UCSB LIBRARY ^(yjtuL oL. ^ cu^ LESSONS ELEMENTARY PHYSIOLOGY. R', The Ciirtilages of the R; 5. The Sacrum Cx. The Coccyx. Sep. The Scapula, or Sh.mld a. The Clavicle, or CoUar ] H. The Humerus. \ EXPLANATION OF THE PLATE. . I.— The Hpman Skeleton in PROFini Mc. Tlic MetacnrpiiJ Digits of t > Vertical Section of . The Occipital Foramea III.— The Right Sca: S&. Bcaphoides. Tpm- Trapezium. A front view of the S dm. The Acetabulum e Cartilages of the Ribs. R; and part o ■A FitoNT View of tub Pelvis. II. Fb. Is. as before. Fio VII.— The Dorsal LESSONS ELEMENTARY PHYSIOLOGY. THOAIAS H. HUXLEY, LLD. F.R.S. ELEVENTH EDITION. MACMILLAN AND CO. 1878. [Zyzd- Rigid of Tianslation and Reprodiiclion is resciirdA LONDON : R. CLAY, SONS, AND TAYLOR. PRINTKRS, BREAD STREET HILL, QUEEN VICTORIA STREET. PREFACE TO THE SIXTH EDITION. A CONSIDERABLE iiumber of illustrations have been added to this edition ; and several of them have been taken, not from the Human subject, but from the Rabbit, the Sheep, the Dog, and the Frog, in order to aid those, who, in accordance with the recommendation contained in the Preface to the Second Edition, attempt to make their knowledge real, by acquiring some practical acquaintance with the facts of Anatomy and Physiology. My thanks are again due to my friend Dr. Foster, F.R.S., for many valuable suggestions, and, more especially, for the trouble he has taken in super- intending the execution of the new woodcuts. LONUON, September 1872. PREFACE TO THE SECOND EDITION. The present edition of the " Lessons in Elementary Physiology," has been very carefully revised. A few woodcuts have been added ; others have been re- placed by better ones, as in the case of the figures of the retina, which embody the results of Schultze's latest researches. Some additions (but as few as possible, lest the book should insensibly lose its elementary character) have been made ; among the most important I count the very useful " Table of Anatomical and Physi- ological Constants " drawn up for me by Dr. Michael Foster, for whose friendly aid I am again glad to express my thanks. It will be well for those who attempt to study Elementary Physiology, to bear in mind the impor- tant truth that the knowledge of science which is attainable by mere reading, though infinitely better than ignorance, is knowledge of a very different kind from that which arises from direct contact with fact ; viii PREFACE TO SECOAD EDITIOM and that the worth of the pursuit of science as an intellectual discipline is almost lost by those who seek it only in books. As the majority of the readers of these Lessons will assuredly have no opportunity of studying ana- tomy or physiology upon the human subject, these remarks may seem discouraging. But they are not so in reality. For the purpose of acquiring a prac- tical, though elementary, acquaintance with physi- ological anatomy and histology, the organs and tissues of the commonest domestic animals afford ample materials. The principal points in the structure and mechanism of the heart, the lungs, the kidneys, or the eye, of man, may be perfectly illustrated by the corresponding parts of a sheep ; while the phenomena of the circulation, and many of the most important properties of living tissues, are better shown by the common frog than by any of the higher animals. Under these circumstances there really is no reason why the teaching of elementary physiology should not be made perfectly sound and thorough. But it should be remembered that, unless the learner has previously acquired a knowledge of the elements of Physics and of Chemistry, his path will be beset with difficulties and delays. T. H. H. London, July 1868. PREFACE TO THE FIRST EDITION. The following " Lessons in Elementary Physiology " are primarily intended to serve the purpose of a text-book for teachers and learners in boys' and girls' schools. i\Iy object in writing them has been to set down, in plain and concise language, that which any person who desires to become acquainted with the prin- ciples of Human Physiology may learn, with a fair prospect of having but httle to unlearn as our know- ledge widens. It is only by inadvertence, or from an error in judgment, therefore, that the book contains any statement, or doctrine, which cannot be regarded as the common property of all physiologists. I have endeavoured simply to play the part of a sieve, and to separate the well-estabUshed and essential from the doubtful and the unimportant portions of the vast mass of knowledge and opinion we call Human Physiology. X PREFACE TO FIRST EDITION. The originals of the woodcuts are, for the most part, to be found in the works of Bourgery, Gray, Henle, and KolUker. A few are new. I am particularly indebted to my accomplished friend, Dr. Michael Foster, for the pains and trouble he has bestowed upon the Lessons in their passage through the press. The Royal School of Mines, London, Octobe)- 1866. CONTENTS. LESSON I. A General View of the Structure and Functions OF the Human Body. Pp. 1—20. § I. Modes of studying the actions of man's body. 2. Purpose of these Lessons. 3. Experimental p7'ocf that a living active man gives out heat, exerts mechanical force, and loses substance in the form of carbonic acid, zcafer, and other matters. 4, 5. These losses made good by the taking in of air, drink, and food. 6. Balance of bodily income and expenditure. 7. Work and Waste; the body compared to a steam-engine. 8. General build of the body — head, trunk, and limbs. 9. The vertebm and spinal cord. The cavities of the trunk. 10. The Jiuman body a double tube. 11. The tissues. Integument. 12. Connective tissue. 13. Muscle. 14. The skeleton. 15. The maintenance of an upright position the result of many combined actions. 16. The relation of the mind to the action of the muscles. 17. The spinal cord capable of converting impressions from witho7it into muscular contractions. 18. Special sensations. 19. The tissues are constantly beijig renetoed. The rennval is effected by means of the alimentary appa- ratus, which converts food into nutriment ; and by the 20. xii COXTEiVTS. %2i, 22. Organs of circulation, 70/1 ich distribiilc (he m.triincnt over the body. 23. The excretory organs drain ivaste matters from the body. 24. Double function of the lungs. 25. The nervous system combines the action of the various organs. 26. Life and death. 27. Local death constantly going on in the body. 28. General death— death of the body as a whole, ami death of the tissues. 29. A/odes of death. 30. Decomposition and diffusion. LESSON II. The Vascular System and the Cjkculatiun. Pp. 21—57. § I. 77/^ nature and arrangment of the capillaries. 2. Structure and properties of arteries and veins. 3. Differences behveen arteries and veins. 4. Strticture and function of the valves of the veins. 5. The Lymphatics. 6. The Lacttals. 7. A general vieiu of the way in which the vessels are arranged in the body and are connected 7vith the heart. 8, 9. The Heart, its connexions and structure ; the pericardium and endocardium ; the auricles and ventricles. 10. Lts valves, their structtire, action, and pmpose. 11. Its systole and diastole. 12. The 7Jorking of the heart ; the mechanism by which the heart, thvugh its contractions, drives the blood ahi'ays in one direction, explained. 1 3. The working of the arteries. 14. The beat of the heart. 1 5. The sotinds of the heart. 1 6. The pulse in the arteries. 17. Why blood Jlozus i)i Jerks from a act artery. 18 — 20. Why no pulse can befell in the capillaries. 21,22. The circulation traced in its whole course. 23. The ner^'ous system regulates the calibre of the small arteries and z-eins, and thereby controls the f 020 of blood through various parts : blushing, &^c. CONTE.VTS. xiii § 24. Experimental proof of this. 25. Relation of this controlling poiver to disease. 26, 27. The movements of the heart are also under the control of the tiervous system. 28. The proofs of the circulation. Direct observation of the circidation of the blood in the ii>eb of a f-og' s foot. LESSON III. The Blood and the Lymph. Pp. 58—73. §1—3. The properties of a drop of blood, corpuscles, plasma, coagulation. 4. Red corpuscles. 5. 6. Colourless corpuscles ; their contractility. 7. Development of corpuscles ; the red corpuscles are probably derived from the colourless ones. 5. Red corpuscles of shed blood tend to stick together in rolls. 9. Blood-crystals. io, II. Coagulation of blood ; fibrin, crassamentiun or clot, serum 1 2. Buffy coat. 1 3. Influence of circumstances on the rapidity of coagulation. 14. N'ature of the process of coagulation; globulin, fibrin- ogen. 15. The physical qualities of the blood. 16. The chemical composition of the blood. 1 7. Infliience of age, sex, food, &^c. on the blood. 18. Total quantity of blood in the body. 19. The vivifying influence of blood over the tissues : trans- fitsion. 20. The Lymph. LESSON IV. Respiration. Pp. 74—100. § I. The blood a highly complex product derived from all parts of the body. 2. Blood rendered venous in the capillaries. 3. Difference betivcen arterial and venous blood. 4. Diffusion of gases. 5. Ca?fse of change in colour of blood. xiv CONTENTS. § 6. Blood is changed from arterial to venous in the systemic, and from venons to arterial in the pulmo)Uiry capillaries. 7. The essence of respiration. 8. Machinery of respiration. The air-passages and chambers. 9. Necessity for the renewal of the air in the lungs. 10. The respiratory act ; inspiration, expiration. 1 1. Differences between inspired and expired air. 12. The amount of ivork done by the lungs. 1 3 . The nucha nism of the respiratory moi •enunts. The elasticity of the lungs. 14. Contractility of the walls of the bronchial tubes. Ciliary action. 15. Movements of the chest walls. The intercostal muscles. 16. The diaphragm. 1 7. Actions of the diaphragm and intercostal muscles compared. 18. Accessory muscles. 19. Sighing, coughing, ^c. 20. The chest compared to a bellows. Residual, supplemental, complemental, tidal, and stationary air. 21. The stationary air plays the part of a middle man. 22. Composition of stationary air. 23. The respiratory mechanism under the control of the nervous system. 24, 25. Respiration and cijxulation compared. 26. The respiratory murtmirs. 27. Inspiration assists the circulation. 28, 29. E^ect of expiration on the circulation. Stoppage of the heart by distension of the lungs. 30. The activity of the respiratory process modified by the circtimstances of life. 31. Asphyxia. 32. Respiratory poisons. 33. SI01.V asphyxiation. 34. Necessity for an abundance of fresh air. LESSON V. The Sources of Loss and of Gain to the Blood. Pp. 101 — 132. § I . Distribution of artenal blood. I — 4. The blood in various ways meets 'with constant or in termittent gains and losses of matei-ial and heat. 5. Tabular view of the sources of loss and gain. COXTENTS. XV § 6. T/ie loss by the kidneys. The urinary apparatus. 7. Composition of urine. 8. Kidneys and lungs cotnpared. 9. The stnicture of the kidney. 10, 1 1. Changes in the blood while passing through the kidney. 12. The ne>'z'ous sy stein controls the excretion of urine. 13. The loss by the skin. Sensible and insensible peispiration. 14. Quantity a>id composition of sweat. 