +restored, and a natural color of the lips and skin replaces the pallor +caused by disease. + +[Sidenote: 13. Soda, potash, and magnesia? How do they occur?] + +13. OTHER INORGANIC SUBSTANCES.--In addition to the substances mentioned, +the mineral kingdom supplies compounds of soda, potash, and magnesia, which +are essential for the use of the body. They occur in small quantities in +the body, and enter it in combination with the various articles of diet. + +[Sidenote: 14. Organic substances, whence derived? What do they comprise? +Groups?] + +14. ORGANIC SUBSTANCES.--These substances are derived from the vegetable +and animal creations. They comprise all those articles which are commonly +spoken of as "food," and which are essential to sustain the body in life +and strength. They are divided into three groups, namely: the Albuminoid +substances, the Fats, and Sugars. + +[Sidenote: 15. The Albuminoid class, includes what? These compounds +constitute what? The food? Their importance? Their properties?] + +15. THE ALBUMINOIDS.--This class includes three important nutritive +substances--(1) _Albumen_, which gives it its name; (2) _Fibrin_, including +_gluten_; and (3) _Casein_. These compounds constitute a large part of the +human body, and the food contains them in proportionally large quantities. +Their importance is so great, and the system so promptly suffers from their +absence, that they have been styled the "_nutritious_ substances." The +properties which they hold in common are, that they do not crystallize, and +have a jelly-like form, except when heat is applied to them, when they +harden, or _coagulate_. {58} + +[Sidenote: 16. Decomposition? Effect of cold? Illustrations? Elephants?] + +16. They likewise decompose, or _putrefy_, under the influence of warmth +and moisture. Hence the decay of all dead animal tissues. Cold arrests this +process. It is well known that milk, eggs, and the like, "keep" much longer +in winter than at other seasons. The bodies of elephants, caught in the ice +many hundred years ago, are occasionally borne by the icebergs to the coast +of Siberia, completely frozen, but preserved almost perfectly in form and +limb. + +[Sidenote: 17. In what substances does albumen exist? What further is said +of the egg?] + +17. ALBUMEN exists in milk, meat, the grains, and the juices of many +plants; but the purest form is obtained from the white of egg. When we +consider that an egg is composed chiefly of albumen and water--namely, six +parts in seven; and when we also consider the numerous, diverse, and +complex tissues--the muscles, bones, internal organs, bill, claws, and +feathers--with which the chick is equipped on leaving his shell, we are +impressed with the importance of these apparently simple constituents of +the food and body. + +[Sidenote: 18. Fibrin, gluten, clotting of the blood?] + +18. FIBRIN is derived from meats, and exists in the blood both of man and +the lower animals. _Gluten_, or vegetable fibrin, resembles closely true +fibrin, and is abundantly furnished in wheat and other grains from which +flour is commonly made. Animal fibrin coagulates spontaneously when it is +removed from the body, and thus causes the "clotting" of the blood. + +[Sidenote: 19. Casein? Its coagulation? Effect of rennet? Making of +cheese?] + +19. CASEIN is the curdy ingredient of milk, and a highly important +food-substance. Its coagulation in milk takes place not from heat, but by +the addition of an acid, and also when milk becomes sour from exposure to +the air. It is commonly effected, however, by introducing a piece of +_rennet_, a preparation made from a calf's stomach. The _curds_, or casein, +may then be separated from the _whey_, {59} and made into cheese, by +pressing it sufficiently to drive off the water. + +[Sidenote: 20. What are the fats? The oils? How supplied? How alike? +Emulsifying? Example? How do we know it?] + +20. THE FATS OR OILS.--This is the second group of organic foods. Those +which are more solid are called _fats_: the more fluid ones are the _oils_. +Oleaginous substances are supplied in both animal and vegetable food; but, +from whatever source derived, they are chemically much alike. They are +insoluble in water, and yet they unite readily with the watery fluids of +the body, and are by them conveyed to its various parts for their +nourishment. This is due to their property of "emulsifying;" that is, they +are held in suspension, in a finely divided state, in water. Ordinary milk +is an example of an _emulsion_. We know that it contains fat; for butter is +obtained from it, and, under the microscope, the minute oil-globules may be +distinctly seen. + +[Sidenote: 21. Whence are fatty articles of food derived?] + +21. In our country and climate, and also in colder climates, fatty articles +of food are principally derived from the animal creation, such as meat or +flesh, milk and butter. But most of the bread-stuffs contain more or less +fat or oil; Indian meal as much as nine parts in a hundred. + +[Sidenote: 22. Appetite of persons in cold climates? What do they require? +Upon what must they rely? Why? The Esquimaux? Laplander? Olive and palm?] + +22. Among persons living in cold climates, the appetite for oleaginous food +is especially eager; and they require large quantities of it to enable them +to resist the depressing influences of cold. Since vegetation is scanty and +innutritious, and the waters of the frozen regions abound in animal life, +they must rely wholly upon a diet derived from the latter source. The +Esquimaux consumes daily from ten to fifteen pounds of meat or blubber, a +large proportion of which is fat. The Laplander will drink train-oil, and +regards tallow-candles as a great delicacy. In hot climates, on the +contrary, where flourish the olive {60} and the palm, this kind of food may +be obtained from vegetable sources in abundant quantities. + +[Sidenote: 23. Which are the third of the organic groups? What do they +embrace? Points of resemblance?] + +23. THE SUGARS, OR THE SACCHARINE SUBSTANCES.--These constitute the third, +and last, group of the organic substances, which are employed as food. This +group embraces, in addition to the different kinds of _Sugar_, the +varieties of starch and gum, from whatever source derived. The two +substances last named do not, at first sight, present many points of +similarity to sugar; but they closely resemble it in respect to their +ultimate chemical composition, being made up of the same elements, in +nearly the same proportions. And their office in the system is the same, +since they are all changed into sugar by the processes of digestion. + +[Sidenote: 24. Origin of the sugars? Ordinary sugar? Beetroot? Maple-sugar? +Grape-sugar? Cane-sugar?] + +24. SUGAR is chiefly of vegetable origin; the animal varieties being +obtained from honey and milk. The most noticeable characteristic of this +substance is its agreeable, sweet taste, which makes it everywhere a +favorite article of food. But this quality of sweetness is not possessed by +all the varieties of sugar in the same degree; that obtained from milk, for +instance, has a comparatively feeble taste, but rather imparts a gritty +feeling to the tongue. The other important properties of sugar are, its +power to crystallize when evaporated from watery solutions, such as the +juices of many plants; a tendency to ferment, by which process alcohol is +produced; and a ready solubility in water. This latter quality renders it +very easy of digestion, and more so than any other of the saccharine group. +It is computed that the annual production of sugar, in all parts of the +world, is more than one million of tons. The kind of sugar that is in +ordinary use, in this country, is prepared from the juice of the +sugar-cane, which contains eighteen per cent. of sugar. In France it is +manufactured from the {61} beet root, which holds about nine per cent.; the +maple-tree of our climate yields a similar sugar. The sweet taste of fruits +is due to the presence of grape-sugar: the white grains seen on raisins +belong to this variety. Cane-sugar is more soluble than the latter, and has +twice the sweetening power. + +[Illustration: FIG. 15.--GRANULES OF POTATO STARCH.] + +[Sidenote: 25. Starch, how widely distributed? Its qualities? Its +constituents? Its solubility?] + +25. STARCH.--This is the most widely distributed of the vegetable +principles. It is tasteless, inodorous, and does not crystallize. It +consists of minute rounded granules, which, under the microscope, reveal a +somewhat uniform structure (Fig. 15). Starch will not dissolve in cold +water, but in boiling water the small grains burst open, and may then be +dissolved and digested. + +[Sidenote: 26. How much starch in bread-stuffs? In rice? Unripe fruits? +Ripe fruits?] + +26. The bread stuffs, wheat, corn, and rye flours, are more than one-half +starch. Rice, which is the "staff of life" to one-third of the human +family, contains eighty per cent. Unripe fruits have much starch in them, +which renders them indigestible when eaten uncooked; for the grains of raw +starch are but slightly acted upon within the body. But, under the potent +chemistry of the sun's ray, this crude material is converted into sugar. +Thus are the fruits prepared by the careful hand of Nature, so that when +ripe they may be freely used without further preparation. + +[Sidenote: 27. Gum, where found? Its composition? Gum Arabic?] + +27. GUM is commonly found in those articles which {62} also contain starch; +and has the same chemical composition as the latter, but is much less +nutritious. In the East, gum-arabic and similar substances are largely +employed as food. Persons who travel by caravan across vast, sandy deserts, +find such substances well adapted to their wants, since they are not +perishable, and are easily packed and carried. + +[Sidenote: 28. The three classes of food principles? What besides? What is +said of them? Name the articles not nutritious.] + +28. STIMULATING SUBSTANCES.--The three classes of food-principles already +considered--the Albuminoids, the Fats, and the Sugars--comprise all the +more important organic ingredients of our food. There are, besides, a great +variety of coloring and flavoring matters that stimulate or increase the +appetite for food by appealing to the eye and taste; but they are not +nutritious, and are quickly separated from the truly useful substances, and +do not long remain in the body. Among these may be classed spices, flavors +of fruits, tea, coffee, and vegetable acids. + +[Sidenote: 29. What is said of experiments that have been tried?] + +29. NECESSITY OF A REGULATED DIET.--A great variety of experiments have +been tried in order to test the relative value of the different nutritive +principles. They have been practised to some extent upon man, but chiefly +upon those inferior animals which require a similar diet to man. + +[Sidenote: 30. What has been demonstrated in the first place? Example? +Second demonstration? Example? Give the illustration in relation to +convertibility.] + +30. By this means it has been demonstrated that--first, when any one of +these substances is eaten exclusively, the body is imperfectly nourished, +and life is shortened. Dogs fed exclusively upon either albumen, fat, or +sugar, soon die of starvation. Second, a diet long deprived of either of +these principles, is a fertile cause of disease; for example, on +ship-board, where fresh vegetables are not dealt out for a long period, +_scurvy_ becomes prevalent among the sailors. They are, however, to a +certain extent mutually convertible, and thus the missing article is +indirectly supplied. For {63} instance, sugar changes to fat in the body; +and hence, as is well known, the "hands" on a sugar plantation grow fat +during the sugar season, by partaking freely of the ripened juices of the +cane. + +31. That is the best diet therefore which contains some of each of these +principles, in due proportion; and that is the worst which excludes the +most of them. The cravings and experience of man had unerringly guided him +to a correct regulation of his diet, long before the chemistry of food was +understood; so that his ordinary meals long ago combined these various +principles, the necessity and value of which are now explained. + +QUESTIONS FOR TOPICAL REVIEW. + + PAGE + 1. What is understood by the term food? 53 + 2. What can you state in relation to the source of food? 53 + 3. What discriminations and selections are necessary? 53, 54 + 4. How can you tell the organic from the inorganic substances? 54 + 5. What relative position does water hold as an article of food? 54 + 6. In what parts of the body is water found? 54, 55 + 7. In what articles that we eat is it found? 55 + 8. If you were required to go without water or solid food for a + number of days, which would you prefer to have, and why? 55 + 9. What can you state of the importance of salt as an article + of food? 55, 56 + 10. How abundant is salt, and how does it find its way into the + human system? 55, 56 + 11. What can you state of the importance of lime in the body? 56 + 12. What, of the importance of iron? 56, 57 + 13. What further is stated of other inorganic substances? 57 + 14. What in relation to organic substances? 57 + 15. What can you state in relation to the albuminoids? 57, 58 + 16. What, in relation to albumen? 58 + 17. What, in relation to casein? 58, 59 + 18. In relation to the fats or oils, and how generally consumed? 59, 60 + 19. What do we understand by the sugars or saccharine substances? 60 + 20. State what you can of sugar--its origin and various qualities. 60, 61 + 21. Of starch--its varieties and qualities. 61 + 22. Of the abundance of starch, and its importance as a food + principle. 61 + 23. What is stated in relation to stimulating substances? 62 + 24. Of the necessity for regulation in diet? 62, 63 + + * * * * * + + +{64} + +CHAPTER V. + +FOOD AND DRINK. + +_Necessity for Food--Waste and Repair--Hunger and Thirst--Amount of +Food--Renovation of the Body--Mixed +Diet--Milk--Eggs--Meat--Cooking--Vegetable Food--Bread--The +Potato--Fruits--Purity of Water--Action of Water upon Lead--Coffee, Tea, +and Chocolate--Effects of Alcohol._ + +[Sidenote: 1. What follow activity? Examples? Necessity for food?] + +1. NECESSITY FOR FOOD.--Activity is everywhere followed by waste. The +engine uses up coal and water to produce motion, the stream wears away its +bank, the growing corn-blade draws tribute from the soil. When the human +body acts, and it is always in action during life, some of its particles +are worn out and thrown off. This waste must constantly be repaired, or the +body suffers. In this fact is seen the necessity for food. The particles, +thus worn out, being henceforth useless, are removed from the body. Our +_food_ and _drink_ are rapidly transformed into a new supply of living, +useful material, to be in turn used up and replaced by a fresher supply. + +[Sidenote: 2. Give the theory in relation to waste and repair.] + +2. WASTE AND REPAIR.--In this way the healthful body, though always +wasting, is always building up, and does not greatly change in size, form, +or weight. At two periods of life the processes of waste and repair are not +exactly balanced. In early life the process of building up is more active, +and in consequence the form is plump, and the stature increases. Repair now +exceeds waste. On the other hand, when old age comes on, the wasting +process is more active, the flesh and weight diminish, the skin falls in +wrinkles, and the senses become dull. Only during the prime of life--from +about twenty to sixty years of age--is the balance exact between loss and +gain. {65} + +[Sidenote: 3. System deprived of food? Warning? What is the pain? How +proved?] + +3. HUNGER AND THIRST.--When the system is deprived of its supply of solid +food during a longer time than usual, nature gives warning by the sensation +of hunger, to repair the losses that have taken place. This sensation or +pain appears to be located in the stomach, but it is really a distress of +the system at large. Let a sufficient quantity of nourishment be introduced +into the system in any other way than by the mouth, and it will appease +hunger just as certainly as when taken in the usual manner. + +[Sidenote: 4. Feeling of thirst? Seat of the pain? How proved? Time a +person can exist without food?] + +4. The feeling of thirst, in like manner, is evidence that the system is +suffering from the want of water. The apparent seat of the distress of +thirst is in the throat; but the injection of water into the blood-vessels +is found to quench thirst, and by the immersion of the body in water, the +skin will absorb sufficient to satisfy the demands of the system. The +length of time that man can exist without food or drink is estimated to be +about seven days. If water alone be supplied, life will last much longer; +there being cases recorded where men have lived twenty days and over, +without taking any solid food. + +[Sidenote: 5. Amount of food required? The young and others? Those living +in hot and cold climates? Habits?] + +5. QUANTITY OF FOOD.--The quantity of food required varies greatly, +according to the individual and his mode of life. The young, and others who +lead active lives, or who live in the open air, require more food than the +old, the inactive, or the sedentary. Those who live in cold regions require +more than the inhabitants of hot climates. Habit, also, has much to do with +the quantity of food required. Some habitually eat and drink more than they +actually need, while a few eat less than they should. + +[Sidenote: 6. Quantity of food daily? How divided? Compare with the weight +of the body?] + +6. The average daily quantity of food and drink for a healthy man of active +habits is estimated at six pounds. This amount may be divided in about the +following {66} proportions: the mineral kingdom furnishes three and +one-half pounds, including water and salt; the vegetable kingdom, one and +one-half pounds, including bread, vegetables, and fruits; the animal +kingdom, one pound, comprising meat, eggs, butter, and the like. This +quantity is about one twenty-fourth the weight of the body, as it is +generally computed; the average weight of an adult man being placed at 140 +pounds. A man, therefore, consumes an amount of solid and liquid nutriment +every twenty-four days equal in weight to that of his body, a corresponding +amount being _excreted_, or removed from the system in the same time. + +[Sidenote: 7. How often then might the body be renewed? Why is it not? +Opinion? How correct? What further is stated?] + +7. RENOVATION OF THE BODY.--By this process, so far as weight is concerned, +the body might be renewed every twenty-four days; but these pounds of food +are not all real nutriment. A considerable portion of that which we eat is +innutritious, and though useful in various ways, is not destined to repair +the losses of the system. An opinion has prevailed that the body is renewed +throughout once in seven years; how correct this may be it is not easy to +decide, but probably the renovation of the body takes place in a much +shorter period. Some parts are very frequently renewed, the nutritive +fluids changing more or less completely, several times during the day. The +muscles, and other parts in frequent exercise, change often during a year; +the bones not so often, and the enamel of the teeth probably never changes +after being once fully formed. + +[Sidenote: 8. Habits of nations? Give the different cases.] + +8. MIXED DIET.--The habits of different nations in respect to diet exhibit +the widest and strangest diversity. The civilized, cook their food, while +savages often eat it in a raw state. Some prefer it when fresh, others +allow it to remain until it has become tainted with decay. Those dwelling +in the far north subsist almost wholly on {67} animal food, while those +living in hot climates have bountiful supplies of delicious fruits with +which to satisfy all their bodily wants. One race subsists upon the banana, +another upon the blubber of seals. In temperate climates, a diet composed +partly of vegetable and partly of animal food is preferred. + +[Sidenote: 9. The point to consider? Vegetable diet? Louis Cornaro? John +the Baptist?] + +9. The important point to consider is, however, not one of origin, but +whether the chemical principles (mentioned in the last chapter) enter into +the composition of the diet. A purely vegetable diet may be selected which +would contain all the principles necessary to sustain life. It is recorded +of Louis Cornaro, a Venetian noble, that he supported himself comfortably +for fifty-eight years on a daily allowance of twelve ounces of vegetable +food, and about a pint of light wine. On the other hand, the food of John +the Baptist, consisting of "locusts and wild honey," is an example of the +sustaining power of a diet chiefly animal in its origin. + +[Sidenote: 10. What has been found in our climate? Exclusive vegetable +diet?] + +10. In our climate, those who lead active lives crave an allowance of +animal food; and it has been found by experience that with it they can +accomplish more work and are less subject to fatigue, than without it. +Among nations where an exclusively vegetable diet is employed, indigestion +is a disorder especially prevalent. + +[Sidenote: 11. Necessity for change in diet? Continuous use of the same +diet? Exception? Why? Too rich diet? Horses?] + +11. The necessity for occasionally changing or varying the diet, is seen in +the fact that no single article comprises all the necessary principles of +food, and that the continuous use of any one diet, whether salt or fresh, +is followed by defective nutrition and disease. There is one exception to +this rule: in infancy, milk alone is best calculated to support life; for +then the digestive powers are incompletely developed, and the food must be +presented in the simplest form possible. It should also be remembered {68} +that too rich diet is injurious, just as truly as one that is inadequate. +When the food of horses is too nutritious, instinct leads them to gnaw the +wood-work of their mangers. + +[Sidenote: 12. Milk as a model food? Cow's milk? The constituents when +separated?] + +12. DIFFERENT ARTICLES OF DIET--MILK.--Milk is the earliest nutriment of +the human race, and in the selection and arrangement of its constituents, +may be regarded as a model food, no other single article being capable of +sustaining life so long. Cow's milk holds casein, one of the albuminoids, +about five parts in one hundred; a fatty principle, when separated, known +as butter, about four parts; sugar of milk four parts; water and salts +eighty-seven parts. The casein and fatty substance are far more digestible +in milk, than after they have been separated from it in the form of cheese +and butter. + +[Sidenote: 13. Milk as a beverage? Milk sold in cities? How to detect the +cheat?] + +13. Since milk, in itself, is so rich an article of food, the use of it as +a beverage is unwise, unless the quantity of the other articles consumed be +reduced at the same time. The milk sold in cities is apt to be diluted with +water. The way to detect the cheat is by testing the specific gravity of +the article. Good milk is about 1030; skimmed milk 1035; but milk diluted +one-fifth is 1024. An instrument called the lactometer is also used, by +which the amount of cream present is ascertained. + +[Sidenote: 14. Composition of eggs? Yolk? How should eggs be eaten? Why? +How boiled? Why?] + +14. EGGS.--The egg is about two-thirds water, the balance is pure albumen +and fat in nearly equal proportions. The fat is in the yolk, and gives it +its yellow color. Eggs contain none of the sugar-principles, and should be +eaten with bread or vegetables that contain them. Soft-boiled eggs are more +wholesome than those which are hard-boiled or fried, as the latter require +longer time to digest. + +[Sidenote: 15. Meats, whence derived? Why important? Flesh of young +animals?] + +15. MEATS.--The meats, so called, are derived from the muscular parts of +various animals. They are most {69} important articles of food for adults, +inasmuch as they are richly stored with albuminoid substances, and contain +more or less fat. Such food is very nourishing and easily digested if eaten +when fresh,--veal and pork being exceptions. The flesh of young animals is +more tender and, in general, more digestible than that of older ones. All +meat is more tough immediately after the killing of the animal, but +improves by being kept a certain length of time. + +[Sidenote: 16. Preference of persons? Venison? Mutton? Cheese? Uncooked +flesh?] + +16. Some persons prefer flesh that has begun to show signs of +decomposition, or is unmistakably putrid. By some, venison is not +considered to have its proper flavor until it is tainted. In England, +people prefer mutton that is in a similar condition, just as on the +continent of Europe many delight in cheese that is in a state of +decomposition. In certain less civilized countries flesh is not only eaten +uncooked, but in a mouldy, rotten condition. The use of such food is not +always immediately injurious, but it predisposes to certain diseases, as +indigestion and fevers. + +[Sidenote: 17. Cold as a preserver? Meat in Russia? Beef and pork, how +preserved? Salted meat as food? Scurvy?] + +17. Cold is one means of preserving meat from decay. In the markets of +northern Russia, the frozen carcases of animals stand exposed for sale in +the winter air for a considerable time, and are sawn in pieces, like sticks +of wood, as the purchases are made; such meat, when thawed, being entirely +fit for food. Beef and pork are preserved by salting down in brine, and in +this condition may be carried on long voyages or kept for future use. +Salted meat is not as nutritious as fresh, since the brine absorbs its rich +juices and hardens its fibres. Long continued use of salt meats, without +fresh vegetables, gives rise to the disease called scurvy, formerly very +prevalent on shipboard and in prisons; but now scarcely known. + +[Sidenote: 18. The antiquity of the custom of cooking food? Object of +cooking? The oyster? Raw meat as an occasional food?] + +18. COOKING.--The preparation of food by the agency {70} of fire is of +almost universal practice, even among the rudest nations. The object of +cooking is to render food more easy of digestion by softening it, to +develop its flavor, and to raise its temperature more nearly to that of the +body. A few articles of flesh-food are eaten uncooked in civilized lands, +the oyster being an instance. Raw meat is occasionally eaten by invalids +with weak digestive powers, and by men training for athletic contests. + +[Sidenote: 19. Effect of boiling meat? How may the cooking be done? The +proper method? Effect? Making of soup?] + +19. In boiling meat, the water in which it is placed tends to dissolve its +nutrient juices. In fact, the cooking may be so conducted as to rob the +meat of its nourishment, its tenderness, and even of its flavor. The proper +method, in order to preserve or promote these qualities, is to place the +meat in boiling water, which, after a few minutes, should be reduced in +temperature. In this way the intense heat, at first, coagulates the +exterior layers of albumen, and imprisons the delicate juices; after that, +moderate heat best softens it throughout. When soup is to be made, an +opposite course should be pursued; for then the object is to extract the +juices and reject the fibre. Meat, for such purpose, should be cut in small +pieces and put into cold water, which should then be gradually raised to +boiling heat. + +[Sidenote: 20. Roasting? How should it be done? Give the philosophy of the +process. Frying?] + +20. Roasting is probably the best method of cooking meat, especially +"joints" or large pieces, as by this process the meat is cooked in its own +juices. Roasting should begin with intense heat, and be continued at a +moderate temperature, in order to prevent the drying out of the nutritious +juices, as by this process an outer coating or crust of coagulated albumen +is formed. During this process the meat loses one-fourth of its weight, but +the loss is almost wholly water, evaporated by the heat. Too {71} intense +or prolonged heat will dry the meat, or burn it. Frying is the worst +possible method, as the heated fat, by penetrating the meat, or other +article placed in it, dries and hardens it, and thus renders it +indigestible. + +[Sidenote: 21. What is "Trichina?" How guarded against?] + +21. TRICHINA.--It should be remembered that ham, sausages, and other forms +of pork, should never be eaten in a raw or imperfectly cooked condition. +The muscle of the pig is often infested by a minute animal parasite, or +worm, called _trichina spiralis_. This worm may be introduced alive into +the human body in pork food, where it multiplies with great rapidity, and +gives rise to a painful and serious disease. This disease has been +prevalent in Germany, and cases of it occur from time to time in this +country. + +[Sidenote: 22. What part of fish is eaten? What does it resemble? Fish as +food for digestion? Fish as a diet?] + +22. FISH.--The part of fish that is eaten is the muscle, just as in the +case of the meats and poultry. It closely resembles flesh in its +composition, but is more watery. Some varieties are very easy of digestion, +such as salmon, trout, and cod; others are quite indigestible, especially +lobsters, clams, and shell-fish generally. A diet in which fish enters as +the chief article, is ill adapted to strengthen mind or body, while its +continued use is said to be the fertile source of nearly every form of +disease of the skin. Some persons are so constituted that they can eat no +kind of fish without experiencing unpleasant results. + +[Sidenote: 23. List of vegetable articles? Usefulness of the different +vegetables? Strychnia? What further is said in relation to the nourishing +and other qualities of vegetables?] + +23. VEGETABLE FOOD.--The list of vegetable articles of diet is a very long +one, including the grains from which our bread-stuffs are made, the +vegetables from the garden, and the fruits. All the products of the +vegetable kingdom are not alike useful. Some are positively hurtful; +indeed, the most virulent poisons, as strychnia and prussic acid, are +obtained from certain vegetables. Again, of such {72} articles as have been +found good for food, some are more nourishing than others: some require +very little preparation for use, while others are hard and indigestible, +and can only be used after undergoing many preparatory processes. Great +care must therefore be exercised, and many experiments made, before we can +arrive at a complete knowledge in reference to these articles of diet. Tea, +coffee, and other substances from which drinks are made, are of vegetable +origin. + +[Sidenote: 24. Wheat? "Staff of life?" White flour? Hard-grain wheats? +Bolting? Graham bread?] + +24. BREAD.--Wheat is the principal and most valuable kind of grain for the +service of man. Bread made from wheat-flour has been in use for many +hundreds of years, and on this account, as well as because of its highly +nourishing properties, has been aptly called "the staff of life." We never +become tired of good bread as an article of daily food. + +The white kinds of flour contain more starch and less gluten than the +darker, and are therefore less nutritious. The hard-grain wheat yields the +best flour. In grinding wheat, the chaff or bran is separated by a process +called "bolting." Unbolted flour is used for making brown or Graham bread. + +[Sidenote: 25. Leavened bread? Unleavened? Hot bread?] + +25. The form of bread most easily digested is that which has been +"leavened," or rendered porous by the use of yeast, or by some similar +method. Unleavened bread requires much more mastication. Hot bread is +unwholesome, because it is not firm enough to be thoroughly masticated, but +is converted into a pasty, heavy mass that is not easily digested. + +[Sidenote: 26. Wheaten bread? Bread and butter? Experiment on the dog?] + +26. Wheaten bread contains nearly every principle requisite for sustaining +life, except fat. This is commonly added in other articles of diet, +especially in butter,--"bread and butter," consequently, forming an almost +perfect article of {73} food. The following experiment is recorded: "A dog +eating _ad libitum_ of white bread, made of pure wheat, and freely supplied +with water, did not live beyond fifty days. He died at the end of that time +with all the signs of gradual exhaustion." Death took place, not because +there was anything hurtful in the bread, but because of the absence of one +or more of the food-principles. + +[Sidenote: 27. State what is said of the Irish potato?] + +27. THE POTATO.--The common or Irish potato is the vegetable most +extensively used in this country and Great Britain. Among the poorer +classes in Ireland it is the main article of food. While it is not so rich +in nutritious substances as many others, it has some very useful qualities. +It keeps well from season to season, and men do not weary of its continuous +use. It is more than two-thirds water, the balance being chiefly starch, +with a little albumen. + +[Sidenote: 28. Sweet potato? Nightshades? Potatoes when germinating?] + +28. The sweet potato differs from the white or common, in containing more +water and a small proportion of sugar. The common potato and the tomato +belong to the same botanical order as the "nightshades," but do not possess +their poisonous qualities, unless we except potatoes that are in the +process of germination or sprouting, when they are found injurious as food. + +[Sidenote: 29. Fruits? Use of ripe fruit? Nutriment they contain? Starch in +unripe fruits? Cooking of unripe fruits?] + +29. FRUITS.--These are produced, in this country, in great abundance, and +are remarkable alike for their variety and delicious flavor; consequently +they are consumed in large quantities, especially during the warmer months. +The moderate use of ripe fruits, in their season, is beneficial, because +they offer a pleasant substitute for the more concentrated diet that is +used in cold weather. The amount of solid nutriment they contain is, +however, small. The percentage of water in cherries is seventy-five, in +grapes eighty-one, in apples eighty-two. Unripe fruits contain starch, +which, during the process of ripening, {74} is converted into sugar. Such +fruits are indigestible and should be avoided: cooking, however, in part +removes the objections to them. + +[Sidenote: 30. How should drinking-water be as regards color and smell? +Chemically pure water? How obtained? Agreeableness of perfectly pure +water?] + +30. PURE WATER.--It is important that the water we drink and use in the +preparation of food should be pure. It should be clear and colorless, with +little or no taste or smell, and free from any great amount of foreign +ingredients. Chemically pure water does not occur in nature: it is only +obtained by the condensation of steam, carefully conducted, and is not as +agreeable for drinking purposes as the water furnished by springs and +streams. Rain-water is the purest occurring in nature; but even this +contains certain impurities, especially the portion which falls in the +early part of a shower; for in its descent from the clouds, the particles +floating in the air are caught by the falling drops. + +[Sidenote: 31. Spring and well water? Whence the sparkle, or life? The +water supply of cities? Croton water? Ridgewood?] + +31. Water from springs and wells always contains more or less foreign +matter of mineral origin. This imparts to the drink its pleasant taste--the +sparkle, or "life," coming from the gases absorbed by the water during its +passage under ground. The ordinary supply of cities is from some pure +stream or pond conveyed from a distance through pipes, the limpid fluid +containing generally only a small amount of impurity. Croton water, the +supply of New York City, is very pure, and contains only four and a half +grains to a gallon: the Ridgewood water of Brooklyn holds even less foreign +matter. + +[Sidenote: 32. Impurities in drinking-water? Mineral springs?] + +32. Drinking-water may contain as large a proportion as sixty to seventy +grains per gallon of impurity, but a much larger quantity renders it +unwholesome. The mineral spring waters, used popularly as medicines, are +highly charged with mineral substances. Some of them, such as {75} the +waters at Saratoga, contain three hundred grains and more to the gallon. + +[Sidenote: 33. What is stated of the action of water upon lead?] + +33. ACTION OF WATER UPON LEAD.--The danger of using water that has been in +contact with certain metals is well known. Lead is one of the most readily +soluble, and probably the most poisonous of these substances in common use. +When pure water and an untarnished surface of lead come in contact, the +water gradually corrodes the metal, and soon holds an appreciable quantity +of it in solution. When this takes place the water becomes highly +injurious: the purer the water, and the more recent the use of the metal, +the greater will be the danger. + +[Sidenote: 34. Lead in pipes and other things? Advice? What takes place +after the articles of lead have been used much? What is wise?] + +34. In cities, lead pipes are commonly used to convey water through the +houses; lead being also used in the construction of roofs, cisterns, and +vessels for keeping water and other liquids. After the articles of lead +have been in use several months, the danger of lead-poisoning diminishes. +An insoluble coating of the sulphate of lead forms upon the exposed +surface, thus protecting it from further corrosion. It is, however, a wise +precaution, at all times to reject the water or other fluid that has been +in contact with leaden vessels over night, or for a number of hours. Allow +the water in pipes to run freely before using. + +[Sidenote: 35. Coffee as an article of diet? Of what does it consist? How +does the water affect the coffee? The peculiar stimulant? How does it +affect most persons?] + +35. COFFEE.--This is an important addition to diet, and if moderately used +is beneficial to persons of adult age. As commonly employed, it consists of +an infusion in boiling water of the roasted and ground berry. The water +extracts certain flavoring and coloring matters, but that which gives it +its peculiar stimulant qualities is the alkaloid _caffein_. With most +persons its action is that of a gentle stimulant, without any injurious +reaction. It produces a restful feeling after exhausting efforts of mind or +{76} body; it tranquilizes, but does not disqualify for labor; and hence it +is highly esteemed by persons of literary pursuits. + +[Sidenote: 36. Another property of coffee? Miners of Belgium? The Caravans? +Among armies? Taken with meals?] + +36. Another property of coffee is, that it diminishes the waste of the +tissues, and consequently permits the performance of excessive labor upon +an economical and inadequate diet. This has been tested among the miners of +Belgium. Their allowance of solid food was below that found necessary in +prisons and elsewhere; but, with the addition of about four pints of coffee +daily, they were enabled to undergo severe labor without reducing their +muscular strength. The caravans which traverse the deserts are supported by +coffee during long journeys and lengthened privation of food. Among armies +it is indispensable in supplementing their imperfect rations, and in +relieving the sense of fatigue after great exposure and long marches. When +taken with meals, coffee is also thought to promote digestion. + +[Sidenote: 37. Effects of tea-drinking? Peculiar principle? The tea +beverage, how made? Black and green tea? Excessive use of tea or coffee?] + +37. TEA.--The effects of tea-drinking are very similar to those of coffee, +and are due to a peculiar principle called _thein_. This principle is +probably the same as that found in coffee, _caffein_, since the chemical +composition of both is precisely alike. Tea, as a beverage, is made from +the dried leaves of the plant by the addition of hot water; if the tea is +boiled, the oil which gives it its agreeable flavor is driven off with the +steam. There are two kinds of tea--the black and the green: the latter is +sometimes injurious, producing wakefulness and other nervous symptoms. The +excessive use of either coffee or tea will cause wakefulness. + +[Sidenote: 38. Experiments made during Kane's expedition?] + +38. During Dr. Kane's expedition in the Arctic regions, the effects of +these articles were compared. "After {77} repeated trials, the men took +most kindly to coffee in the morning and tea in the evening. The coffee +seemed to continue its influence throughout the day, and they seemed to +grow hungry less rapidly than after drinking tea, while tea soothed them +after a day's hard labor, and the better enabled them to sleep. They both +operated upon fatigued men like a charm, and their superiority over +alcoholic stimulants was very decided." + +[Sidenote: 39. State what is said of chocolate.] + +39. Chocolate is made from the seeds of the cocoa-tree, a native of +tropical America. Its effects resemble somewhat those of tea and coffee, +but it is very rich in nutriment. Linnaeus, the botanist, was so fond of +this beverage, that he gave to the cocoa-tree the name, _Theobroma_--"the +Food of the Gods." Its active principle is _theobromin_. + +[Sidenote: 40. Use of alcoholic drinks, how general? The rule given?] + +40. ALCOHOL.--The list of beverages that are consumed for the sake of the +alcohol they contain is a very long one. Their use is almost universally +prevalent, every civilized nation, and nearly every barbarous one, having +its favorite alcoholic drink; and, as a general rule, the nations which +stand the highest in civilization have the greatest varieties of these +beverages,--at the same time using them the most intelligently and wisely. + +[Sidenote: 41. The beverages produced by fermentation? The ardent spirits? +Grains and fruits employed? Long use of wine? Of distilled liquors?] + +41. The wines and malt liquors that contain a small amount of alcohol are +produced by fermentation. The beverages that hold a large proportion of +alcohol, the "ardent spirits," are made by distillation. Enormous +quantities of grains and fruits are thus yearly diverted from their proper +uses as food; some of these being corn, wheat, rye, barley, potatoes, and +rice; also the grape, apple, pear, peach, sugar-cane, cherry, fig, and +orange. Wine, the fermented juice of the grape, has been in use from time +immemorial, while the introduction of distilled liquors dates from a +comparatively recent period. {78} + +[Sidenote: 42. Describe the action of alcohol upon the human system? +Experience of Dr. Hayes and others?] + +42. What is the physiological action of alcohol? Its first and most evident +action is stimulation: this effect is transient, and is followed by a +variable degree of depression. At first it sharpens the appetite and +quickens digestion, but its habitual use impairs both. This stimulation is +efficient in giving the system an artificial strength during some temporary +derangement, and in enabling the endurance of unusual fatigue or exposure. +The experience of Dr. Hayes, and other explorers of the polar regions, is +that alcohol does not enable the body to resist the influence of cold, but, +on the contrary, is always injurious. + +[Sidenote: 43. Another property of alcohol? How do we explain the +restorative influence of wines and liquors?] + +43. Another property it has in common with tea and coffee. It supports the +powers of life, economizes food, and retards the waste of tissues; in other +words, it "banks the fires," and prevents their burning wastefully. On this +principle we explain the restorative influence of wines or liquors during +exhausting diseases, in convalescence, and after excessive labors of mind +or body. + +[Sidenote: 44. Alcohol, a poison? Moderate stimulants? Prevailing opinion? +Hence?] + +44. Pure alcohol, or an excessive quantity of ardent spirits, is an +undoubted poison, and has been frequently known to produce fatal results. +Stimulants in moderate quantities have been thought to increase strength, +and in this view they have been called "alcoholic foods." This is not now +conceded by scientific men. The prevailing opinion is, that they serve no +useful purpose as an article of diet, and that their beneficial influence +is limited to cases where the system is enfeebled, where some unnatural +demand is made upon the vital powers, or where the supply of food is +insufficient. Hence, while alcohol has not the power to build up, it may +obstruct the process of pulling down. {79} + +QUESTIONS FOR TOPICAL REVIEW. + + PAGE + 1. How is the necessity for food shown? 64 + 2. To what process of waste and repair is the body constantly + subjected? 64 + 3. How do you account for the sensations of hunger and thirst? 65 + 4. What further can you state having relation to the subject? 65 + 5. What can you state in regard to the quantity of food required + for the support of life? 65, 66 + 6. What circumstances change the needs of persons, old and young, + as regards food and drink? 65, 67 + 7. What becomes of all the food and drink we consume? 66 + 8. What further can you state in relation to the process of + renovation through which the body passes? 66 + 9. What can you state of the habits of nations in respect to + diet? 66, 67, 69 + 10. What in relation to the selection of articles for food? 67 + 11. What as respects the necessity for changing or varying the diet? 67 + 12. What has been proved as regards animal food? 67 + 13. Of what importance is milk as an article of food? 67, 68 + 14. What are the constituents of milk? 68 + 15. What can you state of eggs as an article of food? 68 + 16. Of the meats, so called, as an article of food? 68, 69 + 17. What effect does cold have upon meats? 69 + 18. In what other way may beef and pork be preserved? 69 + 19. What can you state of salted meat as food, and of its + continued use? 69 + 20. What change does meat undergo in the cooking? 70, 71 + 21. What directions are given for boiling meat? 70 + 22. What for roasting, and with what results? 70, 71 + 23. What is said about the frying of meats? 71 + 24. Give the statement in relation to trichina. 71 + 25. State what is said in relation to fish. 71 + 26. What is stated of the usefulness and other properties of the + products of the vegetable kingdom? 71, 72 + 27. What further is said of vegetable food? 71, 72 + 28. Why is bread made of wheat flour so important as an article + of food? 72 + 29. State whatever else you can in relation to bread. 72, 73 + 30. Give the statement respecting the potato. 73 + 31. What is stated of fruits, the use of them, their nutritious + qualities, etc.? 73, 74 + 32. How general is the existence of perfectly pure water? 74 + 33. What is stated in relation to drinking water? 74, 75 + 34. How does the action of water upon lead affect lead? 75 + 35. What further can you state on the subject? 75 + 36. What properties has coffee as an article of diet? 75, 76 + 37. In what circumstances has coffee been found peculiarly + beneficial? 76 + 38. What comparison is made between coffee, tea, and chocolate? 76 + 39. How are the wines, and malt and other alcoholic beverages + produced? 77 + 40. What articles are employed in their production? 77 + 41. Describe the physiological action of alcohol. 78 + 42. What comparison is made between tea, coffee, and alcohol? 78 + 43. What can you state of alcohol, as a poison, a stimulant, and + article of diet? 78 + 44. What, then, can be said of alcohol as a recommendation? 78 + + * * * * * + + +{80} + +CHAPTER VI. + +DIGESTION. + +_The Principal Processes of Nutrition--The General Plan of +Digestion--Mastication--The Teeth--Preservation of the +Teeth--Insalivation--The Stomach and the Gastric Juice--The Movements of +the Stomach--Gastric Digestion--The Intestines--The Bile and Pancreatic +Juice--Intestinal Digestion--Absorption by means of Blood-vessels and +Lacteals--The Lymphatic or Absorbent System--The Lymph--Conditions which +affect Digestion--The Quality, Quantity, and Temperature of the Food--The +Influence of Exercise and Sleep._ + +[Sidenote: 1. Design of food? How accomplished?] + +1. NUTRITION.--The great design of food is to give _nutriment_ or +nourishment to the body. But this is not accomplished directly, as the food +must first pass through certain preparatory changes, as follows: (1), +_Digestion_, by which the food is reduced to a soluble condition; (2), +_Absorption_, by which, when digested, it is imbibed into the blood; (3), +_Circulation_, which carries the enriched blood to the various parts of the +system; and (4), _Assimilation_, by which each tissue derives from the +blood the materials necessary for its support. + +[Sidenote: 2. Sustaining power of food? Simile of the engine? Operation in +the human body?] + +2. By these means the sustaining power of food is gradually developed and +employed, and the vital machinery kept in working order, somewhat after the +manner of the steam-engine. To operate the latter, the force imprisoned +within the coal and water is set free and converted into motion by the +combustion of the fuel and the vaporization of the water. It will be seen, +however, when we come to study these operations in the human body, that +they are conducted silently and harmoniously, with marvellous delicacy and +completeness, and without that friction, and {81} consequent loss of power, +which attend the working of the most perfect machinery of man's invention. + +[Sidenote: 3. Change of food in digestion? Process of digestion? Describe +the alimentary canal.] + +3. GENERAL PLAN OF DIGESTION.--The great change which food undergoes in +digestion is essentially a reforming process, reducing articles of diet, +which are at first more or less solid, crude, and coarse, to a liquid and +finely comminuted condition, suitable for absorption into the blood. The +entire process of digestion takes place in what is called the alimentary +canal, a narrow, tortuous tube, about thirty feet in its entire length. +This canal begins in the mouth, extends thence downward through the gullet +to the stomach (a receptacle in which the principal work of digestion is +performed), and thence onward through the small and large intestines. + +[Illustration: FIG. 16.--SECTION OF THE TRUNK SHOWING THE CAVITIES OF THE +CHEST AND ABDOMEN. + +A, Cavity of Chest; B, Diaphragm; C, Abdomen; D, E, Spinal Column.] + +[Sidenote: 4. Situation of the stomach and intestines? Action of the food? +Mechanical action? Chemical?] + +4. The stomach and intestines are situated in the cavity of the abdomen +(Fig. 16, C, and Fig. 22), and occupy about two-thirds of its space. The +action to which the food is subjected in these organs is of two +kinds--mechanical and chemical. By the former it is crushed, agitated, and +carried onward from one point to another; by the latter it is changed in +form through the solvent power of the various digestive juices. {82} + +[Sidenote: 5. Describe the process of mastication? How many and what +movements?] + +5. MASTICATION.