15. Perspiration by simple transudation. 1 6. Sweat-gla nds. 1 7. These glands are controlled by the nervous system. 18. Variations in the quantity of matter lost by perspiration. 19. The lungs, skin, and kidneys compared together. 20. The liver, its connexions and structure. 21. The active paioers of the liver-cells. 22. The bile. Its quantity and composition. 23. Bile is formed in the liver-cells. 24. Sources of gain of matter. Gain of oxygen through the lungs. 25. Gain of corpuscles and of sugar through the liver. 26. Experimental proof of the formation of sugar in the liver. Glycogen. 27. Gain by the lymphatics. 28. The spleen. 29. Gain of heat. Generation of heat by oxidation. 30. Distribution of heat by the blood current. 31. Temperature of the body kept dotvn by evaporation. Adjustjnent by means of the nei-vous systetfi. 32. The glands are intermittently active sources of loss. Structtire and functions of glands, '■^'i^. Gain of waste products from the ?nuscles. LESSON VI. The Function of Alimentation, Pp. 133 — 155. § I. The alirnentary canal, the chief source of gain. 2. The quantity of dry, solid, and gaseous aliment daily taken in by a man. 3. The quantity of dry solid matter daily lost by a man. 4. Classification of aliments. Proteids, Fats, Amyloids, • Minerals. The chief vital food-stuffs. 5. Their ultimate analysis. The presence oj Proteids and Minerals in food indispensable. i COXTEXTS. i 6. X'o absolute necessity Jor other foodstuffs. 7. XitrogeJi starvation. 8. Disadvantages of a purely nitrogenous diet. , 9^ Economy of a mixed diet. 10. Advantage of combining different articles of Jood. 11. Intermediate changes undergone by food in the body. 12. Division of food-stuff^s into heat -producers and tissue- formers jnisleading. 13. Function of the alimentary apparatus. The mouth a)ui pharynx. 14. The salivary glands. 15. The teeth. 16. Eating and suHillounng. 17. Drinking. 18. The stomach and tJie gastric juics. 19. Artificial digestion. 20. Chyme. Absorption from tJie stonuich. 2 1 . The large and small intestines. 22. The intestinal glands a>ul Juice. The valvule conniveiites and villi. J^eristaltic contractio)i. 23. Entrance of bile a}Hl pancreatic Juice. 24. Chyle. Absorption from the intestines. 25. Digestion in the large intestine. LESSON VII. Motion and Locomotion. Pp. 156—186, ^i. The vital eddy. The source of the acti've pc'oers of the eco)uvny. 2. The organs of motion are cilia and muscles. 3. Cilia. 4. Muscles. Muscular contractio)i. Kigor nuvtis. 5. HolUriv muscles. 6. Muscles attached to Irvers. The three orders of h\c?s. 7. Examples, in the body, of Irvcrs of the first oi'der. 8. Examples of levers of the second order. 9. PI xn tuples of le^'ers of the third order. JO. The same parts may represent, in turn, each of the three orders. 11. Joints or articulations. Imperfect Joints. 1 2 . Structure of perfect Joints. COiVTEXTS. xvii § 13. Ball and socket pints. 14. Hinge joints. 15. Pivot Joints. The atlas and axis. TJie roiiius and ubh pronation and supination. 16. Ligaments. 17. Various kinds of movetnents 0/ Joints. 18. Means of effecting them. 19. Tendons. 20. Walking, running, Jumping. 21. Conditions of the production of the Wtice. 2.1. The vocal chords. 23. The cartilages of the larynx, 24. The muscles of the larynx. The action of the several parts of the larynx. 25. High and lozv notes ; range and quahty of voice. 26, 27. Speech. Production if vowel sounds and co}itinuous consoiuxnts. 28. Explosive consonants. 29. Speaking ma-chines. 30. Tongueless Speech. LESSON VIII. Sensations and Sensorv Organs. Pp. 187—213. 1,2. Animal movemerits the results of a series of changes usually originated by external impressions. 3. Reflex action. Sensations and consciousness. 4. Subjective setisations. 5. The muscular sense. 6. The higher senses. 7. General plan of a sensoiy organ. 8. ToJCH. Papilla. Tactile it impulses, by the grey matter. The crossing of afferent impulses, 1 5 . Vaso-motor centres. 16. The bi-ain ; the outlines of its anatomy. 17. The arrangement of its white and grey mat.'er. 18. The ne>-ves given off from the brain. 19. The olfactory and optic nemes in reality processes of the brain. 20. Effect of injuries to the medulla oblongata. 21. The crossing of efferent impulses in the medulla oblongata. 22. The functions of different parts of the brain. Intelligence and Will reside in the cerebral hemisphe? es. 23. Reflex actions take place ez>en when the brain is zchole and sound. 24. Many ordinary and very complicated muscular acts are mere reflex processes. 25. Artiflcial reflex actions. Education. 26. The sympathetic system. CONTENTS. LESSON XII. Histology, or the Minute Structure of the Tissues. Pp. 272—296. § r. The microscopical analysis of the body. 2. Nuclei, cells. 3. EpitJielium and epidermis. 4. Nails. 5. Hairs. 5. The cryslalli/ie lens. 7. Cartilage. 8. Connective tissue. 9. Fat cells. 10. Pigment cells. 11. Bone. Gro'cuth of bone. Ossification. 12. Teeth. 13, 14. Druelopment of teeth. Cutting of teeth. 1 5. Muscle, striated and smooth. 16. Nervous tissue ; nej-ve fibres and ganglionic corpuscles. 1 7. Tactile corpuscles. 18. The olfactojy ne}i-es. 19. Ganglionic corpuicles. APPENDIX A. Table of x-\xatomical and Physiological Constants. Pp- 297—301. T. General statistics. V. Cutaneous excretion. II. Digestion. VI. Renal excretion. III. Circjdaciun. VII. xYervous action. IV. Respiration. VIII. Histology. APPENDIX B. Case of Mrs. A. Pp. 302 — 306. LESSONS IN ELEMENTARY PHYSIOLOGY. LESSON I. A GENERAL VIEW OF THE STRUCTURE AND FUNCTIONS OF THE HUMAN BODY. I. The body of a living man performs a great diversity of actions, some of which are quite obvious ; others re- quire more or less careful observation ; and yet others can be detected only by the employment of the most delicate appliances of science. Thus, some part of the body of a living man is plainly always in motion. Even in sleep, when the limbs, head, and eyelids may be still, the incessant rise and fall of the chest continue to remind us that we are viewing slumber and not death. More careful observation, however, is needed to detect the motion of the heart ; or the pulsation of the arteries ; or the changes in the size of the pupil of the eye with vary- ing light ; or to ascertain that the air which is breathed out of the body is hotter and damper than the air which is taken in by breathing. And lastly, when we try to ascertain what happens in the eye when that organ is adjusted to dififerent distances ; B 2 ELEMENTARY PHYSIOLOGY. [less. or what in a nerve when it is excited : or of what materials flesh and blood are made : or in virtue of what mechanism it is that a sudden pain makes one start — we have to call into operation all the methods of inductive and deductive logic ; all the resources of physics and chemistry ; and all the delicacies of the art of experiment. 2. The sum of the facts and generahzations at which we arrive by these various modes of inquir>', be they simple or be they refined, concerning the actions of the body and the manner in which those actions are brought about, con- stitutes the science of Human Physiology. An elementary outline of this science, and of so much anatomy as is inci- dentally necessary, is the subject of the following Lessons ; of which I shall devote the present to an account of so much of the structure and such of the actions (or, as they are technically called, "functions "; of the body, as can be ascertained by easy observation ; or might be so ascer- tained if the bodies of men were as easily procured, exa- mined, and subjected to experiment, as those of animals. 3. Suppose a chamber with walls of ice, through which a current of pure ice-cold air passes ; the walls of the chamber will of course remain unmelted. Now, having weighed a healthy living man with great care, let him walk up and down the chamber for an hour. In doing this he will obviously exercise a great amount of mechanical force ; as much, at least, as would be required to lift his weight as high and as often as he has raised him- self at every step. But, in addition, a certain quantity of the ice will be melted, or converted into water ; showing that the man has given off heat in abundance. Furthermore, if the air which enters the chamber be made to pass through lime-water, it will cause no cloudy white precipitate of carbonate of lime, because the quantity of carbonic acid in ordinary air is so small as to be inappreciable in this way. But if the air which passes out is made to take the same course, the lime-water will soon become milky, from the precipitation of carbonate of lime, showing the pre- sence of carbonic acid, which, like the heat, is given off by the man. Again, even if the air be quite dry as it enters the cham- ber (and the chamber be lined with some material so as to shut out all vapour from the melting ice walls), that which I.] IVORK AXD WASTE. 3 is breathed out of the man, and that which is given off from his skin, will exhibit clouds of vapour ; which vapour, therefore, is derived from the body. After the expiration of the hour during which the expe- riment has lasted, let the man be released and weighed once more. He will be found to have lost weight. ^T Thus a living, active, man constantly exerts mechanical force, gives off hcat^ evolves carbonic acid and 'K'atcr, and undergoes a loss of snbstance. 4. Plainly, this state of things could not continue for an unlimited period, or the man would dwindle to nothing. But long before the effects of this gradual diminution of substance become apparent to a bystander, they are felt by the subject of the experiment in the fonii of the two imperious sensations called hunger and thirst. To still these cravings, to restore the weight of the body to its former amount, to enable it to continue giving out heat, water and carbonic acid, at the same rate, for an indefinite period, it is absolutely necessary that the body should be supplied with each of three things, and with three only. These are, firstly, fresh air ; secondly, drink — consisting of water in some shape or other, however much it may be adulterated ; thirdly, food. That compound known to chemists as proteid matter, and which contains carbon, hydrogen, oxygen, and nitrogen, must form a part of this food, if it is to sustain life indefinitely ; and fatty, starchy, or saccharine matters ought to be contained in the food, if it is to sustain life conveniently. 5. A certain proportion of the matter taken in as food either cannot be, or at any rate is not, used ; and leaves the body, as cxcrcnicntitious matter., ha^•ing simply passed through the alimentary canal without undergoing much change, and without ever being incorporated with the actual substance of the body. But, under healthy con- ditions, and when only so much food as is necessary is taken, no important proportion of either proteid matter, or fat, or starchy or saccharine food, passes out of the body as such. Almost all real food leaves the body in the form either of water., or of carbonic acid, or of a third substance called urea, or of certain saline compounds. Chemists have determined that these products which are thrown out of the body and arc called excretions, contain, 13 2 4 ELEMENTARY PHYSIOLOGY. [less. if taken altogether, far more oxygen than the food and water taken into the body. Now, the only possible source whence the body can obtain oxygen, except from food and water, is the air which surrounds it.^ And careful inves- tigation of the air which leaves the chamber in the imagi- nary experiment described above would show, not only that it has gained carbonic acid _//'' 100 parts nearly 21 of oxygen and 79 of nitrogen gas, together with a small fraction of a part of carbonic acid, a minute uncertain poportion of ammonia, and a variable quantity of watery vapour. (See Lesson IV. § 11.) I.] THE BCILD OF THE BODY. $ extra food be supplied to him. Let the temperature of the air fall, and the same result will occur, if his body remains as warm as before. On the other hand, diminish his exertion and lower his production of heac, and either he will gain weight, or some of his food will remain unused. Thus, in a properly nourished man, a stream of food is constantly entering the body in the shape of complex compounds containing comparatively little oxygen ; as constantly, the elements of the food (whether before or after they have formed part of the living substance) are leaving the body, combined with more oxygen. And the incessant breaking down and oxidation of the complex compounds which enter the body are definitely propor- tioned to the amount of force the body exerts, whether in the shape of heat or otherwise ; just in the same way as the amount of work to be got out of a steam-engine, and the amount of heat it and its furnace give off, bear a strict proportion to its consumption of fuel. 8. From these general considerations regarding the nature of life, considered as physiological work, we may turn for the purpose of taking a like broad survey f f the apparatus which does the work. We have seen the general performance of the engine, we may now look at its build. The human body is obviously separable into head., inenk, and iifubs. In the head, the brain-case or skull is distinguishable from the face. The trunk is naturally divided into the chest or Ihorax, and the belly or abdo- men. Of the limbs there are two pairs — the upper, or arms, and the lower, or legs; and legs and arms again are subdivided by their joints into parts which obviously exhibit a rough con'espondence— ////^// and upper arm., leg and fore-arm, ankle and iK-'rist, fingers and ioes, plainly answering to one another. And the two last, in fact, are so similar that they receive the same name of digits J while the several joints of the fingers and toes have the common denomination oi phalanges. The whole body thus composed (without the viscera) is seen to be bilaterally symmetrical ; that is to say, if it were split lengthways by a great knife, which should be made to pass along the middle line of both the dorsal and 6 ELEMENTAR V PHYSIOLOGY. [less. ventral (or back and front) aspects, the two halves vvou.ld almost exactly resemble one another. 9. One-half of the body, divided in the manner de- scribed (Fig. I, A), would "exhibit, in the trunk, the cut faces of thirty-three bones, joined together by a very strong and tough substance into a long column, which lies much nearer the dorsal (or back) than the ventral (or front) aspect of the body. The bones thus cut through are called the bodies of the vertebrcB. They separate a long, narrow canal, called the spinal canal, which is placed upon their dorsal side, from the spacious chamber of the chest and abdomen, which lies upon their ventral side. There is no direct communication between the dorsal canal and the ventral cavity. The spinal canal contains a long white cord — the spiiial cord — which is an important part of the nervous system. The ventral chamber is divided into the two subordinate cavities of the thorax and abdomen by a remarkable, partly fleshy and partly membranous, partition, the dia- phragm (Fig. I, Z)), which is concave towards the abdo- men, and convex towards the thorax: The alimentary cajial (Fig. i, Al) traverses these cavities from one end to the other, piercing the diaphragm. So does a long double series of distinct masses of nervous substance, which are called ganglia, are connected together by nervous cords, and constitute the so-caWcd. sympathetic (Fig. i, Sy.). The abdomen contains, in addition to these parts, the two kidneys, one placed against each side of the vertebral column, the liver, the. pancreas or "sweetbread,"' and the spleen. The thorax encloses, besides its segment of the alimentary canal and of the sympathetic, the hea?'t and the two Inngs. The latter are placed one on each side of the heart, which lies nearly in the middle of the thorax. Where the body is succeeded by the head, the upper- most of the thirty-three vertebral bodies is followed by a continuous mass of bone, which extends through the whole length of the head, and, like the spinal column, separates a dorsal cham_ber from a ventral one. The dorsal cham- ber, or cavity of the skull, opens into the spinal canal. It contains a mass of nervous matter called the brain, which is continuous with the spinal cord, the brain and the spinal cord together constituting what is termed the cerebrospinal !■] THE TISSUES. Fig. I. A. A diagrammatic section of the human body taken vertically through the median plane. C-S. the cerebro-spinal nervous system ; X, the cavity of tlie nose ; J/, that of the mouth ; Al. Al. the alimentary- canal represented as a simple straight tube ; H, the heart ; D, the diaphragm ; Sy. the sympathetic ganglia. B. A transverse vertical section of the head taken along the line a b ; letters as before. C. A transverse section taken along the line c d; letters as before. 8 ELEMENTARY PHYSIOLOGY. [less. axis (Fig. i, C.S.^ C.S.). The ventral chamber, or cavity of the face, is almost entirely occupied by the mouth and phary?ix', into which last the upper end of the alimentary canal (called gullet or ccsophagits) opens. 10. Thus, the study of a longitudmal section shows us that the human body is a double tube, the two tubes being completely separated by the spinal column and the bony axis of the skull, which form the floor of the one tube and the roof of the other. The dorsal tube contains the cere- bro-spinal axis ; the ventral, the alimentary canal, the sympathetic nervous system, and the heart, besides other organs. Transverse sections, taken perpendicularly to the axis of the vertebral column, or to that of the skull, show still more clearly that this is the fundamental structure of the human body, and that the great apparent difference be- tween the head and the trunk is due to the different size of the dorsal cavity relatively to the ventral. In the head the former cavity is very large in proportion to the size of the latter (Fig. i, B) ; in the thorax, or abdomen, it is very small (Fig. i, C). The limbs contain no such chambers as are found in the body and the head ; but, with the exception of certain branching tubes filled with fluid, which are called blood- vessels and lymphatics, are solid or semi-solid, throughout. 11. Such being the general character and arrangement of the parts of the human body, it will next be well to con- sider into what constituents it may be separated by the aid of no better means of discrimination than the eye and the anatomist's knife. With no more elaborate aids than these, it becomes easy to separate that tough membrane which invests the whole body, and is called the skin, or integument, from the parts which lie beneath it. Furthermore, it is readily enough ascerained that this integument consists of two portions : a superficial layer, which is constantly being shed in the form of powder or scales, composed of minute particles of horny matter, and is called the epide?'mis; and the deeper part, the dermis, which is dense and fibrous 'Fig. 32}. The epidermis, if wounded, neither gives rise to pain nor bleeds. The dermis, under like circum- stances, is very tender, and bleeds freely. A practical I.] THE TISSUES. 9 distinction is drawn between the two in shaving, in the course of which operation the razor ought to cut only epidermic structures ; for if it go a shade deeper, it gives rise to pain and bleeding. The skin can be readily enough removed from all parts of the exterior, but at the margins of the apertures of the boiy it seems to stop, and to be replaced by a layer which is much redder, more sensitive, bleeds more readily, and which keeps itself continually moist by giving out a more or less tenacious fluid, called viucus. Hence, at these apertures, the skin is said to stop, and to be replaced by mucous jjisnibrane, which lines all those interior cavities, such as the alimentary canal, into which the apertures open. But, in truth, the skin does not really come to an end at these points, but is directly continued into the mucous membrane, which last is simply an integument of greater delicacy, but consisting fundamentally of the same two layers, — a deep, fibrous layer, containing blood-vessels and nerves, and a superficial, insensible, and bloodless one, now called the epithelium. Thus every part of the body might be said to be contained between the walls of a double bag, formed by the epidermis, which invests the outside of the body, and the epithelium, its continuation, which lines the internal cavities. 12. The dermis, and the deep, sanguine layer, which answers to it in the mucous membranes, are chiefly made up of a filamentous substance, which yields abundant gelatine on being boiled, and is the matter which tans when hide is made into leather. This is called areolar, Jibrous, or, better, connective tissue.-^ The last name is the best, because this tissue is the great connecting medium by which the different parts of the body are held together. Thus it passes from the dermis between all the other organs, ensheathing the muscles, coating the bones and cartilages, and eventually reaching and entering into the mucous membranes. And so completely and thoroughly does the connective tissue permeate almost all parts of the body, that if every other tissue could be dissected away, a complete model of all the organs would be left composed of this tissue. Connective tissue varies very » Every such constituent of the body, as epidermis, cartilage, or muscle, is called a "tissue." (See Lesion XII.) lo ELEMENTARY PHYSIOLOGY. [less. much in character ; sometimes being very soft and tender, at others— as in the tendons and ligaments, which are almost wholly composed of it — attaining great strength and density. 