--As soon as solid food is taken into the mouth, it +undergoes mastication, or chewing. It is caught between the opposite +surfaces of the teeth, and by them is cut and crushed into very small +fragments. In the movements of chewing, the lower jaw plays the chief part; +the upper jaw, having almost no motion, acts simply as a point of +resistance, to meet the action of the former. These movements of the lower +jaw are of three sorts: a vertical or cutting, a lateral or grinding, and a +_to-and-fro_ or gnawing motion. + +[Illustration: FIG. 17.--SECTION OF A TOOTH. + +_a_, Enamel; _b_, Cavity; _c c_, Roots; _d_, Body of the Tooth.] + +[Sidenote: 6. Composition of the teeth? Enamel of the teeth? Interior of +teeth?] + +6. The teeth are composed of a bone-like material, and are held in place by +roots running deeply into the jaw. The exposed portion, or "crown," is +protected by a thin layer of enamel (Fig. 17, _a_), the hardest substance +in the body, and, like flint, is capable of striking fire with steel. In +the interior of each tooth is a cavity, containing blood-vessels and a +nerve, which enter it through a minute opening at the point of the root +(Fig. 19). + +[Sidenote: 7. The milk teeth? The permanent teeth?] + +7. There are two sets of teeth; first, those belonging to the earlier years +of childhood, called the milk teeth, which are twenty in number and small. +At six or eight years of age, when the jaw expands, and when the growing +body requires a more powerful and numerous set, the roots of {83} the milk +teeth are absorbed, and the latter are "shed," or fall out, one after +another (Fig. 18), to make room for the permanent set. + +[Illustration: FIG. 18.--SECTION OF THE JAWS. + +1' 2' 3' 4' 5', The Milk Teeth; 1" to 8", The Germs of the Permanent Set.] + +[Sidenote: 8, 9. Number of teeth? How distributed?] + +8. There are thirty-two teeth in the permanent set, as many being in one +jaw as the other. Each half-jaw has eight teeth, similarly shaped and +arranged in the same order: thus, two incisors, one canine, two bicuspids, +and three molars. The front teeth are small, sharp, and chisel-edged, and +are well adapted for cutting purposes; hence their name incisors. The +canines stand next, one on each side of the jaw; these receive their name +from their resemblance to the long, pointed tusks of the dog (Fig. 19). +{84} + +[Illustration: FIG. 19.--SECTION OF THE JAWS--RIGHT SIDE. + +V, A, N, Veins, Arteries, and Nerves of the Teeth. The root of one tooth in +each jaw is cut vertically to show the cavity and the blood-vessels, etc., +within it. 1 to 8, Permanent Teeth.] + +9. The bicuspids, next in order, are larger and have a broader crown than +the former; while behind them are the molars, the largest and most powerful +of the entire set. These large back teeth, or "grinders," present a broad, +rough surface, suitable for holding and crushing the food. The third molar, +or "wisdom tooth," is the last to be cut, and does not appear until about +the twenty-first year. {85} The order of arrangement of the teeth is +indicated by the following dental formula:-- + +[Illustration] + +[Sidenote: 10. Different forms of teeth? Human teeth? The inference?] + +10. It is interesting, at this point, to notice the different forms of +teeth in different animals, and observe how admirably their teeth are +suited to the respective kinds of food upon which they subsist. In the +_carnivora_, or flesh-feeders, the teeth are sharp and pointed, enabling +them both to seize their prey, and tear it in pieces; while the +_herbivora_, or vegetable-feeders, have broad, blunt teeth, with rough +crowns, suitable for grinding the tough grasses and grains upon which they +feed. Human teeth partake of both forms; some of them are sharp, and others +are blunt; they are therefore well adapted for the mastication of both +flesh and vegetables. Hence we argue that, although man may live +exclusively upon either vegetable or animal food, he should, when possible, +choose a diet made up of both varieties. + +[Sidenote: 11. Cleaning of teeth? Effects of not cleaning?] + +11. PRESERVATION OF THE TEETH.--In order that the teeth shall remain in a +sound and serviceable condition, some care is of course requisite. In the +first place, they require frequent cleansing; for every time we take food, +some particles of it remain in the mouth; and these, on account of the heat +and moisture present, soon begin to putrefy. This not only renders the +breath very offensive, but promotes decay of the teeth. {86} + +[Sidenote: 12. Effects upon the saliva? Formation of tartar? How prevented? +How destroyed?] + +12. The saliva, or moisture of the mouth, undergoes a putrefactive change, +and becomes the fertile soil in which a certain minute fungus has its +growth. This fluid, too, if allowed to dry in the mouth, collects upon the +teeth in the form of an unsightly, yellow concretion, called tartar. To +prevent this formation, and to remove other offensive substances, the teeth +should be frequently cleaned with water, applied by means of a soft +tooth-brush. The destruction of the tartar fungus is best effected by the +use of a weak solution of carbolic acid. + +[Sidenote: 13. Destruction of the enamel? How guarded against?] + +13. Again, it should be borne in mind that the enamel, Nature's protection +for the teeth, when once destroyed, is not formed anew; and the body of the +tooth thus exposed, is liable to rapid decay. On this account, certain +articles are to be guarded against; such as sharply acid substances that +corrode the enamel, and hard substances that break or scratch it--as gritty +tooth powders, metal tooth picks, and the shells of hard nuts. Sudden +alternations from heat to cold, when eating or drinking, also tend to crack +the enamel. + +[Sidenote: 14. Mixing of food with the saliva? What is the saliva? How +secreted? The salivary glands?] + +14. INSALIVATION.--When the morsel of food is cut and ground by the teeth, +it is at the same time also intimately mixed with the saliva, or fluids of +the mouth. This constitutes the second step of digestion, and is called +insalivation. The saliva, the first of the digestive solvents, is a +colorless, watery, and frothy fluid. It is secreted (_i. e._ separated from +the blood) partly by the mucous membrane which lines the mouth; but chiefly +by the salivary glands, of which there are three pairs situated near the +mouth. + +[Sidenote: 15. The flow of saliva? The thought of food? Anxiety and grief? +Animals fed upon dry and coarse food?] + +{87} 15. These glands consist of clusters of very small pouches, around +which a delicate network of blood-vessels is arranged: they empty into the +mouth by means of little tubes, or ducts. The flow from these glands is at +all times sufficient to maintain a soft and moist condition of the tongue +and mouth; but when they are excited by the presence and taste of food, +they pour forth the saliva more freely. Even the mere thought of food will +at times cause the saliva to flow, as when the appetite is stimulated by +the sight or smell of some savory article; so that the common expression is +correct that "the mouth waters" for the favorite articles of food. Anxiety +and grief prevent its flow, and cause "the tongue to cleave to the roof of +the mouth." In the horse and other animals, that feed upon dry and coarse +fodder, and require an abundant supply of saliva, we find large salivary +glands, as well as powerful muscles of mastication. + +[Illustration: FIG. 20.--STRUCTURE OF A SALIVARY GLAND.] + +[Illustration: FIG. 21.--THE HEAD OF A HORSE, showing the large salivary +gland (_a_), its duct (_b_), the muscles of mastication (_c_, _d_, _e_, +_f_, and _g_).] + +[Sidenote: 16. Importance of the process? The first place? The second? The +third?] + +16. The mingling of the saliva with the food seems a simple process, but it +is one that plays an important part {88} in digestion. In the first place, +it facilitates the motions of mastication, by moistening the food and +lubricating the various organs of the mouth. Secondly, it prepares the way +for other digestive acts: by the action of the teeth, the saliva is forced +into the solid food, softens the harder substances, and assists in +converting the whole morsel into a semi-solid, pulpy mass, that can be +easily swallowed, and readily permeated by other digestive fluids. The +saliva also, by dissolving certain substances, as sugar and salt, develops +the peculiar taste of each; whereas, if the tongue be dry and coated, they +are tasteless. Hence, if substances are insoluble, they are devoid of +taste. + +[Sidenote: 17. Its final importance? Starch? How effected? Ptyalin?] + +17. Finally, the saliva has the property of acting chemically upon the +food. As we have before stated (Chap. IV.), starch, as starch, cannot enter +the tissues of the body; but, in order to become nutriment, must first be +changed to grape sugar. This change is, in part, effected by the saliva, +and takes place almost instantly, whenever it comes in contact with cooked +starch. This important function is due to an organic ingredient of the +saliva called _ptyalin_. This substance has been extracted from the saliva +by the chemist, and has been found, by experiment, to convert into sugar +two thousand times its own weight of starch. + +[Sidenote: 18. Each of the processes? Why is a knowledge of the digestive +functions important? How shown?] + +18. IMPORTANCE OF MASTICATION AND INSALIVATION.--Each of these processes +complements the other, and makes the entire work available; for, by their +joint action, they prepare the food in the best possible manner for further +digestive changes. The knowledge of these preliminary functions will appear +the more important, when we reflect that they are the only ones which we +can regulate by the will. For, as soon as the act of swallowing begins, the +food not only passes out of sight, but beyond {89} control; and the +subsequent acts of digestion are consequently involuntary and unconsciously +performed. + +[Sidenote: 19. Rapid eating? Describe the process and effects.] + +19. It is generally known that rapid eating interferes with digestion. How +does this occur? In the first place, in rapid eating, the flow of the +saliva is insufficient to moisten the solid parts of the food, so that they +remain too hard and dry to be easily swallowed. This leads to the free and +frequent use of water, or some other beverage, at meals, to "wash down" the +food,--a most pernicious practice. For these fluids, not only cannot take +the place of the natural digestive juices, but, on the contrary, dilute and +weaken them. + +[Sidenote: 20. Loss of taste? Another effect of rapid eating? Mistakes?] + +20. Secondly, the saliva being largely the medium of the sense of taste, +the natural flavors of the food are not developed, and consequently it +appears comparatively insipid. Hence the desire for high-seasoned food, and +pungent sauces, that both deprave the taste and over excite the digestive +organs. Rapid eating also permits the entrance of injurious substances +which may escape detection by the taste, and be unconsciously received into +the system. In some instances, the most acrid and poisonous substances have +frequently been swallowed "by mistake," before the sense of taste could +act, and demand their rejection. + +[Sidenote: 21. Effect of imperfectly broken food in the stomach? Dyspepsia? +Overeating?] + +21. Thirdly, the food, being imperfectly broken up by the teeth, is hurried +onward to the stomach, to be by it more thoroughly divided. But the task +thus imposed upon the stomach, it is not at all adapted to perform; so that +the crude masses of food remain a heavy burden within the stomach, and a +source of distress to that organ, retarding the performance of its +legitimate duty. Hence persons who habitually eat too rapidly, frequently +fall victims to dyspepsia. Rapid eating also conduces to overeating. The +food is introduced so rapidly, that the system has not {90} time to +recognize that its real wants are met, and that its losses have been made +good; and hence the appetite continues, although more nutriment has been +swallowed than the system requires, or can healthfully appropriate. + +[Illustration: FIG. 22.--SECTION OF CHEST AND ABDOMEN. + +A, Heart. B, The Lungs. C, Stomach. D, The Liver. E, Large Intestine. +G, Small Intestine.] + +[Sidenote: 22. Gullet? Describe the stomach and its location. Effects of +gormandizing?] + +22. THE STOMACH.--As soon as each separate portion of food is masticated +and insalivated, it is swallowed; that is, it is propelled downward to the +stomach, through a narrow muscular tube about nine inches in length, called +the _oesophagus_, or gullet (Fig. 23). The stomach is the only large +expansion of the digestive canal, and is the most important organ of +digestion. It is a hollow, pear-shaped a pouch, having a capacity of three +pints, in the adult. Its walls are thin and yielding, and may become +unnaturally distended, as in the case of those who subsist on a bulky, +innutritious diet, and of those who habitually gormandize. + +[Sidenote: 23. Heart-orifice? Gatekeeper? Coins, etc.? Indication of the +soft and yielding texture of the stomach?] + +23. The stomach has also two openings; that by which food enters, being +situated near the heart, is called the _cardiac_, or heart orifice; the +other is the _pylorus_, or "gatekeeper," which guards the entrance to the +intestines, {91} and, under ordinary circumstances, permits only such +matters to pass it as have first been properly acted upon in the stomach. +Coins, buttons, and the like are, however, readily allowed to pass, because +they can be of no use if retained. The soft and yielding texture of this +organ--the stomach--indicates that it is not designed to crush and +comminute solid articles of food. + +[Illustration: FIG. 23.--THE ORGANS OF DIGESTION. + +O, Oesophagus. S, Stomach. L, Liver. M, Pylorus. C, Large Intestines. +P, Pancreas. I, Small Intestines. N, Spleen. G, Gall-bladder. ] + +[Sidenote: 24. What is meant by the gastric juice?] + +24. THE GASTRIC JUICE.--We have seen how the presence of food in the mouth +excites the salivary glands, causing the saliva quickly to flow. In the +same manner, when food reaches the stomach, its inner lining, the mucous +membrane, is at once excited to activity. (At first its surface, which +while the stomach is empty presents a pale pink hue, turns to a bright red +color, for the minute blood-vessels which course through it, are filled +with blood. Presently a clear, colorless, and acid fluid exudes, drop by +drop, from its numerous minute glands or "tubules," until finally the +surface is moistened in every part, and the fluid begins to mingle with the +food. This fluid is termed the gastric juice. {92} + +[Sidenote: 25. What is the office of the gastric juice? Acidity of the +gastric juice? Quantity of gastric juice used? What becomes of it?] + +25. The gastric juice is the proper solvent of certain articles of food, +especially those belonging to the albuminoid class. This solvent power is +due to its peculiar ingredient, _pepsin_; in digestion, this substance acts +like a ferment; that is, it induces changes in the food simply by its +presence, but does not itself undergo change. The acidity of the gastric +juice, which is due to _lactic acid_, is not accidental; for we find that +the pepsin cannot act in an alkaline solution--that is, one which is not +acid or neutral. The quantity of gastric juice secreted daily is very +large, probably not less than three or four pints at each meal. Though this +fluid is at once used in the reduction of the food, it is not lost; since +it is soon re-absorbed by the stomach, together with those parts of the +food which it has digested and holds in solution. + +[Sidenote: 26. Muscular coat of the stomach? Expansion and contraction of +its fibres? Action of the fibres?] + +26. MOVEMENTS OF THE STOMACH.--The inner coating of the stomach is the +mucous membrane, which, as we have seen, furnishes the gastric juice. Next +to this coating lies another, called the muscular coat, composed of +involuntary muscular fibres, some of which run circularly, and others in a +longitudinal direction. These expand to accommodate the food as it is +introduced, and contract as it passes out. In addition, these fibres are in +continual motion while food remains in the stomach, and they act in such +manner that the contents are gently turned round from side to side, or from +one end of it to the other. + +[Sidenote: 27. Peristaltic movements? What is said of our consciousness of +and power over these movements? Describe the movements of the pylorus.] + +27. By these incessant movements of the stomach, called the _peristaltic_ +movements, the gastric juice comes in contact with all parts of the food. +We are, however, not conscious that these movements take place, nor have we +the power to control them. When such portions of the food as are +sufficiently digested approach the pylorus, it {93} expands to allow them +to pass out, and it closes again to confine the residue for further +preparation. + +[Sidenote: 28. How has the knowledge and the workings of the stomach been +ascertained? St. Martin? How else?] + +28. The knowledge of these and other interesting and instructive facts has +been obtained by actual observation; the workings of the stomach of a +living human being have been laid open to view and examined--the result of +a remarkable accident. Alexis St. Martin, a Canadian _voyageur_, received a +gun-shot wound which laid open his stomach, and which, in healing, left a +permanent orifice nearly an inch in diameter. Through this opening the +observer could watch the progress of digestion, and experiment with +different articles of food. Since that occurrence, artificial openings into +the stomach of the inferior animals have been repeatedly made, so that the +facts of stomach-digestion are very well ascertained and verified. + +[Sidenote: 29. What was formerly thought? What do we now know? What else do +we now know? Water, salt, and sugar? Absorption?] + +29. GASTRIC DIGESTION.--What portions of the food are digested in the +stomach? It was formerly thought that all the great changes of digestion +were wrought here, but later investigation has taught us better. We now +know that the first change in digestion takes place in the mouth, in the +partial conversion of starch into sugar. We also know that, of the three +organic food principles (considered in Chapter IV.) two--the fats and the +sugars--are but slightly affected by the stomach; but that its action is +confined to that third and very important class, from which the tissues are +renewed, the albuminoids. A few articles need no preparation before +entering the system, as water, salt, and grape-sugar. These are rapidly +taken up by the blood-vessels of the stomach, which everywhere underlie its +mucous membrane in an intricate and most delicate network. In this way the +function of absorption begins. + +[Sidenote: 30. Albuminose? The process? Chyme?] + +30. The albuminoid substances are speedily attacked and {94} digested by +the gastric juice. From whatever source they are derived, vegetable or +animal, they are all transformed into the same digestive product, called +_albuminose_. This is very soluble in water, and is readily absorbed by the +blood-vessels of the stomach. After a longer or shorter time, varying from +one to five hours, according to the individual and the quantity and quality +of his food, the stomach will be found empty. Not only has the digested +food passed out, but also those substances which the stomach could not +digest or absorb have passed little by little through the pylorus, to +undergo further action in the intestines. At the time of its exit, the +digested food is of a pulpy consistence, and dark color, and is then known +as the _chyme_. + +[Sidenote: 31. What are the intestines? The small intestines? The large +intestines? Their structure?] + +31. THE INTESTINES.--The intestines, or "bowels," are continuous with the +stomach, and consist of a fleshy tube, or canal, twenty-five feet in +length. The small intestine, whose diameter is about one inch and a half, +is twenty feet long and very tortuous. The large intestine is much wider +than the former, and five feet long (Fig. 23). The general structure of +these organs resembles that of the stomach. Like it, they are provided with +a mucous membrane, or inner lining, whence flow their digestive juices; +and, just outside of this, a muscular coat, which propels the food onward +from one point to another. + +[Sidenote: 32. Peritoneum? The work of digestion?] + +32. Moreover, both the intestines and stomach are enveloped in the folds of +the same outer tunic or membrane, called the _peritoneum_. This is so +smooth and so well lubricated, that the intestines have the utmost freedom +of motion. In the small intestines, the work of digestion is completed, the +large intestine receiving from them the indigestible residue of the food, +and in time expelling it from the body. + +[Sidenote: 33. The presence of food in the intestines? Bile?] + +33. INTESTINAL DIGESTION.--As soon as the food passes the pylorus and +begins to accumulate in the upper {95} part of the intestines, it excites +the flow of a new digestive fluid, which enters through a small tube, or +_duct_, about three inches below the stomach. It is formed by the union of +two distinct fluids--the _bile_ and the _pancreatic_ juice. The bile is +secreted by the liver, which is the largest gland of the body, and is +situated on the right side and upper part of the abdomen (Fig. 22). The +bile is constantly formed, but it flows most rapidly during digestion. +During the intervals of digestion it is stored in the _gall-bladder_, a +small membranous bag attached to the under side of the liver. This fluid is +of a greenish-yellow color, having a peculiar smell, and a very bitter +taste. + +[Sidenote: 34. The pancreatic juice? The joint action of these fluids?] + +34. The pancreatic juice is the product of a gland called the _pancreas_, +situated behind the stomach. This fluid is colorless, viscid, and without +odor. Like the digestive juices previously described, it owes its solvent +power to its peculiar ferment principle, called _pancreatin_. By the joint +action of these fluids, the fatty parts of the food are prepared for +absorption. By previous steps of digestion the fats are merely separated +from the other components of the food; but here, within the intestines, +they are reduced to a state of minute division, or _emulsion_, resembling +the condition of butter in milk, before it has been churned. There results +from this action a white and milky fluid, termed the _chyle_, which holds +in solution the digestible portions of the food, and is spread over the +extensive absorbent surface of the small intestines. + +[Sidenote: 35. The mucous membrane? Experiments on inferior animals?] + +35. The mucous membrane of the intestines, also, secretes or produces, a +digestive fluid by means of numerous "follicles," or minute glands; this is +called the intestinal juice. From experiments on the inferior animals, it +has been ascertained that this fluid exerts a solvent influence over each +of the three organic food principles, and in this way may supplement and +complete the action of the {96} fluids previously mentioned, viz.:--of the +saliva in converting starch into sugar, of the gastric juice in digesting +the albuminoids, and of the pancreatic juice and bile in emulsifying the +fats. + +[Sidenote: 36, 37. How much thus far has been done with the food? The next +process? Give the first way.] + +36. ABSORPTION.--With the elaboration of the chyle, the work of digestion +is completed; but, in a certain sense, the food is yet outside of the body; +that is, the blood is not yet enriched by it, and it is not in a position +to nourish the tissues. The process by which the liquefied food passes out +of the alimentary canal into the blood is called absorption. This is +accomplished in two ways; first, by the _blood-vessels_. We have seen how +the inner membrane of the stomach is underlaid by a tracery of minute and +numerous vessels, and how some portions of the food are by them absorbed. +The supply of blood-vessels to the intestines is even greater; particularly +to the small intestines, where the work of absorption is more actively +carried on. + +37. The absorbing surface of the small intestines, if considered as a plane +surface, amounts to not less than half a square yard. Besides, the mucous +membrane is formed in folds with an immense number of thread-like +prolongations, called _villi_, which indefinitely multiply its absorbing +capacity. These minute processes, the villi, give the surface the +appearance and smoothness of velvet; and during digestion, they dip into +the canal, and, by means of their blood-vessels, absorb its fluid contents, +just as the _spongioles_ which terminate the rootlets of plants, imbibe +moisture from the surrounding soil. + +[Sidenote: 38. How is absorption effected in another way? Describe it. Name +of the lacteals? Thoracic duct?] + +38. Secondly, absorption is also effected by the _lacteals_, a set of +vessels peculiar to the small intestines. These have their beginnings in +the little villi just mentioned, side by side with the blood-vessels. These +two sets of absorbents run in different courses, but their destination is +the same, {97} which is the right side of the heart. The lacteals receive +their name from their milky-white appearance. After a meal containing a +portion of fat, they are then distended with chyle, which they are +specially adapted to receive: at other times they are hardly discernible. +The lacteals all unite to form one tube, the _thoracic duct_, which passes +upward through the _thorax_, or chest, and empties into a large vein, +situated just beneath the left collar-bone. + +[Illustration: FIG. 24.--THE LACTEALS. + +A, Small Intestine. B, Lacteals. C, Thoracic Duct. D, Absorbents. +E, Blood-vessel. ] + +[Sidenote: 39. The absorbents? Lymph? What further of the lymph?] + +39. THE ABSORBENTS.--The lacteals belong to a class of vessels known as +absorbents, or lymphatics, which ramify in nearly all parts of the body, +except the brain and spinal cord. The fluid which circulates through the +lymphatics of the limbs, and all the organs not concerned in digestion, is +called _lymph_. This fluid is clear and colorless, like water, and thus +differs from the milky chyle which the lacteals carry after digestion: it +consists chiefly of the watery part of the blood, which was not required by +the tissues, and is returned to the blood by the absorbents or lymphatics. + +[Sidenote: 40. What can you state as to the time required for digestion?] + +40. CIRCUMSTANCES AFFECTING DIGESTION.--What length of time is required for +the digestion of food? From observations made, in the case of St. Martin, +the Canadian {98} already referred to, it has been ascertained that, at the +end of two hours after a meal, the stomach is ordinarily empty. How much +time is needed to complete the digestion of food, within the small +intestines, is not certain; but, from what we have learned respecting their +functions, it must be evident that it largely depends upon the amount of +starch and fat which the food contains. + +[Sidenote: 41. Circumstances affecting duration of digestion? Fresh food?] + +41. In addition to the preparations which the food undergoes in cooking, +which we have already considered, many circumstances affect the duration of +digestion; such as the quality, quantity, and temperature of the food; the +condition of the mind and body; sleep, exercise, and habit. Fresh food, +except new bread and the flesh of animals recently slain, is more rapidly +digested than that which is stale; and animal food more rapidly than that +from the vegetable kingdom. + +[Sidenote: 42. Food in concentrated form? A large quantity of food? +Experiment on the dog? Ice-water? Variety of articles?] + +42. Food should not be taken in too concentrated a form, the action of the +stomach being favored when it is somewhat bulky; but a large quantity in +the stomach often retards digestion. If the white of one egg be given to a +dog, it will be digested in an hour, but if the white of eight eggs be +given it will not disappear in four hours. A wineglassful of ice-water +causes the temperature of the stomach to fall thirty degrees; and it +requires a half-hour before it will recover its natural warmth--about a +hundred degrees--at which the operations of digestion are best conducted. A +variety of articles, if not too large in amount, is more easily disposed of +than a meal made of a single article; although a single indigestible +article may interfere with the reduction of articles that are easily +digested. + +[Sidenote: 43. Strong emotion? The tongue of the patient?] + +43. Strong emotion, whether of excitement or depression, checks digestion, +as do also a bad temper, anxiety, long fasting, and bodily fatigue. The +majority of these {99} conditions make the mouth dry, that is, they +restrain the flow of the saliva; and without doubt they render the stomach +dry also, by preventing the flow of the gastric juice. And, as a general +rule, we may decide, from a parched and coated tongue, that the condition +of the stomach is not very dissimilar, and that it is unfit for the +performance of digestive labor. This is one of the points which the +physician bears in mind when he examines the tongue of his patient. + +[Sidenote: 44. Eating between meals? Severe exercise? Sleep after meals?] + +44. The practice of eating at short intervals, or "between meals," as it is +called, has its disadvantage, as well as rapid eating and over-eating, +since it robs the stomach of its needed period of entire rest, and thus +overtasks its power. With the exception of infants and the sick, no persons +require food more frequently than once in four hours. Severe exercise +either directly before or directly after eating retards digestion; a period +of repose is most favorable to the proper action of the stomach. The +natural inclination to rest after a hearty meal may be indulged, but should +not be carried to the extent of sleeping; since in that state the stomach, +as well as the brain and the muscles, seeks release from labor. + +{100} + +QUESTIONS FOR TOPICAL REVIEW. + + PAGE + + 1. What do you understand by nutrition? 80 + 2. How is the process of nutrition carried on? 80 + 3. What further can you state on the subject? 80, 81 + 4. Describe the general plan of digestion. 81 + 5. How is the process of mastication carried on? 80, 82 + 6. State what you can in relation to the formation of the + teeth. 82, 86 + 7. What, in relation to their arrangement? 83, 84 + 8. What, in relation to the process of "shedding?" 82, 83, 84 + 9. In relation to the different forms of teeth in different + animals? 85 + 10. What causes operate to injure or destroy the teeth? 85, 86 + 11. What suggestions and directions are given for the + preservation of the teeth? 85, 86 + 12. What do you understand by insalivation? 80, 86 + 13. How is the process of insalivation carried on? 86, 87, 88 + 14. Of what importance is the saliva to the process? 87, 88 + 15. Of what importance are mastication and insalivation? 88, 89 + 16. Describe the consequences of rapid eating. 89, 90 + 17. What becomes of the food directly after it has undergone + mastication and insalivation? 90 + 18. Describe the location and formation of the stomach. 90, 91, 92 + 19. Describe the process by which the gastric juice is formed. 91 + 20. What are the properties and uses of the gastric juice? 92 + 21. What are the movements of the stomach, and what their + uses? 92, 93 + 22. What further can you state on the subject? 93 + 23. What portions of the food are digested in the stomach? 93, 94 + 24. What are the first changes of digestion? 93 + 25. Describe the location and formation of the stomach. 94 + 26. What further can you state in relation to the stomach? 94 + 27. Describe the process of intestinal digestion. 94, 95, 96 + 28. What do you understand by absorption? 80, 96 + 29. How is the process of absorption effected? 96, 97 + 30. What are the lacteals and of what use are they? 96, 97 + 31. What length of time is required for the digestion of + food? 97, 98 + 32. What circumstances, of food, affect digestion? 98 + 33. What circumstances, of emotion, affect digestion? 98, 99 + 34. What suggestions and directions are given upon the subject of + eating and drinking? 98, 99 + + * * * * * + + +[Illustration: CIRCULATION OF THE BLOOD. + +[Heart, Lungs, Arteries & Veins.] + +{101} + +CHAPTER VII. + +THE CIRCULATION. + +_The Blood--Its Plasma and Corpuscles--Coagulation of the Blood--The Uses +of the Blood--Transfusion--Change of Color--The Organs of the +Circulation--The Heart, Arteries, and Veins--The Cavities and Valves of the +Heart--Its Vital Energy--Passage of the Blood through the Heart--The +Frequency and Activity of its Movements--The Pulse--The Sphygmograph--The +Capillary Blood-vessels--The Rate of the +Circulation--Assimilation--Injuries to the Blood-vessels._ + +[Sidenote: 1. What is required by every living organism? In plants? +Insects? Reptiles? Man?] + +1. THE BLOOD.--Every living organism of the higher sort, whether animal or +vegetable, requires for the maintenance of life and activity, a circulatory +fluid, by which nutriment is distributed to all its parts. In plants, this +fluid is the sap; in insects, it is a watery and colorless blood; in +reptiles and fishes, it is red but cold blood; while in the nobler animals +and man, it is the red and warm blood. + +[Sidenote: 2. Importance and abundance of blood? Dependence of life? Abel? +Mosaic law? In what part of the body is blood not found? Quantity of blood +in the body?] + +2. The blood is the most important, as it is the most abundant, fluid of +the body; and upon its presence, under certain definite conditions, life +depends. On this account it is frequently, and very properly, termed "the +vital fluid." The importance of the blood, as essential to life, was +recognized in the earliest writings. In the narration of the death of the +murdered Abel, it is written, "the voice of his _blood_ crieth from the +ground." In the Mosaic law, proclaimed over thirty centuries ago, the +Israelites were forbidden to eat food that contained blood, for the reason +that "the life of the flesh is in the blood." With the exception of a few +tissues, such as the hair, the nails, and the _cornea_ of the eye, blood +everywhere pervades the body, as may be proven by puncturing any part with +a {102} needle. The total quantity of blood in the body is estimated at +about one-eighth of its weight, or eighteen pounds. + +[Sidenote: 3. Color of blood? Its consistence? Odor?] + +3. The color of the blood, in man and the higher animals, as is well known, +is red; but it varies from a bright scarlet to a dark purple, according to +the part whence it is taken. "Blood is thicker than water," as the adage +truly states, and has a glutinous quality. It has a faint odor, resembling +that peculiar to the animal from which it is taken. + +[Sidenote: 4. What is stated of the blood as viewed under the microscope?] + +4. When examined under the microscope, the blood no longer appears a simple +fluid, and its color is no longer red. It is then seen to be made up of two +distinct parts: first, a clear, colorless fluid, called the _plasma_; and +secondly, of a multitude of minute solid bodies, or corpuscles, that float +in the watery plasma. The plasma, or nutritive liquid, is composed of water +richly charged with materials derived from the food, viz., albumen, which +gives it smoothness and swift motion; fibrin; certain fats; traces of +sugar; and various salts. + +[Illustration: FIG 25.--THE BLOOD CORPUSCLES.] + +[Sidenote: 5. State what you can of the little bodies called corpuscles.] + +5. THE BLOOD CORPUSCLES.--In man, these remarkable "little bodies," as the +word _corpuscles_ signifies, are of a yellow color, but by their vast +numbers impart a red hue to the blood. They are very small, having a +diameter of about 1/3500 of an inch, and being one-fourth of that fraction +in thickness; so that if 3,500 of them were placed in line, side by side, +they would only extend one inch; or, if {103} piled one above another, it +would take at least 14,000 of them to stand an inch high. Although so small +in size, they are very regular in form. As seen under the microscope, they +are not globular or spherical, but flat, circular, and disc-like, with +central depressions on each side, somewhat like a pearl button that has not +been perforated. In freshly-drawn blood they show a disposition to arrange +themselves in little rolls like coins (Fig. 25). + +[Illustration: FIG. 26. + +_a_, Oval Corpuscles of a fowl. _b_, Corpuscles of a frog. _c_, Those of a +shark. + +The five small ones at the upper part of the figure, represent the human +corpuscles magnified 400 times.] + +[Sidenote: 6. The size and shape of corpuscles? Why is the fact important?] + +6. The size and shape of the blood corpuscles vary in different animals, so +that it is possible to discriminate between those of man and the lower +animals (Fig. 26). This is a point of considerable practical importance. +For example, it is sometimes desirable to decide in a court of justice the +source, whether from man or an inferior animal, of blood stains upon the +clothing of an accused person, or upon some deadly weapon. This may be done +by a microscopical examination of a minute portion of the dried stain, +previously refreshed by means of gum-water. Certain celebrated cases are +recorded in which the guilt of criminals has been established, and they +have been condemned and punished upon the evidence which science rendered +on this single point, the detecting of the human from other blood. + +[Sidenote: 7. The character of the blood of dead animals? Means of +detecting such blood?] + +7. The character of the blood of dead, extinct, and even fossil animals, +such as the mastodon, has been ascertained by obtaining and examining +traces of it which had been shut up, perhaps for ages, in the circulatory +canals of bone. A means of detecting blood in minute quantities is found +{104} in the spectroscope, the same instrument by which the constitution of +the heavenly bodies has been studied. If a solution containing not more +than one-thousandth part of a grain of the coloring matter of the +corpuscle, be examined, this instrument will detect it. + +[Sidenote: 8. White corpuscles? Total number of corpuscles in the body?] + +8. The corpuscles, just described, are known as the red blood corpuscles. +Besides these, and floating along in the same plasma, are the white +corpuscles. These are fewer in number, but larger and globular in form. +They are colorless, and their motion is less rapid than that of the other +variety. The total number of both varieties of these little bodies in the +blood is enormous. It is calculated that in a cubic inch of that fluid +there are eighty-three millions, and at least five hundred times that +number in the whole body. + +[Sidenote: 9. The blood in its natural condition in the body? Describe the +process by which the coagulation of blood takes place?] + +9. COAGULATION.--The blood, in its natural condition in the body, remains +perfectly fluid; but, within a few minutes after its removal from its +proper vessels, whether by accident or design, a change takes place. It +begins to coagulate, or assume a semi-solid consistence. If allowed to +stand, after several hours it separates into two distinct parts, one of +them being a dark red jelly, the coagulum, or clot, which is heavy and +sinks; and the other, a clear, straw-colored liquid, called serum, which +covers the clot. This change is dependent upon the presence in the blood of +fibrin, which possesses the property of solidifying under certain +circumstances; one of these circumstances being when the blood is separated +from living tissues. The color of the clot is due to the entanglement of +the corpuscles with the fibrin. + +[Sidenote: 10. If coagulation were impossible? How is it in fact?] + +10. In this law of the coagulation of the blood is our safeguard against +death by haemorrhage, or against undue loss of blood. If coagulation were +impossible, the {105} slightest injury in drawing blood would prove fatal. +Whereas now, in vastly the larger proportion of cases, bleeding ceases +spontaneously, because the blood, as it coagulates, stops the mouths of the +injured blood-vessels. In another class of cases, where larger vessels are +cut or torn, it is simply necessary to close them by a temporary pressure; +for in a few minutes the clot will form and seal them up. In still more +serious cases, where the blood-vessel is of large size, the surgeon is +obliged to tie a "ligature" about it, and thus prevent the force of the +blood-current from washing away the clots, which, forming within and around +the vessel, would close it effectually. + +[Sidenote: 11. What is worthy of remark? Coagulation of the blood of +inferior animals? Of the blood of birds?] + +11. It is worthy of remark that this peculiarity is early implanted in the +blood, even before birth, and in advance of any existing necessity for it; +thus anticipating and guarding against danger. But this is not all. Of most +of the inferior animals, which, as compared with man, are quite helpless, +the blood coagulates more rapidly, and in the case of the birds, almost +instantly. The relative composition of fluid and coagulated blood may be +thus represented: + + _Fluid Blood._ _Coagulated Blood._ + + Plasma----------Serum---------Serum + \ + \ + --------Fibrin-------- + \ + \ + Corpuscles------Corpuscles-------Clot. +[Sidenote: 12. The blood, as a provider and purifier? What uses does the +blood subserve? Experiments? Transfusion?] + +12. THE USES OF THE BLOOD.--The blood is the great provider and purifier of +the body. It both carries new materials to all the tissues, and removes the +worn out particles of matter. This is effected by the plasma. It both +conveys oxygen and removes carbonic acid. This is done through the +corpuscles. Some singular experiments have {106} been tried to illustrate +the life-giving power of the blood. An animal that has bled so freely as to +be at the point of dying, is promptly brought back to life by an operation +called transfusion, by which fresh blood from a living animal is injected +into the blood-vessels of his body. + +[Sidenote: 13. The case of the deaf and feeble dog? Horse? Dead dog?] + +13. It is related that a dog, deaf and feeble from age, had hearing and +activity restored to him by the introduction into his veins of blood taken +from a young dog; and, that a horse, twenty-six years old, having received +the blood of four lambs acquired new vigor. And further, that a dog, just +dead from an acute disease, was so far revived by transfusion, as to be +able to stand and make a few movements. + +[Sidenote: 14. Transfusion, as a fashionable remedy? What further of +transfusion?] + +14. Transfusion has been practised upon man. At one time, shortly after +Harvey's discovery of the "Circulation of the Blood," it became quite a +fashionable remedy, it being thought possible by it to cure all forms of +disease, and even to make the old young again. But these claims were soon +found extravagant, and many unhappy accidents occurred in its practice; so +that being forbidden by government and interdicted by the Pope, it rapidly +fell into disuse. At the present time, however, it is sometimes resorted to +in extreme cases, when there has been a great and rapid loss of blood; and +there are upon record several instances where, other means having failed, +life has been restored or prolonged by the operation of transfusion. + +[Sidenote: 15. The seat of the reviving power of the blood? What further is +related?] + +15. This reviving power of the blood seems to reside in the corpuscles; for +transfusion, when attempted to be performed with the serum alone, has, in +every case, proved fruitless. Now, though so much depends upon the blood +and its corpuscles, it is a mistake to suppose that in them alone is the +seat of life, or that they are, in an exclusive manner, alive. All the +organs and parts of the body are mutually dependent one upon the other; and +the complete usefulness {107} of the blood, or of any other part, flows out +of the harmonious action of all the parts. + +[Sidenote: 16. Changes in the blood? What further is stated?] + +16. CHANGE OF COLOR.--The blood undergoes a variety of changes in its +journey through the system. As it visits the different organs it both gives +out and takes up materials. In one place it is enriched, in another it is +impoverished. By reason of these alterations in its composition, the blood +also changes its color. In one part of the body it is bright red, or +arterial; in another it is dark blue, or venous. In the former case it is +pure and fit for the support of the tissues; in the latter, it is impure +and charged with effete materials. (The details of the change from dark to +bright will be given in the chapter on Respiration.) + +[Sidenote: 17. Motion of the blood? What is meant by the circulation of the +blood? How confined? Discovery made by Harvey?] + +17. CIRCULATION.--The blood is in constant motion during life. From the +heart, as a centre, a current is always setting toward the different +organs; and from these organs a current is constantly returning to the +heart. In this way a ceaseless circular movement is kept up, which is +called the Circulation of the Blood. This stream of the vital fluid is +confined to certain fixed channels, the blood-vessels. Those branching from +the heart are the arteries; those converging to it are the veins. The true +course of the blood was unknown before the beginning of the seventeenth +century. In 1619 it was discovered by the illustrious William Harvey. Like +many other great discoverers, he suffered persecution and loss, but unlike +some of them, he was fortunate enough to conquer and survive opposition. He +lived long enough to see his discovery universally accepted, and himself +honored as a benefactor of mankind. + +{108} + +[Illustration: FIG. 27.--THE ORGANS OF CIRCULATION.] + +{109} [Illustration: FIG. 28.--THE HEART AND LARGE VESSELS. + +A, Right Ventricle. B, Left Ventricle. C, Right Auricle. D, Left Auricle. +E, Aorta. F, Pulmonary Artery.] + +[Sidenote: 18. Office of the heart? Location of the heart? Its beat? Its +shape? Protection to the heart? What else is said in relation to the +heart?] + +18. THE HEART.--The heart is the central engine of the circulation. In this +wonderful little organ, hardly larger than a man's fist, resides that +sleepless force by which, during the whole of life, the current of the +blood is kept in motion. It is placed in the middle and front part of the +chest, inclining to the left side. The heartbeat may be felt and heard +between the fifth and sixth ribs, near the breast-bone. The shape of the +heart is conical, with the apex or point downward and in front. The base, +which is upward, is attached so as to hold it securely in its place, while +the apex is freely moveable. In order that loss of power from friction may +be obviated, the heart is enclosed between two layers of serous membrane, +which forms a kind of sac. This membrane is as smooth as satin, and itself +secretes a fluid in sufficient quantities to keep it at all times well +lubricated. The lining membrane of the heart, likewise, is extremely +delicate and smooth. + +{110} [Illustration: FIG. 29.--SECTION OF THE HEART. + +A, Right Ventricle. B, Left Ventricle. C, Right Auricle. D, Left Auricle. +E, F, Inlets to the Ventricles. G, Pulmonary Artery. H, Aorta.] + +[Sidenote: 19. Formation of the heart? Right and left heart?] + +19. THE CAVITIES OF THE HEART.--The heart is hollow, and so partitioned as +to contain four chambers or cavities; two at the base, known as the +_auricles_, from a fancied resemblance to the ear of a dog, and two at the +apex or point, called _ventricles_. An auricle and a ventricle on the same +side, communicate with each other, but there is no opening from side to +side. It is customary to regard the heart as a double organ, and to speak +of its division into the right and left heart. For while both halves act +together in point of time, each half sustains an entirely distinct portion +of the labor of the circulation. Thus, the right heart always carries the +dark or venous blood, and the left always circulates the bright or arterial +blood. + +[Sidenote: 20. Capacity of the chambers of the heart? What wise provision +is mentioned? The auricles?] + +20. If we examine the heart, we at once notice that though its various +chambers have about the same capacity, the walls of the ventricles are +thicker and stronger than those of the auricles. This is a wise provision, +for it is by the powerful action of the former that the blood is forced to +the most remote regions of the body. The auricles, on the contrary, need +much less power, for they simply discharge their contents into the cavities +of the heart near at hand and below them--into the ventricles. {111} + +[Sidenote: 21. Substance of the heart? Its fibres? Its movements? The +advantage of such movements? Action of the heart? Its period of repose?] + +21. ACTION OF THE HEART.--The substance of the heart is of a deep red +color, and its fibres resemble those of the voluntary muscles by which we +move our bodies. But the heart's movements are entirely involuntary. The +advantage of this is evident; for if it depended upon us to will each +movement, our entire attention would be thus engaged, and we would find no +time for study, pleasure, or even sleep. The action of the heart consists +in alternate contractions and dilatations. During contraction the walls +come forcibly together, and thus drive out the blood. In dilatation, they +expand and receive a renewed supply. These movements are called _systole_ +and _diastole_. The latter may be called the heart's period of repose; and +although it lasts only during two-fifths of a heart-beat, or about a third +of a second, yet during the day it amounts to more than nine hours of total +rest. + +[Sidenote: 22. Remarkable property of the tissue of the heart? How shown? +How interesting? In cold-blooded animals? Heart of a turtle? Of a frog? +Alligator?] + +22. A remarkable property of the tissue of the heart is its intense +vitality. For while it is more constantly active than any other organ of +the body, it is the last to part with its vital energy. This is especially +interesting in view of the fact that after life is apparently extinguished, +as from drowning, or poisoning by chloroform, there yet lingers a spark of +vitality in the heart, which, by continued effort, may be fanned into a +flame so as to revivify the whole body. In cold-blooded animals this +irritability of the heart is especially remarkable. The heart of a turtle +will pulsate, and the blood circulate for a week after its head has been +cut off; and the heart will throb regularly many hours after being cut out +from the creature's chest. The heart of a frog or serpent, separated +entirely from the body, will contract at the end of ten or twelve hours: +that of an alligator has been known to beat twenty-eight hours after the +death of the animal. {112} + +[Sidenote: 23. Course of the blood through the heart? Course of +heart-currents?] + +23. PASSAGE OF THE BLOOD THROUGH THE HEART.