13. Among the most important of the tissues imbedded in and ensheathed by the connective tissue, are some the presence and action of which can be readily determined during life. If the upper arm of a man whose arm is stretched out be tightly grasped by another person, the latter, as the former bends up his fore-arm, will feel a great soft mass which lies at the fore part of the upper arm, swell, harden, and become prominent. As the arm is extended again, the swelling and hardness vanish. On removing the skin, the body which thus changes its configuration is found to be a mass of red flesh, sheathed in connective tissue. The sheath is continued at each end into a tendon, by which the muscle is attached, on the one hand, to the shoulder-bone, and, on the other, to one of the bones of the fore-ami. This mass of flesh is 'the muscle called biceps^ and it has the peculiar property of changing its dimensions — shortening and becoming thick in proportion to its decrease in length — when influenced by the will as well as by some other causes,^ and of returning to its original form when let alone. This temporary change in the dimensions of a muscle, this shortening and becoming thick, is spoken of as its con- i7-actio)i. It is by reason of this property that muscular tissue becomes the great motor agent of the body ; the muscles being so disposed between the systems of levers which support the body, that their contraction necessi- tates the motion of one lever upon another. 14. These levers form part of the system of hard tissues which constitute the skeleton. The less hard of these are the cartilages., composed of a dense, firm sub- stance, ordinarily known as " gristle." The harder are the bones., which are masses either of cartilage, or of connec- tive tissue, hardened by being impregnated w'nh phosphate and carbonate of lime. They are animal tissues which have become, in a manner, naturally petrified ; and when the salts of lime are extracted, as they may be, by the * Such causes are called stimuli. ^?^ I.] THE SKELETO.V. n action of acids, a model of the bone in soft and flexible animal matter remains. More than 200 separate bones are ordinarily reckoned in the human body, though the actual number of distinct bones varies at different periods of hfe, many bones which are separate in youth becoming united together in old age. Thus there are originally, as we have seen, thirty-three separate, bodies of vertebrae in the spinal column, and the upper twenty-four of these commonly remain distinct throughout life. But the twenty-fifth, twenty-sixth, twenty- seventh, twenty-eighth, and twenty-ninth early unite into one great bone, called the sacruinj and the four remain- ing vertebras often run into one bony mass called the coccyx. In early adult life, the skull contains twenty-two naturally separate bones, but in youth the number is much greater, and in old age far less. Twenty-four ribs bound the chest laterally, twelve on each side, and most ' them are connected by cartilages with the breast-bone. In the girdle which supports the^shoulder, two bones are always distinguishable as the sca'piila _and the clavicle. ThQ pelvis, to which the legs are attached, consists of two separate bones called the ossa ifuioniiiiata in the adult ; but each os innoniinatum is separable into three (called p7(bis, isc/iiin/i, and iliinn) in the young. There are thirty bones in each of the arms, and the same number in each of the legs, counting \.h.Q patella, or knee pan. "H-^ll these bones are fastened together by ligaments, or by cartilages ; and where they play freely over one another, a coat of cartilage furnishes the surfaces which come into contact. The cartilages which thus form part of a joint are called articular cartilages, and their free surfaces, by which they rub against each other, are lined by a delicate synovial membrane, which secretes a lubri- cating fluid — the synoiyia. 1 5. Though the bones of the skeleton are all strongly enough connected together by ligaments and cartilages, the joints play so freely, and the centre of gravity of the body, when erect, is so high up, that it is impossible to make a skeleton or a dead body support itself in the upright position. That position, easy as it seems, is the result of the contraction of a multitude of muscles which oppose and balance one another. Thus, the foot aftbrding ELEMENT A R V PH VSIOL OGV. [less. the surface of support, the muscles of the calf (Fig. 2, I) must contract, or the legs and body would fall forward. ^: yiy^ FiG. 2. — A Diagram illustrating the Attachments of some of the MOST important Mcscles which keep the Body in the erect Po^TlRE. I. The muscles of the calf. II. Those of the back of the thigh. III. Those ot the spine. These tend to keep the body from falling forward. I. The muscles of the front of the leg. 2. Those of the front of the thigh. ? Those of the front of the abdomen. 4, 5. Those of the front of the neck. T.iese tend to keep the body from falling backwards. The arrows indicate the direction of action of the muscles, the foot being fixed. I . ] THE COMB IX A TIOX OF A C TIOXS. 1 3 But this action tends to bend the leg ; and to neutralize this and keep the hg straight, the great muscles in front of the thigh (Fig, 2, 2) must come into play. But these, by the same action, tend to bend the body forward on the legs ; and if the body is to be kept straight, they must be neutralized by the action of the muscles of the buttocks and of the back (Fig, 2, III). _ The erect position, then, which we assume so easily and without thinking about it, is the result of the comlDined and accurately propoitioned action of a vast number of muscles. What is it that makes them work together in this way t 16. Let any person in the erect position receive a violent blow on the head, and you know what occurs. On the instant he drops prostrate, in a heap, with his limbs relaxed and powerless. What has happened to him .^ The blow may have been so inflicted as not to touch a single muscle of the body ; it may not cause the loss of a drop of blood : and, indeed, if the " concussion," as it is called, has not been too severe, the sufferer, after a few moments of unconsciousness, will come to himself, and be as well as ever again. Clearly, therefore, no per- manent injury has been done to any part of the body, least of all to the muscles, but an influence has been exerted upon a something which governs the muscles. And this irifluence may be the effect of very subtle causes. A strong mental emotion, and even a very bad smell, will, in some people, produce the same effect as a blow. These observations might lead to the conclusion that it is the mind which directly governs the muscles, but a little further inquiry will show that such is not the case. For people have been so stabbed, or shot in the back, as to cut the spinal cord, without any considerable injury to other parts : and then they have lost the power of stand- ing upright as much as before, though their minds may have remained perfectly clear. And not only have they lost the power of standing upright under these circum- stances, but they no longer retain any power of either feeling what is going on in their legs, or, by an act of their volition, causing motion in them, v^ 17. And yet, though the mind is thus cut off from the \ { 14 ELEMENTARY PHYSIOLOGY. [less. lower limbs, a controlling and governing power over them still remains in the body. For, if the soles of the disabled feet be tickled, though no sensation will reach the body, the legs will be jerked up, just as would be the case in an uninjured person. Again, if a series of galvanic shocks be sent along the spinal cord, the legs will per- form movements even more powerful than those which the will could produce in an uninjured person. And, finally, if the injury is of such a nature that the cord is crushed or profoundly disorganized, all these phenomena cease ; tickling the soles, or sending galvanic shocks along the spine, will produce no effect upon the legs. By examinations of this kind carried still further, we arrive at the remarkable result that the brain is the seat of all sensation and mental action, and the primary source of all voluntary muscular contractions ; while the spinal cord is capable of receiving an impression from the exterior, and converting it not only into a simple muscular contraction, but into a combination of such actions. Thus, in general terms, we may say of the cerebro- spinal nervous centres, that they have the power, when they receive certain impressions from without, of giving rise to simple or combined muscular contractions. 1 8. But you will further note that these impressions from without are of very different characters. Any part of the surface of the body may be so affected as to give rise to the sensations of contact, or of heat or cold ; and any or every substance is able, under certain circum- stances, to produce these sensations. But only very few and comparatively small portions of the bodUy frame- work are competent to be affected, in such a manner as to cause the sensations of taste or of smell, of sight or of hearing : and only a few substances, or particular kinds of vibrations, are able so to affect those regions. These very limited parts of the body, which put us in relation with particular kinds of substances, or forms of force, are what are termed sensory or(^a?is. There are two such- organs for sight, two for hearing, two for smell, and one, or more strictly speaking two, for taste. 19. And now that we have taken this brief view of the structure of the body, of the organs which support it, I.] THE ORGANS. 15 of the organs which move it, and of the organs which put it in relation with the surrounding world, or, in other words, enable it to move in harmony with influences from without, we must consider the means by \vhicl>- all this "-y wonderful apparatus is kept in working order, ^^^r / ^ . All work, as we have seen, implies waste. The work '^^of the nervous system and that of the muscles, therefore, implies consumption either of their own substance, or of something else. And as the organism can make nothing, it must possess the means of obtaining from without that which it wants, and of throwing off from itself that which it wastes ; and we have seen that, in the gross, it does these things. The body feeds, and it excretes. But we must now pass from the broad fact to the mechanism by which the fact is brought about. The organs which convert food into nutriment are the organs of alimentation; those which distribute nutriment all over the body are organs of cit'cidation; those which get rid of the waste products are organs of excretion. 20. The organs of alimentation are the mouth, pharynx, gullet, stomach, and intestines, with their appendages. What they do is, first to receive and grind the food. They then act upon it with chemical agents, of which they possess a store which is renewed as fast as it is wasted ; and in this way separate it into a fluid contain- ing nutritious matters in solution or suspension, and in- nutritious dregs or fceces. 21. A system of minute tubes, with very thin walls, termed capillaries, is distributed through the whole or- ganism except the epidermis and its products, the epithe- hum, the cartilages, and the substance of the teeth. On all sides, these tubes pass into others, which are called arteries and veins; while these, becoming larger and larger, at length open into the heart, an organ which, as we have seen, is placed in the thorax. During life, these tubes and the chambers of the heart, with which they are connected, are all full of hquid, which is, for the most part, that red fluid with which we are all familiar as blood. The walls of the heart are muscular, and contract rhythmically, or at regular intervals. By means of these contractions the blood which its cavities contain is driven 1 6 ELEMENTAR V PHYSIOL OGY. [less. in jets out of these cavities into the arteries, and thence into the capillaries, whence it returns by the veins back into the heart. This is the circ7ilatio7i of the blood. 22. Now the fluid containing the dissolved or suspended nutritive matters which are the result of the process of digestion, traverses the veiy thin layer of soft and per- meable tissue which separates the cavity of the alimentary canal from the cavities of the innumerable capillary vessels which lie in the walls of that canal, and so enters the blood, with which those capillaries are filled. Whirled away by the torrent of the circulation, the blood, thus charged with nutritive matter, enters the heart, and is thence propelled into the organs of the body. To these organs it supplies the nutriment with which it is charged ; from them it takes their waste products, and, finally, re- turns by the veins, loaded with useless and injurious ex- cretions, which sooner or later take the form of water, carbonic acid, and urea. 23. These excretionary matters are separated from the blood by the excretory organs^ of which there are three — the skilly the lungs., and the kid?ieys. Different as these organs may be in appearance, they are constructed upon one and the same principle. Each, in ultimate analysis, consists of a very thin sheet of tissue, like so much delicate blotting-paper, the one face of which is free, or lines a cavity in communication with the ex- terior of the body, while the other is in contact with the blood which has to be purified. The excreted matters are, as it were, strained from the blood, through this delicate layer of filtering-tissue, and on to its free surface, whence they make their escape. Each of these organs is especially concerned in the elimination of one of the chief waste products — water, carbonic acid, and urea — though it may at the same time be a means of escape for the others. Thus the lungs are especially busied in getting rid of carbonic acid, but at the same time they give off a good deal of water. The duty of the kidneys is to excrete urea (together with other saline matters), but at the same time they pass away a large quantity of water and a trifling amount of carbonic acid ; while the skin gives off much water, some amount t.] EXCRETION AXD OX ID ATI OX. 17 of carbonic acid, and a certain quantity of saline matter, among which urea is, at all events, sornetimes present. 24. Finally, the lungs play a double part, being not merely eliminators of waste, or excretionaiy, products, but importers into the economy of a substance which is not exactly either food or drink, but something as im- portant as either, — to wit, oxygen. It is oxygen which is the great sweeper of the economy. Introduced by the blood, into which it is absorbed, into all corners of the organism, it seizes upon those organic molecules which are disposable, lays hold of their elements, and combines with them into the new and simpler forms, carbonic acid, water, and urea. The oxidation, or, in other words, the burning of these matters, gives rise to an amount of heat which is as effi- cient as a tire to raise the blood to a temperature of about 100'' ; and this hot tluid, incessantly renewed in all parts of the economy by the torrent of the circulation, warms \ the body, as a house is warmed by a hot-water apparatus. H*^ 25. But these alimentar>', distributive or circulatory, ex- 7 cretory, and combustive processes would be worse' than useless if they were not kept in strict proportion one to another. If the state of physiological balance is to be maintained, not only must the quantity of aliment taken be at least equivalent to the quantity of matter excreted ; but that ahment must be distributed with due rapidity to the seat of each local waste. The circulator}- system is the commissariat of the physiological army. Again, if the body is to be maintained at a tolerably even temperature, while that of the air is constantly vary- ing, the condition of the hot-water apparatus must be most carefully regulated. In other words, a combining organ must be added to the organs already mentioned, and this is found in the nervous system, which not only possesses the function al- ready described of enabling us to move our bodies and to know what is going on in the external world ; but makes us aware of the need of fcod, enables us to discriminate nutritious from innutritions matters, and to exert the muscular actions needful for seizing, killing, and cooking ; guides the hand to the mouth, and governs all the move- ments of the jaws and of the alimentar>' canal. By it, c 1 8 ELEMENTAR V PHYSIO LOG Y. [less. the working of the heart is properly adjusted and the cahbres of the distributing pipes are regulated, so as in- directly to govern the excretory and combustive processes. And these are more directly affected by other actions of the nervous system. 26. The various functions which have been thus briefly indicated constitute the greater part of what are called the vital actions of the human body, and, so long as they are performed, the body is said to possess life. The ces- sation of the performance of these functions is what is ordinarily called death. But there are really several kinds of death, which may, in the first place, be distinguished from one another under the two heads of local and of general death. 27. Local death is going on at every moment, and in most, if not in all, parts of the living body. Individual cells of the epidermis and of the epithelium are inces- santly dying and being cast off, to be replaced by others which are, as constantly, coming into separate existence. The like is true of blood-corpuscles, and probably of many other elements of the tissues. This form of local death is insensible to ourselves, and is essential to the due maintenance of life. But, occa- sionally, local death occurs on a larger scale, as the re- sult of injur)-, or as the consequence of disease. A burn, for example, may suddenly kill more or less of the skin ; or part of the tissues of the skin may die, as in the case of the slough which lies in the midst of a boil;, or a whole limb may die, and exhibit the strange phenomena of mortification. The local death of some tissues is followed by their regeneration. Not only all the forms of epidermis and epithelium, but nerve, connective tissue, bone, and, at any rate, some muscles, may be thus reproduced, even on a large scale. Cartilage once destroyed is said not to be restored. 28. General death is of two kinds, death of the body as a 7uhole, and death of the tissues. B}- the former term is implied the absolute cessation of the functions of the brain, of the circulatory, and of the respiratoiy organs ; by the latter, the entire disappearance of the vital actions of the ultimate structural constituents of the body. T.] LOCAL AXD GENERAL DEATH. 19 When death takes place, the body, as a whole, dies first, the death of the tissues sometimes not occurring until after a considerable interval. Hence it is that, for some httle time after what is ordi- narily called death, the muscles of an executed criminal may be made to contract by the application of proper stimuli. The muscles are not dead, though the man is. 29. The modes in which death is brought about appear at first sight to be extremely varied. We speak of natural death by old age, or by some of the endless forms of dis- ease ; of violent death by starvation, or by the innumer- able va*-ieties of injury, or poison. But, in reality, the immediate cause of death is always the stoppage of the functions of one of three organs ; the cerebro-spinal ner- vous centre, the lungs, or the heart. Thus, a man may be instantly killed by such an injury to a part of the brain which is called the medulla oblongata (see Lesson XI.) as may be produced by hanging, or breaking the neck. Or death may be the immediate result of suffocation by strangulation, smothering, or drowning. — or, in other words, of stoppage of the respiratory functions. Or, finally, death ensues at once when the heart ceases to propel blood. These three organs -the brain, the lungs, and the heart — have been fancifully termed the tripod of life. In ultimate analysis, however, life has but two legs to stand upon, the lungs and the heart, for death through the brain is always the elTect of the secondary action of the injur}- to that organ upon the lungs or the heart. The functions of the brain cease, when either respiration or circulation is at an end. But if circulation and respira- tion are kept up artificially, the brain may be removed without causing death. On the other hand, if the blood be not aerated, its circulation by the heart cannot pre- serve life ; and, if the circulation be at an end, mere aeration of the blood in the lungs is equally ineiTectual for the prevention of death. 30. With the cessation of life, the everyday forces of the inorganic world no longer remain the servants of the bodily frame, as they were during life, but become its masters. Oxygen, the sweeper of the living organism, becomes the lord of the dead body. Atom by atom, the c 2 20 ELEMENTAR V PHYSIOL OGY. [less. complex molecules of the tissues are taken to pieces and reduced to simpler and more oxidized substances, until the soft parts are dissipated chiefly in the form of car- bonic acid, ammonia, water, and soluble salts, and the bones and teeth alone remain. But not even these dense and earthy structures are competent to offer a permanent resistance to water and air. Sooner or later the animal basis which holds together the earthy salts decomposes and dissolves — the solid structures become friable, and break down into powder. Finally, they dissolve and are diffused among the waters of the surface of the globe, just as the gaseous products of decomposition are dissi- pated through its atmosphere. It is impossible to follow, with any degree of certainty, wanderings more varied and more extensive than those imagined by the ancient sages who held the doctrine of transmigration ; but the chances are, that sooner or later, some, if not all, of the scattered atoms will be gathered into new forms of life. The sun's rays, acting through the vegetable world, build up some of the wandering molecules of carbonic acid, of water, of ammonia, and of salts, into the fabric of plants. The plants are devoured by animals, animals devour one another, man devours both plants and other animals ; and hence it is very possible that atoms which once formed an integral part of the busy brain of Julius Caesar may now enter into the composition of Csesar the negro in Alabama, and of Caesar the house-dog in an English homestead. And thus there is sober tmth in the words which Shake- speare puts into the mouth of Hamlet — " Imperial Csesar, dead and turned to clay. Might stop a hole to keep the cold away : Oh that that earth, which kept the world in awe, Should patch a wall, t' expel the winter's flasv !" Li.j THE CAPILLARIES. LESSON II. THE VASCULAR SYSTEM AND THE CIRCULATION. I. Almost all parts of the body are vascular; that is to say, they are traversed by minute and very close-set canals, which open into one another so as to constitute