--Let us now trace the course of +the blood through the several cavities of the heart. In the first place, +the venous blood, rendered dark and impure by contact with the changing +tissues of the body, returns to the right heart by the veins. It enters and +fills the right auricle during its dilatation: the auricle then contracts +and fills the right ventricle. Almost instantly, the ventricle contracts +forcibly and hurries the blood along the great artery of the lungs, to be +purified in those organs. Secondly, having completed the circuit of the +lungs, the pure and bright arterial blood enters the left auricle. This now +contracts and fills the left ventricle, which cavity, in its turn, +contracts and sends the blood forth on its journey again through the +system. This general direction from right to left is the uniform and +undeviating course of heart-currents. + +[Sidenote: 24. Openings of the ventricles? How guarded? How do the valves +operate? The consequence? Heart-sounds?] + +24. The mechanism which enforces and regulates it, is as simple as it is +beautiful. Each ventricle has two openings, an inlet and an outlet, each of +which is guarded by strong curtains, or valves. These valves open freely to +admit the blood entering from the right, but close inflexibly against its +return. Thus, when the auricle contracts, the inlet valve opens; but as +soon as the ventricle begins to contract, it closes promptly. The contents +are then, so to speak, cornered, and have but one avenue of escape, that +through the outlet valve into the arteries beyond. As soon as the ventricle +begins to dilate again, this valve shuts tightly and obstructs the passage. +The closing of these valves occasions the two heart-sounds, which we hear +at the front of the chest. + +[Sidenote: 25. Heart-beats? The heart as a susceptible organ? Heat, +exercise, etc.? Posture?] + +25. FREQUENCY OF THE HEART'S ACTION.--The alternation of contraction and +dilation constitutes the {113} heartbeats. These follow each other not only +with great regularity, but with great rapidity. The average number in an +adult man is about seventy-two in a minute. But the heart is a susceptible +organ, and many circumstances affect its rate of action. Heat, exercise, +and food will increase its action, as cold, fasting, and sleep will +decrease it. Posture, too, has a curious influence; for if while sitting, +the beats of the heart number seventy-one; standing erect will increase +them to eighty-one, and lying down will lower them to sixty-six. + +[Sidenote: 26. Mental emotions? Sudden excitement? Excessive joy? The +heart-beat rate? Bonaparte and Wellington?] + +26. The modifying influence of mental emotions is very powerful. Sudden +excitement of feeling will cause the heart to palpitate, or throb +violently. Depressing emotions sometimes temporarily interrupt its +movements, and the person faints in consequence. Excessive joy, grief, or +fear, has occasionally suspended the heart's action entirely, and thus +caused death. The rate of the heart-beat may be naturally above or below +seventy-two. Thus it is stated that the pulse of the savage is always +slower than that of the civilized man. Bonaparte and Wellington were very +much alike in their heart's pulsations, which were less than fifty in the +case of each. + +[Sidenote: 27. Average number of heart-beats? In one hour? Year? Lifetime?] + +27. ACTIVITY OF THE HEART.--The average number of heart-beats during a +lifetime may be considered as at the rate of seventy-two per minute, +although this estimate is probably low; for during several years of early +life the rate is above one hundred a minute. In one hour, then, the heart +pulsates four thousand times; in a day, one hundred thousand times; and in +a year, nearly thirty-eight million times. If we compute the number during +a lifetime, thirty-nine years being the present average longevity of +civilized mankind, we obtain as the vast aggregate, fourteen hundred +millions of pulsations. {114} + +[Sidenote: 28. Amount of blood expelled? Theories of the ancients?] + +28. Again, if we estimate the amount of blood expelled by each contraction +of the ventricles, at four ounces, then the weight of the blood moved +during one minute will amount to eighteen pounds. In a day it will be about +twelve tons; in a year, four thousand tons; and in the course of a +lifetime, over one hundred and fifty thousand tons. These large figures +indicate, in some measure, the immense labor necessary to carry on the +interior and vital operations of our bodies. In this connection, we call to +mind the fanciful theories of the ancients in reference to the uses of the +heart. They regarded it as the abode of the soul, and the source of the +nobler emotions--bravery, generosity, mercy, and love. The words courage +and cordiality are derived from a Latin word signifying heart. Many other +words and phrases, as hearty, heart-felt, to learn by heart, and +large-hearted, show how tenaciously these exploded opinions have fastened +themselves upon our language. + +[Sidenote: 29. The tendency at the present time? Why is this view +inadequate?] + +29. At the present time the tendency is to ascribe purely mechanical +functions to the heart. This view, like the older one, is inadequate; for +it expresses only a small part of our knowledge of this organ. The heart is +unlike a simple machine, because its motive power is not applied from +without, but resides in its own substance. Moreover, it repairs its own +waste, it lubricates its own action, and it modifies its movements +according to the varying needs of the system. It is more than a mere +force-pump, just as the stomach is something more than a crucible, and the +eye something more than an optical instrument. + +[Sidenote: 30. What are the arteries? Their walls? Their membrane?] + +30. THE ARTERIES.--The tube-like canals which carry the blood away from the +heart are the arteries. Their walls are made of tough, fibrous materials, +so that they sustain the mighty impulse of the heart, and are not ruptured. +In common with the heart, the arteries have a {115} delicately smooth +lining membrane. They are also elastic, and thus re-enforce the action of +the heart: they always remain open when cut across, and after death are +always found empty. + +[Sidenote: 31. Early anatomists? The service of the illustration?] + +31. The early anatomists observed this phenomenon, and supposing that the +same condition existed during life, came to the conclusion that these tubes +were designed to act as air-vessels, hence the name artery, from a Greek +word which signifies containing air. This circumstance affords us an +illustration of the confused notions of the ancients in reference to the +internal operations of the body. Cicero speaks of the arteries as +"conveying the breath to all parts of the body." + +[Sidenote: 32. The arterial system? The branches and sub-branches of the +arteries?] + +32. The arterial system springs from the heart by a single trunk, like a +minute and hollow tree, with numberless branches. As these branches leave +the heart they divide and subdivide, continually growing smaller and +smaller, until they can no longer be traced with the naked eye. If, then, +we continue the examination by the aid of a microscope, we see these small +branches sending off still smaller ones, until all the organs of the body +are penetrated by arteries. + +[Sidenote: 33. Successive undulations from the heart? Course of the +arteries? Protection of the arteries? General location of the arteries?] + +33. THE PULSE.--With each contraction of the left heart, the impulse causes +a wave-like motion to traverse the entire arterial system. If the arteries +were exposed to view, we might see successive undulations speeding from the +heart to the smallest of the branches, in about one-sixth part of a second. +The general course of the arteries is as far as possible from the surface. +This arrangement is certainly wise, as it renders them less liable to +injury, the wounding of an artery being especially dangerous. It also +protects the arteries from external and unequal pressure, by which the +force of the heart would be {116} counteracted and wasted. Accordingly, we +generally find these vessels hugging close to the bones, or hiding behind +the muscles and within the cavities of the body. + +[Sidenote: 34. Where do the arteries lie? If we apply the finger? Pulse? +Where felt?] + +34. In a few situations, however, the arteries lie near the surface; and if +we apply the finger to any of these parts, we will distinctly feel the +movement described, taking place in harmony with the heart-beat. This is +part of the wave-motion just mentioned, and is known as the pulse. All are +more familiar with the pulse at the wrist, in the _radial_ artery; but the +pulse is not peculiar to that position, for it may be felt in the _carotid_ +of the neck, in the _temporal_ at the temple, and elsewhere, especially +near the joints. + +[Illustration: FIG. 30.--THE FORM OF THE PULSE.] + +[Sidenote: 35. The pulse as an index? Of what does it inform the physician? +Instrument for recording pulsation?] + +35. Since the heart-beat makes the pulse, whatever affects the former +affects the latter also. Accordingly, the pulse is a good index of the +state of the health, so far as the health depends upon the action of the +heart. It informs the physician of the condition of the circulation in four +particulars: its rate, regularity, force, and fullness; and nearly every +disease modifies in some respect the condition of the pulse. A very +ingenious instrument, known as the sphygmograph, or pulse-writer, has +recently been invented, by the aid of which the pulse is made to write upon +paper its own signature, or rather to sketch its own profile. This +instrument shows with great accuracy the difference between the pulses of +health and those of disease. In Fig. 30 is traced the form of the pulse in +health, which should be read from left to right. That part of the trace +{117} which is nearly perpendicular coincides with the contraction of the +ventricles; while the wavy portion marks their dilatation. + +[Sidenote: 36. What are the veins? How do they form? What do they +resemble?] + +36. THE VEINS.--The vessels which convey the blood on its return to the +heart are the veins. They begin in the several organs of the body, and at +first are extremely small; but uniting together as they advance, they +constantly increase in size, reminding us of the way in which the fine +rootlets of the plant join together to form the large roots, or of the +rills and rivulets that flow together to form the large streams and rivers. +In structure, the veins resemble the arteries, but their walls are +comparatively inelastic. They are more numerous, and communicate with each +other freely in their course, by means of interlacing branches. + +[Illustration: FIG. 31.--THE VALVES OF THE VEINS.] + +[Sidenote: 37. Valves in the veins? What are they? Their position? +Experiment with the cord?] + +37. But the chief point of distinction is in the presence of the valves in +the veins. These are little folds of membrane, disposed in such a way, that +they only open to receive blood flowing toward the heart, and close against +a current in the opposite direction. Their position in the veins on the +back of the hand may be readily observed, if we first obstruct the return +of blood by a cord tied around the forearm or wrist. In a few minutes the +veins will appear swollen, and upon them will be seen certain prominences, +about an inch apart. These latter indicate the location of the valves, or, +rather, they show that the vessels in front of the valves are distended by +the blood, which cannot force a passage back through them. + +[Sidenote: 38. What will be proved by the experiment? What inference is +drawn?] + +38. This simple experiment proves that the true direction of the venous +blood is toward the heart. That the color {118} of the blood is dark, will +be evident, if we compare the hand thus bound by a cord with the hand not +so bound. It also proves that the veins lie superficially, while the +arteries are beneath the muscles, well protected from pressure; and that +free communication exists from one vein to another. If now we test the +temperature of the constricted member by means of a thermometer, we will +find that it is colder than natural, although the amount of blood is larger +than usual. From this fact we infer, that whatever impedes the venous +circulation tends to diminish vitality; and hence, articles of clothing or +constrained postures, that confine the body or limbs, and hinder the +circulation of the blood, are to be avoided as injurious to the health. + +[Sidenote: 39. Capillaries? How regarded? Harvey?] + +39. THE CAPILLARIES.--A third set of vessels completes the list of the +organs of the circulation, namely, the _capillary_ vessels, so called (from +the Latin word _capillaris_, hair-like), because of their extreme fineness. +They are, however, smaller than any hair, having a diameter of about 1/3000 +of an inch, and can only be observed by the use of the microscope. These +vessels may be regarded as the connecting link between the last of the +arteries and the first of the veins. The existence of these vessels was +unknown to Harvey, and was the one step wanting to complete his great work. +The capillaries were not discovered until 1661, a short time after the +invention of the microscope. + +[Sidenote: 40. The circulation of the blood in the web of a frog's foot? +Describe it. How general is the existence of the tissues?] + +40. The circulation of the blood, as seen under the microscope, in the +transparent web of a frog's foot, is a spectacle of rare beauty, possessing +more than ordinary interest, when we consider that something very similar +is taking place in our own bodies, on a most magnificent scale. It is like +opening a secret page in the history of our own frames. We there see +distinctly the three classes of vessels with their moving contents; first, +the artery, {119} with its torrent of blood rushing down from the heart, +secondly, the vein, with its slow, steady stream flowing in the opposite +direction; and between them lies the network of capillaries, so fine that +the corpuscles can only pass through "in single file." The current has here +an uncertain or swaying motion, hurrying first in one direction, then +hesitating, and then turning back in the opposite direction, and sometimes +the capillaries contract so as to be entirely empty. Certain of the tissues +are destitute of capillaries; such are cartilage, hair, and a few others on +the exterior of the body. In all other structures, networks of these +vessels are spread out in countless numbers: so abundant is the supply, +that it is almost impossible to puncture any part with the point of a +needle without lacerating tens, or even hundreds of these vessels. + +[Illustration: FIG. 32.--WEB OF A FROG'S FOOT, slightly magnified.] + +[Illustration: FIG. 33.--MARGIN OF FROG'S WEB magnified 30 diameters.] + +[Sidenote: 41. Elasticity of the capillaries? Grain of sand in the eye? +Blush? Other cases?] + +41. The capillaries are elastic, and may so expand as to produce an effect +visible to the naked eye. If a grain of sand, or some other foreign +particle, lodge in the eye, it will become irritated, and in a short time +the white of the eye will be "blood-shot." This appearance is due to an +{120} increase in the size of these vessels. A blush is another example of +this, but the excitement comes through the nervous system, and the cause is +some transient emotion, either of pleasure or pain. Another example is +sometimes seen in purplish faces of men addicted to drinking brandy; in +them the condition is a congestion of the capillary circulation, and is +permanent, the vessels having lost their power of elastic contraction. + +[Sidenote: 42. Show what time is required for a given portion of blood to +travel once around the body.] + +42. RAPIDITY OF THE CIRCULATION.--That the blood moves with great rapidity +is evident from the almost instant effects of certain poisons, as prussic +acid, which act through the blood. Experiments upon the horse, dog, and +other inferior animals, have been made to measure its velocity. If a +substance, which is capable of a distinct chemical reaction (as _potassium +ferrocyanide_, or _barium nitrate_), be introduced into a vein of a horse +on one side, and blood be taken from a distant vein on the other side, its +presence may be detected at the end of twenty or thirty-two seconds. In +man, the blood moves with greater speed, and the circuit is completed in +twenty-four seconds. + +[Sidenote: 43. Time required for all the blood to circulate completely +around?] + +43. What length of time is required for all the blood of the body to make a +complete round of the circulation? This question cannot be answered with +absolute accuracy, since the amount of the blood is subject to continual +variations. But, if we assume this to be one-eighth of the weight of the +body, about eighteen pounds, it will be sufficiently correct for our +purpose. Now to complete the circuit, this blood must pass once through the +left ventricle, the capacity of which is two ounces. Accordingly, we find +that, under ordinary circumstances, all the blood makes one complete +rotation every two minutes; passing successively through the heart, the +capillaries of the lungs, the arteries, the capillaries of the extremities, +and through the veins. {121} + +[Sidenote: 44. What is meant by assimilation? What can you say of its use, +etc.? Time?] + +44. ASSIMILATION.--The crowning act of the circulation, the furnishing of +supplies to the different parts of the body, is effected by means of the +capillaries. The organs have been wasted by use; the blood has been +enriched by the products of digestion. Here, within the meshes of the +capillary network, the needy tissues and the needed nutriment are brought +together. By some mysterious chemistry, each tissue selects and withdraws +from the blood the materials it requires, and converts them into a +substance like itself. This conversion of lifeless food into living tissue +is called assimilation. The process probably takes place at all times, but +the period especially favorable for it is during sleep. Then the +circulation is slower, and more regular, and most of the functions are at +rest. The body is then like some trusty ship, which after a long voyage is +"hauled up for repairs." + +[Sidenote: 45. What is stated of the injuries to the blood-vessels?] + +45. INJURIES TO THE BLOOD-VESSELS.--It is important to be able to +discriminate between an artery and a vein, in the case of a wound, and if +we remember the physiology of the circulation we may readily do so. For, as +we have already seen, haemorrhage from an artery is much more dangerous +than that from a vein. The latter tends to cease spontaneously after a +short time. The arterial blood flows away from the heart with considerable +force, in jets; its color being bright scarlet. The venous blood flows +toward the heart from that side of the wound furthest from the heart; its +stream being continuous and sluggish; its color dark. In an injury to an +artery, pressure should be made between the heart and the wound; and in the +case of a vein that persistently bleeds, it should be made upon the vessel +beyond its point of injury. {122} + +QUESTIONS FOR TOPICAL REVIEW. + + PAGE + 1. In what organisms is the so-called circulatory fluid found? 101 + 2. How is it designated in the different organisms? 101 + 3. What can you state of the importance of blood to the body? 101, 105 + 4. Of its great abundance, color, and composition? 101, 102, 107 + 5. Describe the corpuscles of the human blood. 102, 103, 104 + 6. What is said of them in comparison with those of the lower + animals? 103 + 7. Of the importance of sometimes detecting human from other blood? 103 + 8. What means have we of detecting blood in spots or stains? 103, 104 + 9. What is meant by coagulation of the blood? 104 + 10. What wisdom is there in the law of the blood's coagulation? 104, 105 + 11. How is this wisdom made manifest? 105 + 12. In what cases is the aid of the surgeon required? 105 + 13. What are the two great uses of the blood? 105 + 14. Through what mediums is the blood provided with new material and + relieved of the old material? 105 + 15. What do you understand by the operation called transfusion? 106 + 16. What cases of transfusion are reported of the lower animals? 106 + 17. What can you state of transfusion as practised upon man? 106 + 18. What further can you say on the subject? 106, 107 + 19. What changes take place in the color of the blood in its journey + through the system? 107 + 20. State all you can in relation to the circulation of the blood. 107 + 21. All, in relation to the size, shape, and location of the + heart. 107, 109 + 22. How is the loss of power in the heart movements obviated? 109 + 23. Give a description of the formation of the heart. 109, 110, 111 + 24. What can you state of the ventricles and auricles of the heart? 110 + 25. Describe the action of the heart. 111 + 26. What special vitality does the tissue of the heart possess? 111 + 27. State all you can on the subject. 111 + 28. Describe the course of the blood through the cavities of the + heart. 112 + 29. Describe the mechanism that regulates the heart-currents. 112 + 30. How do you account for the two heart-sounds at the front of + the chest? 112 + 31. State what you can of the frequency of the heart's action. 112, 113 + 32. Of the activity of the heart. 113, 114 + 33. What do you understand by the arteries? 114, 115 + 34. State what you can of the arteries and the arterial system. 114, 115 + 35. What do you understand by the pulse? 115, 116 + 36. In what part of the body may the pulse be felt? 116 + 37. What further can you state of the pulse? 116, 117 + 38. What are the veins? 117 + 39. Where do they exist, and how are they formed? 117 + 40. Describe the valves of the veins and their uses. 117 + 41. Now give a full description of the construction of the veins. 117 + 42. What further can you state of the veins? 117, 118 + 43. What do you understand by the capillaries? 118, 119 + 44. What service do the capillaries perform? 118, 119, 121 + 45. Describe the circulation of the blood in the region of + the heart. 118, 119 + 46. What can you state of the rapidity of the blood's circulation? 120 + 47. Of the process known as assimilation? 121 + 48. Of injuries to the blood-vessels? 121 + + * * * * * + + +{123} + +CHAPTER VIII. + +RESPIRATION. + + _The Objects of Respiration--The Lungs--The Air-Passages--The Movements + of Respiration--Expiration and Inspiration--The Frequency of + Respiration--Capacity of the Lungs--The Air we breathe--Changes in the + Air from Respiration--Changes in the Blood--Interchange of Gases in the + Lungs--Comparison between Arterial and Venous Blood--Respiratory + Labor--Impurities of the Air--Dust--Carbonic Acid--Effects of Impure + Air--Nature's Provision for Purifying the Air--Ventilation--Animal + Heat--Spontaneous Combustion._ + +[Sidenote: 1. Difference between the two sets of capillaries? Change +effected by respiration or breathing?] + +1. THE OBJECT OF RESPIRATION.--In one set of capillaries, or hair-like +vessels, the blood is impoverished for the support of the different members +and organs of the body. In another capillary system the blood is refreshed +and again made fit to sustain life. The former belongs to the greater or +_systemic_ circulation; the latter to the lesser or _pulmonary_, so called +from _pulmo_, the lungs, in which organs it is situated. The blood, as sent +from the right side of the heart to the lungs, is venous, dark, impure, and +of a nature unfit to circulate again through the tissues. But, when the +blood returns from the lungs to the left side of the heart, it has become +arterial, bright, pure, and no longer hurtful to the tissues. This +marvellous purifying change is effected by means of the very familiar act +of respiration, or breathing. + +[Sidenote: 2. What are the lungs? How many lungs are there? Lung-substance? +Its properties? The pleura?] + +2. THE LUNGS.--The lungs are the special organs of respiration. There are +two of them, one on each side of the chest, which cavity they, with the +heart, almost wholly occupy. The lung-substance is soft, elastic, and +sponge-like. Under pressure of the finger, it _crepitates_, or crackles, +and floats when thrown into water; these properties being {124} due to the +presence of air in the minute air-cells of the lungs. To facilitate the +movements necessary to these organs, each of them is provided with a double +covering of an exceedingly smooth and delicate membrane, called the +_pleura_. One layer of the pleura is attached to the walls of the chest, +and the other to the lungs; and they glide, one upon the other, with utmost +freedom. Like the membrane which envelops the heart, the pleura secretes +its own lubricating fluid, in quantities sufficient to keep it always +moist. + +[Illustration: FIG. 34.--ORGANS OF THE CHEST. + +A, Lungs. B, Heart. D, Pulmonary Artery. E, Trachea.] + +{125} + +[Illustration: FIG. 35.--LARYNX, TRACHEA, AND BRONCHIAL TUBES.] + +[Illustration: FIG. 36.--DIAGRAM AND SECTION OF THE AIR-CELLS.] + +[Sidenote: 3. Communication of the lungs with the external air? Bronchial +tubes?] + +3. THE AIR-PASSAGES. --The lungs communicate with the external air by means +of certain air-tubes, the longest of which, the _trachea_, or windpipe, +runs along the front of the neck (Fig. 34, E, and 35). Within the chest +this tube divides into two branches, one entering each lung; these in turn +give rise to numerous branches, or bronchial tubes, as they are called, +which gradually diminish in size until they are about one-twenty-fifth of +an inch in diameter. Each of these terminates in a cluster of little +pouches, or "air-cells," having very thin walls, and covered with a +capillary network, the most intricate in the body (Fig. 36). + +[Sidenote: 4. Office of the bronchial tubes? What further can you state of +them?] + +4. These tubes are somewhat flexible, sufficiently so to bend when the +parts move in which they are situated; but they are greatly strengthened by +bands or rings of cartilage which keep the passages always open; otherwise +there would be a constantly-recurring tendency to collapse after every +breath. The lung-substance essentially consists of these bronchial tubes +and terminal air-cells, with the blood-vessels ramifying about them (Fig. +37). At the top of the trachea is the larynx, a sort of {126} box of +cartilage, across which are stretched the vocal cords. Here the voice is +produced chiefly by the passage of the respired air over these cords, +causing them to vibrate. + +[Illustration: FIG. 37.--SECTION OF THE LUNGS.] + +[Sidenote: 5. The epiglottis? When it does not close in time, what is the +consequence?] + +5. Over the opening of the larynx is found the _epiglottis_, which fits +like the lid of a box at the entrance to the lungs, and closes during the +act of swallowing, so that food and drink shall pass backward to the +oesophagus, or gullet (Fig. 38). Occasionally it does not close in time, +and some substance intrudes within the larynx, when we at once discover, by +a choking sensation, that "something has gone the wrong way," and, by +coughing, we attempt to expel the unwelcome intruder. The epiglottis is one +of the many safeguards furnished by nature for our security and {127} +comfort, and is planned and put in place long before these organs are +brought into actual use in breathing and in taking food. + +[Illustration: FIG. 38.--SECTION OF MOUTH AND THROAT. + +A, The Tongue. B, The Uvula C, Vocal Cord. E, Epiglottis. L, Larynx. +N, Trachea. O, Oesophagus.] + +[Sidenote: 6. Lining of the air-passages? Ciliated cells? Their uses? The +three diseases of the lungs?] + +{128} + +[Illustration: FIG. 39.--CILIATED CELLS.] + +6. The air-passages are lined through nearly their whole extent with mucous +membrane, which maintains these parts in a constantly moist condition. This +membrane has a peculiar kind of cells upon its outer surface. If examined +under a powerful microscope, we may see, even for a considerable time after +their removal from the body, that these cells have minute hair-like +processes in motion, which wave like a field of grain under the influence +of a breeze (Fig. 39). This is a truly beautiful sight; and since it is +found that these little _cilia_, as they are called, always produce +currents in one direction, from within outward, it is probable that they +serve a useful purpose in catching and carrying away from the lungs dust +and other small particles drawn in with the breath (Fig. 39). The three +diseases which more commonly affect the lungs, as the result of exposure, +are pneumonia, or inflammation of the lungs, implicating principally the +air-cells; bronchitis, an inflammation of the large bronchial tubes; and +pleurisy, an inflammation of the investing membrane of the lungs, or +pleura. Among the young, an affection of the trachea takes place, known as +croup. + +[Sidenote: 7. The act of breathing? Extension of the chest by breathing?] + +7. THE MOVEMENTS OF RESPIRATION.--The act of breathing has two parts--(1), +_inspiration_, or drawing air into the lungs, and (2), _expiration_, or +expelling it from the lungs again. In inspiration, the chest extends in its +length, breadth, and height, or width. We can prove that this is the case +as regards the two latter, by observing the effect of a deep breath. The +ribs are elevated by means of numerous muscles, some of which occupy the +entire spaces between those bones. But the increase in length, or +vertically, is not so apparent, as it is caused by a muscle within the body +called the _diaphragm_, it being the thin partition which separates the +chest from the abdomen, rising like a dome within the chest. (Fig. 16). +{129} + +[Sidenote: 8. Contraction of the diaphragm? Power of the diaphragm? Effects +of extending the walls of the chest? The habit of taking frequent and deep +inspirations?] + +8. With every inspiration, the diaphragm contracts, and in so doing, +approaches more nearly a plane, or horizontal, surface, and thus enlarges +the capacity of the chest. Laughing, sobbing, hiccoughing, and sneezing are +caused by the spasmodic or sudden contraction of the diaphragm. The special +power of this muscle is important in securing endurance, or "long wind," as +it is commonly expressed; which may be obtained mainly by practice. It is +possessed in a marked degree by the mountaineer, the oarsman, and the +trained singer. As the walls of the chest extend, the lungs expand, and the +air rushes in to fill them. This constitutes an inspiration. The habit of +taking frequent and deep inspirations, in the erect position, with the +shoulders thrown back, tends greatly to increase the capacity and power of +the organs of respiration. + +[Sidenote: 9. Expiration? The mechanism of expiration?] + +9. EXPIRATION is a less powerful act than inspiration. The diaphragm +relaxes its contraction, and ascends in the form of a dome; the ribs +descend and contract the chest; while the lungs themselves, being elastic, +assist to drive out the air. The latter passes out through the same +channels by which it entered. At the end of each expiration there is a +pause, or period of repose, lasting about as long as the period of action. + +[Sidenote: 10. Frequency of respiration? Effect of hurried action of the +heart?] + +10. FREQUENCY OF RESPIRATION.--It is usually estimated that we breathe once +during every four beats of the heart, or about eighteen times in a minute. +There is, of course, a close relation between the heart and lungs, and +whatever modifies the pulse, in like manner affects the breathing. When the +action of the heart is hurried, a larger amount of blood is sent to the +lungs, and, as the consequence, they must act more rapidly. Occasionally, +the heart beats so very forcibly that the lungs cannot keep pace with it, +and then we experience a peculiar sense of {130} distress from the want of +air. This takes place when we run until we are "out of breath." At the end +of every fifth or sixth breath, the inspiration is generally longer than +usual, the effect being to change more completely the air of the lungs. + +[Sidenote: 11. Respiration controlled by the will? Advantage of the +knowledge to us?] + +11. Although, as a general rule, the work of respiration goes on +unconsciously and without exertion on our part, it is nevertheless under +the control of the will. We can increase or diminish the frequency of its +acts at pleasure, and we can "hold the breath," or arrest it altogether for +a short time. From twenty to thirty seconds is ordinarily the longest +period in which the breath can be held; but if we first expel all the +impure air from the lungs, by taking several very deep inspirations, the +time may be extended to one and a half or even two minutes. This should be +remembered, and acted upon, before passing through a burning building, or +any place where the air is very foul. The arrest of the respiration may be +still further prolonged by training and habit; thus it is said, the +pearl-fishers of India can remain three or four minutes under water without +being compelled to breathe. + +[Sidenote: 12. Capacity of the lungs? Time required to renovate the air in +the lungs? In tranquil respiration? Importance of the provision?] + +12. CAPACITY OF THE LUNGS.--The lungs are not filled and emptied by each +respiration. For while their full capacity, in the adult, is three hundred +and twenty cubic inches, or more than a gallon, the ordinary breathing air +is only one-sixteenth part of that volume, or twenty cubic inches, being +two-thirds of a pint. Accordingly, a complete renovation, or rotation, of +the air of the lungs does not take place more frequently than about once in +a minute; and by the gradual introduction of the external air, its +temperature is considerably elevated before it reaches the delicate +pulmonary capillaries. In tranquil respiration, less than two-thirds of the +breathing power is {131} called into exercise, leaving a reserve capacity +of about one hundred and twenty cubic inches, equivalent to three and one +half pints. This provision is indispensable to the continuation of life; +otherwise, a slight embarrassment of respiration, by an ordinary cold, for +instance, would suffice to cut off the necessary air, and the spark of life +would be speedily extinguished. + +[Sidenote: 13. The atmosphere? How high or deep? How essential to life? +Marine life in perfectly pure water and air?] + +13. THE AIR WE BREATHE.--The earth is enveloped on all sides by an +invisible fluid, called the atmosphere. It forms a vast and shoreless ocean +of air, forty-five miles deep, encircling and pervading all objects on the +earth's surface, which is absolutely essential for the preservation of all +vegetable and animal life,--in the sea, as well as on the land and in the +air. At the bottom, or in the lower strata of this aerial ocean, we move +and have our being. Perfectly pure water will not support marine life, for +a fish may be drowned in water from which the air has been exhausted, just +as certainly as a mouse, or any other land animal, will perish if put +deeply into the water for a length of time. The cause is the same in both +cases: the animal is deprived of the requisite amount of air. It is also +stated, that if the water-supply of the plant be deprived of air, its vital +processes are at once checked. + +[Sidenote: 14. Composition of the air? Properties of the two gases?] + +14. The air is not a simple element, as the ancients supposed, but is +formed by the mingling of two gases, known to the chemist as oxygen and +nitrogen, in the proportion of one part of the former to four parts of the +latter. These gases are very unlike, being almost opposite in their +properties: nitrogen is weak, inert, and cannot support life; while oxygen +is powerful, and incessantly active; and is the essential element which +gives to the atmosphere its power to support life and combustion. The +discovery of this fact was made by the French chemist, Lavoisier, in 1778. +{132} + +[Sidenote: 15. Air once breathed? An animal in it? A candle? Analysis of +expired air? Change in volume?] + +15. CHANGES IN THE AIR FROM RESPIRATION.--Air that has been once breathed +is no longer fit for respiration. An animal confined within it will sooner +or later die; so too, a lighted candle placed in it will be at once +extinguished. If we collect a quantity of expired air and analyze it, we +shall find that its composition is not the same as that of the inspired +air. When the air entered the lungs it was rich in oxygen; now it contains +twenty-five per cent. less of that gas. Its volume, however, remains nearly +the same; its loss being replaced by another and very different gas, which +the lungs exhaled, called _carbonic acid_, or, as the chemist terms it, +_carbon dioxide_. + +[Sidenote: 16. What else has the expired air gained? When and where +noticed?] + +16. The expired air has also gained moisture. This is noticed when we +breathe upon a mirror, or the window-pane, the surface being tarnished by +the condensation of the watery vapor exhaled by the lungs. In cold weather, +this causes the fine cloud which is seen issuing from the nostrils or mouth +with each expiration, and contributes in forming the feathery crystals of +ice which decorate our window-panes on a winter's morning. + +[Sidenote: 17. Nature of the watery vapor? Its effects upon animals?] + +17. This watery vapor contains a variable quantity of animal matter, the +exact nature of which is unknown; but when collected it speedily putrefies +and becomes highly offensive. From the effects, upon small animals, of +confinement in their own exhalations, having at the same time an abundant +supply of fresh air, it is believed that the organic matters thrown off by +the lungs and skin are direct and active poisons; and that to such +emanations from the body, more than to any other cause, are due the +depressing and even fatal results which follow the crowding of large +numbers of persons into places of limited capacity. {133} + +[Sidenote: 18. Give some of the instances furnished by history.] + +18. History furnishes many painful instances of the ill effects of +overcrowding. In 1756, of one hundred and forty-six Englishmen imprisoned +in the Black Hole of Calcutta, only twenty-three, at the end of eight +hours, survived. After the battle of Austerlitz, three hundred prisoners +were crowded into a cavern, where, in a few hours, two-thirds of their +number died. On board a steam-ship, during a stormy night, one hundred and +fifty passengers were confined in a small cabin, but when morning came, +only eighty remained alive. + +[Sidenote: 19. Change in the blood from blue to red. Upon what does the +change depend? How shown?] + +19. CHANGES IN THE BLOOD FROM RESPIRATION.--The most striking change which +the blood undergoes by its passage through the lungs, is the change of +color from a dark blue to bright red. That this change is dependent upon +respiration has been fully proved by experiment. If the trachea, or +windpipe, of a living animal be so compressed as to exclude the air from +the lungs, the blood in the arteries will gradually grow darker, until its +color is the same as that of the venous blood. When the pressure is removed +the blood speedily resumes its bright hue. Again, if the animal be made to +breathe an atmosphere containing more oxygen than atmospheric air, the +color changes from scarlet to vermilion, and becomes even brighter than +arterial blood. This change of color is not of itself a very important +matter, but it indicates a most important change of composition. + +[Sidenote: 20. What does the air lose and gain by respiration? What, the +blood? Air as food?] + +20. The air, as we have seen, by respiration loses oxygen and gains +carbonic acid: the blood, on the contrary, gains oxygen and loses carbonic +acid. The oxygen is the food of the blood corpuscles; while the articles we +eat and drink belong more particularly to the plasma of the blood. The air, +then, it is plain, is a sort of food, and we should {134} undoubtedly so +regard it, if it were not for the fact that we require it constantly, +instead of taking it at stated intervals, as is the case with our articles +of diet. Again, as the demand of the system for food is expressed by the +sensation of hunger, so the demand for air is marked by a painful sensation +called suffocation. + +[Sidenote: 21. Moist animal membranes? How shown with the bladder?] + +21. INTERCHANGE OF GASES IN THE LUNGS.--As the air and the blood are not in +contact, they being separated from each other by the walls of the air-cells +and of the blood-vessels, how can the two gases, oxygen and carbonic acid, +exchange places? Moist animal membranes have a property which enables them +to transmit gases through their substance, although they are impervious to +liquids. This may be beautifully shown by suspending a bladder containing +dark blood in a jar of oxygen. At the end of a few hours the oxygen will +have disappeared, the blood will be brighter in color, and carbonic acid +will be found in the jar. + +[Sidenote: 22. Gaseous diffusion? If oxygen be not received? If carbonic +acid be retained?] + +22. If this interchange takes place outside of the body, how much more +perfectly must it take place within, where it is favored by many additional +circumstances! The walls of the vessels and the air-cells offer no obstacle +to this process, which is known as gaseous diffusion. Both parts of the +process are alike of vital importance. If oxygen be not received, the +organs cease to act; and if carbonic acid be retained in the blood, its +action is that of a poison; unconsciousness, convulsions, and death +following. + +[Sidenote: 23. Difference in the appearance and composition of the blood? +Temperature of the blood? The blood while passing through the lungs? The +consequence?] + +23. DIFFERENCE BETWEEN ARTERIAL AND VENOUS BLOOD.--The following table +presents the essential points of difference in the appearance and +composition of the blood, before and after its passage through the lungs:-- +{135} + + _Venous Blood._ _Arterial Blood._ + + Color, Dark blue, Scarlet. + Oxygen, 8 per cent., 18 per cent. + Carbonic Acid, 15 to 20 per cent., 6 per cent., or less. + Water, More, Less. + +The temperature of the blood varies considerably; but the arterial stream +is generally warmer than the venous. The blood imparts heat to the air +while passing through the lungs, and consequently the contents of the right +side of the heart has a higher temperature than the contents on the left +side. + +[Sidenote: 24. What do we learn by means of the spectroscope? "Carriers of +oxygen?" Blue blood in the system?] + +24. By means of the spectroscope, we learn that the change of color in the +blood has its seat in the corpuscles; and that, according as they retain +oxygen, or release it, they present the spectrum of arterial or venous +blood. There evidently exists, on the part of these little bodies, an +affinity for this gas, and hence they have been called "carriers of +oxygen." It was long ago thought that blue blood was a trait peculiar to +persons of princely and royal descent, and boastful allusions to the "_sang +azure_" of kings and nobles are quite often met with. Physiology, however, +informs us that blue blood flows in the veins of the low as well as the +high, and that so far from its presence indicating a mark of purity, it, in +reality, represents the waste and decay of the system. + +[Sidenote: 25. The amount of air that passes in and out of the lungs?] + +25. AMOUNT OF RESPIRATORY LABOR.--During ordinary calm respiration, we +breathe eighteen times in a minute; and twenty cubic inches of air pass in +and out of the lungs with every breath. This is equivalent to the use of +three hundred and sixty cubic inches, or more than ten pints of air each +minute. From this we calculate that the quantity of air which hourly +traverses the lungs is about thirteen cubic feet, or seventy-eight gallons; +and daily, not {136} less than three hundred cubic feet, an amount nearly +equal to the contents of sixty barrels. + +[Sidenote: 26. Air absorbed in its transit through the lungs? The loss? +Carbonic acid exhaled? Effect of excitement or exertion? What estimate?] + +26. Of this large volume of air five per cent. is absorbed in its transit +through the lungs. The loss thus sustained is almost wholly of oxygen, and +amounts to fifteen cubic feet daily. The quantity of carbonic acid exhaled +by the lungs during the day is somewhat less, being twelve cubic feet. +Under the influence of excitement or exertion, the breathing becomes more +frequent and more profound; and then the internal respiratory work +increases proportionately, and may even be double that of the above +estimate. It has been estimated that in drawing a full breath, a man exerts +a muscular force equal to raising two hundred pounds placed upon the chest. + +[Sidenote: 27. Importance of the oxygen in the atmosphere? Injurious +character of gases?] + +27. IMPURITIES OF THE AIR.--The oxygen in the atmosphere is of such prime +importance, and its proportion is so nicely adjusted to the wants of man, +that any gas or volatile substance which supplants it must be regarded as a +hurtful impurity. All gases, however, are not alike injurious. Some, if +inhaled, are necessarily fatal; _arsenuretted hydrogen_ being one of these, +a single bubble of which destroyed the life of its discoverer, Gehlen. +Others are not directly dangerous, but by taking the place of oxygen, and +excluding it from the lungs, they become so. Into this latter class we +place carbonic acid. + +[Sidenote: 28. Pungency of gases? The inference? Our safeguard?] + +28. Most of the actively poisonous gases have a pungent or offensive odor; +and, as may be inferred, most repugnant odors indicate the presence of +substances unfit for respiration. Accordingly, as we cannot see or taste +these impurities, the sense of smell is our principal safeguard against +them; and we recognize the design which has planted this sense, like a +sentinel at the proper entrance of the {137} air-passages, the nostrils, to +give us warning of approaching harm. Take, as an example, the ordinary +illuminating gas of cities, from which so many accidents happen. How many +more deaths would it cause if, when a leak occurs, we were not able to +discover the escape of the gas by means of its disagreeable odor. + +[Sidenote: 29. The air of rooms in which fever-sick persons are confined?] + +29. Organic matters exist in increased measure in the expired breath of +sick persons, and impart to it, at times, a putrid odor. This is especially +true in diseases which, like typhus and scarlet fever, are referable to a +blood poison. In such cases the breath is one of the means by which nature +seeks to expel the offending material from the system. Hence, those who +visit or administer to fever-sick persons should obey the oft-repeated +direction, "not to take the breath of the sick." At such times, if ever, +fresh air is demanded, not alone for the sick, but as well for those who +are in attendance. + +[Sidenote: 30. Animalcula in the water? Dust in the air?] + +30. DUST IN THE AIR.--Attention has lately been directed to the dust, or +haze, that marks the ray of sunshine across a shaded room. Just as, many +years ago, it was discovered that myriads of animalcula infested much of +the water we drank, so now the microscope reveals "the gay motes that dance +along a sunbeam" to be, in part, composed of multitudes of animal and +vegetable forms of a very low grade, the germs of fermentation and +putrefaction, and the probable sources of disease. + +[Sidenote: 31. The best air filter? The remarks of Prof. Tyndall?] + +31. It is found that the best filter by which to separate this floating +dust from the air is cotton wool, although a handkerchief will imperfectly +answer the same purpose. In a lecture on this subject by Prof. Tyndall, he +remarks that, "by breathing through a cotton wool respirator, the noxious +air of the sick room is restored to practical purity. Thus filtered, +attendants may breathe the air unharmed. {138} In all probability, the +protection of the lungs will be the protection of the whole system. For it +is exceedingly probable that the germs which lodge in the air-passages are +those which sow epidemic disease in the body. If this be so, then disease +can certainly be warded off by filters of cotton wool. By this means, so +far as the germs are concerned, the air of the highest Alps may be brought +into the chamber of the invalid." + +[Sidenote: 32. Carbonic acid in volcanic regions? In Java? At Lake Avernus? +In mines?] + +32. CARBONIC ACID IN THE AIR.--We have already spoken of this gas as an +exhalation from the lungs, and a source of impurity; but it exists +naturally in the atmosphere in the proportion of one half part per +thousand. In volcanic regions it is poured forth in enormous quantities +from fissures in the earth's surface. Being heavier than air, it sometimes +settles into caves and depressions in the surface. It is stated that in the +island of Java, there is a place called the "Valley of Poison," where the +ground is covered with the bones of birds, tigers, and other wild animals, +which were suffocated by carbonic acid while passing. The Lake Avernus, the +fabled entrance to the infernal regions, was, as its name implies, +bird-less, because the birds, while flying over it, were poisoned by the +gas and fell dead into its waters. In mines, carbonic acid forms the +dreaded _choke-damp_, while carburetted hydrogen is the _fire-damp_. + +[Sidenote: 33. In the open air? Amount of carbonic acid exhaled by a man? A +gas-burner? A room fire? From furnaces?] + +33. In the open air, men seldom suffer from carbonic acid, for, as we shall +see presently, nature provides for its rapid distribution, and even turns +it to profitable use. But its ill effects are painfully evident in the +abodes of men, in which it is liable to collect as the waste product of +respiration and of that combustion which is necessary for lighting and +warming our homes. A man exhales, during repose, not less than one-half +cubic foot of carbonic acid per hour. One gas-burner liberates five cubic +feet in the {139} same time, and spoils about as much air as ten men. A +fire burning in a grate or stove emits some gaseous impurity, and at the +same time abstracts from the air as much oxygen as twelve men would consume +in the same period, thus increasing the relative amount of carbonic acid in +the air. From furnaces, as ordinarily constructed, this gas, with other +products of combustion, is constantly leaking and vitiating the air of +tightly-closed apartments. + +[Sidenote: 34. Effects of inhaling carbonic acid alone? In small +quantities?] + +34. EFFECTS OF IMPURE AIR.