a small-meshed network, and confer upon these parts a spongy texture. The canals, or rather tubes, are pro- vided with distinct but very dehcate walls, composed of a structureless membrane (Fig. 3 A, a), in which at inter- vals small oval bodies (Fig. 3 A, b)^ termed fiuclti (see Lesson XII. § 2), are imbedded. These tubes are the capillaries. They vary in diameter from 0(700^^ ^*^ r/do^h of an inch ; they are sometimes disposed in loops, sometimes in long, sometimes in wide, sometimes in narrow meshes : and the diameters of these meshes, or, in other words, the interspaces between the capillaries, are sometimes hardly wider than the diameter of a capillar)', sometimes many times as wide (see Figs. 16, 20, 32, 33, and 37). These interspaces are occupied by the substance of the tissue which the capillaries per- meate (Fig. 3 A, c), so that the ultimate anatomical components of every part of the body are, strictly speak- ing, outside the vessels, or extra-vascular. But there are certain parts which, in another and broader sense, are also said to be extra-vascular or non- vascular. These are the epidermis and epithelium, the nails and hairs, the substance of the teeth, and the car- tilages ; which may and do attain a very considerable thickness or length, and yet contain no vessels. How- ever, as we have seen that all the tissues are really extra- 2^ ELEMENTA R Y PHYSIOL OGY. [less. vascular, these differ only in degree from the rest. The circumstance that all the tissues are outside the vessels by no means interferes with their being bathed by the fluid which is inside the vessels. In fact, the walls of the Fig. 3. A. Diagrammatic representation of a capillary seen from above and in section : a, the wall of the capillary with b, the nuclei ; f , nuclei belonging to the connective tissue in which the capillary is supposed to be lying ; d, the canal of the capillary. 15. niagrammatic representation of the structure of a small arter>- : a, epithelium ; b, the so-called basement membrane ; c, the circular non-striated muscular fibres, each with nucleus^; e, the coat of fibrous tissue with nuclei yi capillaries are so exceedingly thin that their fluid contents readily exude through the delicate membrane of which thev are composed, and irrigate the tissues in which thev lie.' 2. Of the capillary tubes thus described, one kind con- tains, during life, the red fluid, bloody while the others arc lilled with a pale, watery, or milky fluid, termed lymph, or c/iyle. The capillaries, which contain blood, are con- II. J THE MUSCLES OF THE VESSELS. 23 tinued on different sides into somewhat larger tubes, with thicker walls, which are the smallest arteries and veins. The mere fact that the walls of these vessels are thicker than those of the capillaries constitutes an important difference between the capillaries and the small arteries and veins ; for the walls of the latter are thus rendered far less permeable to fluids, and that thorough irrigation of the tissues, which is effegted by the capillaries, cannot be performed by them, /i/l <^ •'"•'- The most important difference between these vessels and the capillaries, however, lies in the circumstance that their walls are *not only thicker, but also more complex, being composed of several coats, one, at least, of which is muscular. The number, arrangement, and even nature of these coats differ according to the size of the vessels, and are not the same in the veins as in the arteries, though the smallest veins and arteries tend to resemble each other. If we take one of the smallest arteries, we find, first, a ver)- delicate ilning of cells constituting a sort of epi- thelium (Fig. 3 B, a). Outside this (separated from it by a structureless membrane, Fig. 3 B, b) comes the mus- cular coat of the kind called plain or non-striated muscle (see Lesson XII.), made up of flattened spindle-shape bands or fibres which are wrapped round the vessel (Fig. 3 B, c\ Outside the muscular coat is a sheath of fibrous or connective tissue (Fig. 3 B,/";. In the smallest arteries there is but a single layer of these muscular fibres encircling the vessel like a series of rings ; but in the larger arteries there are several layers of circular muscular fibres variously bound together w^ith fibrous and elastic tissue, though as the vessels get larger the c[uantity of muscular tissue in them gets relatively less. Now these plain muscular fibres possess that same power of contraction, or shortening in the long, and broadening in the narrow, directions which, as was stated in the preceding Lesson, is the special property of mus- cular tissue. And when they exercise this power, they, of course, narrow the calibre of the vessel, just as squeezing it in any other way would c'o ; and this con- 24 ELEMEXTARY PHYSIOLOGY. [less. traction may go so far as, in some cases, to reduce the cavity of the vessel almost to nothif^g, and to render it practically impervious. The state of contraction of these muscles of the small arteries and veins is regulated, like that of other muscles, by their nerves ; or, in other words, the nerves supplied to the vessels determine whether the passage through these tubes should be wide and free, or narrow and obstructed. Thus while the small arteries and veins lose the function, which the capillaries possess, of directly irrigating the tissues by transudation, they gain that of regulating the supply of tluid to the irrigators, or capillaries themselves. The contraction, or dilatation, of the arteries which supply a set of capillaries, comes to the same result as lowering or raising the sluice-gates of a system of irrigation-canals. 3. The smaller arteries and veins severally unite into, or are branches of, larger arterial or venous trunks, which again spring from or unite into still larger ones, and these, at length, communicate by a few principal arterial and venous trunks with the heart. The smallest arteries and veins, as we have seen, are similar in structure, but the larger arteries and veins differ widely ; for the larger arteries have walls so thick and stout that they do not sink together v/hen empty ; and this thickness and stoutness arises from the circumstance that not only is the muscular coat very thick, but that, in addition, and more especially, several layers of a highly elastic, strong, fibrous substance become mixed up with •the muscular layers. Thus, when a large arter}' is pulled out and let go, it stretches and returns to its primitive dimensions almost like a piece of india-rubber. The larger veins, on the other hand, contain but little of either elastic or muscular tissue. Hence, their walls are thin, and they collapse when empty. This is one great difference between the larger arteries and the veins ; the other is the presence of what are termed valves in a great many of the veins, especially in those which lie in muscular parts of the body. They are absent in the largest trunks, and in the smallest branches, and in all the divisions of the portal, pulmonary, and cerebral veins. 4. These valves are pouch-like folds of the inner wall II.] THE VALVES OF THE VEINS. 25 of the vein. The bottom of the pouch is turned towards those capillaries from which the vein springs. The free edge of the pouch is directed the other way, or towards the heart. The action of these pouches is to impede the passage of any fluid from the heart towards the capillaries, while they do not interfere with fluid passing in the oppo- site direction (Fig. 4). The working of some of these valves may be ver}^ easily demonstrated in the living body. When the arm is bared, blue veins may be seen running from the hand, under the skin, to the upper arm. The diameter of these veins is pretty even, and diminishes regularly towards the hand, so long as the current of the blood, which is running in them, from the hand to the upper arm, is uninterrupted. Fig. 4. — Diagrammatic Sections of Veins \vith Valves. In the upper, the blood is supposed to be flowing in the direction of the prrow, towards the heart ; in the lower, the reverse way. C, capillary side ; H, heart side. But if a finger be pressed upon the upper part of one of these veins, and then passed downwards along it, so as to drive the blood which it contains backwards, sundry swellings, like little knots, will suddenly make their ap- pearance at several points in the length of the vein, where nothing of the kind was visible before. These swellings are simply dilatations of the wall of the vein, caused by the pressure of the blood on that wall, above a valve which opposes its backward progress. The moment the backward impulse ceases the blood flows on again ; the valve, swinging back towards the wall of the vein, affords no obstacle to its progress, and the distension caused by its pressure disappears (Fig. 4). %\x\ /'/I ^(yiA^^ 26 ELEMENTARY PHYSIOLOGY. [less. The only arteries which possess valves are the primary trunks — the aorta and pulmonary artery — which spring from the heart, and they will be best considered with the latter organ. -_ ^. Besides the capillary network and the trunks connected with it, which constitute the blood-vascular system, all parts of the body which possess blood capillaries — except the brain and spinal cord, the eyeball, the gristles, tendons, and perhaps the bones ^ — also contain another set of what are termed lymphatic capilla- ries, mixed up with those of the blood- vascular system, but not directly com- municating with them, and, in addi- tion, differing from the blood capilla- ries in being connected with larger vessels of only one kind. That is to say, they open only into trunks \vhich carry fluid away from them, there being no large vessels which bring anything to them. These trunks further resemble the small veins in being abundantly pro-^ vided with valves which freely allo\\ri- of the passage of liquid from the lym- phatic capillaries, but obstruct the Fig. 5.— The Lympha- flow of anything the other way. But TICS OF THE Front OF the -lymphatic trunks differ from the THE Right Arm. . ^ . ^ ,^ . ,1 1 ^ -ji vems, in that they do not rapidly unite ^J^.X!;r'a.sth"ey\reMnto larger and larger trunks, which sometimes called. These present a continually increasing cali- guiglia^r^ not to be ^^j-g ^^^^ ^^^^^^ ^^ ^ flo^y without intcr- c )nfounded with ner- ' . , , y .\i-> ganglia. ruption to the heart. On the contrary, remaining nearly of the same size, they, at intervals, enter and ramify in rounded bodies called lymphatic glands., whence new lymphatic trunks arise (Fig. 5). In these glands the ' Tt is probable that the.se exceptions are apparent rather than real, bu the question is not yet satisfactorily decided. Y ., M II.] ^M' LYMPHATICS AXlD LAC TEALS. 27 The Thoracic Duct. The Thoracic Duct occupies the middle of the figure. It lies upon the spinal column, at the sides of which are seen portions of the ribs i). a, the receptacle of the chyle ; b. the trunk of the thoracic duct, opening: at c into the junction of the left jugular ' f" and subclavian (?