--Carbonic acid, in its pure form, is +irrespirable, causing rapid death by suffocation. Air containing forty +parts per thousand of this gas (the composition of the expired breath) +extinguishes a lighted candle, and is fatal to birds; when containing one +hundred parts, it no longer yields oxygen to man and other warm-blooded +animals; and is of course at once fatal to them. In smaller quantities, +this gas causes headache, labored respiration, palpitation, +unconsciousness, and convulsions. + +[Sidenote: 35. Effects of the air in crowded and badly ventilated rooms?] + +35. In crowded and badly ventilated apartments, where the atmosphere +relatively contains from six to ten times the natural amount of carbonic +acid, the contaminated air causes dulness, drowsiness, and faintness; the +dark, impure blood circulating through the brain, oppressing that organ and +causing it to act like a blunted tool. This is a condition not uncommon in +our schools, churches, court-rooms, and the like, the places of all others +where it is desirable that the mind should be alert and free to act; but, +unhappily, an unseen physiological cause is at work, dispensing weariness +and stupor over juries, audience, and pupils. + +[Sidenote: 36. A cause of consumption? How was the fact illustrated?] + +36. Another unmistakable result of living in and breathing foul air is +found in certain diseases of the lungs, especially consumption. For many +years the barracks of {140} the British army were constructed without any +regard to ventilation; and during those years the statistics showed that +consumption was the cause of a very large proportion of deaths. At last the +government began to improve the condition of the buildings, giving larger +space and air-supply; and as a consequence, the mortality from consumption +has diminished more than one-third. + +[Sidenote: 37. How, in the case of the lower animals? Tendency of certain +occupations?] + +37. The lower animals confined in the impure atmosphere of menageries, +contract the same diseases as man. Those brought from a tropical climate, +and requiring artificial warmth, generally die of consumption. In the +Zoological gardens of Paris, this disease affected nearly all monkeys, +until care was taken to introduce fresh air by ventilation; and then it +almost wholly disappeared. The tendency of certain occupations to shorten +life is well known; disease being occasioned by the fumes and dust which +arise from the material employed, in addition to the unhealthful condition +of the workshop or factory where many hours are passed daily. + +[Sidenote: 38. Give the fact as set forth in the table.] + +38. The following table shows the comparative amount of carbonic acid in +the air under different conditions and the effects sometimes produced:-- + + PROPORTION OF CARBONIC ACID. In 1000 parts of Air. + + Air of country. .4 + " " city. .5 + In hospital, well ventilated. .6 + In school, church, etc., fairly ventilated. 1.2 to 2.5 + In court-house, factory, etc., without ventilation. 4. to 40. + In bedroom, before being aired. 4.5 + In bedroom, after being aired. 1.5 + Constantly breathed, causing ill health. 2. + Occasionally breathed, causing discomfort. 3. + Occasionally breathed, causing distress. 10. + Expired air. 40. + Air no longer yielding oxygen 100. + +{141} + +[Sidenote: 39. What can you state of the diffusive power of gases? The +added influence of the winds?] + +39. NATURE'S PROVISION FOR PURIFYING THE AIR.--We have seen that carbonic +acid is heavier than air, and is poisonous. Why, then, does it not sink +upon and overwhelm mankind with a silent, invisible wave of death? Among +the gases there is a more potent force than gravity, which forever +precludes such a tragedy. It is known as the diffusive power of gases. It +acts according to a definite law, and with a resistless energy compelling +these gases, when in contact, to mingle until they are thoroughly diffused. +The added influence of the winds is useful, by insuring more rapid changes +in the air; air in motion being perfectly wholesome. The rains also wash +the air. + +[Sidenote: 40. How is the constant purity of the air secured? Explain the +process?] + +40. We have seen that the whole animal creation is constantly abstracting +oxygen from the atmosphere, and as constantly adding to it vast volumes of +a gas injurious alike to all, even in small quantities. How, then, does the +air retain, unchanged, its life-giving properties? The constant purity of +the air is secured by means of the vegetable creation. Carbonic acid is the +food of the plants, and oxygen is its waste product. The leaves are its +lungs, and under the stimulus of sunlight a vegetable respiration is set in +motion, the effects of which are just the reverse of the function we have +been considering. Thus nature purifies the air, and at the same time builds +up beautiful and useful forms of life from elements of decay. + +[Sidenote: 41. What process occurs in the sea? How is the fact +illustrated?] + +41. In the sea, as in the air, the same circle of changes is observed. +Marine animals consume oxygen and give off carbonic acid; while marine +plants consume carbonic acid and liberate oxygen. Taking advantage of this +fact, we may so arrange aquaria with fishes and sea-plants, in their proper +combinations, so that each supplies the needs of the other, and the water +requires seldom to be renewed. This {142} affords us, on a small scale, an +illustration of the mutual dependence of the two great kingdoms of nature; +as well as of those compensating changes which are taking place on such a +grand scale in the world about us. + +[Sidenote: 42. Character of the external air? Of the air in our dwellings? +What becomes imperative? Imperfect ventilation of our dwellings?] + +42. VENTILATION.--Since the external atmosphere, as provided by nature, is +always pure, and since the air in our dwellings and other buildings is +almost always impure, it becomes imperative that there should be a free +communication from the one to the other. This we aim to accomplish by +ventilation. As our houses are ordinarily constructed, the theory of +ventilation, "to make the internal as pure as the external air," is seldom +carried out. Doors, windows, and flues, the natural means of replenishing +the air, are too often closed, almost hermetically, against the precious +element. Special means, or special attention, must therefore be used to +secure even a fair supply of fresh air. This is still more true of those +places of public resort, where many persons are crowded together. + +[Sidenote: 43. What hints are given for the ventilation of our dwellings?] + +43. If there are two openings in a room, one as a vent for foul air, and +the other an inlet for atmospheric air, and if the openings be large, in +proportion to the number of air consumers, the principal object will be +attained. Thus, a door and window, each opening into the outer air, will +ordinarily ventilate a small apartment; or a window alone will answer, if +it be open both above and below, and the open space at each end be not less +than one inch for each occupant of the room, when the window is about a +yard wide. The direction of the current is generally from below upward, +since the foul, heated air tends to rise; but this is not essential. Its +rate need not be rapid; a "draught," or perceptible current, is never +necessary to good ventilation. The temperature of the air admitted may be +warm or cold. It is thought by many that if the {143} air is cold, it is +pure; but this is an error, since cold air will receive and retain the same +impurities as warm air. + +[Sidenote: 44. State what Florence Nightingale says about inhaling night +air?] + +44. Shall we open our bedrooms to the night air? Florence Nightingale says, +in effect, that night air is the only air that we can then breathe. "The +choice is between pure air without and impure air within. Most people +prefer the latter,--an unaccountable choice. An open window, most nights in +the year, can hurt no one. In great cities, night air is the best and +purest to be had in twenty-four hours. I could better understand, in towns, +shutting the windows during the day than during the night." + +[Sidenote: 45. Warmth of the bird as compared with that of the air? Of the +fish and the water? Heat in animals and plants? How illustrated with the +thermometer?] + +45. ANIMAL HEAT.--Intimately connected with respiration is the production +of animal heat, or the power of maintaining the temperature of the body +above that of the medium in which the creature moves; thus, the bird is +warmer than the air, and the fish than the water. This elevation of +temperature is a result of the various chemical changes which are +constantly taking place in the system. Although common to all animals, in a +greater or less degree, heat is not peculiar to them; since plants also +generate it, especially at the time of sprouting and flowering. If a +thermometer be placed in a cluster of geranium flowers, it will indicate a +temperature several degrees above that of the surrounding air. + +[Sidenote: 46. Amount of heat in animals, how apportioned? As regards the +birds? Frogs, and other sluggish animals? Arrangement made by zoologists?] + +46. Among animals great differences are noticed in this respect, but the +degree of heat produced is always proportional to the activity of +respiration and the amount of oxygen consumed. Accordingly, the birds, +whose habits are extremely active, and whose breathing capacity is the +greatest, have uniformly the highest temperature. Sluggish animals, on the +contrary, as frogs, lizards, and snakes, have little need for oxygen, and +have incompletely {144} developed lungs; these animals are cold to the +touch, that is, they have relatively a lower temperature than man, and +their positive temperature is but little above that of the external air. +Accordingly, zoologists have so arranged the animal kingdom that +_warm-blooded_ animals, including man, the birds, and the quadrupeds, are +classified together; while the _cold-blooded_ animals, such as the fish, +tortoise, frog, and all that have no vertebral column, are classed by +themselves. + +[Sidenote: 47. State what is said respecting the temperature of the human +body.] + +47. The temperature of the human body is about 100deg Fahrenheit, and +remains about the same through winter and summer, in the tropics as well as +in the frozen regions of the north. It may change temporarily within the +range of about twelve degrees; but any considerable, or long-continued +elevation or diminution of the bodily heat is certain to result +disastrously. + +[Sidenote: 48. Ability of man to adapt himself to different climates? In +what does the power to resist cold consist? What is said about warm +clothing?] + +48. Man is able to adapt himself to all extremes of climate; and, in fact, +by means of clothing, shelter, and food, is able to create for himself an +artificial climate where-ever he choses to reside. The power to resist cold +consists chiefly in preventing the heat which is generated by the vital +processes of the body from being lost by radiation. Warm clothing, such as +we wear in winter, has, in reality, the same temperature as that which is +worn in summer; but, by reason of being thick and porous, it is a bad +conductor of heat, and thus prevents the escape of that produced by the +body. If woollen fabrics were intrinsically warm, no one would wrap a piece +of flannel, or blanket, around a block of ice to prevent its melting in +summer. + +[Sidenote: 49. Men in an atmosphere above the boiling-point? In foundries +and glass works?] + +49. The faculty of generating heat explains how it is that we are enabled +to resist the effects of cold; but how does the body withstand a +temperature higher than its {145} own? Men have been known to remain +several minutes in an atmosphere heated above the boiling-point of water, +and yet the temperature of their own bodies was not greatly elevated. Those +who labor in foundries and glass-works are habitually subjected to very +high degrees of temperature, but they do not suffer in health more than +those engaged in many other occupations. + +[Sidenote: 50. The regulation of the temperature of the body. Give the +explanation.] + +50. The regulation of the temperature of the body is effected by means of +perspiration, and by its evaporation. So long as the skin acts freely and +the air freely absorbs the moisture, the heat of the body does not +increase, for whenever evaporation takes place, it is attended by the +abstraction of heat--that is, the part becomes relatively colder. This may +be tested by moistening some part of the surface with cologne, ether, or +other volatile liquid, and then causing it to evaporate rapidly by fanning. +The principle that evaporation produces cold has been ingeniously and +practically employed, in the manufacture of ice, by means of freezing +machines. + +[Sidenote: 51, 52. State what is said of spontaneous combustion.] + +51. SPONTANEOUS COMBUSTION.--Is it possible that the temperature of the +living body can be so increased, that its tissues will burn spontaneously? +From time to time, cases have been reported in which, by some mysterious +means, considerable portions of the human body have been consumed, +apparently by fire, the victim being found dead, or incapable of explaining +the occurrence. Hence, the theory has been current that, under certain +conditions, the tissues of the body might become self-ignited; and the fact +that this so-called _spontaneous combustion_ has ordinarily taken place in +those who had been addicted to the use of alcoholic drinks, has given a +color of probability to the opinion. It has been supposed that the flesh of +these unfortunate persons becoming saturated with the inflammable +properties of the alcohol thus taken into the {146} system, took fire upon +being exposed to a flame, as of a lighted candle, or, indeed, without any +external cause. But, whether this be possible or not, one thing is certain, +this strange kind of combustion has never been actually witnessed by any +one competent to give a satisfactory account of it. + +52. The results that have been observed may be satisfactorily explained by +the accidental ignition of the clothes, or other articles near the body, +and by the supposition that the individual was at the time too much +stupefied by intoxication, to notice the source of danger, and provide for +his safety. The highest temperature that has been observed in the body, +about 112deg Fahrenheit, is too low to ignite the vapor of alcohol; much +less will it cause the burning of animal tissues. It is undoubtedly true +that when the tissues are filled with alcohol, combustion will more easily +take place than when the body is in a normal state; but, under any +condition, the combustion of the body requires a higher degree of heat than +can be generated by the body itself, or the mere _proximity_ of a lighted +candle, or any cause of a similar character. {147} + +QUESTIONS FOR TOPICAL REVIEW. + + PAGE + 1. What is the object of respiration? 123 + 2. What are the special organs of respiration? 123 + 3. In what organs does a change in the blood take place? 123 + 4. What is the nature of the change? 123, 133 + 5. Where are the lungs situated, and what is the character of the + substance of which they are composed? 123, 125 + 6. Describe the facilities provided for the lung movements. 124 + 7. Describe the trachea, or windpipe. 124, 125, 127, 128 + 8. Describe the bronchial tubes, and their uses. 125, 126 + 9. What can you state in relation to the epiglottis? 126, 127 + 10. What are the cilia and what use do they probably serve? 128 + 11. How may the lungs be affected by not being properly protected? 128 + 12. Describe the movements necessary to the act of perfect + respiration. 128, 129 + 13. What is the diaphragm, and what is its office? 128, 129 + 14. How may the organs of respiration be so improved as to + increase their capacity and power? 129, 137 + 15. What is stated in relation to the frequency of respiration? 129, 130 + 16. To what extent may the act of respiration be subjected to + our wills? 130 + 17. What may be said to be the capacity of the lungs? 130, 131 + 18. How long does it take every particle of air in the lungs to + be expelled and new air to take its place? 130 + 19. What would be the consequences, if the entire capacity of + the lungs were constantly used? 130, 131 + 20. What would be the consequences to a fish put into water from + which the air had been completely exhausted? Why? 131 + 21. What is the air, and what are its parts? 131, 136, 138 + 22. What is the character of the air that has been just breathed? 132 + 23. Why is it that such air is not fit for respiration? 132, 139 + 24. What are the effects, as recorded in notable cases, of confinement + in places the air of which has been breathed "over and over?" 133 + 25. What can you state of changes in the blood from respiration? 133 + 26. What of the air, as an article of food? 133, 134 + 27. What, on the subject of interchange of gases in the lungs? 134 + 28. Explain the difference between arterial and venous blood. 134, 135 + 29. Explain, if you can, the cause of the difference. 135 + 30. State what you can in relation to blue blood. 135 + 31. In relation to the amount of labor exerted in respiration. 135, 136 + 32. In relation to the deleterious properties of different + gases. 136, 137 + 33. In relation to the dust that floats in the air. 137, 138 + 34. What are the properties of carbonic acid gas? 132, 138, 141 + 35. In what places is carbonic acid gas commonly found? 132, 138, 139 + 36. Describe the effects of carbonic acid gas. 132, 138, 139, 141 + 37. What are the general effects of breathing any impure + atmosphere? 139, 140 + 38. What are Nature's provisions for purifying the air? 141, 142 + 39. What hints and directions are given on the subject of + ventilation? 142, 143 + 40. How does the temperature of the body compare with the medium + in which it lives? 143 + 41. How is temperature of the body regulated and sustained? 143, 144, 145 + 42. State what you can on the subject of spontaneous combustion. 145, 146 + + * * * * * + + +{148} + +CHAPTER IX. + +THE NERVOUS SYSTEM. + + _Animal and Vegetative Functions--Sensation, Motion, and Volition--The + Structure of the Nervous System--The White and Gray Substances--The + Brain--Its Convolutions--The Cerebellum--The Spinal Cord and its System + of Nerves--The Anterior and Posterior Roots--The Sympathetic System of + Nerves--The Properties of Nervous Tissue--Excitability of Nervous + Tissues--The Functions of the Spinal Nerves and Cord--The Direction of + the Fibres of the Cord--Reflex Activity, and its Uses--The Functions of + the Medulla Oblongata and the Cranial Ganglia--The Reflex Action of the + Brain._ + +[Sidenote: 1. What processes are known as the vegetative functions? Why so +called? What properties and functions does the plant possess? Their +object?] + +1. ANIMAL FUNCTIONS.--The vital processes which we have been considering, +in the three previous chapters, of digestion, circulation, and +respiration--belong to the class of functions known as _vegetative_ +functions. That is, they are common to vegetables as well as animals; for +the plant, like the animal, can originate nothing, not even the smallest +particle of matter; and yet it grows, blossoms, and bears fruit, by reason +of obtaining and digesting the nutriment which the air and soil provide. +The plant has its circulatory fluid and channels, by which the nutriment is +distributed to all its parts. It has, also, a curious apparatus in its +foliage, by which it abstracts from the air those gaseous elements so +necessary to its support; and thus it accomplishes vegetable respiration. +These vegetative functions have their beginning and end within the organism +of the plant; and their object is the preservation of the plant itself, as +well as of the entire species. + +[Sidenote: 2. What second set of powers has the animal? What functions are +mentioned? The advantage they give?] + +2. The animal, in addition to these vegetative functions, has another set +of powers, by the use of which he becomes conscious of a world external to +himself, and brings {149} himself into active relations with it. By means +of the vegetative processes, his life and species are maintained; while, by +means of certain animal functions, he feels, acts, and thinks. These +functions, among which are sensation, motion, and volition, not only +distinguish the animal from the plant, but, in proportion to their +development, elevate one creature above another; and it is by virtue of his +pre-eminent endowment, in these respects, that man holds his position at +the head of the animal creation. + +[Sidenote: 3. Animals whose structure is simple? As we approach man? +Dependence of the animal functions of man?] + +3. Among animals whose structure is very simple, the hydra, or fresh-water +polyp, being an example, no special organs are empowered to perform +separate functions; but every part is endowed alike, so that if the animal +be cut into pieces, each portion has all the properties of the entire +original; and, if the circumstances be favorable, each of the pieces will +soon become a complete hydra. As we approach man, in the scale of beings, +we find that the organs multiply, and the functions become more complete. +The function of motion, the instruments of which--the muscles and +bones--have been considered in former chapters, and all the other animal +functions of man, depend upon the set of organs known as the nervous +system. + +[Sidenote: 4. The nervous tissues, of what composed? When examined by the +aid of the microscope? The white substance? The gray substance?] + +4. THE NERVOUS SYSTEM.--The intimate structure of this system differs from +any tissue which we have before examined. It is composed of a soft, pulpy +substance, which, early in life, is almost fluid, but which gradually +hardens with the growth of the body. When examined under the microscope, it +is found to be composed of two distinct elements:--(1) the white substance, +composing the larger proportion of the nervous organs of the body, which is +formed of delicate cylindrical filaments, about 1/6000 of an inch in +diameter, termed the nerve-fibres; and (2) the gray substance, composed of +grayish-red, or {150} ashen-colored cells, of various sizes, generally +possessing one or more off-shoots, which are continuous with the +nerve-fibres just mentioned. + +[Sidenote: 5. Nervous centres and ganglia? Nerves? What do they serve? +Cerebro-spinal system?] + +5. The gray, cellular substance constitutes the larger portion of those +important masses, which bear the name of _nervous centres_ and _ganglia_ +(from _ganglion_, a knot), and in which all the nerve-fibres unite. These +white nerve-fibres are found combined together in long and dense cords, +called _nerves_ (from _neuron_, a cord), which serve to connect the nervous +centres with each other, and to place them in communication with all the +other parts of the body which have sensibility or power of motion. That +part of the nervous system which is concerned in the animal functions, +comprises the brain, the spinal cord, and the nerves which are derived +therefrom; these are, together, called the _cerebro-spinal_ system (Fig. +40); while that other set of organs, which presides over, and regulates the +vegetative functions, is called the sympathetic system of nerves. + +[Sidenote: 6. Location of the brain? Its weight? Its shape? Of what it +consists? What organs at the base?] + +6. THE BRAIN.--The brain is the great volume of nervous tissue that is +lodged within the skull. It is the largest and most complex of the nervous +centres, its weight, in the adult, being about fifty ounces, or +one-fortieth of that of the whole body. The shape of the brain is oval, or +egg-shaped, with one extremity larger than the other, which is placed +posteriorly in the skull, to the concavity of which it very closely +conforms. The brain consists chiefly of two parts; the _cerebrum_, or brain +proper, and the _cerebellum_, or "little brain." In addition to these, +there are several smaller organs at the base, among which is the +commencement or expansion of the spinal cord, termed the _medulla +oblongata_, or oblong marrow. + +{151} + +[Illustration: FIG. 40.--THE CEREBRO-SPINAL SYSTEM.] + +{152} [Sidenote: 7. The tissue of the brain? What, therefore, is required? +Blows on the head? Membranes of the brain? Blood sent to the brain?] + +7. The tissue of the brain is soft and easily altered in shape by pressure; +it therefore requires to be placed in a well-protected position, such as is +afforded by the skull, or _cranium_, which is strong without being +cumbrous. In the course of an ordinary lifetime, this bony box sustains +many blows, with little inconvenience; while, if they fell directly upon +the brain, they would at once, and completely, disorganize that structure. +Within the skull, the brain is enveloped by certain membranes, which at +once protect it from friction, and furnish it with a supply of nutrient +vessels; they are called the _arachnoid_, or "spider's web," the _dura +mater_ and the _pia mater_, or the "tough" and "delicate coverings." The +supply of blood sent to the brain is very liberal, amounting to one-fifth +of all that the entire body possesses. The brain of man is heavier than +that of any other animal, except the elephant and whale. + +[Sidenote: 8. Size of the brain proper? How divided? The exterior of the +hemispheres? The interior?] + +8. THE CEREBRUM.--The brain proper, or _cerebrum_, is the largest of the +intracranial organs, and occupies the entire upper and front portion of the +skull. It is almost completely bisected, by a fissure, or cleft, running +through it lengthwise, into two equal parts called _hemispheres_. The +exterior of these hemispheres is gray in color, consisting chiefly of +nerve-cells, arranged so as to form a layer of gray matter one-fifth of an +inch in thickness, and is abundantly supplied with blood-vessels. The +interior of the brain, however, is composed almost wholly of white +substance, or nerve-fibres. + +[Sidenote: 9. The surface of the cerebrum, how marked? The gray matter of +the surface? Extent of the entire brain surface? Source of nervous power? +What further?] + +9. The surface of the cerebrum is divided by a considerable number of +tortuous and irregular furrows, about an inch deep, into "convolutions," as +shown in Fig. 41. Into these furrows the gray matter of the surface is +extended, and, in this manner, its quantity is vastly increased. The extent +of the entire surface of the brain, {153} with the convolutions unfolded, +is computed to be equal to four square feet; and yet it is easily enclosed +within the narrow limits of the skull. When it is stated that the gray +matter is the true source of nervous power, it becomes evident that this +arrangement has an important bearing on the mental capacity of the +individual. And it is noticed that in children, before the mind is brought +into vigorous use, these markings or furrows on the surface are +comparatively shallow and indistinct; the same fact is true of the brain in +the less civilized races of mankind and in the lower animals. It is also +noticeable, that among animals, those are the most capable of being +educated which have the best development of the cerebrum. + +[Illustration: FIG. 41.--UPPER SURFACE OF THE CEREBRUM. + +A, Longitudinal Fissure. B, The Hemispheres.] + +[Sidenote: 10. Location of the "little brain?" How divided? Its surface and +interior? Its subdivisions? Its size?] + +10. THE CEREBELLUM.--The "little brain" is placed beneath the posterior +part of the cerebrum, and, like the latter, is divided into hemispheres. +Like it, also, the surface of the cerebellum is composed of gray matter, +and its interior is chiefly white matter. It has, however, no convolutions, +but is subdivided by many crescentic, parallel ridges, which, sending down +gray matter deeply into the {154} white, central portion, gives the latter +a somewhat branched appearance. This peculiar appearance has been called +the _arbor vitae_, or the "tree of life," from the fact that when a section +of the organ is made, it bears some resemblance to the trunk and branches +of a tree (Fig. 42, F). In size, this cerebellum, or "little brain," is +less than one-eighth of the cerebrum. + +[Illustration: FIG. 42.--VERTICAL SECTION OF THE BRAIN. + +A, Left Hemisphere of Cerebrum. B, Corpus Callosum. C, Optic Thalamus. +D, The Pons Varolii. E, Upper extremity of the Spinal Cord. F, The Arbor +Vitae.] + +[Sidenote: 11. Medulla oblongata? Cranial nerves? Their shape and +position?] + +11. From the under surface of the cerebrum, and from the front margin of +the cerebellum, fibres collect together to form the _medulla oblongata_ +(Fig. 43, MA), which, on issuing from the skull, enters the spinal column, +and then becomes known as the spinal cord. From the base of the brain, and +from the sides of the medulla originate, also, the _cranial nerves_, of +which there are twelve pairs. These nerves are round cords of glistening +white appearance, and, {155} like the arteries, generally lie remote from +the surface of the body, and are well protected from injury. + +[Illustration: FIG. 43.--THE BASE OF THE BRAIN.] + +[Sidenote: 12. The spinal cord? Of what composed? How divided? Each half?] + +12. THE SPINAL CORD.--The spinal cord, or "marrow," is a cylindrical mass +of soft nervous tissue, which occupies a chamber, or tunnel, fashioned for +it in the spinal column (Fig. 44). It is composed of the same substances as +the brain; but the arrangement is exactly reversed, the white matter +encompassing or surrounding the gray matter instead of being encompassed by +it. The amount of the white substance is also greatly in excess of the +other material. A vertical fissure partly separates the cord into two +lateral halves, and each half is composed of two separate bundles of +fibres, which are named the anterior and posterior columns. + +{156} [Illustration: FIG. 44. + +A, Cerebrum. B, Cerebellum. D, D, Spinal Cord.] + +[Sidenote: 13. Uses of these columns? Importance of this part of the +nervous system? How protected?] + +13. These columns have entirely different uses, and each of them unites +with a different portion of the nerves which have their origin in the +spinal cord. The importance of this part of the nervous system is apparent +from the extreme care taken to protect it from external injury. For, while +a very slight disturbance of its structure suffices to disarm it of its +power, yet so staunch is its bony enclosure, that only by very severe +injuries is it put in peril. The three membranes that cover the brain are +continued downward so as to envelope and still further shield this delicate +organism. + +[Sidenote: 14. The spinal nerves? The posterior root? The nerves, how +arranged? Their office?] + +14. THE SPINAL NERVES.--The spinal nerves, thirty-one pairs in number, +spring from each side of the cord by two roots, an anterior and a posterior +root, which have the same functions as the columns bearing similar names. +The posterior root is distinguished by possessing a ganglion of gray +matter, and by a somewhat larger size. The successive points of departure, +or the off-shooting of these nerves, occur at short and nearly regular +intervals along the course of the spinal cord. Soon after leaving these +points, {157} the anterior and posterior roots unite to form the trunk of a +nerve, which is distributed, by means of branches, to the various organs of +that part of the body which this nerve is designed to serve. The spinal +nerves supply chiefly the muscles of the trunk and limbs and the external +surface of the body. + +[Sidenote: 15. The nerve tissue? Its character? Course of each nerve +fibre?] + +15. The tissue composing the nerves is entirely of the white variety, or, +in other words, the nerve-fibres; the same as we have observed forming a +part of the brain. But the nerves, instead of being soft and pulpy, as in +the case of the brain, are dense in structure, being hardened and +strengthened by means of a fibrous tissue which surrounds each of these +delicate fibres, and binds them together in glistening, silvery bundles. +Delicate and minutely fine as are these nerve-fibres, it is probable that +each of them pursues an unbroken, isolated course, from its origin, in the +brain or elsewhere, to that particular point which it is intended to serve. +For, although their extremities are often only a hair's breadth distant +from each other, the impression which any one of them communicates is +perfectly distinct, and is referred to the exact point whence it came. + +[Sidenote: 16. How may we illustrate the fact? The fibre connecting the +brain with a point in the foot?] + +16. This may be illustrated in a simple manner, thus: if two fingers be +pressed closely together, and the point of a pin be carried lightly across +from one to the other, the eyes may be closed, and yet we can easily note +the precise instant when the pin passes from one finger to the other. If +the nerve-fibres were less independent, and if it were necessary that they +should blend with and support each other, all accuracy of perception would +be lost, and all information thus afforded would be pointless and confused. +These silvery threads must, therefore, be spun out with an infinite degree +of nicety. Imagine, for instance, the fibre which {158} connects the brain +with some point on the foot,--its length cannot be less than one hundred +thousand times greater than its diameter; and yet it performs its work with +as much precision as fibres that are comparatively much stronger and less +exposed. + +[Sidenote: 17. The sympathetic system of nerves? Of what does it consist?] + +17. THE SYMPATHETIC SYSTEM.--The _sympathetic system_ of nerves remains to +be described. It consists of a double chain of ganglia, situated on each +side of the spinal column, and extending through the cavities of the trunk, +and along the neck into the head. These ganglia are made up for the most +part of small collections of gray nerve-cells, and are the nerve-centres of +this system. From these, numerous small nerves are derived, which connect +the ganglia together, send out branches to the cranial and spinal nerves, +and form networks in the vicinity of the stomach and other large organs. A +considerable portion of them also follows the distribution of the large and +small blood-vessels, in which the muscular tunic appears. Branches also +ascend into the head, and supply the muscles of the eye and ear, and other +organs of sense. + +[Sidenote: 18. Association of the various regions of the body? If one +member suffers? Blushing?] + +18. In this manner, the various regions of the body are associated with +each other by a nervous apparatus, which is only indirectly connected with +the brain and spinal cord; and thus it is arranged that the most widely +separated organs of the body are brought into close and active sympathy +with each other, so that, "if one member suffers, all the other members +suffer with it." From this fact, the name _sympathetic system_, or the +_great sympathetic nerve_, has been given to the complicated apparatus we +have briefly described. Blushing and pallor are caused by mental emotions, +as modesty and fear, which produce opposite conditions of the capillaries +of the face by means of these sympathetic nerves. {159} + +[Sidenote: 19. Properties of nervous tissue? Office of the gray substance? +Of the white? The nervous centres? White fibres?] + +19. THE PROPERTIES OF NERVOUS TISSUE.--We have seen that in all parts of +this system, there are only two forms of nervous tissue; namely, the gray +substance and the white substance, so called from their difference of color +as seen by the naked eye; or the nerve-cell, and the nerve-fibre, so called +from their microscopic appearance. Now these two tissues are not commonly +mingled together, but either form separate organs, or distinct parts of the +same organs. This leads us to the conclusion that their respective uses are +distinct. And this proves to be the simple fact; wherever we find the gray +substance, we must look upon it as performing an active part in the system, +that is, it originates nervous impulses; the white matter, on the contrary, +is a passive agent, and serves merely as a conductor of nervous influences. +Accordingly, the nervous centres, composed so largely of the gray cells, +are the great centres of power, and the white fibres are simply the +instruments by which the former communicate with the near and distant +regions of the body under their control. + +[Sidenote: 20. What comparison is made between the brain and the nation's +capitol? The vital property, excitability? What example is given?] + +20. We may compare the brain, then, to the capital, or seat of government, +while the various ganglia, including the gray matter of the cord, like so +many subordinate official posts, are invested with authority over the +outlying provinces; and the nerves, with the white matter of the cord, are +the highways over which messages go and return between these provinces and +the local or central governments. But both forms of nervous tissue possess +the same vital property, called excitability; by which term is meant, that +when a nerve-cell or fibre is stimulated by some external agent, it is +capable of receiving an impression and of being by it excited into +activity. A ray of light, for example, falling upon one extremity of a +fibre in the eye, excites it throughout its whole length; and its {160} +other extremity, within the brain, communicating with a nerve-cell, the +latter, in its turn, is excited, and the sensation of sight is produced. + +[Sidenote: 21. Change in the nervous tissues? Nerve force and electricity?] + +21. What sort of change takes place in the nervous tissue when its +excitability is aroused, is not known; certainly none is visible. On this +account, it has been thought by some, that the nerve-fibre acts after the +manner of a telegraph wire; that is, it transmits its messages without +undergoing any material change of form. But, though the comparison is a +convenient one, it is far from being strictly applicable; and the notion +that nerve-force is identical with electricity has been fully proved to be +incorrect. + +[Sidenote: 22. Functions of the nerves? In the case of the nerve of a +living animal? Of the human body?] + +22. THE FUNCTIONS OF THE NERVES.--The nerves are the instruments of the two +grand functions of the nervous system, Sensation and Motion. They are not +the true centres of either function, but they are the conductors of +influences which occasion both. If the nerve in a limb of a living animal +be laid bare, and irritated by pinching, galvanizing, or the like, two +results follow, namely: the animal experiences a sensation, that of pain, +in the part to which the nerve is distributed, and the limb is thrown into +convulsive action. When a nerve in a human body is cut by accident, or +destroyed by disease, the part in which it ramifies loses both sensation +and power of motion; or, in other words, it is paralyzed. We accordingly +say that the nerves have a twofold use, a _sensory_ and _motor_ function. + +[Sidenote: 23. If an exposed nerve be divided? What is proved? The course +of the sensory set of fibres? Of the motor set? To what are they likened?] + +23. If a nerve that has been exposed be divided, and the inner end, or that +still in connection with the nerve-centres, be irritated, sensation is +produced, but no movement takes place. But if the outer end, or that still +connected with the limb, be irritated, then no pain is felt, but {161} +muscular contractions are produced. Thus we prove that there are two +distinct sets of fibres in the nerves; one of which, the _sensory_ fibres, +conduct toward the brain, and another, the _motor_ fibres, conduct to the +muscles. The former may be said to begin in the skin and other organs, and +end in the brain; while the latter begin in the nervous centres and end in +the muscles. They are like a double line of telegraph wires, one for +inquiries, the other for responses. + +[Sidenote: 24. The two roots of the spinal nerves? What has been found? +Difference of the two sorts of fibres? Result of their union?] + +24. We have already spoken of the two roots of the spinal nerves, called +from their points of origin in the spinal cord, the anterior and posterior +roots. These have been separately cut and irritated in the living animal, +and it has been found that the posterior root contains only sensory fibres, +and the anterior root has only motor fibres. So that the nerves of a limb +may be injured in such a way that it will retain power of motion and yet +lose sensation; or the reverse condition, feeling without motion, may +exist. Between these two sorts of fibres, no difference of structure can be +found; and where they have joined to form a nerve it is impossible to +distinguish one sort from the other. + +[Sidenote: 25. Transient paralysis? When such is the case with the leg? +What other fact is observed?] + +25. Occasionally a nerve is so compressed as to be temporarily unable to +perform its functions: a transient paralysis then takes place. This is the +case when the leg or arm "gets asleep," as it is expressed. When such is +the condition with the leg, and the person suddenly attempts to walk, he is +liable to fall, inasmuch as the motor fibres cannot convey orders to the +muscles of the limb. Another fact is observed: there is no sensation in +this nerve at the point of its compression; but the whole limb is numb, and +tingling sensations are felt in the foot, the point from which the sensory +fibres arise. + +[Sidenote: 26. What does this illustrate? Sensation? The feeling after a +limb has been amputated? Striking of the "funny bone?"] + +26. This illustrates the manner in which the brain {162} interprets all +injuries of the trunk of a nerve. Sensation or pain is not felt at the +point of injury, but is referred to the outer extremities of the nerve, +where impressions are habitually received. This is the reason why, after a +limb has been amputated by the surgeon, the patient appears to suffer pain +in the member that has been severed from the body; while some form of +irritation at the end of the nerve in the wound, or stump, is the real +source of his distress. Again, when the "funny-bone"--that is, the ulnar +nerve at the elbow,--is accidentally struck, the tingling sensations thus +produced are referred to the outer side of the hand and the little finger, +the parts to which that nerve is distributed. + +[Sidenote: 27. The spinal nerves, and two from the brain? Of the remainder? +Difference in the nerves? How accounted for? The rate of conduction along a +nerve? As compared with electricity?] + +27. All the spinal nerves, and two from the brain, are concerned in both +sensation and motion. Of the remainder of the cranial nerves, some are +exclusively motor, others exclusively sensory; and still others convey, not +ordinary sensations, but special impressions, such as sight, hearing, and +smell, which we have yet to consider. However much the functions of the +nerves seem to vary, there is but little difference discoverable in the +nerves themselves, when examined under the microscope. Whatever difference +exists must be accounted for in consequence of the nerves communicating +with different portions of the gray matter of the brain. The rate of motion +of a message, to or from the brain along a nerve, has been measured by +experiment upon the lower animals, and estimated in the case of man at +about two hundred feet per second. As compared with that of electricity, +this is a very slow rate, but, in respect to the size of the human body, it +is practically instantaneous. + +[Sidenote: 28. Functions of the anterior and posterior columns of the cord? +If the cord be divided?] + +28. THE FUNCTIONS OF THE SPINAL CORD.--As the {163} anterior and posterior +roots of the spinal nerves have separate functions, so the anterior and +posterior columns of the cord are distinct in function. The former are +concerned in the production of motion, the latter in sensation. If the cord +be divided, as before in the case of the nerve, it is found that the parts +below the point of injury are deprived of sensation and of the power of +voluntary motion on both sides of the body, a form of paralysis which is +called _paraplegia_. + +[Sidenote: 29. Paraplegia? Result and danger to life? When the injury +occurs in the neck?] + +29. This form of disease, paraplegia, is sometimes seen among men, +generally as the result of a fall, or some other severe accident, by which +the bones of the spine are broken, and the cord is crushed, or pierced by +fragments of bone. The parts which are supplied by nerves from the cord +above the point of injury are as sensitive and mobile as before. The +results are similar, whether the division happens at a higher or lower +portion of the spinal cord; but the danger to life increases proportionally +as the injury approaches the brain. When it occurs in the neck, the muscles +of inspiration are paralyzed, since they are supplied by nerves issuing +from that region; and as a result of this paralysis, the lungs are unable +to act, and life is speedily brought to a close. + +[Sidenote: 30. Experiment of cutting the spinal cord of an animal? What +inference is drawn?] + +30. When the spinal cord of an animal has been cut, in experiment, it may +be irritated in a manner similar to that alluded to when considering the +nerves. If, then, the upper cut surface be excited, it is found that pain, +referable to the parts below the cut, is produced; but when the lower cut +surface is irritated, no feeling is manifested. So we conclude that in +respect to sensation, the spinal cord is not its true centre, but that it +is merely a conductor, and is therefore the great sensory nerve of the +body. When the lower surface of the cut is irritated, the muscles of the +{164} parts below the section are violently contracted. Hence, we conclude +that, in respect to the movements ordered by the will, the spinal cord is +not their source; but that it acts only as a conductor, and is, +accordingly, the great motor nerve of the body. + +[Sidenote: 31. What singular fact is noticed? What does the result show?] + +31. DIRECTION OF THE FIBRES OF THE CORD.--If one lateral half of the spinal +cord be cut, or injured, a very singular fact is observed. All voluntary +power over the muscles of the corresponding half of the body is lost, but +the sensibility of that side remains undiminished. This result seems to +show that the motor fibres of the cord pursue a direct course, while its +sensory fibres are bent from their course. And this has been proved to be +the fact; for immediately after the posterior roots--the conductors of +sensory impressions--join the posterior columns, they enter the gray matter +of the cord, and passing over, ascend to the brain on the opposite side. +Accordingly, the sensory fibres from the right and left sides interlace +each other in the gray matter; this arrangement has been termed the +_decussation_, or crossing of these fibres. This condition serves to +explain how a disease or injury of the cord may cause a paralysis of motion +in one leg, and a loss of sensation in the other. + +[Sidenote: 32. Direction of the anterior or motor columns? In the cord +itself? In the medulla oblongata? The decussation?] + +32. The direction of the anterior, or motor columns of the cord, is +downward from the brain. In the cord itself, the course of the motor fibres +is for the most part, a direct one; but in the medulla oblongata, or upper +extremity of the cord, and therefore early in their career, these fibres +decussate, or cross from side to side in a mass; and not separately, as in +the case of the posterior fibres just mentioned. This arrangement is termed +the _decussation_ of the anterior columns of the medulla. + +[Sidenote: 33. Result of the double interlacing of fibres? Where is the +seat of pain when the right hand is hurt? The moving of the foot? Loss of +sensation in one side of the body?] + +33. From this double interlacing of fibres results a {165} crossed action +between the original and terminal extremity of all nerve-fibres which pass +through the medulla; namely, those of all the spinal nerves. Consequently, +if the right hand be hurt, the left side of the brain feels the pain; and +if the left foot move, it is the right hemisphere which dictates its +movement. For the same reason, when a loss of sensation and power of motion +affecting the right side of the body alone is observed, the physiologist +understands that the brain has been invaded by disease upon its left side. +This affection is termed _hemiplegia_, or the "half-stroke." The +full-stroke, which often follows the rupture of a blood-vessel in the +brain, is commonly called _paralysis_. + +[Sidenote: 34. What other important use has the cord? What is the activity +denominated?] + +34. THE REFLEX ACTION OF THE CORD.--We have already considered the cord as +the great motor and sensory nerve of the body, but it has another and +extremely important use. By virtue of the gray matter, which occupies its +central portion, it plays the part of an independent nerve centre. The +spinal cord not only conducts some impressions to the brain, but it also +arrests others; and, as it is expressed, "reflects" them into movements by +its own power. This mode of nervous activity is denominated the _Reflex +Action_ of the cord. + +[Sidenote: 35. Example of the fowl? Centipede? Frog? What do they prove?] + +35. A familiar example of this power of the cord is found in the violent +movements which agitate a fowl after its head has been cut off. The +cold-blooded animals also exhibit reflex movements in an astonishing +degree. A decapitated centipede will run rapidly forward, and will +seemingly strive to overturn, or else climb over obstacles placed in its +way. A frog similarly mutilated will sustain its headless body upon its +feet, in the standing posture, just as it might do if it were still alive. +If pushed over, it will regain its feet; and if the feet are irritated, it +will {166} jump forward. There can be no doubt that, in the lower animals, +movements may take place which are completely divorced from the will, +sensation, and consciousness; for in those animals, as well as in man, +these faculties have their principal seat within the brain. + +[Sidenote: 36. What is necessary in most cases to awaken reflex movements? +In the case of the fowl? Convulsions which follow decapitation?] + +36. An irritation is necessary, in most instances, to awaken reflex +movements. In the case of the decapitated fowl, its muscles are excited to +convulsive action by reason of its being thrown upon the hard ground and +roughly handled. Let it be treated differently, and the convulsions will +not take place: let it be laid gently upon soft cotton, and the body will +remain comparatively quiet. It may comfort some people to know that the +convulsions which follow decapitation are not attended with pain; nor are +they a necessary part of the "act of death," as some suppose. + +[Sidenote: 37. Actions in the human body distinct from voluntary efforts?] + +37. In the human body, likewise, actions are excited that are entirely +distinct from the ordinary voluntary efforts. It is not permissible, +desirable, nor even necessary to decapitate a man that the body may be +disconnected from his brain, in order to test the effect of irritation upon +the spinal cord; although the bodies of beheaded criminals have been +experimented upon, and caused to move by powerful galvanic batteries. The +resort to such means of experiment is rendered unnecessary by the +occurrence of certain deplorable cases of disease and injury, which +effectually sever all communication between the brain and a large part of +the body. + +[Sidenote: 38. Reflex action after injury of the cord? Why not due to the +muscles?] + +38. Thus, the cord may be so far injured, as the result of accident, as to +terminate all sensation and voluntary motion in the lower half of the body, +the patient seemingly becoming lifeless and powerless from the waist +downward. And yet, by tickling or pinching either foot, the leg {167} of +the same side may be made to jerk, or even to kick with considerable force; +but, unless the patient is observing his limbs, he is wholly unconscious of +these movements, which are, therefore, performed independently of the +brain. And they are in nowise due to the muscles of the limb; for, if the +cord itself becomes diseased below the point of injury, the muscles cease +to contract. + +[Sidenote: 39. What are the requisites for the production of this form of +nervous action?] + +39. For the production of this form of nervous action three things are +requisite--(1) a nerve to conduct messages from the surface of the body, +one of that variety formerly described as sensory, but which are now +incapable of awakening sensation; (2) a portion of uninjured spinal cord +which shall reflect or convert impressions into impulses; and (3) a motor +nerve to conduct impulses outward to the muscles. The power of the cord to +enforce reflex acts resides in the gray matter, into which the reflex +nerves enter and from which they depart, by means of their posterior and +anterior roots respectively. + +[Sidenote: 40. Why do we not readily recognize the reflex activity of the +cord in our own bodies? How best studied in others? Example?] + +40. THE USES OF THE REFLEX ACTION.