■) veins as they unite into the left innominate vein, which has been cut across to show the thoracic duct nmning behind it ; d, lymphatic glands placed in the lumbar regions ; h, the superior vena cava formed by the junction of the right a^d left innominate veins. \ Y % 28 ELEMENTARY PHYSIOLOGY. [less. lymphatic capillaries and passages are closely interlaced with blood capillaries. Sooner or later, however, the great majority of the smaller lymphatic trunks pour their contents into a tube, which is about as large as a crow-quill, lies in front of the backbone, and is called the thoracic duct. This opens at the root of the neck into the conjoined trunks of the great veins which bring back the blood from the left side of the head and the left arm (Fig. 6). The remaining lym- phatics are connected by a common canal with the corre- sponding vein on the right side. Where the principal trunks of the lymphatic system open into the veins, valves are placed, which allow of the passage of fluid only from the lymphatic to the vein. Thus the lymphatic vessels are, as it were, a part of the venous system, though, by reason of these valves, the fluid which is contained in the veins cannot get into the lymphatics. On the other hand, every facility is afibrded for the passage into the veins of the fluid contained in the lymphatics. Indeed, in consequence of the numerous valves in the lymphatics, every pressure on, and contrac- tion of, their walls, not being able to send the fluid back- ward, must drive it more or less forward, towards the veins. 6. The lower part of the thoracic duct is dilated, and is termed the receptacle^ or cistern., of the chyle {a, Fig. 6). In fact, it receives the lymphatics of the intestines, which, though they differ in no essential respect from other lym- phatics, are called lacteals, because, after a meal con- taining much fatty matter, they are filled with a milky fluid, which is termed the chyle. The lacteals, or l}m- phatics of the small intestine, not only form networks in its walls, but send blind prolongations into the little velvety processes termed vilh\ wdth which the mucous membrane of that intestine is beset (see Lesson VI.). The trunks which open into the network lie in the viesentery (or membrane which suspends the small intestine to the back wall of the abdomen), and the glands through which these trunks lead are hence termed the mesenteric glands. 7. It will now be desirable to take a general view of the arrangement of all these different vessels, and of their ^. II.] THE VASCULAR SYSTEM. 29 relations to the great central organ of the vascular system the heart (Fig. 7). All the veins of every part of the body, except the lungs, the heart itself, and certain viscera of the abdomen, join together into larger veins, which, sooner or later, open into one of two great trunks (Fig. 7, l^.C.S. V.C.I.) termed the superior and the inferior ve/ia cava, which debouch into the upper, or broad end of the right half of the heart. All the arteries of every part of the body, except the lungs, are more or less remote branches of one great trunk — the aorta (Fig. 7, Ao.), which springs from the lower division of the left half of the heart. The arteries of the lungs are branches of a great trunk (Fig. 7, P. A.) springing from the lower division of the right side of the heart. The veins of the lungs, on the contrary, open by four trunks into the upper part of the left side of the heart (Fig. 7, P. V.). Thus the venous trunks open into the upper division of each half of the heart — those of the body in general into that of the right half ; those of the lungs into that of the left half : while the arterial trunks spring from the lower moieties of each half of the heart — that for the body in general from the left side, and that for the lungs from the right side. Hence it follows that the great artery of the body, and the great veins of the body, are connected with opposite sides of the heart ; and the great arter)- of the lungs and the great veins of the lungs also with opposite sides of ,hat organ. On the other hand, the veins of the body pen into the same side of the heart as the artery of the lungs, and the veins of the lungs open into the same side of the heart as the artery of the body. The arteries which open into the capillaries of the sub- stance of the heart are called coronary arteries, and arise, like the other arteries, from the aorta, but quite close to its origin, just beyond the semilunar valves. But the coronary vein, which is formed by the union of the small veins which arise from the capillaries of the heart, does not open into either of the venae cavas, but pours the blood which it contains directly into the division of the \ heart into which these cavce open — that is to say, into the %ight upper division (Fig. 14 b). so ELEMENT A R V PH YSIOL 0 G Y. [less. Fig. 7. — Diagram of the Heart and Vessels, with the Course of THE CiRCTLATION, VIEWED FROM BEHIND SO THAT THE PROPER LEFT OF THE Observer corresponds with the left side of the Heart IN the Diagram. L.A. left auricle \ LV. left ventricle ; Ao. aorta ; A^. arteries to the upper part of the body ; A^. arteries to the lower part of the body ; H.A. hepatic artery, which supplies the liver with part of its blood ; V^. veins of the upper c^ oCt^t' IT.] THE VASCULAR SYSTEM: 31 The abdominal viscera referred to above, the veins of which do not take the usual course, are the stomach, the intestines, the spleen, and the pancreas. These veins all combine into a single trunk, which is termed the vena ported (Fig. 7, V.P.), but this trunk does not open into the 7'ena cava infcrio)'. On the contrary, having reached the liver, it enters the substance of that organ, and breaks up into an immense multitude of capillaries, which ramify through the liver, and become connected with those into which the artery of the li\'er, called the hepatic artoy (Fig. 7, H.A.), branches. From this common capillary mesh- work veins arise, and unite, at length, into a single trunk, the hepatic vein (Fig. 7, H.V.), which emerges from the liver, and opens into the ijiferior vena cava. The portal vein is the only great vein in the body which branches out and becomes continuous with the capillaries of an organ, like an arteiy. 8. The heart (Figs. 8 and to), to which all the vessels in the body have now been directly or indirectly traced, IS an organ, the size of which is usually roughly estimated as equal to that of the closed fist of the person to whom it belongs, and which has a broad end turned upwards and backwards, and rather to the right side, called its base : and a pointed end which is called its apex^ turned downwards and forwards, and to the left side, so as to lie opposite the interval between the fifth and sixth ribs. It is lodged between the lungs, nearer the front than the back wall of the chest, and is enclosed in a sort of double bag— the pericardium , (Fig. 9, /.). One-half of the double bag is closely adherent to the heart itself, forming a thin coat upon its outer surface. At the base of the hear?, this half of the bag passes on to the great vessels which spring from, or open into, that organ ; and becomes con- tinuous with the other half, which loosely envelopes the heart and the adherent half of the bag. Between the two part of the hody : V^. veins of the lower part of the body ; V.P. vena portae ; H. V. hepatic vein ; I'.C.I. inferior vena cava ; V.C.S' superior vena cava; R.A. right auricle; R.V. right ven ride ; P. A. pulmonary artery; Lg-. lung; P.l^. pulmonary vein; Let. lacteals; Ly. lymphatics; T/i.'l). thoracic duct; Al. alimentary canal; Lr. liver. The arrows indicate the 'ourse of the blood, h-mph, and chyle. The vessels which contain arterial nlood have dark contours, while those which carry venous blood have li-'ht Contours. * ELEMENTAR V PHYSIOLOGY. [less. Tr Fig. 8. — Heart of Sheev. as seen after Removal from the Body, LYING UPON THE TwO Ll'NGS. ThE PeRICARDILM HAS BEEN CUT AWAY, BUT NO OTHER DISSECTION MADE. R.A. Auricular appendage of right auricle; L.A. auricular appendage of left auricle ; R. V. right ventricle ; L. V. left ventricle ; S. V.C. superior vena cava ; I. V.C. inferior vena cava; P. A. pulmonary artery ; Ao, aorta ; A'o', innominate branch from aorta dividing into subclavian and carotid arteries ; THE STRUCTURE OF THE HEART _J!: layers of the pericardium, consequently, there is a com- pletely closed, narrow cavity, lined by an epithelium, and secreting into its interior a small quantity of clear fluid, ^ The outer layer of the pericardium is tirmly connected ' elow with the upper surface of the diaphragm. But the heart cannot be said to depend altogether upon the diaphragm for support, inasmuch as the great vessels which issue from or enter it — and for the most part pass I W^' % ,// Fig. 9.— Transverse Section of the Chest, with the Heart and Lungs in Place. (A little diagrammatic j D.y. dorsal vertebra, or joint of the backbone; Ao. Ao . aorta, the top of its arch being cut away in this section; S.C. superior vena cava; P. A. pulmonary' artery-, divided into a branch for each lung; L.P. R.P. left and right pulmonary veins; Br. Bronchi; R.L.L.L. right and left lungs ; CE. the gullet or ossophagus ; p, outer bag of pericardium ; //, the two layers of pleura ; v, azygos vein. L. lung ; Tr. trachea, i, solid cord often present, the remnant of a once open communication between the pulmonary artery and aorta. 2, masses of fat at the bases of the ventricle hiding from view the greater part of the auricles. 3, line of fat marking the division between the two ventricles. 4, mass of fat covering end of trachea. * This fluid, like that contained in the peritoneum, pleura, and other shut sacs of a similar character to the pericardium, is sometimes called serum ; whence the membranes forming the wails of these sacs are frequently terme<.l strous nietnbranes, 34 ELEMENTAR V FHYSIOLOG Y. [less. upwards from its base — help to suspend and keep it in place. Thus the heart is coated, outside, by one layer of the pericardium. Inside, it contains two great cavities or " divisions," as they have been termed above, completely separated by a fixed partition which extends from the base to the apex of the heart ; and, consequently, having no direct communication with one another. Each of these two great cavities is further subdivided, not longitudinally 7?. U.M Fig, lo.— The Heart, Great Vessels, and Lungs. (Front View.) R.V. right ventricle; L.V. left ventricle; R.A. right auricle; Z./4. left auricle; Ao. aorta; P. A. pulmonary artery; P.V. pulmonary' veins; R.L. right lung; L.L. left lung; VS. vena cava superior; S.C. sub- clavian vessels; C. carotids; R.J.V. and L.J.V. right and left jugular veins ; V.I. vena cava inferior ; T. trachea ; B. bronchi. All the great vessels but those of the lungs are cut. but transversely, by a moveable partition. The cavity above the transverse partition, on each side, is called the auricle ; the cavity below, the vejitj'icle — right or left as the case may be. Each of the four cavities has the same capacity, and is capable of containing from 4 to 6 cubic inches of water. II.] THE STRUCTCRE OF THE HEART -rp Fig. -Right Side or the Heart of a Sheep. R.A. cavity of right auricle; S.V.C. superior vena cava, I.V.C. inferior vena cava ; ^'a style has been passed through each of these ;) a, a style parsed from the auricle to the ventricle through the auriculo-ventricular orifice ; h, a style passed into the coronary vein, A'. V. ca^•'ty of right ventricle ; tz', ti', two flaps of the tricuspid valve : the third is dimly seen behind them, the style a passing between the three. Between the two flaps, and attached to them by chorda" fendifiecp, is seen a papillary muscle, //, cut away from its attachment to that portion of the wall of the ventricle which has been removed. Above, the ventricle ter- minates somewhat like a fimnel in the pulmonary artery, P. A. One of the pockets of the semilunar valve, .