--The reflex activity of the cord is +exhibited in the healthy body in many ways, but since it is never +accompanied with sensation, we do not readily recognize it in our own +bodies. Reflex movements are best studied in the cases of other persons, +when the conditions enable us to distinguish between acts that are +consciously, and those that are unconsciously performed. For example, if +the foot of a person soundly asleep be tickled or pinched, it will be +quickly withdrawn from the irritation. + +[Sidenote: 41. Similar movements? Arm of a person? Melted wax or heated +coin on the hand?] + +41. Similar movements may be observed in cases where the consciousness and +sensation are temporarily obliterated by disease, or by means of narcotic +poisons. If the arm of a person who has been rendered insensible by {168} +chloroform, be raised, and then allowed to fall, it will be noticed that +the limb does not drop instantly, like a lifeless member, but a certain +amount of rigidity remains in its muscles, which resists or breaks the +force of its descent. Again, when a substance like melted sealing-wax, or a +heated coin, falls upon the hand, the limb is snatched away at once, even +before the feeling of pain has been recognized by the brain. When jolted in +a rapidly moving car, we involuntarily step forward or backward, so as to +preserve the centre of gravity of the body. + +[Sidenote: 42. Result of healthful reflex activity? When may the reflex +energy be deficient?] + +42. These and similar acts are executed by the same mechanism as that +previously described in the case of paralysis from an injury of the spinal +cord. The muscles thus called into play, are those which are ordinarily +under the sway of the will, but which in these cases act through this +reflex action of the cord, altogether independently of the will. A +healthful reflex activity produces an elasticity, or "tone," of the +voluntary muscular system, which, in a great measure, explains the +existence in the young and vigorous of a feeling of buoyancy and reserve +power. Its possessor is restlessly active, and it may appropriately be said +of him, "he rejoiceth as a strong man to run a race." But this reflex +energy may be deficient. This is true when the blood is poor and wanting in +its solid ingredients, or the circulation is feeble; the muscles, then, are +flabby and weak, and the person himself is said to be "nerveless," or +indisposed to exertion. Shivering from cold, and trembling from fear, may, +in part, be referred to a temporary loss of tone, resulting from a powerful +impression upon the brain. + +[Sidenote: 43. Excess of this activity in disease? Hydrophobia, etc.? The +difference in severity of the convulsions?] + +43. An excess of this activity may also be observed in disease. In this +condition, the excitability of the cord is unnaturally aroused, and +frequent and violent movements {169} of the limbs and body, called +convulsions, are the result. The convulsions of young children, and the +nervous agitation of _chorea_, or St. Vitus's dance, are reflex in +character; as are also the symptoms attending poisoning by strychnine, and +those terrible diseases, _tetanus_, or "locked jaw," and _hydrophobia_. The +severity of the convulsions is not the same in all cases of these +disorders; but, in those last mentioned the most violent spasmodic +movements are provoked by the slightest form of irritation--such as the +sound of pouring water, the sight of any glittering object, the glancing of +a mirror, the contact of cool air, or even the touch of the bedclothes. + +[Sidenote: 44. Another variety of reflex motions? What are they? What is +stated of the mind in connection with these movements?] + +44. Another variety of reflex motions takes place in certain involuntary +muscles, and over these the cord exercises supreme control. They are +principally those movements which aid the performance of digestion and +nutrition, the valve-action of the pylorus, and other movements of the +stomach and intestines. In these movements the mind shares no part. And it +is well that this is so; for since the mind is largely occupied with +affairs external to the body, it acts irregularly, becomes fatigued, and +needs frequent rest. The spinal cord, on the contrary, is well fitted for +the form of work on which depends the growth and support of the body, as it +acts uniformly, and with a machine-like regularity. + +[Sidenote: 45. Consciousness in these operations? Physical wants?] + +45. These operations are not accompanied by consciousness; for, as a +general rule, the attention is only called to them when they become +disordered. Many a person does not know where his stomach is situated, +until he discovers its position by reason of a feeling of distress within +it, produced by giving that organ improper work to perform. In this manner +the higher and nobler faculties of the mind are liberated from the simply +routine duties of the {170} body; and we are thus left to direct the +attention, the reason, and the will to the accomplishment of the great ends +of our existence. If it were otherwise, we could only find time to attend +to our ordinary physical wants. + +[Sidenote: 46. How many objects may the reflex activity be said to have? +State the first. The second. The third.] + +46. The objects of the reflex activity of the cord are threefold. In the +first place, it acts as the protector of man, in his unconscious moments. +It is his unseen guardian, always ready to act, never growing weary, and +never requiring sleep. Nor does its faithful action wholly cease with the +cessation of life in other parts. In the second place, it is the regulator +of numerous involuntary motions that are necessary to the nutrition of the +body. Here its actions are entirely independent of the brain, and are +performed in a secret and automatic manner. And, thirdly, it acts as a +substitute, and regulates involuntary movements in the muscles usually +under the influence of the will. It thus takes the place of the higher +faculties in performing habitual acts, and permits them to extend their +operations more and more beyond the body and its material wants. + +[Sidenote: 47. How does the medulla oblongata resemble the cord?] + +47. THE FUNCTIONS OF THE MEDULLA OBLONGATA.--The prolongation of the spinal +cord, within the skull, has been previously spoken of as the medulla +oblongata. It resembles the cord, in being composed of both white and gray +matter, and in conducting sensory and motor influences. It likewise gives +rise to certain nerves, which are here called cranial nerves (from +_cranium_, the skull). All except two of these important nerves spring from +the medulla, or the parts immediately adjoining it; the exceptions are the +two nerves taking part in the special senses of sight and smell, which +nerves have their origin at the base of the cerebrum. + +[Sidenote: 48. What final fact is observed in the crossing of the motor +columns?] + +48. The decussation, or crossing of the motor columns, has been previously +described, when treating of the {171} direction of the nerve-fibres of the +cord; and the singular fact has been alluded to, that when one side of the +brain is injured, its effects are limited to the opposite side of the body. +One more fact remains to be observed in this connection, namely, this +crossed action does not usually take place in the cranial nerves. +Accordingly, when apoplexy, or the rupture of a blood-vessel, occurs in the +right hemisphere of the cerebrum, the left side of the body is paralyzed, +but the right side of the face is affected; this is because that part of +the body is supplied by the cranial nerves. + +[Sidenote: 49. The pneumogastric nerve? The feelings aroused by it? The +"vital knot?"] + +49. A portion of the medulla presides over the important function of +respiration, and from it arises the _pneumogastric_ nerve, so called +because its branches serve both the lungs and stomach. The feelings of +hunger, thirst, and the desire for air are aroused by means of this nerve. +The wounding of the gray matter of the medulla, even of a small portion of +it, near the origin of the pneumogastric nerve, at once stops the action of +the lungs and causes death. In consequence of the importance of this part, +it has been termed the "vital knot." We find, also, that its location +within the skull is exceedingly well protected, it being quite beyond the +reach of any ordinary form of harm from without. + +[Sidenote: 50. The uses of the smaller gray masses at the base of the +brain?] + +50. THE FUNCTIONS OF THE CRANIAL GANGLIA.--The uses of the smaller gray +masses lying at the base of the brain are not well ascertained; and, on +account of their position, so remote from the surface, it would, at first, +seem well-nigh impossible to study them. But, from the results following +diseases in these parts, and from experiments upon inferior animals, they +are becoming gradually better understood; and there is reason to believe +that eventually the physiological office of each part will be clearly +ascertained and defined. It is believed, however, but not {172} absolutely +proven, that the anterior masses, like the anterior roots of the spinal +nerves and the anterior columns of the cord, are concerned in the +production of motion; in fact, that they are the central organs of that +function. The posterior gray masses are, on the contrary, supposed to be +the seat of sensation. + +[Sidenote: 51. Function of the cerebellum? When it is diseased?] + +51. THE FUNCTION OF THE CEREBELLUM.--The function of the cerebellum, or +"little brain," is the direction of the movements of the voluntary muscles. +When this organ is the seat of disease or injury, it is usually observed +that the person is unable to execute orderly and regular acts, but moves in +a confused manner as if in a state of intoxication. Like the larger brain, +or cerebrum, it appears to be devoid of feeling; but it takes no part in +the operations of the mind. + +[Sidenote: 52. Where is the seat of the mind? The subordination of the +other organs? The gray matter?] + +52. THE FUNCTION OF THE CEREBRUM.--The cerebrum, or brain proper, is the +seat of the mind; or, speaking more exactly, it is the material instrument +by which the mind acts; and, as it occupies the highest position in the +body, so it fulfils the loftiest uses. All the other organs are subordinate +to it: the senses are its messengers, which bring it information from the +outer world, and the organs of motion are its servants, which execute its +commands. Here, as in the nervous apparatus of lower grade already +considered, the gray matter is the element of power; and, in proportion as +this substance increases in extent, and in proportion to the number of +convolutions in the hemispheres, do the mental faculties expand. + +[Sidenote: 53. What is stated of men in connection with the size of their +brain? With the brains of other animals?] + +53. There have been a few, but only a few, men of distinguished ability +whose brains have been comparatively small in size; the rule being that +great men possess large brains. The relative weight of the brain of man, as +{173} compared with the weight of the body, does not, in all instances, +exceed that of the inferior animals; the canary and other singing-birds +have a greater relative amount of nervous matter than man; but man +surpasses all other creatures in the size of the hemispheres of the +cerebrum, and in the amount of gray substance which they contain. + +[Sidenote: 54. Sensitiveness of the brain substance? The removal of a +portion of the brain? State the remarkable case mentioned?] + +54. It is a singular fact that this cerebral substance is insensitive, and +may be cut without causing pain. The removal of a considerable quantity of +the brain has taken place, as the result of accident, without causing +death, and without even affecting seriously the intellect. A remarkable +case of injury of the brain is recorded, in which, from the accidental +explosion of gunpowder used in blasting a rock, the "tamping-iron" was +driven directly through the skull of a man. This iron rod, three feet and +seven inches long, an inch and a quarter in diameter, and weighing more +than thirteen pounds, entered the head below the ear and passed out at the +top of the skull, carrying with it portions of the brain and fragments of +bone. The man sustained the loss of sight on one side, but otherwise +recovered his health and the use of his faculties. Moreover, disease has +occurred, compromising a large portion of the brain, without impairing the +faculties of the mind, when the disease was limited to one side only. + +[Sidenote: 55. Thought, emotion, and will? What power do they give us?] + +55. Impressions conveyed to the hemispheres from the external world arouse +the mental operations called thought, emotion, and the will. These are the +godlike attributes which enable man to subjugate a world, and afterward +cause him to "sigh for other worlds to conquer;" which enable him to +acquaint himself with the properties of planets millions of miles distant +from him, and which give him that creative power by which he builds and +peoples the new worlds of poetry and art. {174} + +[Sidenote: 56. Are the brain and the mind identical?] + +56. All these mental acts, and many others, are developed through the +action of the brain; not that the brain and the mind are the same, or that +the brain secretes memory, imagination, or the ideas of truth and justice, +as the stomach secretes the gastric juice. But rather, as the nerve of the +eye, stimulated by the subtile waves of light, occasions the notion of +color, so the brain, called into action by the mysterious influences of the +immaterial soul, gives rise to all the intellectual, emotional, and +voluntary activities of mankind. + +[Sidenote: 57. What do we know of the cerebrum and its powers?] + +57. The cerebrum, according to our present knowledge of it, must be +regarded as a single organ, which produces different results, according as +it is acted upon by the immaterial mind in different ways. Recent +investigations, however, seem to prove that the faculty of language is +dependent upon a small part of the left hemisphere of the cerebrum, near +the temple. At least, in almost every instance where this part is diseased, +the patient can no longer express himself in speech and writing. + +[Sidenote: 58. The reflex function of the organs within the skull? The +reflex power of the medulla? Respiration?] + +58. THE REFLEX ACTION OF THE BRAIN.--The reflex function of the organs +within the skull is very active and important. Like that of the cord, it +protects the body by involuntary movements, it regulates the so-called +vegetative acts, and it takes the place of the will in controlling the +voluntary muscles, when the attention is turned in other directions. The +reflex power of the medulla governs the acts of respiration, which are +absolutely and continuously essential to life. Respiration is, as we have +seen, partly under the influence of the will; but this is due in part to +the fact that respiration is indirectly concerned in one of the animal +functions, that of speech. + +[Sidenote: 59. What else does reflex action occasion? Winking? Other +examples?] + +59. Reflex action also occasions coughing and sneezing, {175} whenever +improper substances enter the air-passages. Winking is an act of the same +sort, and serves both to shield the eyes from too great glare of light, and +to preserve them by keeping the cornea moist. Looking at the sun or other +strong light, causes sneezing by reflex action. Laughing, whether caused by +tickling the feet or by some happy thought, and also sobbing, are reflex +acts, taking place by means of the respiratory muscles. + +[Sidenote: 60. Muscles called into play by certain reflex movements? The +somnambulist?] + +60. Certain of the protective reflex movements call into play a large +number of muscles, as in the balancing of the body when walking along a +narrow ledge, or on a slippery pavement. The dodging motion of the recruit, +when the first cannon ball passes over his head, is reflex and involuntary. +The fact that these involuntary, reflex acts are performed with great +precision, will explain why it is that accidents seldom befall the +somnambulist, or sleep-walker, although he often ventures in most perilous +places. + +[Sidenote: 61. What is said of walking and other acts in connection with +the office performed by the medulla and spinal cord?] + +61. Walking, sitting, and other acts of daily life, become automatic, or +reflex, from habit: the mind is seldom directed to them, but delegates +their control to the medulla and spinal cord. Thus a person in walking, may +traverse several miles while absorbed in thought, or in argument with a +companion, and yet be conscious of scarcely one in a thousand of the acts +that have been necessary to carry his body from one point to another. By +this admirable and beautiful provision, the mind is released from the +charge of the ordinary mechanical acts of life, and may devote itself to +the exercise of its nobler faculties. And it is worthy of notice, that the +greater the use of these faculties, the more work does the reflex function +assume and perform; and thus the employment of the one insures the +improvement of the other. {176} + +QUESTIONS FOR TOPICAL REVIEW. + + PAGE + 1. State fully what is meant by the term vegetable function. 148 + 2. To what is man indebted for his position as the head of the + animal creation? 148, 149 + 3. What can you state on the subject of special organs for + separate functions? 149 + 4. Describe, as fully as you can, the structure of the nervous + system. 149, 150 + 5. Describe the brain, its location, size, shape, and structure. 150, 152 + 6. Describe the brain proper, or cerebrum. 152, 153, 174 + 7. What connection is noticed between the cerebrum and mental + power? 153, 172, 174 + 8. Describe the little brain, or cerebellum. 153, 154, 172 + 9. Describe the spinal cord. 154, 155, 156 + 10. What are the spinal nerves, and how are they arranged? 156, 157 + 11. What is the character and substance of their tissues? 157 + 12. State how the nerve-fibres perform their office, and give + the illustration. 157, 158 + 13. Describe the sympathetic system of nerves. 158 + 14. State what is meant by the properties of nervous tissue, + and give the illustration. 159, 160 + 15. Explain what is meant by the functions of the nerves, and + give the illustration. 160, 161, 162 + 16. What is meant by a transient paralysis of a nerve? Give the + illustration. 161, 162 + 17. What can you state of the rate of message-motion along a nerve? 162 + 18. What are the functions of the spinal cord? 162, 163, 164, 165 + 19. State what you can of the form of paralysis known as + paraplegia. 163 + 20. What experiments, with results, upon the spinal cord are + noted? 163, 164 + 21. Explain how injury of the cord may produce paralysis of + motion in one leg, and at the same time a loss of sensation + in the other. 164 + 22. Explain how, if the right hand be hurt, the left side of + the brain is made to feel the pain. 165 + 23. Now, explain as fully as you can the direction of the fibres + of the cord. 164, 165 + 24. What is understood by the reflex action of the cord? 165 + 25. What experiments are mentioned to prove this power of the + cord? 165, 166 + 26. What are the uses of the reflex action of the cord? 167-170 + 27. What illustrations are mentioned to show such uses? 167-170 + 28. What is the medulla oblongata? 154, 170 + 29. What are the functions of the medulla oblongata? 170, 171 + 30. What can you state of the functions of the cranial ganglia? 171, 172 + 31. What are the functions of the cerebellum? 172 + 32. What is the function of the cerebrum? 172, 174 + 33. In what way does the size of the brain generally indicate + the character of the man? 172, 173 + 34. What facts show that the gray substance of the brain is + insensitive? 173 + 35. Upon what does the faculty of language seem to depend? 174 + 36. What has been observed in support of this statement? 174 + 37. Of what importance is the reflex action of the brain? 174, 175 + 38. In what ways is this importance made manifest? 174, 175 + + * * * * * + + +{177} + +CHAPTER X. + +THE SPECIAL SENSES. + + _The Production of Sensations--Variety of Sensations--General + Sensibility--Pain and its Function--Special Sensation, Touch, Taste, + Smell, Sight, and Hearing--The Hand, the Organ of Touch--The Sense of + Touch--Delicacy of Touch--Sensation of Temperature and Weight--The + Tongue the Organ of Taste--The Nerves of Taste--The Sense of Taste and + its Relations with the other Senses--The Influence of Education on the + Taste--The Nasal Cavities, or the organs of Smell--The Olfactory + Nerve--The Uses of the Sense of Smell--The Sense of Sight--Light--The + Optic Nerve--The Eyeball and its Coverings--The Function of the + Iris--The Sclerotic, Choroid, and Retina--The Tears and their + Function--The Movements of the Eyeball--The Function of + Accommodation--The Sense of Hearing and Sound--The Ear, or the organ of + Hearing--The External, Middle, and Internal Ear._ + +[Sidenote: 1. True centre of sensation? Place of the mind's impressions? +What is it convenient to say? What further is stated?] + +1. PRODUCTION OF SENSATIONS.--We have already seen that the true centre of +sensation is some organ within the skull, probably among the gray masses at +the base of the brain; but the mind never perceives impressions at that +point; but, on the contrary, always refers them to the external organs of +sensation. Hence, it is convenient to say, that those outer parts possess +the property of sensibility. For instance, we say that we hear with the +ear, taste with the tongue, and feel with the fingers. That this is not the +exact truth is proven by the fact, that whenever the nerve connecting one +of these organs with the brain is severed, it at once loses its capacity +for sensation. + +[Sidenote: 2. Consciousness? During sleep? In profound insensibility?] + +2. Consciousness, another faculty of the brain, is necessary to complete a +sensation. During sleep, and in other unconscious states, the usual +impressions are presented to {178} the ear, the nose, and the skin, but +they fail to excite sensations, because the nerve-centres are inactive. In +profound insensibility, from chloroform or ether, a limb may be removed +without occasioning the least feeling. + +[Sidenote: 3. Sensibility in animals? In the earth-worm? In man?] + +3. VARIETY OF SENSATIONS.--All animals have some degree of sensibility. It +is of course feeble and indistinct in the lower forms of life, but +increases in power and variety as we ascend the scale. In the earth-worm, +the nervous system is very simple, the sensibility being moderate and alike +in all parts: hence, if its body be cut into two pieces, each piece will +have the same degree of feeling as before. As we approach man, however, the +sensations multiply and become more acute; the organs are more complex, and +special parts are endowed with special gifts. These special organs cannot +be separated from the rest of the body without the loss of the functions +they are designed to exercise. + +[Sidenote: 4. The lowest form of sensation? The highest? Sensations, how +modified? What further can you state as to habitual impressions?] + +4. The lowest form of sensation, that of simple contact, is possessed by +the lowest of the animal creation. The highest forms are those by which we +are enabled to know the properties of external objects, such as shape, +size, sound, and color. A variety of means of communicating with the outer +world is the necessary possession of a high intelligence. Sensations are +modified by use. They become more acute and powerful by moderate exercise; +or, they are dulled by undue excitement. The former is shown by the acute +hearing of the Indian, by the sharp sight of the sailor, and by the +delicate touch of the blind. The latter is exemplified by the impaired +hearing of the boiler-maker, and the depraved taste of him who uses pungent +condiments with his food. Again, impressions habitually presented may not +be consciously felt; as is the case with the rumbling of carriages in a +neighboring {179} street, or the regular ticking of a clock. All sensations +become less vivid with the advance of age, especially hearing and vision. + +[Sidenote: 5. General sensibility? What have we seen as regards the brain? +Of what other structures is the same true?] + +5. GENERAL SENSIBILITY.--There is a property possessed by nearly all parts +of the human body which we call general sensibility. We have recently seen +that the brain is wholly insensitive, and may be cut or pinched without +pain. The same is true of the nails, hair, the scarf-skin or external +covering of the body, and a few other structures. In these parts no nerves +are found. On the other hand, the sensibility of the true skin, and of +mucous membranes, as of the eye and nose, is exquisite, these organs having +a large supply of sensory nerve-fibres. The bones and tendons have less of +these fibres, and are only moderately sensitive. + +[Sidenote: 6. The cause of sensibility? Painful part in a surgical +operation? Benumbing the surface? How done by ether?] + +6. The sensibility of any part of the body, then, depends upon the number +of nerves present; and, as a rule, the nervous supply is proportional to +the importance of the part, and to its liability to injury. When, +therefore, a surgical operation is performed, the most painful part of it +is the incision through the skin; the muscles, cartilage, and bone being +comparatively without sensation. Hence, if we could benumb the surface, +certain of the lesser operations might be undergone without great +inconvenience. This is, in fact, very successfully accomplished by means of +the cold produced by throwing a spray of ether, or of some other rapidly +evaporating liquid, upon the part to be cut. + +[Sidenote: 7. Tickling? Internal sensations? The nerves of general +sensibility?] + +7. Tickling is a modification of general sensibility. At first, it excites +a pleasurable sensation, but this soon passes into pain. It is only present +in those parts where the sense of touch is feeble. But all impressions are +not received from without: there are, also, certain internal {180} +sensations, as they are called, which depend upon the condition of the +internal organs, such as appetite, hunger, thirst, the sense of +satisfaction after taking food, dizziness when looking down from some lofty +position, lassitude, drowsiness, fatigue, and other feelings of comfort or +discomfort. General sensibility, whether of the internal or external organs +of the body, chiefly depends upon the sensory fibres of the spinal nerve. +The face, however, is supplied by the sensory cranial nerves. The +sympathetic system has a low grade of feeling in health; but disease in the +parts served by it arouses an intense degree of pain. + +[Sidenote: 8. Connection between pain and sensibility?] + +8. THE SENSATION OF PAIN.--What then is _pain_? Is it identical with +ordinary sensibility? There seems to be some necessary connection between +the two feelings, for they take place through the same channels, and they +are alike intense in the same situations. But sensibility habitually +contributes to our sources of pleasure, the very opposite of pain; hence, +these feelings cannot be identical. + +[Sidenote: 9. Explain the difference between pain and sensibility.] + +9. Pain must, therefore, be a modification of the general sensibility, +which follows an excessive degree of excitement of the nerves; there being +a natural limit to the amount of stimulation which they will sustain. So +long as this limit is observed, the part excited may be said to be simply +sensitive; but when it is exceeded, the impression becomes painful. This +difference between sensibility and pain is well shown by the effects of +sunlight upon the eye. The indirect illumination of the sun arouses only +the former feeling, and is indispensable to our comfort and existence; +while the direct ray received into the eye occasions great pain. + +[Sidenote: 10. Dread of pain? How may its value be appreciated? Example.] + +10. THE USES OF PAIN.--The dread of pain is a valuable monitor to the body. +It puts us on our guard in the presence of danger; teaches moderation in +the use of our {181} powers; indicates the approach of disease; and calls +attention to it when present. The word disease, in fact, according to its +original use, had reference simply to the pain, or want of ease, which +commonly attends disordered health. When we observe the serious mishaps +which occur when sensibility and pain are absent, we cannot fail to +appreciate its value. For example, a paralytic in taking a foot-bath, +forgets to test its temperature, and putting his limbs into water while it +is too hot, is severely scalded without knowing it. + +[Sidenote: 11. The case of the traveller? Grain of sand? The sun and +child?] + +11. A traveller, overcome by cold and fatigue, lies down and falls asleep +near a large fire, and when he is aroused in the morning, it is discovered +that one of his feet has been insensibly destroyed. A grain of sand, +lodging in an insensitive eye, may cause inflammation and even the loss of +sight. If intense light were not painful to the eye, many a child would +innocently gaze upon the glories of the sun to the ruin of his sight. + +[Sidenote: 12. Mission of pain? Painful impressions compared with those of +pleasure?] + +12. Pain is, indeed, a present evil, but its relations with the future +prove its mission merciful. Painful impressions cannot be recollected from +past experience; and they cannot be called into existence by the fancy. +Considered in the light of results, pain has a use above that of pleasure; +for while the immoderate pursuit of the latter leads to harm, the tendency +of pain is to restrict the hurtful courses of life, and in this manner to +protect the body. + +[Sidenote: 13. What does Magendie say of the relation of pain to pleasure?] + +13. The relations of pain to pleasure are thus described by the eminent +physiologist, Magendie:--"By these sensations Nature induces us to concur +in the order which she has established among organized beings. Though it +may appear like sophistry to say that pain is the shadow of pleasure, yet +it is certain that those who have exhausted the ordinary sources of +pleasure have recourse to the {182} causes of pain, and gratify themselves +by their effects. Do we not see in all large cities, that men who are +debauched and depraved find agreeable sensations, where others experience +only intolerable pain?" + +[Sidenote: 14. The law of Nature as regards painful sensations among +animals?] + +14. As to painful sensation among the inferior animals, the plan of Nature +seems to be, that the higher the intelligence of the creature, and the more +complete its power of defence, the more acute is its sensibility. We infer, +therefore, that animals low in the scale of existence, and helpless, are +not very liable to suffer pain. + +[Sidenote: 15. The sensation of contact and pain? Special sensations of +man? How regarded?] + +15. SPECIAL SENSATION.--The sensations of simple contact and pain are felt +by nearly all parts of the system, whether external or internal, and are +the necessary consequence of the general sensibility; but, so far as the +objects which surround us are concerned, these impressions are vague and +passive in character, and inform the mind of none of the properties or +powers of these objects. Besides these feelings, therefore, man is endowed +with certain special sensations, which are positive and distinct in +character, and which he can call into exercise at will, and employ in the +pursuit of knowledge. For reasons relating to the original constitution of +the body, these sensations are to be regarded as modifications of the +general sensibility already alluded to, constructed with special reference +to the different forces of Nature, of which we have any knowledge, such as +heat, motion, gravity, sunlight, and the like. + +[Sidenote: 16. What are the special senses? Special organs for them?] + +16. These distinct and active faculties are termed the special senses, and +are five in number, viz., Touch, Taste, Smell, Sight, and Hearing. For the +exercise of these senses, special organs are furnished, such as the hand, +the tongue, the nose, the eye, and the ear. The manner in which the nerves +of special sense terminate, varies in the {183} case of each organ, so that +each is adapted to one set of sensations alone, and is incapable of +perceiving any other. Thus the nerve of hearing is excited by the +undulations of sound, and not by those of light, while the reverse is true +of the nerve of sight; and the nerve of smell can appreciate neither of +them, being capable only of taking cognizance of the odorous properties of +bodies. + +[Sidenote: 17. What is said in relation to one more than the five senses?] + +17. By some writers six senses are accorded to man; the additional one +being either the sense of temperature, for as we shall presently see this +is not the same as touch; or according to others, the muscular sense by +which we are enabled to estimate the weights of bodies. The latter also +differs in some respects from the sense of touch. + +[Sidenote: 18. The sense of touch, how prevalent? What is said of the +hand?] + +18. ORGANS OF TOUCH.--The sense of touch is possessed by nearly all +portions of the general surface of the body, but it finds its highest +development in the hands. The human hand is properly regarded as the model +organ of touch. The minute structure of the skin fits it admirably for this +form of sensation: the cuticle, or scarf-skin, is fine and flexible, while +the cutis, or true skin, contains multitudes of nerve-filaments, arranged +in rows of _papillae_, or cone-like projections, about one-hundredth of an +inch in length. It is estimated that there are 20,000 of these papillae in +a square inch of the palmar surface of the hand. Now, although the nerves +of the cutis are the instruments by which impressions are received and +transmitted to the brain, yet the cuticle is essential to the sensation of +touch. This is shown by the fact that whenever the true skin is laid bare, +as by a burn or blister, the only feeling that it experiences from contact +is one of pain, not that of touch. + +[Sidenote: 19. Office of the cuticle? Tips of the fingers? The fingers with +thumb?] + +19. The office of the cuticle is thus made evident: it is to shield the +nerve filaments from direct contact with {184} external objects. At the +tips of the fingers, where touch is most delicate, the skin rests upon a +cushion of elastic material, and receives firmness and permanence of shape +by means of the nail placed upon the less sensitive side. Besides these +favorable conditions, the form of the arm is such, and its motions are so +easy and varied, that we are able to apply the test of touch in a great +number of directions. The slender, tapering fingers, with their pliant +joints, together with the strong opposable thumb, enable the hand to mold +itself upon and grasp a great variety of objects; so that great as are the +delicacy and grace of the hand, it is not wanting in the elements of power. + +[Sidenote: 20. What special importance is attributed to the hand?] + +20. Its beauty and adaptation to the wants of man have made the hand an +attractive theme for philosophers. They do not, however, always agree in +their conclusions. One has the opinion that man has acquired his +intelligence and achieved his place as "lord of creation," because he has +this organ. Buffon, in effect, declares that with fingers twice as numerous +and twice as long, we would become proportionally wiser; but Galen long ago +took a more reasonable view, when he taught that "man is the wisest of +animals, not because he possesses the hand; but because he is the wisest +and understands its use, the hand has been given to him; for his mind, not +his hand has taught him the arts." Another has well said, that "no one can +study carefully the human hand and fail to be convinced of the existence of +the Deity." + +[Sidenote: 21. The simplicity of touch? What does it teach us?] + +21. THE SENSE OF TOUCH.--Touch is the simplest of the senses. It is that +which the child first calls into exercise in solving the early problems of +existence; and it is that which is in the most constant use throughout +life. We are brought by the touch into the most intimate relations with +external objects, and by it we learn the greater number, if not the most +important, of the properties of {185} these objects; such as size, figure, +solidity, motion, and smoothness or roughness of surface. + +[Sidenote: 22. Importance of the sense of touch to the development of the +other senses?] + +22. The sense of touch assists the other senses, especially that of sight, +giving foundation and reality to their perceptions. Without it, the +impressions received by the eye would be as vague and unreal as the figures +that float through our dreams. A boy who had been blind from birth, at the +age of twelve years received sight by means of a surgical operation: at +first, he was unable to distinguish between a globe and a circular card, of +the same color, before he had touched them. After that, he at once +recognized the difference in their form. He knew the peculiarities of a dog +and a cat by feeling, but not by sight, until one day, happening to take up +the cat, he recognized the connection of the two sorts of impressions, +those of touch and sight; and then, putting the cat down, he said: "So, +puss, I shall know you next time." + +[Illustration: FIG. 45.] + +[Sidenote: 23. Liability of touch to err? Describe the illustration.] + +23. Touch is considered the least liable to error of all the senses; yet, +if that part of the skin by which the sense is exercised is removed from +its customary position, a false impression may be created in the mind. This +is well illustrated by an experiment, which dates from the time of +Aristotle. If we cross the middle finger behind the {186} forefinger, and +then roll a marble, or some small object, upon the tips of the fingers (see +Fig. 45), the impression will be that two marbles are felt. If the fingers, +thus transposed, be applied to the end of the tongue, two tongues will be +felt. When the nose is accidentally destroyed, the surgeon sometimes +performs an operation for the purpose of forming a new one, by +transplanting a partially removed piece of the skin of the forehead upon +the injured part: then, if the new nose be touched or pinched, the feeling +is referred to the forehead. This fact illustrates one important truth, +that the nerves will re-unite after they have been cut, and feeling will be +restored: if it were otherwise, a succession of slight cuts upon the +fingers would seriously impair their tactile sensibility. + +[Sidenote: 24. The delicacy of touch? Experiments with a pair of +compasses?] + +24. THE DELICACY OF TOUCH.--Although the hand is the proper organ of this +sense, yet it is exercised by various parts of the body, their degree of +sensibility being proportional to the number of papillae they contain. The +varying degrees of tactile delicacy of the different parts of the surface +have been measured, in an ingenious manner, by means of a pair of +compasses, tipped with small pieces of cork. The two points of the +compasses are touched at the same moment to the skin, the eyes being +closed, and it is found that, in sensitive parts, the distance between the +points may be quite slight, and yet each be plainly felt; while, in less +sensitive parts, the points of the compasses are felt as a single point, +although they are separated one or two inches. + +[Sidenote: 25. Further experiments and results?] + +25. At the tips of the fingers, the distance between the points being +one-twentieth of an inch, a double impression is felt. The distance must be +twice as great, for the palm; four times as great, for the lips; and, on +the forehead, it must be twenty times greater. At the middle of the back, +where the touch is least acute, the points must be {187} separated more +than two inches before they can be separately felt. Therefore, the sense of +touch in the fingers is said to be fifty times more delicate than upon the +posterior surface of the body. + +[Sidenote: 26. Exquisite delicacy of touch? The same among the blind?] + +26. Exquisite delicacy of touch is attained by practice. This is shown in +many of the lighter and more graceful employments of daily life. Without +it, the skill of the painter, sculptor, and musician would be rude indeed. +By training, also, the physician acquires the _tactus eruditus_, or +discriminating touch; but among the blind, delicacy of touch is most +remarkable, and it here finds its highest value; for its possession, in a +measure, compensates the loss of sight by enabling them to read, by means +of raised letters, to work with certain tools, and even to play upon +musical instruments. A person born without sight, and without hearing or +voice, may, by the education of the touch, be rescued from apparent +imbecility, and be taught not only to read and write, but even to perform +household and other useful labors. + +[Sidenote: 27. Rival candidates for the sixth sense? Give the two reasons +on the subject.] + +27. SENSATIONS OF TEMPERATURE AND WEIGHT.--Each of these sensations has +been described by the physiologists as a special sense, and they are rival +candidates, so to speak, for the position and title of the sixth sense. In +the sensation of temperature, or the thermal sense, touch bears a part, but +the two feelings appear to be distinct. In proof of this, we observe, +firstly, that they are not alike intense in the same situations; as, for +example, the skin of the face and elbow, where the sense of touch is +feeble, is very sensitive to impressions of heat and cold. Secondly, the +ability to recognize temperature may be lost by paralysis, while the +sensibility of touch remains unaffected. When the skin comes in contact +with a very hot substance, the sensation felt is that of pain, not of +touch. In like manner, a {188} very cold substance causes pain, not the +feeling of cold. So that a red-hot iron, and solid carbonic acid (the +temperature of which is 108deg below zero), feel alike; and each, if +pressed slightly, will produce a blister. + +[Sidenote: 28. The muscular sense? State what is said to illustrate the +subject.] + +28. The _muscular sense_, by some considered distinct from touch, gives +rise to the sensations of weight, and other forms of external resistance. +That this feeling exists, is shown by the following simple experiment. If +the hand be placed flat upon a table, and a somewhat heavy weight be put +into it, touch alone is exercised and a feeling of pressure results; but if +the hand be raised, a certain amount of muscular effort must be put forth, +and thus the sensation of weight is recognized. Through the muscular sense, +precision of effort is rendered possible; for by it we learn to adjust the +force exerted to the weight of the object to be lifted, moved, or carried. +Without it, all our movements would necessarily become ill-regulated and +spasmodic. In cases of disease, where the sensibility of the lower limbs is +lost, while power of motion remains, the patient is able to stand erect so +long as he can see his limbs; but just as soon as his eyes are closed, he +begins to waver, and will fall unless supported. + +[Sidenote: 29. The organ of taste? The tongue? Its powers of motion?] + +29. THE ORGAN OF TASTE.--The _tongue_ is the special organ of the sense of +taste; but the back part of the mouth also possesses this faculty. The +tongue is a muscular organ, the muscles composing it being so numerous and +interwoven as to give it the freedom and variety of motion which it +possesses. It can curve itself upward or downward; it can extend or +contract itself; and, with its point, can sweep the cavity of the mouth, in +all directions, in the search for scattered particles of food. + +[Sidenote: 30. Peculiarities of the tongue? Uses of the papillae?] + +30. The upper surface of the tongue is peculiar, being marked by the +presence of innumerable _papillae_, some of {189} which are of microscopic +size, resembling those that abound in the fingers, and in other parts of +the body that have the sense of touch. Others are much larger, and give to +the tongue its roughness of feeling and appearance. Through the medium of +these papillae, the tongue receives impressions of touch and temperature, +as well as taste: indeed, its extremity is fully as delicate, in respect to +tactile sensations, as the tips of the fingers themselves. It can recognize +the two points of the compasses when separated not more than +one-twenty-fourth of an inch; the back of it is much less sensitive to +touch, while at the same time it is more highly sensitive to impressions of +taste. + +[Sidenote: 31. Resemblance to the parts of the tongue? Powers and functions +of the parts?] + +31. Each lateral half of the tongue resembles the other in structure, and +each receives the same number of nerves--three. One of these regulates +motion, the other two are nerves of special sense. One of the latter +supplies the front half of the tongue, and is called the _gustatory_ nerve. +This is a branch of the great cranial nerve, called the "fifth pair," which +ramifies in all parts of the face. The back of the tongue is endowed with +the power of taste through a nerve known as the _glosso-pharyngeal_, +because it is distributed both to the tongue and throat. This difference in +the nervous supply of the tongue becomes significant, when we learn, as we +shall presently, that each part of it perceives a different class of +flavors. + +[Sidenote: 32. Taste? What are the requisites to taste?] + +32. THE SENSE OF TASTE.--Taste is the special sense by means of which we +discover the savors, or flavoring properties of the substances, which come +in contact with the tongue. Mere contact with the surface of the tongue, +however, is not sufficient, but contact with the extremities of the nerves +of taste within the papillae is required. In order that the substance to be +tasted may penetrate the {190} cells covering the nerves, it must either be +liquid in form, or readily soluble in the watery secretion of the mouth, +the saliva. The tongue must be moist also. If the substance be insoluble, +as glass or sand, or the tongue dry, the sense of taste is not awakened. In +sickness, when the tongue is heavily coated, the taste is very defective, +or, as is frequently expressed, "nothing tastes aright." + +[Sidenote: 33. Portions of the tongue endowed with taste? Where else does +the sense lodge? What is stated in respect to sweet and bitter flavors? +Reflex effects mentioned?] + +33. All portions of the tongue are not alike endowed with the sense of +taste, that function being limited to the posterior third, and to the +margin and tip of this organ. The soft palate, also, possesses the sense of +taste; hence, an article that has an agreeable flavor may very properly be +spoken of as palatable, as is often done. All parts of the tongue do not +perceive equally well the same flavors. Thus, the front extremity and +margin, which is the portion supplied by the "fifth pair" of nerves, +perceives more acutely sweet and sour tastes; but the base of the tongue, +supplied by the _glosso-pharyngeal_ nerve, is especially sensitive to salt +and bitter substances. The nerve of the front part of the tongue, as before +stated, is in active sympathy with those of the face, while the relations +of the other nerve are chiefly with the throat and stomach; so that when an +intensely sour taste is perceived, the countenance is involuntarily +distorted, and is said to wear an acid expression. On the other hand, a +very bitter taste affects certain internal organs, and occasions a +sensation of nausea, or sickness of the stomach. + +[Sidenote: 34. What is stated of the relations of taste with other senses?] + +34. RELATIONS OF TASTE WITH OTHER SENSES.--Taste is not a simple sense. +Certain other sensations, as those of touch, temperature, smell, and pain, +are blended and confused with it; and certain so-called tastes are really +sensations of another kind. Thus an astringent taste, like that of alum, is +more properly an astringent feeling, and {191} results from an impression +made upon the nerves of touch, that ramify in the tongue. In like manner, +the qualities known as smooth, oily, watery, and mealy tastes, are +dependent upon these same nerves of touch. A burning or pungent taste is a +sensation of pain, having its seat in the tongue and throat. A cooling +taste, like that of mint, pertains to that modification of touch called the +sense of temperature. + +[Sidenote: 35. Its dependence on smell? on sight?] + +35. Taste is largely dependent upon the sense of smell. A considerable +number of substances, like vanilla, coffee, and garlic, which appear to +possess a strong and distinct flavor, have in reality a powerful odor, but +only a feeble taste. When the sense of smell is interfered with by holding +the nose, it becomes difficult to distinguish between substances of this +class. The same effect is frequently observed when smell is blunted during +an ordinary cold in the head. Sight also contributes to taste. With the +eyes closed, food appears comparatively insipid; and a person smoking +tobacco in the dark is unable to determine by the taste whether his cigar +is lighted or not. Accordingly, it is not a bad plan to close the nose and +shut the eyes when about to swallow some disagreeable medicine. + +[Sidenote: 36. The chief use of the sense of taste? The position of its +organs? The rule as regards wholesome and unwholesome food? Remarks +respecting the rule?] + +36. INFLUENCE OF EDUCATION ON THE TASTE.--The chief use of the sense of +taste appears to be to act as a guide in the selection of proper food. +Hence its organs are properly placed at the entrance of the digestive +canal. As a general rule, those articles which gratify the taste are +wholesome; while the opposite is true of those which impress it +disagreeably. This statement is more exact in reference to the early years +of life than to later years, when, by reason of mischievous habits, the +sense of taste has become dulled or perverted. The desires of a child are +simple; he is fully satisfied with plain and wholesome articles of diet, +and must usually "learn to like" those {192} which have a strongly marked +flavor. Accordingly, it is far easier at this age to encourage the +preference for plain food, and thus establish healthful habits, than later +in life to uproot habits of indulgence in stimulating substances, after +their ill effects begin to manifest themselves. + +[Sidenote: 37. Diversity in tastes of men? How shown? The education of the +sense of taste?] + +37. The tastes of men present the most singular diversities, partly the +result of necessity and partly of habit or education. The Esquimaux like +the rank smell of whale oil, which is a kind of food admirably suited to +the requirements of their icy climate; and travellers who go from our +climate to theirs are not slow to develop a liking for the same articles +that the natives themselves enjoy. The sense of taste is rendered very +acute by education, as is shown in an especial manner by those who become +professional "tasters" of tea and wine. + +[Sidenote: 38. Location of the sense of smell? The nose? "Roof of the +mouth?"] + +38. THE SENSE OF SMELL--THE NASAL CAVITIES.--The sense of smell is located +in the delicate mucous membrane which lines the interior of the nose. That +prominent feature of the face, the nose, which is merely the front boundary +of the true nasal organ, is composed partly of bone and partly of +cartilage. The upper part of it is united with the skull by means of a few +small bones; to which circumstance is due its permanence of shape. The +lower portion, or tip of the nose, contains several thin pieces of +cartilage, which render it flexible and better able to resist the effects +of blows and pressure. Behind the nose we find quite a spacious chamber, +separated from the mouth by the hard palate, forming the "roof of the +mouth," and by the soft palate (see Fig. 46); and divided into two cavities +by a central partition running from before backward. + +[Sidenote: 39. Cavities of the nose? Obstruction of the passage of air +through them?] + +39. These nasal cavities, constituting the true beginning of the +air-passages, extend from the nose backward to the {193} upper opening of +the throat, and rise as high as the junction of the nose with the forehead. +The inner wall of each cavity is straight and smooth; but from the outer +wall there jut into each cavity three small scroll-like bones. The +structure of these bones is very light, and hence they have been called the +"spongy" bones of the nose. In this manner, while the extent of surface is +greatly increased by the formation of these winding passages, the cavities +are rendered extremely narrow; so much so, in fact, that a moderate +swelling of the mucous membrane which lines them, as from a cold, is +sufficient to obstruct the passage of air through them. + +[Illustration: FIG. 46.--SECTION OF THE RIGHT NASAL CAVITY.] + +[Sidenote: 40. The special nerve of smell? Its location?] + +40. THE NERVE OF SMELL.--The internal surface of the nasal passages is +covered by a delicate and sensitive mucous membrane. Its surface is quite +extensive, following as it does, all the inequalities produced by the +curved spongy bones of the nose. The upper portion of it alone is the seat +of smell, since that part alone receives branches from the "first pair" of +cranial nerves, or the olfactory nerve, which is the special nerve of smell +(see Fig. 43). In Fig. 46 is shown the distribution of this nerve, in the +form of an intricate network upon the two upper spongy bones. The nerve +itself (1) does not issue from the skull, but rests upon a thin bone which +separates it from the cavity of the nose; and the branches which proceed +from it pass through this bone by means of numerous small openings. The +{194} engraving represents the outer surface of the right nasal cavity; the +three wave-like inequalities, upon which the nervous network is spread out, +are due to the spongy bones. The left cavity is supplied in the same +manner. + +[Sidenote: 41. Branches of the "fifth pair" of nerves? Nasal mucus? Birds?] + +41. The nerves which ramify over the lower part of the membrane, and which +endow it with sensibility to touch and pain, are branches of the "fifth +pair" of nerves. An irritation applied to the parts where this nerve is +distributed occasions sneezing, that is, a spasmodic contraction of the +diaphragm; the object of which is the expulsion of the irritating cause. +The manner in which the olfactory nerve-fibres terminate is peculiar. +Unlike the extremities of other nerves, which are covered in by a greater +or less thickness of tissue, these come directly to the surface of the +mucous membrane, and thus come into very close contact with the odorous +particles that are carried along by the respired air. The surface is at all +times kept in a moist condition by an abundant flow of nasal mucus; +otherwise it would become dry, hard, and insensitive from the continual +passage of air to and fro in breathing. Birds, which respire more actively +than men, have a special gland, for secreting a lubricating fluid, located +in the air-passages of the head. + +[Sidenote: 42. Smell? Touch? Taste? Design of smell? Invisible and gaseous +particles? The extreme fineness of the particles? Musk? In other cases?] + +42. THE USES OF THE SENSE OF SMELL.--Smell is the special sense which +enables us to appreciate odors. Touch, as we have seen, is largely +concerned with solid bodies; and taste, with fluids, or with solids in +solution. Smell, on the other hand, is designed to afford us information in +reference to substances in a volatile or gaseous form. Invisible and +subtile particles emanate from odorous bodies, and are brought by the +respired air in contact with the terminal filaments of the olfactory nerve, +upon which an agreeable or disagreeable impression is produced. {195} The +fineness of the particles that constitute odors is often so extreme, that +they elude all attempts to measure or weigh them. A piece of musk, for +instance, may be kept for several years, constantly emitting perfume, +without any appreciable loss of weight. In other cases, a loss of substance +is perceptible, such as the essential oils, which enter into the +composition of the ordinary perfumes. + +[Sidenote: 43. Aid given by smell? The highest use of the sense? Explain +the manner.] + +43. Smell, like taste, aids us in the choice of proper food, leading us to +reject such articles as have a rank or putrid odor, and which are, as a +rule, unfit to be eaten. The highest usefulness of this sense, however, +consists in the protection it affords to the organs of respiration. +Stationed at the gateways of the air-passages, it examines the current of +air as it enters, and warns us of the presence of noxious gases, and of +other and generally invisible enemies to health. Not all dangerous vapors +are offensive, but almost all offensive vapors are unfit to be breathed. A +number of small stiff hairs grow from the margin of the nostrils to prevent +the entrance of dust and other atmospheric impurities, which would be alike +injurious to the olfactory mucous membrane and to the lungs. The benevolent +design of the Maker of our bodies may be observed in all parts of their +mechanism; but, probably, in none is it more clearly displayed than in +connection with the sense of smell. + +[Sidenote: 44. Sense of smell in the inferior animals? How, and in what +cases, illustrated?] + +44. The sense of smell is developed in a remarkable degree in certain of +the inferior animals, and is especially acute in reference to the peculiar +emanations that appear to characterize the different animals. The lion and +other carnivorous beasts scent their prey from a great distance; and the +fox-hound is able to track the fox through thickets and over open country +for many miles; while the timid, helpless herbivora, such as the deer and +sheep, find in the {196} sense of smell a means of protection against their +natural enemies, of whose approach they are in this manner warned. By +training this sense in the dog, and making it subservient to his use, man +is able to hunt with success certain shy and very fleet animals, which +otherwise he could but seldom approach. Among men, individuals differ +greatly in respect to the development of this sense; and especially in +certain savage tribes it is found to be extremely delicate. Humboldt states +that the natives of Peru can, by this sense, distinguish in the dark +between persons of different races. + +[Sidenote: 45. What is sight? What information does it furnish? Composite +visual sensations?] + +45. THE SENSE OF SIGHT.--Sight, or Vision, is the special sense by means of +which we appreciate the color, form, size, distance, and other physical +properties of the objects of external nature. Primarily, this sense +furnishes us with information concerning the different shades of color and +the different degrees of brightness: these are the simple sensations of +sight, such as the yellowness and glitter of a gold coin. In addition to +these, there are composite visual sensations, produced by the joint action +of the other senses and by the use of the memory and judgment; such as, in +the case of the coin, its roundness, solidity, size, its distance and +direction from us. So that many of our sensations, commonly considered as +due to sight, are in reality the results of intellectual processes which +take place instantaneously and unconsciously. + +[Sidenote: 46. Comparison between sight and hearing? Relative capacity of +deaf and blind?] + +46. This faculty not only has value in the practical every-day affairs of +life, but it contributes so largely to the culture of the intellect and to +our higher forms of pleasure, that some writers are disposed to rate it as +the first and most valuable of the senses. Others, however, maintain that +the sense of hearing does not yield in importance to that of sight; and +they cite in support of their position {197} the fact that the blind are +commonly cheerful and gay, while the deaf are inclined to be morose and +melancholy. In respect to the relative capacity for receiving education in +the deaf and blind, it is found that the former learn more quickly, but +their attainments are not profound; while the blind acquire more slowly, +but are able to study more thoroughly. + +[Sidenote: 47. Sight, unlike the other senses? In the case of the stars?] + +47. LIGHT.--THE OPTIC NERVE.--Unlike the senses previously +considered--touch, taste, and smell--sight does not bring us into immediate +contact with the bodies that are examined; but, by it, we perceive the +existence and qualities of objects that are at a greater or less distance +from us. In the case of the stars, the distance is incalculable, while the +book we read is removed but a few inches. Light is the agent which gives to +this sense its wide range. The nature of this mysterious force is not +known, and it is not here to be discussed; since its study belongs more +properly to the province of natural philosophy. + +[Sidenote: 48. The undulatory theory of light? What does the theory +suppose?] + +48. It is sufficient, in this connection, to state that the theory of light +now generally accepted, and which best explains the facts of optics, is +that known as the undulatory theory. This theory supposes that there exists +an intangible, elastic medium, which fills all space, and penetrates all +transparent substances, and which is thrown into exceedingly rapid +undulations or waves, by the sun and every other luminous body; the +undulations being propagated with extreme rapidity, and moving not less +than 186,000 miles in a second. + +[Sidenote: 49. The sensation of light? Optic nerve?] + +49. These waves are thought to produce in the eye the sensation of light, +in the same manner as the sonorous vibrations of the air produce in the ear +the sensation of sound. That part of the eye which is sensitive to these +waves is the expansion of the _optic nerve_. It is sensitive {198} to no +other impression than that of light, and it is the only nerve which is +acted upon by this agent. The optic nerve, also called the "second pair" of +cranial nerves, is the means of communication between the eye and the +brain. + +[Sidenote: 50. The two nerves constituting the pair of nerves?] + +50. The two nerves constituting the pair, arise from ganglia lying at the +base of the cerebrum, one of them on each side; from which points they +advance to the eyes, being united together in the middle of their course in +the form of the letter X (Fig. 43--2). By this union the two eyes are +enabled to act harmoniously, and in some respects to serve as a double +organ. And by reason of this same intimate nervous communication, when +serious disease affects one eye, the fellow-eye is extremely liable to +become the seat of _sympathetic_ inflammation; and this, if neglected, +almost certainly results in hopeless blindness. + +[Sidenote: 51. Why is the eye called the "window of the soul?" Why, the +subject of enthusiastic study?] + +51. THE ORGAN OF SIGHT.--THE EYE.--The proximity of the eye to the brain, +and the important part it performs in giving expression to the emotions, +have given it the name of "the window of the soul." The exceeding beauty of +its external parts, and the high value of its function, have long made this +organ the subject of enthusiastic study. It is chiefly within the last +twenty years, however, that this study has been successful and fruitful of +practical results. Several ingenious instruments have been invented for the +examination of the eye in health and disease, and new operations have been +devised for the relief of blindness and of impaired vision. As a result, it +is now a well-marked fact that, in civilized lands, the number of those who +suffer from loss of sight is proportionally much less than in countries +where science is less known and cultivated. + +[Sidenote: 52. The most obvious fact? The consequence? The next thing +noticed? Its range of view? Of what does the organ of vision consist?] + +52. The most obvious fact in respect to the apparatus of {199} sight is +that there are two eyes, which may either act together as one, and be fixed +upon one object, or one eye may be used independently of the other. In +consequence of this arrangement the loss of one eye does not necessitate +blindness, and, in fact, it not infrequently happens that the sight of one +eye may be long impaired or lost before the fact is discovered. We next +notice that it is placed at the most elevated part of the body, in front, +and near the brain. It also commands a wide range of view, being itself +moved with great rapidity, and being further aided by the free motion of +the head and neck. The organ of vision consists essentially of two parts: +the optical instrument itself--the eyeball--and its enveloping parts, or +the case in which the instrument is kept free from harm. The latter, which +are external, and which we shall first consider, are chiefly the _Orbits_, +the _Eyelids_, and the apparatus for the _Tears_. + +[Sidenote: 53. The protection of the eyeball against injury? The +overhanging brow? The opening for the optic nerve?] + +53. THE ORBITS.--The eyeball, which is a delicate organ, is well defended +against external injury within the orbits or bony sockets of the head. +These are deep conical hollows, bounded in part by the bones of the skull, +and in part by those of the nose and cheek. The orbit juts out beyond the +most exposed portion of the eyeball, as may be seen by laying a book over +the eye, when it will be found that no part of the eyeball, unless it be +very prominent, will be touched by the book; so that the only direction in +which an injury is liable to be received is immediately in front of the +eye. The overhanging brow is itself covered by a layer of thick skin, +studded with short, stout hairs, which are so bent as to prevent the +perspiration from running into the eye and obscuring vision. Through a hole +in the bottom of the orbit, the nerve of sight passes outward from the +brain. The orbit also contains a considerable amount of a fatty tissue, +upon which, as upon an elastic cushion, the eye rests. {200} + +[Illustration: FIG. 47.--FRONT VIEW OF RIGHT EYE. (Natural Size.) + +1. The Lachrymal, or tear gland, lying beneath the upper eyelid. + +2. The Nasal Duct is shown by the dotted line. The * marks the orifice in +the lower lid. + +The central black spot is the _pupil_; surrounding it is the _iris_; and +the triangular white spaces are the visible portion of the _sclerotic_.] + +[Sidenote: 54. What are the eyelids? The upper lid? The lower one? The +mucous membrane of the eye?] + +54. THE EYELIDS.--The eyelids are two movable curtains, or folds, which, +when shut, cover the front part of the orbit, and hide the eye from view. +The upper lid is the larger, has a curved margin, and moves freely, while +the lower lid is comparatively short and straight, and has but a slight +degree of motion (Fig. 47). Skin covers the exterior of the lids, while a +fine mucous membrane lines their inner surface, and is likewise spread out +over the entire front of the eyeball. This membrane, which is called the +_Conjunctiva_, is highly sensitive, and thus plays an important part in +protecting the eye against the lodgment of sand, ashes, chaff, and other +foreign particles that are blown about in the air. This sensitive membrane +will not endure the presence of these particles. If any find access, it +causes a constant winking, a flow of tears, and other signs of irritation, +until it is removed. + +[Sidenote: 55. The eyelashes? The little points within the line of the +lashes? Of what use are these glands?] + +55. The long, silky eyelashes, which garnish the edges of the lids, act +like a sieve to prevent the entry of dust and the like; and together with +the lids, they regulate the amount of light which is permitted to enter the +eye, so that it is shielded from a sudden flood or glare of light. The +little points seen in the figure just within the line of the lashes, +especially on the lower lid, represent the mouths of numerous little +sebaceous glands (Fig. 48, D,D), such as are always {201} found in the +neighborhood of hairs. These glands supply a thick, oily material which +greases the edges of the lids and prevents their adhering together, and +likewise prevents the overflow of the tears upon the cheek. + +[Sidenote: 56. The location of the lachrymal gland? The use of the gland?] + +56. THE LACHRYMAL FLUID, OR THE TEARS.--Just within the outer part of the +bony arch of the brow, where the bone may be felt to be sharper than in +other positions, is lodged a little organ called the lachrymal gland, the +situation of which is indicated in Fig. 47, 1. This is the gland whence +flows the watery secretion, commonly called the _tears_, which is designed +to perform an exceedingly important duty in lubricating the lids, and in +keeping the exposed surface of the eyeball moist and transparent. For, +without this or some similar liquid, the front of the eye would speedily +become dry and lustreless, like that of a fish which has been removed from +the water: the simple exposure of the eye to the air would then suffice to +destroy vision. + +[Sidenote: 57. When does the secretion of the tears occur? The secretion +not used for the eye? Location of the nasal duct? Its use? The overflow of +tears in old people?] + +57. This secretion of the tears takes place at all times, during the night +as well as the day; but it is seldom noticed, except when under the +influence of some strong mental emotion, whether of sorrow or happiness, it +is poured forth in excess, so as to overflow the lids. Strong light or a +rapid breeze will, among many other causes, excite the flow of the tears. +That portion of this secretion which is not used in moistening the eye is +carried off into the nose by a canal situated near the inner angle of the +eye, called the _nasal duct_. This duct is shown in Fig. 47, 2, and is +connected with each lid by delicate tubes, which are indicated by dotted +lines in the figure; the asterisk marks the little opening in the lower +lid, by which the tears enter the nasal duct. By gently turning the inner +part of that lid downward, and looking in a mirror, {202} this small +"lachrymal point" may be seen in your own eye. In old people, these points +become everted, and do not conduct the tears to the nasal cavity, so that +they are inconvenienced by an overflow of tears upon the face. + +[Illustration: FIG. 48.--VERTICAL SECTION OF THE EYE. (Enlarged.) + +C, The Cornea. A, The Aqueous Humor. I, The Iris. P, The Pupil. L, The +Crystalline Lens. H, The Ligament of the Lens. B, The Ciliary Process. +V, The Cavity containing the Vitreous Humor. S, The Sclerotic. Ch, The +Choroid. R, The Retina. N, The Optic Nerve. DD, The Eyelids. X, The Levator +Muscle of the Upper Lid. Y, The Upper Straight Muscle of the Eye. Z, The +Lower Straight Muscle.] + +[Sidenote: 58. The watery fluid passing over the eyeball? Design of the +arrangement? Winking?] + +58. Thus we observe that the gland which forms the tears is placed at the +outer part of the eye, while their means of exit is at the inner angle of +the eye; which fact renders it necessary that this watery fluid shall pass +over the surface of the eyeball before it can escape. This arrangement +cannot be accidental, but evinces design, for it thus secures the perfect +lubrication of the surface of the eye, and cleanses it from the smaller +particles of dust {203} which may enter it, in spite of the vigilance of +the lids and lashes. The act of winking, which is generally unconsciously +performed, and which takes place six or more times in a minute, assists +this passage of the tears across the eye, and is especially frequent when +the secretion is most abundant. + +[Sidenote: 59. Describe the shape of the eyeball. Its structure.] + +59. THE EYEBALL.--The remarkable optical instrument called the eyeball, or +the globe of the eye, upon which sight depends, is, as the name indicates, +spherical in shape. It is not a perfect sphere, since the front part +projects somewhat beyond the rest, and at the posterior part the optic +nerve (Fig. 48, N) is united to it, resembling the junction of the stem +with a fruit. In its long diameter, that is, the horizontal or from side to +side, it measures a little more than an inch; in other directions it is +rather less than an inch. In structure, the ball of the eye is firm, and +its tense round contour may in part be felt by pressing the fingers over +the closed lids. + +[Sidenote: 60. Of what is the eyeball composed? State how.] + +60. The eyeball is composed chiefly of three internal, transparent media, +called _humors_; and three investing coats, or _tunics_. The former are the +_aqueous humor_, Fig. 48, A, the _crystalline lens_ L, and the _vitreous +humor_ V. Of these the lens alone is solid. The three coats of the eyeball +are called the _sclerotic_ S, the _choroid_ CH, and the _retina_ R. This +arrangement exists in respect to five-sixths of the globe of the eye, but +in the anterior one-sixth, these coats are replaced by the _cornea_ C, +which is thin and transparent, so that the rays of light pass freely +through it, as through a clear window-pane. + +[Sidenote: 61. The shape of the cornea? Its structure? The "white of the +eye?"] + +61. In shape, the cornea is circular and prominent, resembling a miniature +watch-glass, about 1/25 of an inch thick. In structure, it resembles horn +(as the name signifies), or the nail of the finger, and is destitute of +{204} blood-vessels. The _Sclerotic_ (from _scleros_, hard) is composed of +dense, white fibrous tissue, and gives to the eyeball its firmness of +figure and its white color; in front, it constitutes the part commonly +called "the white of the eye." It is one of the strongest tissues in the +body; it possesses very few vessels, and is not very sensitive. It affords +protection to the extremely delicate interior parts of the eye; and the +little muscles which effect its movements are inserted into the sclerotic a +short distance behind the cornea (see Fig. 48, Y, Z). It is perforated +posteriorly to admit the optic nerve. + +[Sidenote: 62. The second or middle coat of the eyeball? Its dark color?] + +62. The _Choroid_ is the second or middle coat of the eyeball, and lies +closely attached to the inner surface of the sclerotic. Unlike the latter +tunic, its structure is soft and tender, it is dark in color, and possesses +a great abundance of blood-vessels. Its dark color is due to a layer of +dark brown or chocolate-colored cells spread out over its inner surface. +This dark layer serves to absorb the rays of light after they have +traversed the transparent structures in front of it; if the rays were +reflected from side to side within the eye, instead of being thus absorbed, +confused vision would result from the multitude of images which would be +impressed upon the optic nerve. + +[Sidenote: 63. Similar mechanism in microscopes? The albinos? White +rabbits?] + +63. This mechanism has been unconsciously imitated by the opticians, who, +when they make a microscope or telescope, take care that the interior of +its tube shall be coated with a thick layer of black paint or lamp-black; +for without it, a clear delineation of the object to be viewed is +impossible. The albinos, in whom these dark cells of the choroid are +wanting, have imperfect vision, especially in the daytime and in strong +lights. The dark cells are also wanting in white rabbits, and other animals +that have red or pink eyes; their vision appears to be imperfect in the +presence of a bright light. {205} + +[Sidenote: 64. What is the iris? Its construction? How is the size of the +pupil regulated?] + +64. THE IRIS.--Continuous with the choroid, in the front part of the globe +of the eye, is a thin, circular curtain, which occasions the brown, blue, +or gray color of the eye in different individuals. On account of the +varieties of its color, this membrane has received the name _Iris_, which +is the Greek word for "rainbow" (see Fig. 48, I). A front view of it is +shown in Fig. 47. The iris is pierced in its centre by a round opening, +called the _pupil_ (P), which is constantly varying in size. In olden times +it was spoken of as the "apple of the eye." The hinder surface of the iris, +except in albinos, has a layer of dark coloring matter resembling that of +the choroid. The iris is a muscular organ, and contains two distinct sets +of fibres; one of which is circular, while the other radiates outward from +the pupil. The action of these sets of fibres regulates the size of the +pupil; for when the circular set acts, the pupil contracts, and when the +other set acts, the opening expands. Their action is involuntary, and +depends on the reflex system of nerves, which causes the contraction of the +pupil when a strong light falls upon the eye, and its expansion when the +illumination is feeble. + +[Sidenote: 65. The admission of light to the eye? The action of the iris +under different circumstances? The lustre of the eye, how affected in youth +and old age?] + +65. The iris, accordingly, serves a very useful purpose in regulating the +admission of light to the eye. It, however, does not act instantaneously; +and hence, when we pass quickly from a dark room into the bright sunlight, +the vision is at first confused by the glare of light, but as soon as the +pupil contracts, the ability to see becomes perfect. On the other hand, +when we enter a dark apartment, such as a cellar, for a short time we can +see nothing clearly; but as soon as the pupil expands and admits more +light, we are enabled to distinguish the surrounding objects. Animals of +the cat species, and others which prowl around after nightfall, are enabled +to see in the dark by {206} having the iris very dilatable. The size of the +pupil affects the lustre of the eye. When it is large, as it usually is +during youth, the eye appears clear and brilliant; while in old age the +pupil is small and the eye is dull. The brilliancy of the eye is in part, +at least, dependent upon the reflection of light from the front surface of +the crystalline lens. + +[Sidenote: 66. Means used to increase the beauty of the eye? The injurious +consequences?] + +66. Certain poisonous vegetables have the property of causing the pupil to +dilate, and have been used in small doses to increase the beauty of the +eye. One of these drugs has been so largely used by the ladies for this +purpose, that it has received the name _belladonna_, from the Italian words +meaning "beautiful lady." This hazardous practice has resulted more than +once in the death of the person desiring thus to increase her personal +attraction. The common English name for belladonna is "deadly nightshade." +(In the diagram on page 214 the shape and relations of the iris are more +accurately shown than in the figure referred to above.) + +[Sidenote: 67. What part does the retina constitute? How formed? Its +texture? Color? Sensitiveness?] + +67. THE RETINA constitutes the third and inner coat of the globe of the +eye. This, the important part of the eye that is sensitive to light, is a +kind of nervous membrane, formed by the expansion of the ultimate filaments +of the optic nerve. Its texture is soft, smooth, and very thin; it is +translucent and of an opaline, or grayish-white color. It is sensitive to +light alone; and if any form of mechanical irritation be applied to it, the +sensations of touch and pain are not experienced, but flashes of fire, +sparks, and other luminous appearances are perceived. Thus an electric +shock given to the eye-ball occasions a flash of light; and a sudden fall, +or a blow upon the eye, is often apparently accompanied by the vision of +"stars." + +[Sidenote: 68. Specific energy of the optic nerve? Trial in Germany?] + +68. These phenomena are due to what is termed the {207} "specific energy" +of the optic nerve, which nerve, in common with the other nerves of special +sense, obeys a general law of nature, which requires that, whenever one of +these nerves is stimulated, it shall respond with the sensation peculiar to +itself. These flashes of retinal light have no power to illuminate external +objects, although the opposite of this statement has been maintained. On +the occasion of a remarkable trial in Germany, it was claimed by a person +who had been severely assaulted on a very dark night, that the flashes of +light caused by repeated blows upon the head enabled him to see with +sufficient distinctness to recognize his assailant. But the evidence of +scientific men entirely refuted this claim, by pronouncing that the eye, +under the circumstances named, was incapacitated for vision. Too intense +light occasions a feeling of pain, but it is of a peculiar kind, and is +termed "dazzling." + +[Illustration: FIG. 49.] + +[Sidenote: 69. Sensitiveness of all parts of the retina? Experiment to +prove the existence of the "blind spot."] + +69. All parts of the retina are not equally sensitive, and singularly +enough, the point of entry of the nerve of sight, in the back part of the +eyeball, is entirely insensible to light, and is called the "blind spot." +The existence of this point may be proven by a simple experiment. Hold the +accompanying figure, on page 207, directly in front of and parallel with +the eyes. Close the left eye, and fix the sight steadily on the left-hand +circle; then, by gradually varying the distance of the figure from the eye, +at a certain distance (about six inches), the right-hand circle will +disappear, {208} but nearer or further than that, it will be plainly seen. +The other eye may be also tried, with a similar result: if the gaze be +directed to the right-hand circle, the left one will seem to disappear. The +experiment may be repeated by using two black buttons on the marble top of +a bureau, or on some other white surface. The blind spot does not +practically interfere with vision, since the eye is seldom fixed immovably +on an object, and the insensitive parts of the two eyes can never be +directed upon the same object at the same time. + +[Sidenote: 70. Duration of impressions upon the retina? How illustrated?] + +70. Impressions made upon the retina are not at once lost, but persist a +measurable length of time, and then gradually fade away. Thus, a bright +light or color, gazed at intently, cannot be immediately dismissed from +sight by closing or turning away the eyes. A stick lighted at one end, if +whirled around rapidly in the dark, presents the appearance of an unbroken +luminous ring; and the spokes of a rapidly revolving carriage-wheel seem to +be merged into a plane surface. If an object move too rapidly to produce +this sort of lasting impression, it is invisible, as in the case of a +cannon-ball passing through the air in front of us. + +[Sidenote: 71. What further illustration? Winking, why it is not noticed. +Ease with which the retina is fatigued or deprived of sensibility? How +shown?] + +71. If a card, painted with two primary colors--as red and yellow--be made +to rotate swiftly, the eye perceives neither of them distinctly; but the +card appears painted with their secondary color--orange. The average +duration of retinal images is estimated at one-eighth of a second; and it +is because they thus endure, that the act of winking, which takes place so +frequently, but so quickly, is not noticed and does not interrupt the +vision. The retina is easily fatigued or deprived of its sensibility. After +looking steadfastly at a bright light, or at a white object on a black +ground, a dark spot, corresponding in shape to the bright object, {209} +presents itself in whatever direction we look. This spot passes away as the +retina resumes its activity. + +[Sidenote: 72. How further shown? How is the result accounted for? +"Color-blindness?"] + +72. If a bright color be gazed at intently, and the eyes then be turned to +a white surface, a spot will appear; but its color will be the complement +of that of the object. Fix the eye upon a red wafer upon a white ground, +and on removing the wafer a greenish spot of the same shape takes its +place. This result happens because a certain portion of the retina has +exhausted its power to perceive the red ray, and perceives only its +complementary ray, which is green. The color thus substituted by the +exhausted retina is called a physiological or accidental color. In some +persons the retina is incapable of distinguishing different colors, when +they are said to be affected with "color-blindness." Thus, red and green +may appear alike, and then a cherry-tree, full of ripe fruit, will seem of +the same color in every part. Railroad accidents have occurred because the +engineer of the train, who was color-blind, has mistaken the color of a +signal. + +[Sidenote: 73. The location of the crystalline lens? How supported? Its +color and texture? Shape? Size?] + +73. THE CRYSTALLINE LENS.--Across the front of the eye, just behind the +iris, is situated the _Crystalline lens_, enclosed within its own capsule. +It is supported in its place partly by a delicate circular ligament, and +partly by the pressure of adjacent structures. It is colorless and +perfectly transparent, and has a firm but elastic texture. In shape it is +doubly convex, and may be rudely compared to a small lemon-drop. The front +face of the lens is flatter than the other, and is in contact with the iris +near its pupillary margin, as is represented in the diagram on page 214. It +is only one-fourth of an inch thick. + +[Sidenote: 74. Cataract? Aqueous humor? Vitreous humor?] + +74. When this little body becomes opaque, and no longer affords free +passage to the rays of light, as often happens {210} with the advance of +age, an affection termed "cataract" is produced. Between the crystalline +lens and the cornea is a small space which contains the _aqueous humor_ +(see Fig. 48, A). This humor consists of five or six drops of a clear, +colorless liquid very much like water, as its name implies. That part of +the globe of the eye lying behind the lens is occupied by the _vitreous +humor_, so called from its fancied resemblance to melted glass (Fig. 48, +V). This humor is a transparent, jelly-like mass, enclosed within an +exceedingly thin membrane. It lies very closely applied to the retina, or +nervous membrane of the eye, and constitutes fully two-thirds of the bulk +of the eyeball. + +[Illustration: FIG. 50.--THE RETINAL IMAGE.] + +[Sidenote: 75. What is a lens and its focus? The miniature image, how +produced?] + +75. THE USES OF THE CRYSTALLINE LENS.--A convex lens has the property of +converging the rays of light which pass through it; and the point at which +it causes them to meet is termed its focus. If a lens of this description, +such as a magnifying or burning-glass, be held in front of an open window, +in such a position as to allow its focus to fall upon a piece of paper, it +will be found to depict upon the paper a miniature image of the scene +outside of the window. It will be further noticed that the image is +inverted, or upside down, and that the paper {211} at the place upon which +the image is thrown is much brighter than any other part. + +[Sidenote: 76. How are figures painted upon the retina? How proved?] + +76. Now all the transparent structures of the eye, but especially the +crystalline lens, operate upon its posterior part, or retina, as the convex +lens acts upon the paper; that is, they paint upon the retina a bright +inverted miniature of the objects that appear in front of the eye (Fig. +50). That this actually takes place may be proved by experiment. If the +eyeball of a white rabbit, the walls of which are transparent, be examined +while a lighted candle is held before the cornea, an image of the +candle-flame may be seen upon the retina. + +[Sidenote: 77. What can be said in respect to the form and structure of the +crystalline lens?] + +77. The form and structure of the crystalline lens endow it with a +remarkable degree of refractive power, and enable it to converge all the +rays of light that enter it through the pupil, to a focus exactly at the +surface of the retina. When this lens is removed from the eye, as is +frequently done for the cure of cataract, it is found that the rays of +light then have their focus three-eighths of an inch behind the retina; +that the image is four times larger than in the healthy eye, that it is +less brilliant, and that its outline is very indistinct. From this we learn +that one of the uses of the crystalline lens is to make the retinal image +bright and sharply-defined, at the same time that it reduces its size. +Indeed, the small size of the image is a great advantage, as it enables the +limited surface of the retina to receive, at a glance, impressions from a +considerable field of vision. + +[Sidenote: 78. How is the inverted image upon the retina presented in its +true position to the mind?] + +78. As the image upon the retina is inverted, how does the mind perceive +the object in its true, erect position? Many explanations have been +advanced, but the simplest and most satisfactory appears to be found in the +fact that {212} the retina observes no difference, so to speak, between the +right and left or the upper and lower positions of objects. In fact, the +mind is never conscious of the formation of a retinal image, and until +instructed, has no knowledge that it exists. Consequently, our knowledge of +the relative location of external objects must be obtained from some other +source than the retina. The probable source of this knowledge is the +habitual comparison of those objects with the position of our own bodies: +thus, to see an elevated object, we know we must raise the head and eyes; +and to see one at our right hand, we must turn the head and eyes to the +right. + +[Illustration: FIG. 51.--THE DIFFERENT SHAPES OF THE GLOBE OF THE EYE. + +N, The Natural Eye. M, The Short-sighted Eye. H, The Long-sighted Eye. +S, Parallel Rays from the Sun.] + +[Sidenote: 79. The uniform perfection of the eye? Examples? The most common +imperfection?] + +79. LONG-SIGHT OR HYPEROPIA, AND SHORT-SIGHT OR MYOPIA.--The eye is not in +all cases perfectly formed. For example, persons may from birth have the +cornea too prominent or too flat, or the lens may be too thick or too thin. +In either of these conditions sight will be more or less defective from the +first, and the defect will not tend to disappear as life advances. The most +common imperfection, however, is in the shape of the globe; which may be +short (Fig. 51, H), as compared with the natural eye, N, or it may be too +long, M. + +[Sidenote: 80. How is "long-sight" explained? "Short-sight?"] + +80. When the globe is short, objects can only be clearly {213} seen that +are at a distance, and the condition of the vision is known as +"long-sight," or hyperopia. It will be observed, by reference to Fig. 51, +that the focus of the rays of light would fall behind the retina of this +eye. When the globe is too long, objects can only be clearly seen that are +very near to the eye; and the condition resulting from this defect is +termed "short-sight," or myopia. The focus of the rays of light is, in this +case, formed in the interior of the eye in front of the retina. + +[Sidenote: 81. Long-sight, how common? With what must it not be confounded? +Kind of glasses for short-sight? Why? Squint?] + +81. Long-sight, or hyperopia, is common among schoolchildren, nearly as +much so as short-sight, and must not be confounded with the defect known as +the "far sight" of old people; although in both affections the sight is +improved by the use of convex glasses. Children not infrequently discover +that they see much better when they chance to put on the spectacles of old +persons. For the relief of short-sight, concave glasses should be employed; +as they so scatter the rays of light as to bring the focus to the retina, +and thus cause the vision of remote objects to become at once distinct. +That form of "squint," in which the eyes are turned inward, is generally +dependent upon long-sight, while that rarer form, when they turn outward, +is due to short-sight. + +[Sidenote: 82. What is stated in connection with the opera-glass? +Experiment with pencil and distant object?] + +82. THE FUNCTION OF ACCOMMODATION.--If, after looking through an +opera-glass at a very distant object, it is desired to view another nearer +at hand, it will be found impossible to obtain a clear vision of the second +object unless the adjustment of the instrument is altered; which is +effected by means of the screw. If an object, like the end of a pencil, be +held near the eye, in a line with another object at the other side of the +room, or out of the window, and the eye be fixed first upon one and then +upon the other, it will be found that when the pencil is clearly seen, the +{214} further object is indistinct; and when the latter is seen clearly, +the pencil appears indistinct; and that it is impossible to see both +clearly at the same time. Accordingly, the eye must have the capacity of +adjusting itself to distances, which is in some manner comparable to the +action of the screw of the opera-glass. + +[Illustration: FIG. 52.--THE FUNCTION OF ACCOMMODATION. + +The right half of the diagram shows the eye at rest. The left half shows +the lens accommodated for near vision.] + +[Sidenote: 83. Function of accommodation? In what does it consist? How is +the function explained?] + +83. This, which has been called the function of accommodation, is one of +the most admirable of all the powers of the eye, and is exercised by the +crystalline lens. It consists essentially in a change in the curvature of +the front surface of the lens, partly through its own elasticity, and +partly through the action of the ciliary muscle. When the eye is at rest, +that is, when accommodated for a distant object, the lens is flatter and +its curvature diminished (see Fig. 52); but when strongly accommodated for +near vision the lens becomes thicker, its curvature increases, and the +image on the retina is made more sharp and distinct. Since a strong light +is not required in viewing near objects, the pupil contracts, as is shown +in the left-hand half of the diagram. {215} + +[Sidenote: 84. Change of sight with the approach of old age? Explain the +change?] + +84. OLD-SIGHT, OR PRESBYOPIA.--But this marvellously beautiful mechanism +becomes worn with use; or, more strictly speaking, the lens, like other +structures of the body, becomes harder with the approach of old age. The +material composing the lens becomes less elastic, the power to increase its +curvature is gradually lost, and as a consequence, the person is obliged to +hold the book further away when reading, and to seek a stronger light. In a +word, the function of accommodation begins to fail, and is about the first +evidence that marks the decline of life. By looking at the last preceding +diagram, and remembering that the increased curvature of the lens cannot +take place, it will be at once understood why old-sight is benefitted in +near vision by the convex lens, such as the spectacles of old people +contain. It acts as a substitute for the deficiency of the crystalline +lens. + +[Sidenote: 85. Hearing or audition? What is sound? How propagated commonly? +Stone thrown in water?] + +85. THE SENSE OF HEARING.--SOUND.--Hearing, or audition, is the special +sense by means of which we are made acquainted with _sound_. What is sound? +It is an impression made upon the organs of hearing, by the vibrations of +elastic bodies. This impression is commonly propagated by means of the air, +which is thrown into delicate undulations, in all directions from the +vibrating substance. When a stone is thrown into smooth water, a wave of +circular form is set in motion, from the point where the stone struck, +which constantly increases in size and diminishes in force, as it advances. + +[Sidenote: 86. Sound-wave in the atmosphere? Its shape? Rate of motion? +Sound, in water, air, and solid bodies?] + +86. Somewhat resembling this, is the undulation, or sound-wave, which is +imparted by a sonorous vibration to the surrounding atmosphere. Its shape, +however, is spherical, rather than circular, since it radiates upward, +downward, and obliquely as well as horizontally, like the wave {216} in +water. The rate of motion of this spherical wave of air is about 1050 feet +per second, or one mile in five seconds. In water, sound travels four times +as fast as in air, and still more rapidly through solid bodies; along an +iron rod, its velocity is equal to two miles per second. + +[Sidenote: 87. The earth as a conductor of sound? To what has the western +Indian been taught? Solid substances as conductors? As regards sound, in +what respect is air necessary? Sound in a vacuum?] + +87. The earth, likewise, is a good conductor of sound. It is said that the +Indian of our western prairies can, by listening at the surface of the +ground, hear the advance of a troop of cavalry, while they are still out of +sight, and can even discriminate between their tread and that of a herd of +buffaloes. Solid substances also convey sounds with greater power than air. +If the ear be pressed against one end of a long beam, the scratching of a +pin at the other extremity may be distinctly heard, which will not be at +all audible when the ear is removed from the beam. Although air is not the +best medium for conveying sound, it is necessary for its production. Sound +cannot be produced in a vacuum, as is shown by ringing a bell in the +exhausted receiver of an air-pump, for it is then entirely inaudible. But +let the air be readmitted gradually, then the tones become more and more +distinct, and when the receiver is again full of air, they will be as clear +as usual. + +88. All sonorous bodies do not vibrate with the same degree of rapidity, +and upon this fact depends the _pitch_ of the sounds that they respectively +produce. The more frequent the number of vibrations within a given time, +the higher will be the pitch; and the fewer their number, the lower or +graver will it be. Now, the rate of the successive vibrations of different +notes has been measured, and it has thus been found that if they are less +than sixteen in a second, no sound is audible; while if they exceed 60,000 +per second the sound is very faint, and is painful to the {217} ear. The +extreme limit of the capacity of the human ear may be considered as +included between these points; but the sounds which we ordinarily hear are +embraced between 100 and 3,000 vibrations per second. + +89. The _ear_, which is the proper organ of hearing, is the most +complicated of all the structures that are employed in the reception of +external impressions. The parts of which it is composed are numerous, and +some of them are extremely small and delicate. Nearly all these parts are +located in an irregularly shaped cavity hollowed out in the temporal, or +"temple," bone of each side of the head. That part of the bone in which the +auditory cavity is placed has the densest structure of all bones of the +body, and has therefore been called the "petrous," or rocky part of the +temporal bone. In studying the ear, it is necessary to consider it as +divided into three portions, which are called, from their relative +positions, the _external_ ear, the _middle_ ear, and the _internal_ ear. +(In the diagram, Fig. 53, A, the first is not shaded, the second is lightly +shaded, and the last has a dark background.) + +[Sidenote: 90. Of what does the external portion of the organ of hearing +consist? Describe the portal of that organ known as the ear. Its use?] + +90. THE EXTERNAL EAR.--The external portion of the organ of hearing, +designated in Fig. 53, A, includes, first, that outer part (_a_), which is +commonly spoken of as "the ear," but which in fact is only the portal of +that organ; and, secondly, the _auditory canal_ (_b_). The former consists +of a flat flexible piece of cartilage, projecting slightly from the side of +the head, attached to it by ligaments, and supplied with a few weak +muscles. Its surface is uneven, and curiously curved, and from its +resemblance to a shell, it has been called the _concha_. It probably serves +to collect sounds, and to give them an inward direction; although its +removal is said not to impair the acuteness of hearing more than a few +days. {218} + +[Sidenote: 91. The ear in the animals of delicate hearing? Rabbit? Fishes?] + +91. In those animals whose hearing is more delicate than that of man, the +corresponding organ is of greater importance, it being larger and supplied +with muscles of greater power, so that it serves as a natural kind of +ear-trumpet, which is easily movable in the direction of any sound that +attracts the attention of the animal. Bold, predaceous animals generally +have the concavity of this organ directed forward, while in timorous +animals, like the rabbit, it is directed backward. Fishes have no outer +ear, but sounds are transmitted directly through the solid bones of the +head, to the internal organ of hearing. + +[Illustration: FIG. 53.--THE EAR AND ITS DIFFERENT PARTS. + +A, Diagram of the Ear. + +_a_, _b_, External Ear. _c_, Membrane tympani. _d_, Middle Ear. +_e_, Internal Ear. + +B to B''', Bones of the Middle Ear (magnified). + +C, The Labyrinths, or Internal Ear (highly magnified).] + +[Sidenote: 92. What is the auditory canal? Describe it.] + +92. The _auditory canal_ (Fig. 53, A, _b_), which is continuous with the +outer opening of the ear, is a blind passage, an inch and a quarter in +length, its inner extremity being bounded by a closely-fitting, circular +membrane. This canal is of oval form, is directed forward and inward, {219} +and is slightly curved; so that the inner end is ordinarily concealed from +view. The pouch of the skin which lines this passage is smooth and thin, +especially at the lower end, where it covers the membrane just mentioned. + +[Sidenote: 93. How is it guarded and protected? Ear-wax?] + +93. As in the case of the nostrils, a number of small, stiff hairs garnish +the margin of the auditory canal, and guard it, to some extent, against the +entrance of insects and other foreign objects. The skin, too, covering its +outer half, is furnished with a belt of little glands which secrete a +yellow, viscid, and bitter substance, called "ear-wax," which is especially +obnoxious to small insects. As the outer layer of this wax-like material +loses its useful properties, it becomes dry, and falls out of the ear in +the form of minute, thin scales, a fresh supply being furnished from the +little glands beneath. In its form, the auditory canal resembles the tube +of an ear-trumpet, and serves to convey the waves of sound to the middle +portion of the ear. + +[Sidenote: 94. What is the middle ear? Why called tympanum?] + +94. THE MIDDLE EAR, OR TYMPANUM.--The middle ear is a small cavity, or +chamber, of irregular shape, about one-fourth of an inch across from side +to side, and half an inch long (see Fig. 53, A, _d_). From the peculiar +arrangement of its various parts it has very properly been called the +_tympanum_, or the "drum of the ear." The middle ear, like the external +canal, contains air. + +[Sidenote: 95. What is the membrana tympani? Describe it.] + +95. The circular membrane, already mentioned as closing the auditory canal, +is the partition which separates the middle from the external ear, and is +called the _membrana tympani_ (_c_), and may be considered as the outer +head of the drum of the ear. It is sometimes itself spoken of as the +"drum," but this is incorrect; since a drum is not a membrane, but is the +hollow space across which the membrane is stretched. This membranous +drum-head is very tense and elastic, and so thin as to be almost +transparent; {220} its margin is fastened into a circular groove in the +adjacent bone. Each wave of sound that impinges against this delicate +membrane causes it to vibrate, and it, in turn, excites movements in the +parts beyond. + +[Sidenote: 96. What are the ossicles? Their number and names? Their +arrangement?] + +96. Within the tympanum is arranged a chain of remarkable "little bones," +or _ossicles_. They are chiefly three in number, and from their peculiar +shapes bear the following names: _malleus_, or the mallet; _incus_, or the +anvil; and _stapes_, or the stirrup. A fourth, the smallest bone in the +body, in early life intervenes between the incus and stapes, but at a later +period it becomes a part of the incus. It is called the _orbicular_ bone. +Small as are these ossicles--and they, together, weigh only a few +grains--they have their little muscles, cartilages, and blood-vessels, as +perfectly arranged as the larger bones of the body. One end of the chain of +ossicles, the mallet, is attached to the membrane of the tympanum, or outer +drum-head, while the other end, the stirrup, is firmly joined by its +foot-piece to a membrane in the opposite side of the cavity. The chain, +accordingly, hangs suspended across the drum between the two membranes; and +when the outer one vibrates under the influence of the sound-wave, the +chain swings inward and transmits the vibration to the entrance of the +inner ear. + +[Sidenote: 97. The Eustachian tube? Describe it, and state its use.] + +97. The musical instrument, the drum, is not complete if the air within be +perfectly confined: we therefore find in all instruments of this kind a +small opening in the side, through which air may pass freely. By this means +the pressure of the air upon the vellum which forms the head of the drum is +made equal upon all sides, and the resonance of the drum remains unaffected +by the varying density of the atmosphere. It will, therefore, emit its +proper sound, whether it be struck in the rarified air of the mountain-top, +or in the condensed air of a mine. The tympanum, {221} or drum of the ear, +in like manner has an opening by means of which it communicates freely with +the external air. This opening is a narrow canal, about an inch and a half +long, called the _Eustachian tube_, after the name of its discoverer, +Eustachius. + +[Illustration: FIG. 54.--SECTION OF THE RIGHT EAR. + +A, The Concha. B, Auditory Canal. C, Membrane of the Drum, (the lower +half.) D, A small muscle. E, Incus, or Anvil. M, Malleus, or Mallet. +I, Eustachian Tube. G, Semicircular Canals. H, Cochlea, or snail's shell.] + +[Sidenote: 98. What can you state of the action of the Eustachian tube?] + +98. The course of this passage is indicated in Fig. 54, I, directed +downward and inward: its other extremity opens into the upper part of the +throat. The passage itself is ordinarily closed, but whenever the act of +swallowing or gaping takes place, the orifice in the throat is stretched +open, and the air of the cavity of the tympanum may then be renewed. Air +may at will be made to enter through this tube, by closing the mouth and +nose, and then trying {222} to force air through the latter. When this is +done, a distinct crackle or clicking sound is perceived, due to the +movement of the membranes, and of the little bones of the ear. + +[Sidenote: 99. What other purpose does the Eustachian tube serve? How is +this shown? "Throat-deafness?" Primary use of the Eustachian tube?] + +99. The Eustachian tube serves, also, as an escape-pipe for the fluids +which form within the middle ear; and hence, when its lining membrane +becomes thickened, in consequence of a cold, or sore throat, and the +passage is thus more or less choked up, the fluids are unable to escape as +usual, and therefore accumulate within the ear. When this takes place, the +vibrations of the membrane are interfered with; the sounds heard appear +muffled and indistinct; and a temporary difficulty of hearing, which is +known as "throat-deafness," is the result. This result resembles the effect +produced by interrupting the vibrations of a sonorous body, such as all are +familiar with; if the finger be placed upon a piano-string or bell when it +is struck, the proper sound is no longer fully and clearly emitted. But the +primary use of this tube is to afford a free communication between the +middle ear and the external atmosphere, and thus secure an equal pressure +upon both sides of the membrane of the drum of the ear, however the density +of the atmosphere may vary. If, from undue tension of the membranes, pain +is experienced in the ears, when ascending into a rare atmosphere, as in a +balloon, or descending into a dense one, as in a diving-bell, it may be +relieved by repeating the act of swallowing, from time to time, in order +that the inner and outer pressure may thus be promptly equalized. + +[Sidenote: 100. The essential part of the organ of hearing? Its location? +Formation?] + +100. THE INTERNAL EAR, OR LABYRINTH.--The most essential part of the organ +of hearing is the distribution of the _auditory nerve_. This is found +within the cavity of the internal ear, which, from its exceedingly tortuous +shape, {223} has been termed the _labyrinth_ (see Fig. 53, C). This cavity +is hollowed out in dense bone, and consists of three parts; the _vestibule_ +(_a_), or ante-chamber, which is connected with the other two; the +_cochlea_ (_b_), or snail's shell; and the three _semicircular canals_ +(_c_). The manner in which the nerve of hearing is distributed is +remarkable, and is peculiar to this nerve. In the vestibule and the canals +its fibres are spread out over the inner surface, not of the bony cavity +but of a membranous bag, which conforms to and partially fills that cavity; +and which floats in it, being both filled and surrounded with a clear, +limpid fluid. + +[Sidenote: 101. Where is the "ear-sand" found? Give the theory as to its +use.] + +101. A singular addition to the mechanism of hearing is observed within +this membranous bag of the labyrinth. This consists of two small oval +ear-stones, and a quantity of fine powder of a calcareous nature, which is +called "ear-sand." When examined under the microscope, these sandy +particles are seen to lie scattered upon and among the delicate filaments +of the auditory nerve; and it is probable, that as the tremulous sound-wave +traverses the fluid of the vestibule, the sand rises and falls upon the +nerve filaments, and thus intensifies the sonorous impression. + +[Sidenote: 102. In the cochlea or snail's shell? "Key-board" in the +internal ear? The vestibules? Semicircular canals?] + +102. In the cochlea, or snail's shell, which contains the fluid, but no +membrane, the nerve ramifies upon a spiral shelf, which, like the cochlea +itself, takes two and a half turns, growing continuously smaller as it +winds upward. As many as three thousand nerve fibres of different lengths +have been counted therein; which, it has been thought, form the grand, yet +minutely small key-board, upon which strike all the musical tones that are +destined to be conveyed to the brain. The vestibule, it is also supposed, +takes cognizance of noise as distinguished from musical sounds; while the +office of the semicircular canals is, in part at least, to prevent internal +echoes, or reverberations. {224} + +[Sidenote: 103. With what does the vestibule communicate? What is the +theory by which sound is conducted to the brain?] + +103. The vestibule communicates with the chain of bones of the middle ear +by means of a small opening, called the "oval window," or _fenestra +ovalis_. Across this window is stretched the membrane, which has already +been alluded to as being joined to the stirrup-bone of the middle ear. +Through this window, then, the sound-wave, which traverses the external and +middle ear, arrives at last at the labyrinth. The limpid fluid which the +latter contains, and which bathes the terminal fibres of the nerve of +hearing, is thus agitated, the nerve-fibres are excited, and a sonorous +impression is conducted to the brain, or, as we say, a sound is heard. + +[Sidenote: 104. The formation of the organ of hearing with a view to its +protection?] + +104. PROTECTION OF THE SENSE OF HEARING.--From what has been seen of the +complicated parts which compose the organ of hearing, it is evident that +while many of them possess an exquisite delicacy of structure, Nature has +well and amply provided for their protection. We have observed the +concealed situation of the most important parts of the mechanism of the +ear, the length of its cavity, its partitions, the hardness of its walls, +and its communication with the atmosphere; all these provisions rendering +unnecessary any supervision or care on our part in reference to the +interior of the ear. But in respect to its external parts, which are under +our control and within the reach of harm, it is otherwise. We may both +observe the dangers which threaten them, and learn the means necessary to +protect them. + +[Sidenote: 105. Danger to which the hearing may be subjected? Advice?] + +105. One source of danger to the hearing consists in lowering the +temperature of the ear, especially by the introduction of cold water into +the auditory canal. Every one is familiar with the unpleasant sensation of +distension and the confusion of sounds which accompany the filling of the +ear with water when bathing: the weight of the {225} water within it really +distends the membrane, and the cold chills the adjacent sensitive parts. It +is not surprising, therefore, that the frequent introduction of cold water +and its continued presence in the ear enfeeble the sense of hearing. Care +should be taken to remove water from the ear after bathing, by holding the +head on one side, and, at the same time, slightly expanding the outer +orifice, so that the fluid may run out. For a like reason, the hair about +the ears should not be allowed to remain wet, but should be thoroughly +dried as soon as possible. + +[Sidenote: 106. The general rule as to the use of water for the ear?] + +106. It may be stated as a general rule, to which there are but few +exceptions, that no cold liquid should ever be allowed to enter the ear. +When a wash or injection is rendered necessary, it should always be warmed +before use. The introduction of cold air is likewise hurtful, especially +when it pours through a crevice directly into the ear, as it may often do +through the broken or partially closed window of a car. The avoidance of +this evil gives rise to another almost as great; namely, the introduction +of cotton or other soft substances into the ear to prevent it from +"catching cold." This kind of protection tends to make the part unnaturally +susceptible to changes of temperature, and its security seems to demand the +continued presence of the "warm" covering. As a consequence of its +presence, sounds are not naturally conveyed, and the sensitiveness of the +nerve of hearing is gradually impaired. + +[Sidenote: 107. Chief source of injury to the ear? Directions for removing +foreign objects from the ear? Of a live insect?] + +107. The chief source of injury, however, to the ear is from the +introduction of solid substances into the auditory canal, with the design +of removing insects or other foreign objects that have found their way into +the ear; or with the design of scraping out the ear-wax. For displacing a +foreign object, it is usually sufficient to syringe the ear gently with +warm water, the head being so held that the {226} fluid easily escapes. If +a live insect has gained entrance to the ear, it may first be suffocated by +pouring a little oil upon it, and afterward removed by syringing the ear as +just mentioned. + +108. The removal of ear-wax is generally unnecessary; for, as we have +before seen, Nature provides that the excess of it shall become dry, and +then spontaneously fall out in the form of fine scales. The danger from the +introduction of solid implements into the outer ear is chiefly found in the +fact that the membrane which lies at the bottom of it is very fragile, and +that any injury of it is liable to be permanent, and to permanently impair +the hearing of the injured ear. + +QUESTIONS FOR TOPICAL REVIEW. + +_Give as full statements as you can on the following subjects_: + + 1. Production of sensation 177, 178 + 2. Variety of sensations 178, 179 + 3. General sensibility 179, 180 + 4. The sensation of pain 180 + 5. The uses of pain 180, 181, 182 + 6. Special sensation 182, 183 + 7. Organs of touch 183, 184 + 8. The sense of touch 184, 185, 186 + 9. The delicacy of touch 186, 187 + 10. Sensations of temperature and weight 187, 188 + 11. The organ of taste 188, 189 + 12. The sense of taste 189, 190 + 13. Relations of taste, etc. 190, 191 + 14. Influence of education, etc. 191, 192 + 15. The sense of smell 192, 193 + 16. The nerve of smell 193, 194 + 17. Uses of the sense of smell 194, 195 + 18. The sense of sight 196, 197 + 19. Light, and the optic nerve 197, 198 + 20. The organ of sight 198, 199 + 21. The orbits 199 + 22. The eyelids 200, 201 + 23. The lachrymal fluid 201, 202, 203 + 24. The eyeball 203, 204 + 25. The iris 205, 206 + 26. The retina 206, 207, 208, 209 + 27. The crystalline lens 209, 210 + 28. Uses of the lens 210, 211, 212 + 29. Long and short sight 212, 213 + 30. Function of accommodation 213, 214 + 31. Old sight, or presbyopia 215 + 32. Hearing and sound 215, 216, 217 + 33. The external ear 217, 218, 219 + 34. The middle ear 219-222 + 35. The internal ear 222, 223, 224 + 36. Protection of the sense of hearing 224, 225, 226 + + * * * * * + + +{227} + +CHAPTER XI. + +THE VOICE. + + _Voice and Speech--The Larynx, or the Organ of the Voice--The Vocal + Cords--The Laryngoscope--The Production of the Voice--The Use of the + Tongue--The different Varieties of Voice--The Change of Voice--Its + Compass--Purity of Tone--Ventriloquy._ + +[Sidenote: 1. The uttering of sounds by animals? How produced?] + +1. VOICE AND SPEECH.--In common with the majority of the nobler animals, +man possesses the power of uttering sounds, which are employed as a means +of communication and expression. In man, these sounds constitute the voice; +in the animals, they are designated as the cry. The song of the bird is a +modification of its cry, which is rendered possible from the fact that its +respiratory function is remarkably active. The sounds of the animals are +generally, but not always, produced by means of their breathing organs. +Among the insects, they are sometimes produced by the extremely rapid +vibrations of the wings in the act of flight, as in the case of the +musquito; or they are produced by the rubbing together of hard portions of +the external covering of the body, as in the cricket. Almost all kinds of +marine animals are voiceless. The tambour-fish and a few others have, +however, the power of making a sort of noise in the water. + +[Sidenote: 2. The evidence of man's superior endowment? What is stated of +the idiot? Parrot? Raven?] + +2. But man alone possesses the faculty of speech, or the power to use +articulate sounds in the expression of ideas, and in the communication of +mind with mind. Speech is thus an evidence of the superior endowment of +man, and involves the culture of the intellect. An idiot, while he {228} +may have complete vocal organs and full power of uttering sounds or cries, +is entirely incapable of speech; and, as a rule, the excellence of the +language of any people will be found to be proportional to their +development of brain. Man, however, is not the only being that has the +power to form articulate sounds, for the parrot and the raven may also be +taught to speak by rote; but man alone attaches meaning to the words and +phrases he employs. + +[Sidenote: 3. Speech and hearing? A deaf child? Person having "no ear for +music?" Impaired hearing? What do the examples show?] + +3. Speech is intimately related to the sense of hearing. A child born deaf +is, of necessity, dumb also; not because the organs of speech are +imperfect, for he can utter cries and may be taught to speak, and even to +converse in a rude and harsh kind of language; but because he can form no +accurate notion of sound. And a person, whose hearing is not delicate, or +as it is commonly expressed, who "has no ear for music," cannot sing +correctly. A person who has impaired hearing commonly talks in an +unnaturally loud and monotonous voice. These examples show the necessary +relation of intelligence and the sense of hearing with that form of +articulate voice, which is termed speech. + +[Sidenote: 4. Organ of the voice? Where situated? Of what is its framework +composed?] + +4. THE ORGAN OF THE VOICE.--The essential organ of the voice is the Larynx. +This has been previously alluded to in its relation to the function of +respiration; and, in the chapter on that subject, are figured the front +view of that organ (Fig. 35), and its connection with the trachea, tongue, +and other neighboring parts (Fig. 38). It is situated at the upper part of +the neck, at the top of the trachea, or tube by which air passes into and +out of the lungs. The framework of the larynx is composed of four +cartilages, which render it at once very strong and sufficiently flexible +to enable it to move according to the requirements of the voice. {229} + +[Illustration: FIG. 55. SECTION OF THE LARYNX AND TRACHEA. + +A, The Epiglottis. B, The Thyroid Cartilage. C, Arytenoid Cartilage. +D, Ventricle of the Larynx. E, Cricoid Cartilage. F, Right Vocal Cord. +H, The Trachea.] + +[Sidenote: 5. Names, formation, and situation of the cartilages?] + +5. The names of the cartilages are (1) the _thyroid_, which is a broad thin +plate, bent in the middle and placed in the central line of the front part +of the neck, where it is known as the _pomum Adami_, or Adam's apple (Fig. +55, B), and where it may be felt moving up and down with each act of +swallowing; (2) the _cricoid_, which is shaped like a seal ring, with the +broad part placed posteriorly (Fig. 55, E). At the top of the cricoid +cartilage are situated the two small _arytenoid_ cartilages, the right one +of which is shown in Fig. 55, C. These latter little organs are much more +movable than the other two, and are very important in the production of the +voice. They have a true ball and socket joint, and several small muscles +which contract and relax with as perfect regularity and accuracy as any of +the larger muscles of the body. + +[Sidenote: 6. Lining of the interior of the larynx? The epiglottis?] + +6. The interior of the larynx is lined with a very sensitive mucous +membrane, which is much more closely adherent to the parts beneath than is +usually the case with membranes of this description. The epiglottis (A), +consisting of a single leaf-shaped piece of cartilage, is attached to the +front part of the larynx. It is elastic, easily moved, and fits accurately +over the entrance to the air-passages below it. Its office is to guard +these delicate passages and the lungs against the intrusion of food and +other foreign {230} articles, when the act of swallowing takes place. It +also assists in modifying the voice. + +[Sidenote: 7. Where are the vocal cords? The false cords? The true cords?] + +7. THE VOCAL CORDS.--Within the larynx, and stretched across it from the +thyroid cartilage in front to the arytenoid cartilages behind, are placed +the two sets of folds, called the vocal cords. The upper of these, one on +each side, are the false cords, which are comparatively fixed and +inflexible. These are not at all essential to the formation of vocal +sounds, for they have been injured, in those lower animals whose larynx +resembles that of man, without materially affecting their characteristic +cries. Below these, one on each side, are the true vocal cords (Fig. 55, +F), which pursue a similar direction to the false cords, namely, from +before backward. But they are composed of a highly elastic, though strong +tissue, and are covered with a thin, tightly-fitting layer of mucous +membrane. Their edges are smooth and sharply defined, and when they meet, +as they do in the formation of sounds, they exactly match each other. + +[Sidenote: 8. Where is the ventricle of the larynx? The essentials to the +formation of the tones and modulation of the voice?] + +8. Between the true and false vocal cords is a depression on each side, +which is termed the ventricle of the larynx (Fig. 55, D). The integrity of +these true cords, and their free vibration, are essential to the formation +of the tones and the modulation of the natural voice. This is shown by the +fact that, if one or both of these cords are injured or become diseased, +voice and speech are compromised; or when the mucous membrane covering them +becomes thickened, in consequence of a cold, the vocal sounds are rendered +husky and indistinct. When an opening is made in the throat below the +cords, as not infrequently happens in consequence of an attempt to commit +suicide, voice is impossible except when the opening is closed by external +pressure. {231} + +[Illustration: FIG. 56. A VIEW OF THE VOCAL CORDS BY MEANS OF THE +LARYNGOSCOPE.] + +[Sidenote: 9. Variation in the interval between the true cords of the +voice? Experiment with the mirror?] + +9. The interval or space between the true cords of the voice is constantly +varying, not only when their vocal function is in exercise, but also during +the act of respiration. Every time the lungs are inflated, the space +increases to make wide the entrance for the air; and diminishes slightly +during expiration. So that these little cords move gently to and fro in +rhythm with the expansion and contraction of the chest in breathing. These +movements and others may be seen to take place, if a small mirror attached +to a long handle be placed back into the upper part of the throat; the +handle near the mirror must be bent at an angle of 45deg, so that we may +look "around the corner," so to speak, behind the tongue. The position +which the mirror must assume will be understood by reference to Fig. 38. A +view of what may be seen under favorable circumstances, during tranquil +inspiration is represented in Fig. 56. The vocal cords are there shown as +narrow, white bands, on each side of the central opening, and since the +image is inverted, the epiglottis appears uppermost. The rings partly seen +through the opening belong to the trachea. This little mirror is the +essential part {232} of an instrument, which is called the laryngoscope, +and, simple as it may seem, it is accounted one of the most valuable of the +recently invented appliances of the medical art. + +[Sidenote: 10. The formation of true vocal tones?] + +10. THE PRODUCTION OF THE VOICE.--During ordinary tranquil breathing no +sound is produced in the larynx, true vocal tones being formed only during +forcible expiration, when, by an effort of the will, the cords are brought +close together, and are stretched so as to be very tense. The space between +them is then reduced to a narrow slit, at times not more than 1/100 of an +inch in width; and the column of expired air being forced through it causes +the cords to vibrate rapidly, like the strings of a musical instrument. +Thus the voice is produced in its many varieties of tone and pitch; its +intensity, or loudness, depending chiefly upon the power exerted in +expelling the air from the lungs. When the note is high, the space is +diminished both in length and width; but when it is low, the space is wider +and longer (Fig. 57, B, C), and the number of vibrations is fewer within +the same period of time. + +[Illustration: FIG. 57. THE DIFFERENT POSITIONS OF THE VOCAL CORDS. + +A, The position during inspiration. B, In the formation of low notes. C, In +the formation of high notes.] + +[Sidenote: 11. To what is the personal quality of the voice mainly due? +What aids are there?] + +11. The personal quality of the voice, or that which enables us to +recognize a person by his speech, is mainly due to the peculiar shape of +the throat, nose, and mouth, and {233} the resonance of the air contained +within those cavities. The walls of the chest and the trachea take part in +the resonance of the voice, the air within them vibrating at the same time +with the parts above them. This may be tested by touching the throat or +breast-bone, when a strong vocal effort is made. The teeth and the lips +also are important, as is shown by the unnatural tones emitted by a person +who has lost the former, or by one who is affected with the deformity known +as "hare-lip." The tongue is useful, but not indispensable to speech; the +case of a woman is reported, from whom nearly the whole tongue had been +torn out, but who could, nevertheless, speak distinctly and even sing. + +[Sidenote: 12. Varieties of voice? The baritone? The voice in early youth?] + +12. THE VARIETIES of voice are said to be four in number; two, the bass and +tenor, belonging to the male sex; and two, the contralto or alto, and +soprano, peculiar to the female. The baritone voice is the name given to a +variety intervening between the bass and tenor. In man, the voice is strong +and grave; in woman, soft and high. In infancy and early youth, the voice +is the same in both sexes, being of the soprano variety: that of boys is +both clear and loud, and being susceptible of considerable training, is +highly prized in the choral services of the church and cathedral. At about +fourteen years of age the voice is said to change; that is, it becomes +hoarse and unsteady by reason of the rapid growth of the larynx. In the +case of the girl, the change is not very marked, except that the voice +becomes stronger and has a wider compass; but in the boy, the larynx nearly +doubles its size in a single year, the vocal cords grow thicker, longer, +and coarser, and the voice becomes masculine in character. During the +progress of this change, the use of the voice in singing is injudicious. + +[Sidenote: 13. The range of the voice? Result of careful training of the +vocal organs?] + +13. The ordinary range of each of the four varieties of {234} the voice is +about two octaves; but this is exceeded in the case of several celebrated +vocalists. Madame Parepa-Rosa has a compass of three full octaves. When the +vocal organs have been subjected to careful training, and are brought under +complete control of the will, the tension of the cords become exact, and +their vibrations become exceedingly precise and true. Under these +circumstances the voice is said to possess "purity" of tone, and can be +heard at a great distance, and above a multitude of other sounds. The power +of a pure voice to make itself heard was recently exemplified in a striking +manner: at a musical festival held in an audience-room of extraordinary +size, and amid an orchestra of a thousand instruments and a chorus of +twelve thousand voices, the artist named above also sang; yet such was the +purity and strength of her voice that its notes could be clearly heard +rising above the vast waves of sound produced by the full accompaniment of +chorus and orchestra. + +[Sidenote: 14. The production of the articulate sounds? What experiment is +mentioned?] + +14. In the production of the articulate sounds of speech, the larynx is not +directly concerned, but those sounds really depend upon alterations in the +shape of the air-passages above that organ. That speech is not necessarily +due to the action of the larynx is proven by the following simple +experiment. Let an elastic tube be passed through the nostril to the back +of the mouth. Then, while the breath is held, cause the tongue, teeth, and +lips to go through the form of pronouncing words, and at the same time, let +a second person blow through the tube into the mouth. Speech, pure and +simple, or, in other words, a whisper is produced. Still further continue +the experiment, while permitting vocal sounds to be made, and there will be +produced a loud and whispering speech at the same moment; thus showing that +voice and speech are the result of two distinct acts. Sighing, in like +manner, is {235} produced in the mouth and throat; if, however, a vocal +sound be added, the sigh is changed into a groan. + +[Sidenote: 15. What is ventriloquism? Indication of the original meaning of +the word? How are the ventriloquous sounds produced?] + +15. VENTRILOQUISM is a peculiar modification of natural speech, which +consists in so managing the voice that words and sounds appear to issue, +not from the person, but from some distant place, as from the chimney, +cellar, or the interior of a chest. The original meaning of the word +ventriloquism (that is, speaking from the belly), indicates the early +belief that this mode of speech was dependent upon the possession and use +of some special organ besides the larynx and mouth; but at the present +time, it is known that it is produced by these organs alone, and that the +sources of deception consist on the part of the performer, in the dexterous +management of the voice, together with a talent for mimicry; and, on the +part of the auditory, in the liability of the sense of hearing to error in +respect to the direction of sounds. The ventriloquist not only seems to +"throw his voice," as it is said, or simulates the sound as it usually +appears at a distance with but little motion of the lips and face, but he +imitates the voices of an infant and of a feeble old man, of a drunken man +disputing with an exasperated wife, the broken language of a foreigner, the +cry of an animal in distress, demonstrating that the performer must be +proficient in the art of mimicry. Ventriloquism was known to the ancient +Romans and Greeks; and it is thought that the mysterious responses that +were said to issue from the sacred trees and shrines of the oracles at +Dodona and Delphi, were really uttered by priests who had the power of +producing this form of speech. + + * * * * * + + +{236} + +CHAPTER XII. + +THE USE OF THE MICROSCOPE IN THE STUDY OF PHYSIOLOGY. + +[Sidenote: 1. The will of the Creator, by what obeyed? The power of a +muscle? Amount of duty performed by the liver?] + +1. THE LAW OF THE TISSUES.--The will of an infinite Creator is obeyed by +atoms as well as by worlds. He has seen fit to commit all the functions of +life to structures or tissues so small as to be invisible to the naked eye. +A muscle, for example, as we have already learned, is composed of +innumerable filaments, visible only by the aid of the microscope; and the +power of the muscular mass is but the sum of the contractile power of the +filaments which enter into its composition. Again, each cell of the liver, +invisible to unassisted sight, is a secreting organ, and the liver performs +as much duty as the sum of these minute organs renders possible and no +more. + +[Sidenote: 2. Necessity for using the microscope? The advantages gained by +its use?] + +2. THE NECESSITY OF THE MICROSCOPE.--If, therefore, we would know the real +structure of the human body, we must make use of the microscope. We are not +at liberty either to use it or not; we _must_ have recourse to it in order +to obtain a real knowledge of the human body. Our eyes are constructed for +the common offices of life, to provide for our wants and guard us from the +ordinary sources of danger; but by arming them with _lenses_, the real +structure of plants and animals is revealed to our intelligence; and +enemies, otherwise invisible, that lie in wait in the air we breathe, and +in our daily food and drink, to destroy life, are guarded against. + +[Sidenote: 3. What are convex lenses? Kind of lenses used in microscopes? +Experiment? Picture thrown upon the eye? Derivation of the word +microscope?] + +3. CONVEX LENSES, or magnifying glasses, are disks of glass or other +transparent substance, which have the {237} property of picturing upon the +retina of the eye an image of an object larger than the image produced +there without their aid. The glasses used in microscopes are either double +convex lenses (_a_) or plano-convex lenses (_b_). If a double convex lens +or a plano-convex one be placed over a hole in the shutter of a darkened +room, or over the key-hole of a door, and a piece of paper be held at a +proper distance, a picture of all objects in front of the lens will be +thrown on the paper, as in the camera-obscura or the magic-lantern. Now, in +the same manner, a lens throws a picture of objects to which it is directed +on the retina of the eye, and when that picture is larger than the image +made in the eye by the object, without the aid of the lens, it is +magnified, or the lens has served as a _microscope_, so called, from its +use in seeing small objects, from _mikros_, small, and _skopeo_, to see. + +[Illustration] + +[Sidenote: 4. Kinds of microscope? What are simple microscopes?] + +4. DIFFERENT KINDS OF MICROSCOPES.--Microscopes are either _simple_ or +_compound_. The glasses of magnifying spectacles, like those commonly used +by aged persons, are simple microscopes. Magnifying glasses, mounted in +frames such as are for sale by opticians and others, for the detection of +counterfeit money, are simple microscopes, and are useful in studying the +coarser structure of plants and animals. + +[Sidenote: 5. Construction of the most powerful simple microscopes? In +practice? A doublet? Triplet? Why are compound microscopes superior to +simple ones?] + +5. The most powerful simple microscopes are made by melting in a flame a +thread of spun glass, so as to form a {238} minute globule or bead, which, +when set in a piece of metal and used to examine objects on a plate of +glass held up to the light, gives a high magnifying power. In practice, +however, it is found better to use several magnifying glasses of moderate +power, than a simple lens alone of high power. A combination of two lenses +is called a _doublet_--of three, a _triplet_. All _simple_ microscopes +throw an enlarged image of the object upon the retina. _Compound_ +microscopes are so constructed that the enlarged image of an object is +again magnified by a second lens, and hence their magnifying power is +vastly superior to that of simple microscopes. + +[Sidenote: 6. Explain, by means of the diagram, the action of the compound +microscope.] + +6. The accompanying diagrams will explain the action of the compound +microscope compared with that of the simple microscope. In Fig. 58, which +represents the working of the simple microscope, the rays from the object +(_a b_), passing through the lens (L), form an image (_a' b'_) in the +retina of the eye (E), and as all images are inverted in the eye, the +object is seen as all other objects are, and appears erect. In Fig. 59 is +seen the action of the compound microscope. An inverted image (_a' b'_) of +the object (_a b_) is magnified by the second lens (L'), and an erect image +is thrown upon the retina, which, as all other objects seen erect with the +naked eye are inverted, gives to the image a contrary direction, or inverts +it to the mind. + +[Illustration: FIG. 58.--SIMPLE MICROSCOPE.] + +{239} + +[Sidenote: 7. Portions, in a compound microscope? The glasses?] + +7. A COMPOUND MICROSCOPE consists of two portions: the optical portion, or +the lenses, and the mechanical portion, or the instrument which bears the +lenses. The glasses of a compound microscope are two: the _object-glass_ +(D), Fig. 60, and the lower lens of Fig. 59, and the _ocular_ or +_eye-piece_ (A), Fig. 60, and the upper piece of Fig. 59. Both the +object-glass and the eye-piece may, and usually do, consist of more than +one lens, for, as previously mentioned, better results are obtained by a +combination of lenses of moderate power than by single lenses of high power +and great curvature. + +[Sidenote: 8. How to choose a microscope? How to use it?] + +8. HOW TO CHOOSE AND USE A MICROSCOPE.--No attractiveness in the mechanical +part of a microscope can compensate for inferior lenses; and the very first +consideration in the choice of an instrument should be the excellence of +the optical part of the instrument. In the use of the instrument, care +should be exercised to keep the lenses clean, free from dust, not to press +the object-glass upon the object under observation, and not to wet it in +the water in which most objects are examined. A good microscope requires +its own table; and when not in use should be covered by a bell glass or a +clean linen cloth. + +[Illustration: FIG. 59. COMPOUND MICROSCOPE.] + +{240} [Illustration: FIG. 60. + +A, Eye-piece. B, Body. C, Collar. D, Object-glass. E, Stage. F, Hinge. +G, Mirror. H, Stand.] + +[Sidenote: 9. The characteristics of the best instrument? What special +requisites should be insisted upon? Why, as to a horizontal stage?] + +9. The mechanical portion of the instrument varies greatly in different +instruments. That one is the best which is simplest, the most solid and +easily managed. The stage (E), upon which the object is placed, should not +be movable: it should be solid and firm. The screw by which the focal +distance is adjusted, and which {241} is in constant use, should be so +placed that it can be worked by the hand resting on the table: otherwise +fatigue is soon induced. The direction of the tube carrying the glasses +should be perpendicular, and the stage therefore horizontal. Most objects +in human anatomy are examined in water or in other liquids, or they are +themselves liquids; hence an oblique stage is often inconvenient. + +[Illustration: FIG. 61.] + +{242} + +[Sidenote: 10. Slides? Covers, square and circular? How kept?] + +10. ADDITIONAL APPARATUS.--As almost all objects in human anatomy are +examined by transmitted light thrown up from the mirror (G, Fig. 60) +beneath the stage through the object to the eye, they must be placed upon +strips of clear glass about three inches long and one inch wide, commonly +called "slides." These should be procured with the microscope. Again, most +objects seen with high powers require to be covered with a thin plate of +glass, very properly called a "cover," that the moisture of the specimen +may not tarnish the object-glass. Square or circular covers of very thin +glass are therefore provided; and a good supply of these should be always +on hand. These glasses should be kept in a covered dish filled with a +mixture of alcohol and water. Simple water will not remove the fatty matter +which exists in all animal tissues, and, therefore, the glasses cannot be +thoroughly cleaned with it alone. + +[Sidenote: 11. Cleaning the glasses? Knives, scissors, etc.? Various +liquids?] + +11. When glasses are required for use, they should be removed from the +liquid and wiped clean and dry with a soft linen handkerchief. Delicate +knives, scissors, needles mounted in handles, forceps, pipettes or little +tubes for taking up water, should be obtained; these are essential to all +microscopical study. The table should be supplied with glass-stoppered +bottles containing the various liquids ordinarily used in the study of +physiology. Thus, tincture of iodine is indispensable in studying vegetable +structure, acetic acid in the study of animal tissues; and other articles +will have to be added from time to time, as your progress in study demands +them. {243} + +[Sidenote: 12. Bodies, in air and water? The examination of starch?] + +12. PRELIMINARY STUDIES.--In order to prepare the way for the study of any +department of science with the aid of the microscope--for the microscope is +but an eye, and can be turned in almost any direction for purposes of +investigation--it is necessary to become acquainted with the many objects +which are liable to complicate the examination of particular structures. +Both air and water are full of floating bodies, and the most common of +these should first occupy the attention. In the city, particles of starch +are always floating in the air. Take a very minute portion of wheat flour, +place it in the middle of a clean glass "slide," drop upon it a drop of +pure water, cover it with a plate of thin glass, and examine it with a +power of from one hundred to six hundred diameters. It will be found to be +composed of minute grains or granules, the largest of which are made up of +coats or layers, like an onion, arranged around a central spot called the +_hilum_. + +[Sidenote: 13. The examination with solution of iodine? Advice respecting +other articles?] + +13. Make another preparation in the same manner, and, after adding the +water and before covering with the thin glass cover, add a small drop of a +solution of iodine. Now, upon examining the specimen, every grain will be +seen to be of a beautiful deep blue color. After thus studying wheat +starch, the starch of Indian corn, of arrowroot, and of various grains +should be examined in like manner, and their resemblances and differences +noted. The granules of potato-starch are as distinctly marked as any. (See +Fig. 15, page 61.) + +[Sidenote: 14. Directions for examining cotton and other fibres? Vegetable +hairs?] + +14. Fibres of cotton, lint, and wool are liable to be found in every +specimen prepared for microscopical examination. In order to study these, +any cotton, woollen, or linen fabric, or garment, may be scraped, and the +scrapings placed on a piece of glass moistened with water, covered with the +thin glass plate or cover as before, and {244} examined with the same +magnifying power, namely, from one hundred to six hundred diameters. +Vegetable hairs or down are constantly floating in air and water. These are +of very various forms, are simple or grouped, and form very interesting +objects of study. They are readily procured from the epidermis or outer +membrane of the leaves or stems of plants, by section with a delicate +knife. + +[Sidenote: 15. Directions for examining various tissues? Down of moths, and +other structures?] + +15. The tissues of plants, epidermis, ducts, and woody fibres are +constantly found in microscopic preparations. They may be studied in +delicate sections made with a sharp knife, or by tearing vegetable tissues +apart with needles. The down of moths, the hairs of different animals, the +fibres of paper, the most common animalcules in water, the dust of shelves, +and generally the structures found in all vegetable and animal substances +by which we are surrounded, should be studied as a preliminary to any +special line of microscopical investigation. + +[Sidenote: 16. Directions for examining a drop of blood?] + +16. THE STUDY OF HUMAN TISSUES.--When this has been done and familiarity +with the use of the instrument has been obtained, proceed to the study of +the human body, for human physiology is our subject. If the end of the +finger be pricked with a pin, a drop of blood may be procured for +examination. Place this on one of the glass slides, cover it with a thin +piece of glass, press down the cover so as to make a thin layer, and then +examine with the magnifying power just mentioned. Do not add water, for +that will cause the blood corpuscles to disappear. If the drop of blood is +placed under the microscope at once after being drawn from the finger, most +interesting phenomena will be observed. The red corpuscles will be seen to +arrange themselves in rows, like piles of coin, while the blood is +coagulating. The spherical, white corpuscles will {245} be left out of the +rows of red disks, and, if the highest power be used, will be seen to +change their shape constantly. + +[Sidenote: 17. Examination of the scales of the mouth? Dandruff?] + +17. If you scrape with a dull knife the inside of the cheek, the flattened +scales of "pavement epithelium," or of the insensible covering which, +analogous to the scarf-skin on the outer surface of the body, lines the +cavities of its interior, may be readily studied. They have the appearance +of transparent tiles, each enclosing a round or oval body, called its +nucleus. Dandruff and the scrapings from the skin of the body are composed +of scales like those of the mouth, but they differ somewhat in being +hardened by horny matter, and in having a very faint central body or +nucleus. + +[Sidenote: 18. In what, as respects the tissues, do the warm blooded +animals differ? Statement of Milne Edwards?] + +18. THE TISSUES OF THE INFERIOR ANIMALS.--The warm-blooded animals do not +differ in the tissues, or microscopic structures, that compose them, but +only in the amount and arrangement of these tissues. Milne Edwards says +these tissues "do not differ much in different animals, but their mode of +association varies; and it is chiefly by reason of the differences in the +combination of these associations in various degrees, that each species +possesses the anatomical properties and characters which are peculiar to +it." + +[Sidenote: 19. How to procure materials for the study of the tissues of +man?] + +19. Hence the butcher's stall will furnish all the materials for the study +of the microscopic tissues. The structure of the heart, lungs, liver, +brain, and muscle may all be studied, and well studied, by using minute +pieces of the flesh of the lower animals, especially of the quadrupeds. +Such portions of these animals as are not exposed for sale can be readily +obtained by order from the slaughter-house. To examine with the powers of +which we have been speaking, it is only necessary to cut off {246} +exceedingly small pieces, tear them apart with needles, or make very +delicate sections with a sharp knife. + +20. INCENTIVES TO STUDY.--A complete knowledge of all minute structures is +not to be expected at once, for you are here introduced into a new realm of +Nature, a world of little things as vast, as wonderful, and as carefully +constructed as the starry firmament,--that other realm of grand objects +which the astronomer nightly scans with the telescope. It will not appear +singular, therefore, if, at first, you feel strange and awkward in this new +creation. With a little perseverance, however, and with the attention +directed toward simple objects at the outset, it will not be long before an +increasing experience will engender confidence. + +21. If to all this there be added an enthusiastic study of the standard +authorities on the subject, the rate of progress will be by so much the +more rapid. As compared with similar studies, few possess more interest +than microscopy, and to the one who pursues it with fondness, it constantly +affords sources of pleasure and agreeable surprises; and in the end, often +leads to new and valuable additions to the sum of human knowledge. The +depths which the microscope is employed to fathom are no more completely +known, than are the heights above us explored and comprehended by the +astronomer. + +QUESTIONS FOR TOPICAL REVIEW. + +_State what you can on the following subjects_: + + 1. Voice and speech 227, 228 + 2. The organ of the voice 228, 229 + 3. The vocal cords 230, 231 + 4. The production of the voice 232, 233 + 5. The varieties of voice 233, 234 + 6. Ventriloquism 235 + 7. The law of the tissues 236 + 8. Necessity of the microscope 236 + 9. Convex lenses 236, 237 + 10. Kinds of microscope 237, 238 + 11. Choosing a microscope 239, 242 + 12. Preliminary studies 243, 244 + 13. The study of human tissues 244 + 14. The study of the inferior animals 245 + + * * * * * + + +{247} + +APPENDIX. + +------o------ + +POISONS AND THEIR ANTIDOTES. + +Accidents from poisoning are of such frequent occurrence, that every one +should be able to administer the more common antidotes, until the _services +of a physician can be obtained_. As many poisons bear a close resemblance +to articles in common use, no dangerous substance should be brought into +the household without having the word _poison_ plainly written or printed +on the label; and any package, box, or vial, without a label, should be at +once destroyed, if the contents are not positively known. + +When a healthy person is taken severely and _suddenly_ ill _soon after some +substance has been swallowed_, we may suspect that he has been poisoned. In +all cases where poison has been taken into the stomach, it should be +quickly and thoroughly evacuated by some active emetic, which can be +speedily obtained. This may be accomplished by drinking a tumblerful of +warm water, containing either a tablespoonful of powdered mustard or of +common salt, or two teaspoonfuls of powdered alum in two tablespoonfuls of +syrup. When vomiting has already taken place, it should be maintained by +copious draughts of warm water or mucilaginous drinks, such as gum-water or +flaxseed tea, and tickling the throat with the finger until there is reason +to believe that all the poisonous substance has been expelled from the +stomach. + +The following list embraces only the more common poisons, together with +such antidotes as are usually at hand, to be used until the physician +arrives. + +POISONS. + +ACIDS.--_Hydrochloric acid_; _muriatic acid_ (spirits of salt); _nitric +acid_ (aqua fortis); _sulphuric acid_ (oil of vitriol). + +ANTIDOTE.--An antidote should be given at once to neutralize the acid. +Strong soapsuds is an efficient remedy, and can always be obtained. It +should be followed by copious draughts of warm water or flaxseed tea. +Chalk, magnesia, soda or saleratus (with water) or {248} lime-water, are +the best remedies. When sulphuric acid has been taken, water should be +given sparingly, because, when water unites with this acid, intense heat is +produced. + +_Oxalic acid._ + +ANTIDOTE.--Oxalic acid resembles Epsom salts in appearance, and may easily +be mistaken for it. The antidotes are magnesia, or chalk mixed with water. + +PRUSSIC ACID; _oil of bitter almonds_; _laurel water_; _cyanide of +potassium_ (used in electrotyping). + +ANTIDOTE.--Cold douche to the spine. Chlorine water, or water of ammonia +largely diluted, should be given, and the vapor arising from them may be +inhaled. + +ALKALIES AND THEIR SALTS.--AMMONIA (hartshorn), _liquor or water of +ammonia_. POTASSA:--_caustic potash_, _strong ley_, _carbonate of potassa_ +(pearlash), _nitrate of potassa_ (saltpetre). + +ANTIDOTE.--Give the vegetable acids diluted, as weak vinegar, acetic, +citric, or tartaric acids dissolved in water. Castor oil, linseed oil, and +sweet oil may also be used; they form soaps when mixed with the free +alkalies, which they thus render harmless. The poisonous effects of +saltpetre must be counteracted by taking mucilaginous drinks freely, so as +to produce vomiting. + +ALCOHOL.--_Brandy_, _wine_; _all spirituous liquors_. + +ANTIDOTE.--Give as an emetic ground mustard or tartar emetic. If the +patient cannot swallow, introduce a stomach pump; pour cold water on the +head. + +GASES.--_Chlorine_, _carbonic acid gas_, _carbonic oxide_, _fumes of +burning charcoal_, _sulphuretted hydrogen_, _illuminating or coal-gas_. + +ANTIDOTE.--For poisoning by chlorine, inhale, cautiously, ammonia +(hartshorn). For the other gases, cold water should be poured upon the +head, and stimulants cautiously administered; artificial respiration. (See +_Marshall Hall's Ready Method_, page 250.) + +METALS.--_Antimony_, _tartar emetic_, _wine of antimony_, etc. + +ANTIDOTE.--If vomiting has not occurred, it should be produced by tickling +the throat with the finger or a feather, and the abundant use of warm +water. Astringent infusions, such as common tea, oak bark, and solution of +tannin, act as antidotes. + +ARSENIC.--_White arsenic_, _Fowler's solution_, _fly-powder_, _cobalt_, +_Paris green_, etc. + +ANTIDOTE.--Produce vomiting at once with a tablespoonful or two of powdered +mustard in a glass of warm water, or with ipecac. The antidote is hydrated +peroxide of iron. If Fowler's solution has been taken, lime-water must be +given. {249} + +COPPER.--_Acetate of copper_ (verdigris), _sulphate of copper_ (blue +vitriol), food cooked in dirty _copper vessels_, or pickles made green by +_copper_. + +ANTIDOTE.--Milk or white of eggs, with mucilaginous drinks (flaxseed tea, +etc.), should be freely given. + +IRON.--_Sulphate of iron_ (copperas), etc. + +ANTIDOTE.--Carbonate of soda in some mucilaginous drink, or in water, is an +excellent antidote. + +LEAD.--_Acetate of lead_ (sugar of lead), _carbonate of lead_ (white lead), +water kept in _leaden pipes_ or _vessels_, food cooked in _vessels_ glazed +with _lead_. + +ANTIDOTE.--Induce vomiting with ground mustard or common salt in warm +water. The antidote for soluble preparations of lead is Epsom salts; for +the insoluble forms, sulphuric acid largely diluted. + +MERCURY.--_Bichloride of mercury_ (corrosive sublimate), _ammoniated +mercury_ (white precipitate), _red oxide of mercury_ (red precipitate), +_red sulphuret of mercury_ (vermilion). + +ANTIDOTE.--The white of eggs, or wheat flour beaten up with water and milk, +are the best antidotes. + +SILVER.--_Nitrate of silver_ (lunar caustic). + +ANTIDOTE.--Give a teaspoonful of common salt in a tumbler of water. It +decomposes the salts of silver and destroys their activity. + +ZINC.--_Sulphate of zinc_, etc. (white vitriol). + +ANTIDOTE.--The vomiting may be relieved by copious draughts of warm water. +The antidote is carbonate of soda administered in water. + +NARCOTIC POISONS.--_Opium_ (laudanum, paregoric, salts of morphia, +Godfrey's cordial, Dalby's carminative, soothing syrup, cholera mixtures), +_aconite_, _belladonna_, _hemlock_, _stramonium_, _digitalis_, _tobacco_, +_hyosciamus_, _nux vomica_, _strychnine_. + +ANTIDOTE.--Evacuate the stomach by the most active emetics, as mustard, +alum, or sulphate of zinc. The patient should be kept in motion, and cold +water dashed on the head and shoulders. Strong coffee must be given. The +physician will use the stomach pump and electricity. In poisoning by nux +vomica or strychnine, etc., chloroform or ether should be inhaled to quiet +the spasms. + +IRRITANT VEGETABLE POISONS.--_Croton oil_, _oil of savine_, _poke_, _oil of +tansy_, etc. + +ANTIDOTE.--If vomiting has taken place, it may be rendered easier by +copious draughts of warm water. But if symptoms of insensibility have come +on without vomiting, it ought to be immediately excited by ground mustard +mixed with warm water, or some other active emetic {250} and after its +operation an active purgative should be given. After evacuating as much of +the poison as possible, strong coffee or vinegar and water may be given +with advantage. + +POISONOUS FISH.--_Conger eel_, _mussels_, _crabs_, etc. + +ANTIDOTE.--Evacuate, as soon as possible, the contents of the stomach and +bowels by emetics (ground mustard mixed with warm water or powdered alum), +and castor oil, drinking freely at the same time of vinegar and water. +Ether, with a few drops of laudanum mixed with sugar and water, may +afterward be taken freely. + +POISONOUS SERPENTS.--ANTIDOTE.--A ligature or handkerchief should be +applied moderately tight above the bite, and a cupping-glass over the +wound. The patient should drink freely of alcoholic stimulants containing a +small quantity of ammonia. The physician may inject ammonia into the veins. + +POISONOUS INSECTS.--_Stings of scorpion_, _hornet_, _wasp_, _bee_, etc. + +ANTIDOTE.--A piece of rag moistened with a solution of carbolic acid may be +kept on the affected part until the pain is relieved; and a few drops of +carbolic acid may be given frequently in a little water. The sting may be +removed by making strong pressure around it with the barrel of a small +watch-key. + +DROWNING. + +MARSHALL HALL'S "READY METHOD" of treatment in asphyxia from drowning, +chloroform, coal gas, etc. + +1st. Treat the patient _instantly on the spot_, in the _open air_, freely +exposing the face, neck, and chest to the breeze, except in severe weather. + +2d. In order _to clear the throat_, place the patient gently on the face, +with one wrist under the forehead, that all fluid, and the tongue itself, +may fall forward, and leave the entrance into the windpipe free. + +3d. _To excite respiration_, turn the patient slightly on his side, and +apply some irritating or stimulating agent to the nostrils, as _veratrine_, +_dilute ammonia_, etc. + +4th. Make the face warm by brisk friction; then dash cold water upon it. + +5th. If not successful, lose no time; but, _to imitate respiration_, place +the patient on his face, and turn the body gently, but completely _on the +side, and a little beyond_; then again on the face, and so on, alternately. +Repeat these movements, deliberately and perseveringly, {251} _fifteen +times only_ in a minute. (When the patient lies on the thorax, this cavity +is _compressed_ by the weight of the body, and _ex_piration takes place. +When he is turned on the side, this pressure is removed, and _in_spiration +occurs.) + +6th. When the prone position is resumed, make a uniform and efficient +pressure _along the spine_, removing the pressure immediately, before +rotation on the side. (The pressure augments the _ex_piration: the rotation +commences _in_spiration.) Continue these measures. + +7th. Rub the limbs _upward_, with _firm pressure_ and with _energy_. (The +object being to aid the return of venous blood to the heart.) + +8th. Substitute for the patient's wet clothing, if possible, such other +covering as can be instantly procured, each bystander supplying a coat or +cloak, etc. Meantime, and from time to time, _to excite inspiration_, let +the surface of the body be _slapped_ briskly with the hand. + +9th. Rub the body briskly till it is dry and warm, then dash _cold_ water +upon it, and repeat the rubbing. + +Avoid the immediate removal of the patient, as it involves a _dangerous +loss of time_--also, the use of bellows, or any _forcing_ instrument; also, +the _warm bath_, and _all rough treatment_. + + * * * * * + + +{252} + +GLOSSARY. + +------o------ + +AB-DO'MEN (Latin _abdo_, to conceal). The largest cavity of the body +containing the liver, stomach, intestines, etc.; the belly. + +AB-SOR'BENTS (L. _ab_ and _sorbeo_, to suck up). The vessels which take +part in the process of absorption. + +AB-SORP'TION. The process of sucking up fluids by means of an animal +membrane. + +AC-COM-MO-DA'TION of the Eye. The alteration in the shape of the +crystalline lens, which accommodates or adjusts the eye for near and remote +vision. + +AC'ID, LACTIC (L. _lac_, milk). The acid ingredient of sour milk; the +gastric juice also contains it. + +AL-BU'MEN, or Albumin (L. _albus_, white). An animal substance resembling +white of egg. + +AL-BU'MI-NOSE (from _albumen_). A soluble animal substance produced in the +stomach by the digestion of the albuminoid substances. + +AL-BU'MIN-OID substances. A class of proximate principles resembling +albumen; they may be derived from either the animal or vegetable kingdoms. + +AL'I-MENT (L. _alo_, to nourish). That which affords nourishment; food. + +AL-I-MENT'A-RY CA-NAL (from _aliment_). A long tube in which the food is +digested, or prepared for reception into the system. + +AN-AES-THET'ICS (Greek, [Greek: an], _an_, without, [Greek: aisthesia], +_aisthesia_, feeling).--Those medicinal agents which prevent the feeling of +pain, such as chloroform, laughing-gas, etc. + +AN-I-MAL'CULE (L. _animal'culum_, a small animal). Applied to animals which +can only be seen with the aid of the microscope. Animalculum (plural, +animalcula) is used with the same meaning. + +A-OR'TA (Gr. [Greek: aorteomai], _aorteomai_, to be lifted up). The largest +artery of the body, and main trunk of all the arteries. It arises from the +left ventricle of the heart. The name was first applied to the two large +branches of the trachea, which appear to be lifted up by the heart. + +{253} A'QUE-OUS HUMOR (L. _aqua_, water). A few drops of watery colorless +fluid occupying the space between the cornea and crystalline lens. + +A-RACH'NOID MEM'BRANE (Gr. [Greek: arachne], _arachne_, a cobweb, and +[Greek: eidos], _eidos_, like). An extremely thin covering of the brain and +spinal cord. It lies between the _dura mater_ and the _pia mater_. + +AR'BOR VI'TAE (L.). Literally, "the tree of life;" a name given to the +peculiar appearance presented by a section of the cerebellum. + +AR'TER-Y (Gr. [Greek: aer], _aer_, air, and [Greek: terein], _terein_, to +contain). A vessel by which blood is conveyed away from the heart. It was +supposed by the ancients to contain air; hence the name. + +AR-TIC-U-LA'TION (L. _articulo_, to form a joint). The more or less movable +union of bones, etc.; a joint. + +A-RYT'E-NOID CAR'TI-LA-GES (Gr. [Greek: arutaina], _arutaina_, a pitcher). +Two small cartilages of the larynx, resembling the mouth of a pitcher. + +AS-SIM-I-LA'TION (L. _ad_, to, and _similis_, like). The conversion of food +into living tissue. + +AU-DI'TION (L. _audio_, to hear). The act of hearing sounds. + +AU'DI-TO-RY NERVE. One of the cranial nerves; it is the special nerve of +hearing. + +AU'RI-CLE (L. _auris_, the ear). A cavity of the heart. + +BAR'I-TONE (Gr. [Greek: barus], _barus_, heavy, and [Greek: tonos], +_tonos_, tone). A variety of male voice between the bass and tenor. + +BEL-LA-DON'NA (It. beautiful lady). A vegetable narcotic poison. It has the +property of enlarging the pupil, and thus increasing the brilliancy of the +eye; so called from its use by Italian ladies. + +BI-CUS'PID (L. _bi_, two, and _cuspis_, prominence). The name of the fourth +and fifth teeth on each side of the jaw; possessing two prominences. + +BILE. The gall, or peculiar secretion of the liver; a viscid, yellowish +fluid, and very bitter to the taste. + +BRONCH'I (Gr. [Greek: bronkos], _bronkos_, the windpipe). The two first +divisions or branches of the trachea; one enters each lung. + +BRONCH'I-AL TUBES. The smaller branches of the trachea within the substance +of the lungs, terminating in the air-cells. + +BRONCH-I'TIS (from _bronchia_, and _itis_, a suffix signifying +inflammation). An inflammation of the larger bronchial tubes; a "cold" +affecting the lungs. + +CAL-CA'RE-OUS (L. _calx_, lime). Containing lime. + +CA-NAL' (L.). In the body, any tube or passage. + +CA-NINE' (L. _canis_, a dog). Name given to the third tooth on each {254} +side of the jaw; in the upper jaw it is also known as the eye-tooth: +pointed like the tusks of a dog. + +CAP'IL-LA-RY (L. _capil'lus_, a hair, _capilla'ris_ hair-like). The name of +the extremely minute blood-vessels which connect the arteries with the +veins. + +CAR'BON DIOX-IDE (CO_{2}). Chemical name for carbonic acid gas. + +CAR-BON'IC A-CID. The gas which is present in the air expired from the +lungs; a waste product of the animal kingdom, and a food of the vegetable +kingdom. + +CAR'DI-AC (Gr. [Greek: kardia], _cardia_, the heart). The cardiac orifice +of the stomach is the upper one, and is near the heart; hence its name. + +CAR-NIV'O-ROUS (L. _ca'ro_, flesh, and _vo'ro_, to devour). Subsisting upon +flesh. + +CA-ROT'ID AR-TE-RY. The large artery of the neck, supplying the head and +brain. + +CAR'TI-LAGE. A solid but flexible material, forming a part of the joints, +air-passages, nostrils, etc.; gristle. + +CA'SE-INE (L. _ca'seus_, cheese). The albuminoid substance of milk; it +forms the basis of cheese. + +CER-E-BEL'LUM (diminutive for _cer'ebrum_, the brain). The little brain, +situated beneath the posterior third of the cerebrum. + +CER'E-BRUM (L.). The brain proper, occupying the entire upper portion of +the skull. It is nearly divided into two equal parts, called "hemispheres," +by a cleft extending from before backward. + +CHO'ROID (Gr. [Greek: chorion], _chorion_, a membrane or covering). The +middle tunic or coat of the eyeball. + +CHYLE (Gr. [Greek: chulos], chulos, juice). The milk-like fluid formed by +the digestion of fatty articles of food in the intestines. + +CHYME (Gr. [Greek: chumos], _chumos_, juice). The pulpy liquid formed by +digestion within the stomach. + +CIL'I-A (pl. of _cil'i-um_, an eyelash). Minute, vibratile, hair-like +processes found upon the cells of the air-passages, and other parts that +are habitually moist. + +CIR-CU-LA'TION (L. _cir'culus_, a ring). The circuit, or course of the +blood through the blood-vessels of the body, from the heart to the +arteries, through the capillaries into the veins, and from the veins back +to the heart. + +CO-AG-U-LA'TION (L. _coag'ulo_, to curdle). Applied to the process by which +the blood clots or solidifies. + +COCH'LE-A (L. _coch'lea_, a snail-shell). The spiral cavity of the internal +ear. + +{255} CONCH'A (Gr. [Greek: konche], _konche_, a mussel-shell). The external +shell-shaped portion of the external ear. + +CON-JUNC-TI'VA (L. _con_ and _jun'go_, to join together). A thin layer of +mucous membrane which lines the eyelids and covers the front of the +eyeball; thus joining the latter to the lids. + +CON-TRAC-TIL'I-TY (L. _con_ and _tra'ho_, to draw together). The property +of a muscle which enables it to contract, or draw its extremities closer +together. + +CON-VO-LU'TIONS (L. _con_ and _vol'vo_, to roll together). The tortuous +foldings of the external surface of the brain. + +CON-VUL'SION (L. _convel'lo_, to pull together). A more or less violent +agitation of the limbs or body. + +COR'NE-A (L. _cor'nu_, a horn). The transparent, horn-like substance which +covers the anterior fifth of the eyeball. + +COR'PUS-CLES, BLOOD (L. dim. of _cor'pus_, a body). The small biconcave +disks which give to the blood its red color; the _white_ corpuscles are +globular and larger. + +COS-MET'IC (Gr. [Greek: kosmeo], _kosmeo_, to adorn). Beautifying; applied +to articles which are supposed to increase the beauty of the skin, etc. + +CRA'NI-AL (L. _cra'nium_, the skull). Pertaining to the skull. The nerves +which arise from the brain are called cranial nerves. + +CRI'COID (Gr. [Greek: krikos], _kri'kos_, a ring). A cartilage of the +larynx, resembling a seal-ring in shape. + +CRYS'TAL-LINE LENS (L. _crystal'lum_, a crystal). One of the so-called +humors of the eye; a double convex body situated in the front part of the +eyeball. + +CU'TI-CLE (L. dim. of _cu'tis_, the skin). The scarf-skin; also called the +_epider'mis_. + +CU'TIS (Gr. [Greek: skutos], _skutos_, a skin or hide). The true skin, +lying beneath the cuticle; also called the _der'ma_. + +DE-CUS-SA'TION (L. _decus'sis_, the Roman numeral ten, X). A reciprocal +crossing of fibres from side to side. + +DI'A-PHRAGM (Gr. [Greek: diaphrasso], _diaphrasso_, to divide by a +partition). A large, thin muscle which separates the cavity of the chest +from the abdomen; a muscle of respiration. + +DIF-FUS'ION OF GASES. The power of gases to become intimately mingled, +without reference to the force of gravity. + +DUCT (L. _du'co_, to lead). A narrow tube; the _thoracic duct_ is the main +trunk of the absorbent vessels. + +DU-O-DE'NUM (L. _duode'ni_, twelve). The first division of the small +intestines, about twelve fingers-breadth long. + +{256} DU'RA MA'TER (L.). Literally, the hard mother; the tough membrane +which envelops the brain. + +DYS-PEP'SI-A (Gr. [Greek: dus], _dus_, difficult, and [Greek: pepto], +_pep'to_, to digest). Difficult or painful digestion; a disordered +condition of the stomach. + +E-MUL'SION (L. _emul'geo_, to milk). Oil in a finely divided state +suspended in water. + +EN-AM'EL (Fr. _email_). The dense material which covers the crown of the +tooth. + +EN'ER-GY, Specific, of a Nerve. When a nerve of special sense is excited, +whatever be the cause, the sensation experienced is that peculiar to the +nerve; this is said to be the law of the specific energy of the nerves. + +EP-I-GLOT'TIS (Gr. [Greek: epi], _epi_, upon, and [Greek: glottis], +_glottis_, the entrance to the windpipe). A leaf-shaped piece of cartilage +which covers the top of the larynx during the act of swallowing. + +EX-CRE'TION (L. _excer'no_, to separate). The separation from the blood of +the waste particles of the body; also the materials excreted. + +EX-PI-RA'TION (L. _expi'ro_, to breathe out). The act of forcing air out of +the lungs. + +EX-TEN'SION (L. _ex_, out, and _ten'do_, to stretch). The act of restoring +a limb, etc., to its natural position after it has been flexed, or bent; +the opposite of _Flexion_. + +FE-NES'TRA (L.). Literally, a window; the opening between the middle and +internal ear. + +FI'BRIN (L. _fi'bra_, a fibre). An albuminoid substance found in the blood; +in coagulating it assumes a fibrous form. + +FLEX'ION (L. _flec'to_, to bend). The act of bending a limb, etc. + +FOL'LI-CLE (L. dim. of _fol'lis_, a bag). A little pouch or depression in a +membrane; it has generally a secretory function. + +FUN'GOUS GROWTHS (L. _fun'gus_, a mushroom). A low grade of vegetable life. + +GAN'GLI-ON (Gr. [Greek: ganglion], _ganglion_, a knot). A knot-like +swelling in the course of a nerve; a smaller nerve-centre. + +GAS'TRIC (Gr. [Greek: gaster], _gaster_, stomach). Pertaining to the +stomach. + +GLAND (L. _glans_, an acorn). An organ consisting of follicles and ducts, +with numerous blood-vessels interwoven; it separates some particular fluid +from the blood. + +GLOS'SO-PHAR-YN-GE'AL NERVE (Gr. [Greek: glossa], _glossa_, the tongue, and +[Greek: pharunx], _pharunx_, the throat). The nerve of taste supplying the +posterior third of the tongue; it also supplies the throat. + +GLU'TEN (L.). Literally, glue: the glutinous albuminoid ingredient of +wheat. + +{257} GRAN'ULE (L. dim. of _gra'num_, a grain). A little grain; a +microscopic object. + +GUS-TA'TION (L. _gus'to_, to taste) The sense of taste. + +GUS'TA-TO-RY NERVE. The nerve of taste supplying the front part of the +tongue; a branch of the "fifth" pair. + +HAEM'OR-RHAGE (Gr. [Greek: haima], _hai'ma_, blood, and [Greek: rhegnumi], +_regnumi_, to burst). Bleeding, or the loss of blood. + +HEM-I-PLE'GIA (Gr. [Greek: hemisus], _hemisus_, half, and [Greek: plesso], +_plesso_, to strike). Paralysis, or loss of power, affecting one side of +the body. + +HEM'I-SPHERES (Gr. [Greek: sphaira], _sphaira_, a sphere). Half a sphere, +the lateral halves of the cerebrum, or brain proper. + +HE-PAT'IC (Gr. [Greek: hepar], _hepar_, the liver). Pertaining to the +liver. + +HER-BIV'O-ROUS (L. _her'ba_, an herb, and _vo'ro_ to devour). Applied to +animals that subsist upon vegetable food. + +HU'MOR (L.). Moisture: the humors are transparent contents of the eyeball. + +HY-DRO-PHO'BI-A (Gr. [Greek: hudor], _hudor_, water, and [Greek: phobeo], +_phobeo_, to fear). A disease caused by the bite of a rabid dog or other +animal. In a person affected with it, convulsions are occasioned by the +sight of a glittering object, like water, by the sound of running water, +and by almost any external impression. + +HY'GI-ENE (Gr. [Greek: hugieia], _huygieia_, health). The art of preserving +health and preventing disease. + +HY'PER-O-PI-A. Abbreviated from HY'PER-MET-RO'PI-A (Gr. [Greek: huper], +_huper_, beyond, [Greek: metron], _metron_, the measure, and [Greek: ops], +_ops_, the eye). A defect of vision dependent upon a too short eyeball; so +called because the rays of light are brought to a focus at a point behind +the retina; the true "far sight." + +IN-CI'SOR (L. _inci'do_, to cut). Applied to the four front teeth of both +jaws, which have sharp cutting edges. + +IN'CUS (L). An anvil; the name of one of the bones of the middle ear. + +IN-SAL-I-VA'TION (L. _in_, and _sali'va_, the fluid of the mouth). The +mingling of the saliva with the food during the act of chewing. + +IN-SPI-RA'TION (L. _in_, and _spi'ro_, _spira'tum_, to breathe). The act of +drawing in the breath. + +IN-TEG'U-MENT (L. _in_, and _te'go_, to cover). The skin, or outer covering +of the body. + +IN-TES'TINE (L. _in'tus_, within). The part of the alimentary canal which +is continuous with the lower end of the stomach; also called the +intestines, or the bowels. + +I'RIS (L. _i'ris_, the rainbow). The thin muscular ring which lies {258} +between the cornea and crystalline lens, and which gives the eye its brown, +blue, or other color. + +JU'GU-LAR (L. _ju'gulum_, the throat). The name of the large veins which +run along the front of the neck. + +LAB'Y-RINTH (Gr. [Greek: laburinthos], _laburin'thos_, a building with many +winding passages). The very tortuous cavity of the inner ear, comprising +the vestibule, semicircular canals, and the cochlea. + +LACH'RY-MAL APPARATUS (L. _lach'ryma_, a tear). The organs for forming and +conveying away the _tears_. + +LAC'TE-ALS (L. _lac_, _lac'tis_, milk). The absorbent vessels of the small +intestines; during digestion they are filled with chyle, which has a milky +appearance. + +LA-RYN'GO-SCOPE (Gr. [Greek: larunx], _larunx_, the larynx, and [Greek: +skopeo], _skopeo_, to look at). The instrument by which the larynx may be +examined in the living subject. + +LAR'YNX (Gr.). The cartilaginous tube situated at the top of the windpipe, +or trachea; the organ of the voice. + +LENS (L.). Literally, a lentil; a piece of transparent glass or other +substance so shaped as either to converge or disperse the rays of light. + +LIG'A-MENT (L. _li'go_, to bind). A strong, fibrous material binding bones +or other solid parts together; it is especially necessary to give strength +to joints. + +LIG'A-TURE. A thread of silk or other material used in tying around an +artery. + +LYMPH (L. _lym'pha_, spring-water). The colorless, watery fluid conveyed by +the lymphatic vessels. + +LYM-PHAT'IC VESSELS. A system of absorbent vessels. + +MAL'LE-US (L.). Literally, the mallet; one of the small bones of the middle +ear. + +MAR'ROW. The soft, fatty substance contained in the central cavities of the +bones: the spinal marrow, however, is composed of nervous tissue. + +MAS-TI-CA'TION (L. _mas'tico_, to chew). The act of cutting and grinding +the food to pieces by means of the teeth. + +ME-DUL'LA OB-LON-GA'TA. The "oblong marrow," or nervous cord, which is +continuous with the spinal cord within the skull. + +MEM-BRA'NA TYM'PAN-I (L.). Literally, the membrane of the drum; a delicate +partition separating the outer from the middle ear; it is sometimes +incorrectly called the drum of the ear. + +MEM'BRANE. A thin layer of tissue serving to cover some part of the body. + +MI'CRO-SCOPE (Gr. [Greek: mikros], _mikros_, small, and [Greek: skopeo], +_skopeo_, to {259} look at). An optical instrument which assists in the +examination of minute objects. + +MO'LAR (L. _mo'la_, a mill). The name applied to the three back teeth of +each side of the jaw; the grinders, or mill-like teeth. + +MO'TOR (L. _mo'veo_, _mo'tum_, to move). Causing motion; the name of those +nerves which conduct to the muscles the stimulus which causes them to +contract. + +MU'COUS MEMBRANE. The thin layer of tissue which covers those internal +cavities or passages which communicate with the external air. + +MU'CUS. The glairy fluid which is secreted by mucous membranes, and which +serves to keep them in a moist condition. + +MY-O'PI-A (Gr. [Greek: muo], _muo_, to contract, and [Greek: ops], _ops_, +the eye). A defect of vision dependent upon an eyeball that is too long, +rendering distant objects indistinct; near-sight. + +NA'SAL (L. _na'sus_, the nose). Pertaining to the nose; the _nasal +cavities_ contain the distribution of the special nerve of smell. + +NERVE (Gr. [Greek: neuron], _neuron_, a cord or string). A glistening, +white cord of cylindrical shape, connecting the brain or spinal cord with +some other organ of the body. + +NERVE CELL. A minute, round and ashen-gray cell found in the brain and +other nervous centres. + +NERVE FI'BRE. An exceedingly slender thread of nervous tissue found in the +various nervous organs, but especially in the nerves; it is of a white +color. + +NU-TRI'TION (L. _nu'trio_, to nourish). The processes by which the +nourishment of the body is accomplished. + +OE-SOPH'A-GUS (Gr.). Literally, that which carries food. The tube leading +from the throat to the stomach; the gullet. + +O-LE-AG'I-NOUS (L. _o'leum_, oil). Of the nature of oil: applied to an +important group of food-principles--the fats. + +OL-FAC'TO-RY (L. _olfa'cio_, to smell). Pertaining to the sense of smell. + +OPH-THAL'MO-SCOPE (Gr. [Greek: ophthalmos], _ophthalmos_, the eye, and +[Greek: skopeo], _skopeo_, to look at). An instrument devised for examining +the interior of the globe of the eye. + +OP'TIC (Gr. [Greek: opto], _opto_, to see). Pertaining to the sense of +sight. + +OR'BIT (L. _or'bis_, the socket). The bony socket or cavity in which the +eyeball is situated. + +OS'MOSE (Gr. [Greek: osmos], _osmos_, a thrusting or impulsion). The +process by which liquids are impelled through a moist membrane. + +OS'SE-OUS (L. _os_, a bone). Consisting of, or resembling bone. + +PAL'ATE (L. _pala'tum_, the palate). The roof of the mouth, consisting of +the hard and soft palate. + +{260} PAL'MAR. Relating to the palm of the hand. + +PAN'CRE-AS (Gr. [Greek: pas], [Greek: pantos], _pas_, _pantos_, all, and +[Greek: kreas], _kreas_, flesh). A long, flat gland situated near the +stomach; in the lower animals the analogous organ is called the +sweet-bread. + +PA-PIL'LAE (L. _papil'la_). The minute prominences in which terminate the +ultimate fibres of the nerves of touch and taste. + +PA-RAL'Y-SIS. A disease of the nervous system marked by the loss of +sensation, or voluntary motion, or both; palsy. + +PAR-A-PLE'GI-A (Gr. [Greek: paraplesso], _paraplesso_, to strike amiss). A +form of paralysis affecting the lower half of the body. + +PA-TEL'LA (L. dim. of _pat'ina_, a pan). The knee-pan; a small bone. + +PEL'VIS (L.). Literally a basin; the bony cavity at the lower part of the +trunk. + +PEP'SIN (Gr. [Greek: pepto], _pepto_, to digest). The organic principle of +the gastric juice. + +PER-I-STAL'TIC MOVE'MENTS (Gr. [Greek: peristello], _peristello_, to +contract). The slow, wave-like movements of the stomach and intestines. + +PER-I-TO-NE'UM (Gr. [Greek: periteino], _periteino_, to stretch around). +The investing membrane of the stomach, intestines, and other abdominal +organs. + +PER-SPI-RA'TION (L. _perspi'ro_, to breathe through). The sweat, or watery +exhalation of the skin; when visible, it is called _sensible_ perspiration, +when invisible, it is called _insensible_ perspiration. + +PE'TROUS (Gr. [Greek: petra], _petra_, a rock). The name of the hard +portion of the temporal bone, in which is situated the drum of the ear and +labyrinth. + +PHAR'YNX (Gr. [Greek: pharunx], _pharunx_, the throat). The cavity between +the back of the mouth and gullet. + +PHYS-I-OL'O-GY (Gr. [Greek: phusis], _phusis_, nature, and [Greek: logos], +_logos_, a discourse). The science of the functions of living, organized +beings. + +PI'A MA'TER (L.). Literally, the tender mother; the innermost of the three +coverings of the brain. It is thin and delicate; hence the name. + +PLEU'RA (Gr. [Greek: pleura], a rib). A membrane covering the lung and +lining the chest. There is one for each lung. + +PLEU'RI-SY. An inflammation affecting the pleura. + +PNEU-MO-GAS'TRIC (Gr. [Greek: pneumon], _pneumon_, the lungs, and [Greek: +gaster], _gaster_, the stomach). The name of a nerve distributed to the +lungs and stomach; it is the principal nerve of respiration. + +PNEU-MO'NIA (Gr.). An inflammation affecting the air-cells of the lungs. + +{261} PRES-BY-O'PI-A (Gr. [Greek: presbus], _presbus_, old, and [Greek: +ops], _ops_, the eye). A defect of the accommodation of the eye, caused by +the hardening of the crystalline lens; the "far-sight" of adults and aged +persons. + +PROC'ESS (L. _proce'do_, _proces'sus_, to proceed, to go forth). Any +projection from a surface. Also, a method of performance; a procedure. + +PTY'A-LIN (Gr. [Greek: ptualon], _ptualon_, saliva). The peculiar organic +ingredient of the saliva. + +PUL'MO-NA-RY (L. _pul'mo_, _pulmo'nis_, the lungs). Pertaining to the +lungs. + +PULSE (L. _pel'lo_, _pul'sum_, to beat). The striking of an artery against +the finger, occasioned by the contraction of the heart, commonly felt at +the wrist. + +PU'PIL (L. _pupil'la_). The central, round opening in the iris, through +which light passes into the depths of the eye. + +PY-LO'RUS (Gr. [Greek: puloros], _puloros_, a gate-keeper). The lower +opening of the stomach, at the beginning of the small intestine. + +RE'FLEX ACTION. An involuntary action of the nervous system, by which an +external impression conducted by a sensory nerve is reflected, or converted +into a motor impulse. + +RES-PI-RA'TION (L. _res'piro_, to breathe frequently). The function of +breathing, comprising two acts: _inspiration_, or breathing in, and +_expiration_, or breathing out. + +RET'I-NA (L. _re'te_, a net). The innermost of the three tunics or coats of +the eyeball, being an expansion of the optic nerve. + +SAC'CHA-RINE (L. _sac'charum_, sugar). Of the nature of sugar; applied to +the important group of food substances which embraces the different +varieties of sugar, starch, and gum. + +SA-LI'VA (L.). The moisture or fluids of the mouth, secreted by the +salivary glands, etc. + +SCLE-ROT'IC (Gr. [Greek: skleros], _skleros_, hard). The tough, fibrous +outer tunic of the eyeball. + +SE-BA'CEOUS (L. _sebum_, fat). Resembling fat, the name of the oily +secretion by which the skin is kept flexible and soft. + +SE-CRE'TION (L. _secer'no_, _secre'tum_, to separate). The process of +separating from the blood some essential important fluid; which fluid is +also called a secretion. + +SEM-I-CIR'CU-LAR CANALS. A portion of the internal ear. + +SEN-SA'TION. The perception of an external impression by the nervous +system; a function of the brain. + +SEN-SI-BIL'I-TY, GENERAL. The power possessed by nearly all parts {262} of +the human body of recognizing the presence of foreign objects that come in +contact with them. + +SE'RUM (L.). The watery constituent of the blood, which separates from the +clot during the process of coagulation. + +SKEL'E-TON (Gr.). The bony framework of an animal, the different parts of +which are maintained in their proper relative positions. + +SPEC'TRO-SCOPE (from _spec'trum_ and [Greek: skopeo], _scopeo_, to examine +the spectrum). An instrument employed in the examination of the spectrum of +the sun or any other luminous body. + +SPHYG'MO-GRAPH (Gr. [Greek: sphugmos], _sphugmos_, the pulse, and [Greek: +grapho], _grapho_, to write). An ingenious instrument by means of which the +pulse is delineated upon paper. + +STA'PES (L.). Literally, a stirrup; one of the small bones of the tympanum, +or middle ear, resembling somewhat a stirrup in shape. + +SYM-PA-THET'IC SYSTEM OF NERVES. A double chain of nervous ganglia, +connected together by numerous small nerves, situated chiefly in front of +and on each side of the spinal column. + +SYN-O'VI-A (Gr. [Greek: sun], _sun_, and [Greek: oon], _oon_, resembling an +egg). The lubricating fluid of joints, so called because it resembles the +white of egg. + +SYS'TO-LE (Gr. [Greek: sustello], _sustello_, to contract). The contraction +of the heart, by which the blood is expelled from that organ. + +TAC'TILE (L. _tac'tus_, touch). Relating to the sense of touch. + +TEM'PO-RAL (L. _tem'pus_, time, and _tem'pora_, the temples). Pertaining to +the temples; the name of an artery: so called, because the hair begins to +turn white with age in that portion of the scalp. + +TEN'DON (L. _ten'do_, to stretch). The white, fibrous cord or band by which +a muscle is attached to a bone; a sinew. + +TET'A-NUS (Gr. [Greek: teino], _teino_, to stretch). A disease marked by +persistent contractions of all or some of the voluntary muscles; those of +the jaw are sometimes solely affected: the disorder is then termed +locked-jaw. + +THO'RAX (Gr. [Greek: thorax], _thorax_, a breastplate). The upper cavity of +the trunk of the body, containing the lungs, heart, etc.; the chest. + +THY'ROID (Gr. [Greek: thureos], _thureos_, a shield). The largest of the +cartilages of the larynx; its angular projection in the front of the neck +is called "Adam's apple." + +TRA'CHE-A (Gr. [Greek: trachus], _trachus_, rough). The windpipe, or the +largest of the air-passages; composed in part of cartilaginous rings, which +render its surface rough and uneven. + +TRANS-FU'SION (L. _transfun'do_, to pour from one vessel to another). {263} +The operation of injecting blood taken from one person into the veins of +another; other fluids than blood are sometimes used. + +TRICH-I'NA SPI-RA'LIS. (L.) A minute species of parasite or worm, which +infests the flesh of the hog, and which may be introduced into the human +system by eating pork not thoroughly cooked. + +TYM'PA-NUM (Gr. [Greek: tumpanon], _tumpanon_, a drum). The cavity of the +middle ear, resembling a drum in being closed by two membranes, and in +having communication with the atmosphere. + +U'VU-LA (L. _uva_, a grape). The small pendulous body attached to the back +part of the palate. + +VAS'CU-LAR (L. _vas'culum_, a little vessel). Pertaining to, or containing +blood-vessels. + +VE'NOUS (L. _ve'na_, a vein). Pertaining to, or contained within a vein. + +VEN-TI-LA'TION. The introduction of fresh air into a room or building, in +such a manner as to keep the air within it in a pure condition. + +VEN-TRIL'O-QUISM (L. _ven'ter_, the belly, and _lo'quor_, to speak). A +modification of natural speech by which the voice is made to appear to come +from a distance. The ancients supposed that the voice was formed in the +belly; hence the name. + +VEN'TRI-CLES of the heart. The two largest cavities of the heart, situated +at its apex or point. + +VER'TE-BRAL COLUMN (L. _ver'tebra_, a joint). The back-bone, consisting of +twenty-four separate bones, called vertebrae, firmly jointed together; also +called the spinal column and spine. + +VES'TI-BULE. A portion of the internal ear, communicating with the +semicircular canals and the cochlea; so called from its fancied resemblance +to the vestibule or porch of a house. + +VIL'LI (L. _vil'lus_, the nap of cloth). Minute thread-like projections +found upon the internal surface of the small intestine, giving it a velvety +appearance. + +VIT'RE-OUS (L. _vi'trum_, glass). Having the appearance of glass; applied +to the humor occupying the largest part of the cavity of the eyeball. + +VIV-I-SEC'TION (L. _vi'vus_, alive, and _se'co_, to cut). The practice of +operating upon living animals, for the purpose of studying some +physiological process. + +VOCAL CORDS. Two elastic bands or ridges situated in the larynx; they are +the essential parts of the organs of the voice. + + * * * * * + + +{265} + +INDEX. + +------o------ + + A. + PAGE + Absorbent vessels, 97 + Absorption, 96 + by blood-vessels, 96 + by the lacteals, 96 + of the food, 96 + Accommodation, function of, 213 + Achilles, tendon of, 27 + Adam's apple, 229 + Air, atmospheric, 131 + Changes in, in respiration, 132 + Carbonic acid in, 138 + Composition of, 131 + Dust in the, 137 + Effects of impure, 139 + Impurities in, 136 + Matters in the expired, 132 + Provision for purifying, 141 + Renovation by ventilation, 142 + Air-cells of the lungs, 125 + Air-passages, 125 + Albinos, 44 + Albumen, 58 + of the blood, 102 + Albuminoid substance, 57 + Varieties of, 57 + Properties of, 57-58 + Albuminose, 94 + Alcoholic liquors, 77 + Physiological action of, 78 + Alimentary canal, 81 + Animal functions, 143 + Animal heat, 143 + how produced, 143 + regulated by perspiration, 145 + Animals, relative strength of, 28 + Apoplexy, 171 + Aqueous humor, 210 + Arachnoid membrane, 152 + Arbor vitae, 154 + Arterial blood, 107-135 + differs from venous, 135 + Arteries, 114 + Arrangement of, 115 + Carotid, 116 + Distribution of, 115 + Pulsation of, 115 + Radial, 116 + Temporal, 116 + Arytenoid cartilage, 229 + Asphyxia, 250 + Assimilation, 80, 121 + Audition, 215 + Auditory canal, 218 + nerve, 222 + Auricles of the heart, 109 + + B. + + Back-bone, 21 + Bathing, 47 + Importance of, 47 + Time and manner of, 49 + Baths, 48 + Different kinds of, 48 + Belladonna, 206 + dilates the pupil, 206 + Use as a cosmetic, 206 + Bile, 95 + Secretion of, in the liver, 95 + Accumulation of, in the + gall-bladder, 95 + Uses of, 95 + Biliary duct, 95 + Bladder, Gall-, 95 + Bleeding, how stopped, 121 + Blind-spot, 207 + Blood, 101 + Arterial, 107-135 + Change of color, 107 + Circulation of, 107, 133 + Coagulation of, 105 + Composition of, 102 + corpuscles, 102, 103 + fluid, 105 + Microscopic appearance of, 102 + Respiratory changes in, 133 + Uses of the, 105 + Venous, 107, 135 + Blood-vessels, 114, 118 + Absorption by, 96 + Injuries to the, 121 + Body, renovation of the, 66 + Bones, 15 + Form and composition of, 16 + Growth of, 22 + Microscopic structure of, 17 + Repair of, 23 + Structure of, 17 + Uses of, 15 + Bowels, 94 + Brain, 150 + Anatomical structure of, 152 + Function of the, 172 + Injuries of the, 173 + Membranes of the, 152 + Reflex action of the, 174 + {266} + Bread, 72 + Bronchial tubes, 125 + Bronchitis, 128 + + C. + + Canals, Semicircular, 223 + Capillary blood-vessels, 118 + Circulation in the, 118 + Carbonic acid, 132 + exhaled from the lungs, 132 + in the air, 138 + retention in the blood, 134 + Cartilage, 20 + Arytenoid, 229 + Cricoid, 229 + Thyroid, 229 + Casein, 58 + Cataract, 210 + Cells, Nerve, 150, 159 + Ciliated, 128 + Cerebellum, 153 + Function of the, 172 + Cerebro-spinal nervous system, 150 + Cerebrum, 152 + Function of the, 172 + Cheese, 58 + Chest, Framework of, 19 + Contents of the, 19 + Chocolate, 77 + Chorea, 169 + Choroid coat of the eye, 204 + Chyle, 95 + Chyme, 94 + Cilia, 128 + Circulation, 107 + in the frog's foot, 119 + of the blood, 107 + Rapidity of, 120 + through the heart, 112 + through the lungs, 123 + Clothing, 51 + Coagulation of milk, 58 + of the blood, 104 + Cochlea, 223 + Coffee, 75 + Effects of, 75, 76 + Collar-bone, 19 + Color-blindness, 209 + Column, Spinal, 21 + Combustion, Spontaneous, 145 + Complexion, 44 + Concha of the ear, 217 + Conjunctiva, 200 + Contraction of heart, 111 + of muscles, 27 + Convulsions, 169 + Cooking, 70 + Cords, Vocal, 126, 230 + Cornea, 203 + Corpuscles, Blood, 102 + Cosmetics, 51 + Cranial ganglia, 150 + Functions of, 171 + Cranial nerves, 154 + Cranium, 19 + Cricoid cartilage, 229 + Crystalline lens, 209 + Uses of, 210 + Cuticle, 41 + Function of, 183 + Cutis, 42 + + D. + + Decussation of motor and sensory + fibres of spinal cord, 164 + Dentition of infancy, 82 + Diaphragm, Movements of the, in + respiration, 128, 129 + Diastole of the heart, 111 + Diet, Mixed, 66, 85 + Necessity for changing, 67 + Necessity of a regulated, 62 + The best, 63 + Digestion, 80 + Circumstances affecting, 97 + Gastric, 93 + Intestinal, 94 + Nature of, 81 + Organs of, 81-91 + Drowning, 250 + Duct, Biliary, 95 + Nasal, 201 + Pancreatic, 95 + Thoracic, 97 + Dura Mater, 152 + + E. + + Ear, 217 + External, 217 + Internal, 222 + Middle, 219 + Foreign bodies in, 225 + Drum of the, 219 + Bones of the, 220 + Ear-sand, 223 + Ear-stones, 223 + Ear-wax, 219 + Eggs, 68 + Composition of, 68 + Emulsion of fats, in digestion, 95 + Enamel of the teeth, 82 + Epiglottis, 126, 229 + Uses of, 126, 220 + Eustachian tube, 221 + Exercise, 30 + Different modes of, 31 + Effects of, 30 + Importance of, 30 + Open-air, 33 + Expiration, 128 + Movements of, 129 + Extensor muscles, 26 + Eye, 198 + Eyeball, 203 + Eyelashes, 200 + Eyelids, 200 + + F. + + Fats, 59 + Emulsion of, 59 + Source of, in food, 59 + Fenestra ovalis, 224 + Fibres, Muscular, 25 + {267} + Fibres, Nervous, 149 + Fibrine in food, 58 + of the blood, 102 + Fish, as food, 71 + Flexor muscles, 26 + Food, 53 + Animal, 67 + Daily quantity of, 65 + Ingredients of, 54-62 + Necessity for, 64 + Source of, 53 + Vegetable, 71 + + G. + + Gall-bladder, 95 + Ganglia, cranial, Functions of the, 171 + Gases, Interchange of, in the lungs, 134 + Gastric digestion, 93 + Gastric juice, 91 + Action of, 93 + Daily quantity of, 92 + General sensibility, 179 + Glands, Perspiratory, 45 + Salivary, 86 + Sebaceous, 44 + Glossary, 252 + Glosso-pharyngeal nerve, 189 + Gullet, 90 + Gum, 61 + as food, 62 + Gustatory nerve, 189 + Gymnastics, 33-38 + for schools and colleges, 33 + + H. + + Hair, 42 + Uses of, 44 + Hearing, Sense of, 215 + Protection of, 224 + Heart, 107 + Cavities of the, 109, 110 + Circulation through the, 112 + Frequency of action, 112 + Movements of the, 111 + Valves of the, 112 + Heat, Animal, 143 + Production of, 143 + Regulation of, 145 + Hemiplegia, 165 + Humor, Aqueous, 210 + Crystalline, 209 + Vitreous, 210 + Hunger, 65 + Seat of the sensation of, 65 + Hydra, 149 + Hydrophobia, 169 + Hygiene, 13 + Hyperopia, 212 + + I. + + Incus, 220 + Inorganic substances in food, 54 + Insalivation, 86, 88 + Insensible perspiration, 46 + Inspiration, 128 + Intestinal juice, 95 + Action of, 96 + Intestines, 94 + Complete digestion in the small, 94 + Villi of the, 96 + Iris, 205 + Function of, 205 + Iron, 56 + Proportion in the blood, 57 + Proportion in the food, 57 + + J. + + Joints, 19 + Varieties of, 20 + Juice, gastric, 91 + Intestinal, 95 + Pancreatic, 95 + + L. + + Labyrinth, 222 + Lachrymal canals, 201 + gland, 201 + Lacteals, 96 + Absorption by, 96 + Lactic acid in gastric juice, 92 + Lactometer, 68 + Large intestines, 94 + Laryngoscope, 231 + Larynx, 125, 228 + Production of the voice in + the, 126, 228 + Lens, crystalline, 209 + Ligaments, 19 + Light, theory of, 197 + Lime in the bones, 16 + in the food, 56 + Importance of, 56 + Liver, 95 + Secretion of the, 95 + Locked jaw, 169 + Long-sight, 212 + Lungs, 123 + Capacity of, 130 + Structure of, 125 + Lymph, 97 + Lymphatic vessels, 97 + + M. + + Magendie, on pain, 181 + Magnesia, Compounds of, in food, 57 + Malleus, 220 + Marrow of the bones, 17 + Mastication, 82 + Importance of, 88, 89 + Meats, 68 + The cooking of, 69 + The preservation of, 69 + Membrane of the tympanum, 219 + Medulla oblongata, 154 + Function of the, 171 + Microscope, 236 + The value of the, 236 + Simple, 237 + Compound, 239 + The use of the, 239 + {268} + Milk, 68 + Composition of, 68 + Specific gravity of, 68 + Milk-teeth, 82 + Mucous membrane of air passages, 127 + Muscles, 25 + Function of the, 25 + Flexion and extension of, 26 + Voluntary and involuntary, 26 + Muscular contraction, 27 + fibres, 25 + sense, 188 + Myopia 212 + + N. + + Nails, 42 + Uses of the, 44 + Nasal cavities, 192 + duct, 201 + Nerve, Auditory, 222 + Glossopharyngeal, 189 + Gustatory, 189 + Olfactory, 193 + Optic, 197 + Sympathetic, 158 + Nerve cells, 150, 159 + Nerve fibres, 149 + Nerves, Cranial, 154 + Spinal, 156 + Functions of the, 160 + Sensory, functions of the, 160 + Motor, functions of the, 160 + Sympathetic system of, 158 + Nervous system, 148, 149 + Cerebro-spinal, 150 + Nervous tissue, Properties of, 159 + Nose, 192 + Nutrition, Processes of, 80 + + O. + + Oesophagus, 90 + Oil, Sources of, in food, 59 + Old-sight, 215 + Olfactory nerve, 193 + Optic nerve, 197 + Orbicular bone, 220 + Orbit of the eye, 199 + Organic substances as food, 57-62 + Organs of circulation, 107 + Digestion, 81-91 + Respiration, 123 + Sight, 198 + Voice, 228 + Oxygen, 131 + Amount of, consumed in + respiration, 132 + Continually supplied to the + atmosphere, 141 + + P. + + Pain, Relations of, to pleasure, 181 + Sensation of, 180 + Uses of, 180 + Pancreatic juice, 95 + Uses of, 95 + Pancreatin, 95 + Paraplegia, 163 + Parlor gymnasium, 36 + Passages, Air, 125 + Pelvis, 19 + Pepsin, 92 + Peristaltic action of the stomach, 92 + Peritoneum, 94 + Perspiration, Daily amount of, 46 + Sensible and insensible, 46 + Uses of, 46, 145 + Perspiratory glands, 45 + Physical strength, 29 + Culture, 33 + Physiology, 11 + Animal, 11 + Comparative, 11 + Human, 11 + Vegetable, 11 + Pia mater, 152 + Plasma of the blood, 102 + Pleura, 124 + Pleurisy, 128 + Pneumo gastric nerve, 171 + Pneumonia, 128 + Poisons and their antidotes, 247 + Potash in the blood, 57 + Potato, 73 + Presbyopia, 215 + Preservation of the teeth, 85 + Ptyalin, 88 + Pulsation of the heart, 113 + of the arteries, 116 + Pulse, 115 + Form of the, 116 + Writer, 116 + Pylorus, 90 + + R. + + Radial artery, 116 + Red corpuscles of the blood, 102 + Reflex action of the spinal cord, 165 + Requisites for, 167 + Uses of, 167, 170 + Causing convulsions, 169 + Objects of, 170 + of the brain, 174, 175 + Rennet, 58 + Respiration, 123 + Change of blood in, 123-133 + Frequency of, 129 + Movements of, 128 + Object of, 123 + Organs of, 123 + Respiratory labor, 135 + Rest, necessity for, 38 + Retina, 206 + Retinal light, 207 + Ribs, Movements of, in respiration, 128 + + S. + + Saccharine substances, 60 + Saliva, 86 + Importance of, 88 + Secretion of, 86 + Salivary glands, 86, 87 + {269} + Salt, Common, 55 + Importance of, 56 + Sclerotic coat of the eyeball, 204 + Sebaceous glands, 44 + Secretion of, 45 + Semicircular canals, 223 + Sensation of pain, 180 + Relations of, to pleasure, 181 + of temperature, 187 + of weight, 188 + Modification of, 178 + Production of, 177 + Variety of, 178 + Sense of hearing, 215 + sight, 196 + smell, 192 + taste, 189 + touch, 184 + Sense, muscular, 188 + thermal, 187 + Senses, Special, 177 + Sensibility, General, 179 + Short-sight, 212 + Sinews, 27 + Sight, Sense of, 196 + Organs of, 198 + Skeleton, 19 + Skin, 41 + Structure of, 41 + Skull, 19 + Uses of the, 19 + Sleep, Necessity for, 38 + Amount required, 39 + Small intestines, 94 + Smell, Sense of, 192 + Nerve of, 193 + Uses of, 194 + Soda in the food, 57 + Sound, Production of, 215 + Special senses, 177 + Spectroscope, 104 + Speech, 227 + Relation of, to the sense of + hearing, 228 + Sphygmograph, 116 + Spinal column, 21 + Spinal cord, 155 + Decussation of the, 164 + Direction of fibres in, 164 + Functions of the, 162 + Nerves of, 156 + Reflex action of, 165 + Spontaneous combustion, 145 + Stapes, 220 + Starch, 61 + Its change into sugar, 61 + Different kinds, 61 + Effect of boiling, 61 + Microscopic appearance, 61 + Stimulating substances, 62 + Stomach, 90, 92 + Digestion, 93 + Movements of, 92 + Secretion of, 92 + St. Vitus' dance, 169 + Sugar, 60 + Varieties, 60 + Sources of, 61 + Sun-bath, 50 + Sympathetic system of nerves, 158 + Synovia, 20 + Systole of the heart, 111 + + T. + + Taste, Association of, 190 + Education of, 191 + Organ of, 188 + Sense of, 189 + Tea, Effect of, 76 + Kinds of, 76 + Tears, 201 + Escape of the, 201 + Teeth, 82 + Temporary set of, 82 + Permanent set of, 83 + Bicuspid, 83 + Canine, 83 + Incisor, 83 + Molar, 84 + Arrangement of, 85 + of different animals, 85 + Preservation of, 85 + Temperature of the body, 146 + Extremes of, 146 + Sensations of, 187 + Tendon of Achilles, 27 + Tendons, 27 + Tetanus, 169 + Thermal, 50 + Thermae sense, 187 + Thirst, 65 + Thoracic duct, 97 + Thorax, 19 + Thyroid cartilage, 229 + Tissues, intimate structure of the, 236 + Human, 244 + of the lower animals, 245 + Tongue, 188 + Nerves of, 189 + Sensibility, 189 + Touch, Delicacy of, 186 + Organs of, 183 + Sense of, 184 + Trachea, 125 + Transfusion, 106 + Trichina spiralis, 71 + Trunk, 19 + Tympanum of the ear, 219 + Membrane of, 219 + + V. + + Valves of the heart, 112 + of the veins, 117 + Vapor, Animal, in breath, 132 + Vegetable food, 71 + Vegetative functions, 148 + Veins, 117 + Valves of, 117 + Venous blood, 135 + Changes of, in respiration, 133 + Ventilation, 142 + Ventricles of the larynx, 229 + of the heart, 110 + Ventriloquism, 235 + {270} + Vertebrae, 21 + Vestibule of the internal ear, 223 + Villi of the intestines, 96 + Absorption by, 96 + Vital knot, 171 + Vitreous humor, 210 + Vocal cords, 126, 230 + Observation of, with laryngoscope, 231 + Voice, 227 + Organ of, 228 + Production of, 232 + Varieties of, 233 + + W. + + Water, 74 + Action of, on lead, 75 + Chemically pure, 74 + Croton, 74 + exhaled with the breath, 132 + from springs and wells, 74 + Proportion of, in the blood 55 + " of, in the tissues and + fluids of the body, 54 + Ridgewood, 74 + Walking, as a means of exercise, 31 + White corpuscles of the blood, 104 + Wisdom teeth, 84 + + * * * * * + + +Changes made to printed original + +Page 45, Sect. 12. "(4, Fig. 14)": '(4, Fig. 13)' in original. + +Page 20, Sect. 20, note. "philosophy": 'philosphy' in original. + +Page 101, heading. "Sphygmograph": 'Spygmograph' in original. So also in +Table of Contents, but cf. p. 116 and Index. + +Page 144, Sect. 46, note. "zoologists": 'zooligsts' in original. + +Page 199, Sect. 53. "considerable": 'considera-ale' (on line break) in +original. + +Page 255, s.v. Convolutions. 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