^r-, is seen in its entirety', another partially. 1, the wall of the ventricle cut across ; 2, the position of the auriculo- ventricular ring ; 3, the wall of the auricle ; 4, masses of fat lodged between the auricle and pulmonary artery. 36 ELEMENTARY PHYSIOLOGY. [less. The avails of the auricles are much thinner than those of the ventricles. The wall of the left ventricle is much thicker than that of the right ventricle ; but no such difference is perceptible between the two auricles (Figs. II and 12, I and 3). 9. In fact, as we shall see, the ventricles have more work to do than the auricles, and the left ventricle more to do than the right. Hence the ventricles have more muscular substance than the auricles, and the left ventricle than the right ; and it is this excess of muscular substance \vhich gives rise to the excess of thickness observed in the left ventricle. The muscular fibres of the heart are not smooth, nu- cleated bands, like those of the vessels, but are bundles of transversely-striped fibres, and resemble those of the chief muscles of the body, except that they have no sheath, or sarcolemma, such as we shall find to exist in the latter. Almost the whole mass of the heart is made up of these muscular fibres, which have a very remarkable and complex arrangement. There is, however, an internal membranous and epithelial lining, called the endoca}-- diiun; and at the junction between the auricles and ventricles, the apertures of communication between their cavities, called the auriculo-^'oitriculav apertiurs, are strengthened by fibrous ri)igs. To these rings the move- able partitions, or valves., between the auricles and ventricles, the arrangement of which must next be con- sidered, are attached. 10. There are three of these partitions attached to the circumference of the right auriculo-ventricular aperture, and two to that of the left (Figs. 11, 12, 13, 14, tv, in 7>). Each is a broad, thin, but very tough and strong trian- gular fold of the endocardium, attached by its base, which joins on to its fellow, to the auriculo-ventricular fibrous ring ; and hanging with its point downwards into the ven- tricular cavity. On the right side there are, therefore, three of these broad, pointed membranes, whence the whole apparatus is called the tiicuspid valve. On the left side, there are but two, which, when detached from all their connexions but the auriculo-ventricular ring, look something like a bishop's mitre^ and hence bear the name of the mitral vah^e. II.] THE VALVES OF THE^ HEART. 37 e Fig. 12. — Left Side of the Heart of a Sheep (laid open). P. V. pulmonary' veins opening into the left auricle by four openings, as shown by the styles: a, a style passed from auricle into ventricle through the auriculoventricular orifice ; b, a style passed into the coronary vein, which, though it has no connection with the left auricle, is, from its position, necessarily cut across in thus laying open the auricle. M. V. the two flaps of the mitral valve drawn somewhat diagrammatically) : j>p, papillary muscles, belonging as before to the part of the ventricle cut away ; c, a style passed from ventricle in Ao. aorta ; Ao'^. branch of aorta (see Fig. 8, A'o); P. A. pulmonary artery ; S.V.C. superior vena cava. I, wail of ventricle cut across ; 2, wall of auricle cut away around auriculo- ventricular orifice ; 3, other portions of auricular wall cut across ; 4, mass of fat around base of ventricle (' much greater than that of the aorta. Hence the blood, or a corpuscle, for instance, of the blood being driven by the same force, viz. the heart's beat, over the whole body, must pass much more rapidly through the aorta than through the capillary system or any part of that system. It is not that the greater friction in any capillary causes the blood to flow more slowly there and there only. The resistance caused by the friction in the capillaries is thrown back upon the aorta, which indeed feels the resistance of the whole vascular system ; and it is this total resistance which has to be overcome by the heart before the blood can move on at all. The blood driven everywhere by the same force simply moves more and more slowly as it passes into wider and wider channels. When it is in the capillaries it is slowest ; after escaping from the capillaries, as the veins unite into 50 ELEMEXTARY PHYSIOLOGY. [less. larger and larger trunks, and hence as the total venous area is getting less and less, the blood moves again faster and faster for just the same reason that in the arteries it moved slower and slower. A very similar case is that of a river widening out in a plain into a lake and then contracting into a narrow stream again. The water is driven by one force throughout (that of gravity). The current is much slower in the lake than in the narrower river either before or behind. 21. It is now necessary to trace the exact course of the circulation as a whole. And we may conveniently commence with the portion of the blood contained at any moment in the right auricle. The contraction of the right auricle drives that fluid into the right ventricle ; the ven- tricle then contracts and forces it into the pulmonary artery ; from hence it passes into the capillaries of the lungs. Leaving these, it returns by the four pulmonary veins to the left auricle ; and the contraction of the left auricle drives it into the left ventricle. The systole of the left ventricle forces the blood into the aorta. The branches of the aorta convey it into all parts of the body except the lungs ; and from the capillaries of all these parts, except from those of the intestines and certain other viscera in the abdomen, it is conveyed, by vessels which gradually unite into larger and larger trunks, into either the superior or the inferior vena cava, which carry it to the right auricle once more. But the blood brought to the capillaries of the stomach and intestines, spleen and pancreas, is gathered into veins which unite into a single trunk — the 7'e?ia porta:. The vena portae distributes its blood to the liver, minghng with that supplied to the capillaries of the same organ by the hepatic artery. From these capillaries it is conveyed by small veins, which unite into a large trunk — the hepatic vein, which opens into the inferior vena cava. The flow of the blood from the abdominal viscera, through the liver, to the hepatic vein, is called the portal circulation. The heart itself is supplied with blood by the two coronary arteries which spring from the root of the aorta just above two of the semilunar valves. The blood from II.] COr/^SE OF THE CIRCUIATIOX. 51 the capillaries of the heart is carried back by the coronary vein, not to either vena cava, but to the right auricle. The opening of the coronary vein is protected by a valve, so as to prevent the right auricle from driving the venous blood which it contains back into the vessels of the heart. 22. Thus, the shojiest possible course which any particle of the blood can take in order to pass from one side of the heart to the other,1asto^leavejLhe aorta by one of thei coronary arteries, and relurn to the right auricle by the coronary vein. And in order to pass through the greatest ■possible 7iu7tiber of capillaries and return to the point from which it started, a particle of blood must leave the heart by the aorta and traverse the arteries which supply the alimentary canal, spleen, and pancreas. It then enters, istly, the capillaries of these organs ; 2ndly, the capillaries of the liver ; and, 3rdly. after passing through the right side of the heart, the capillaries of the lungs, from which \t returns to the left side and eventually to the aorta. Furthermore, from what has been said respecting the lymphatic system, it follows that any particle of matter which enters a lacteal of the intestine, will reach the right auricle by the superior cava, after passing through the lymph capillaries and channels of sundry lymphatic glands ; while anything which enters the adjacent blood capillary in the wall of the intestine will reach the right auricle by the inferior cava, after passing through the blood capillaries of the liver. 23. It has been shown above (§ 2) that the small arteries may be directly affected by the nervous system, which controls the state of contraction of their muscidar walls, and so regulates their calibre. The effect of tnis power of the nervous system is to give it a certain control over the circulation in particular spots, and to produce such a state of affairs that, although the force of the heart and the general condition of the vessels remain the same, the state of the circulation may be very dif- ferent in difterent localities. Blushino is a purely local modiftcation of the circu- lation of this kind, and it will be instructive to consider hf)w a blush is brought about.. An emotion — sometimes pleasurable, sometimes painful — takes possession of the E 3 52 ELEMENTARY PHYSIOLOGY. [less. mind : thereupon a hot flush is felt, the skin grows red, and according to the intensity of the emotion these changes are confined to the cheeks only, or extend to the " roots of the hair," or " all over." What is the cause of these changes ? The blood is a red and a hot fluid ; the skin reddens and grows hot, because its vessels contain an increased quantity of this red and hot fluid ; and its vessels contain more, because the small arteries suddenly dilate, the natural moderate contraction of their muscles being superseded by a state of relaxation. In other words, the action of the nerves which cause this muscular contraction is suspended. On the other hand, in many people, extreme terror causes the skin to grow cold, and the face to appear pale and pinched. Under these circumstances, in fact, the supply of blood to the skin is greatly diminished, in con- sequence of an excessive stimulation of the nerv^es of the small arteries, which causes them to contract and so to cut off the supply of blood more or less completely. 24. That this is the real state of the case may be proved experimentally upon rabbits. These animals may be made to blush artificially. If, in a rabbit, the sympathetic nerve which sends branches to the vessels of the head is cut, the ear of the rabbit, which is covered by so delicate an integument that the changes in its vessels can be readily perceived, at once blushes. That is to say, the vessels dilate, fill with blood, and the ear becomes red and hot. The reason of this is, that when the sympathetic is cut, the nervous stimulus which is ordinarily sent along its branches is interrupted, and the muscles of the small vessels, which were slightly contracted, become altogether relaxed. And now it is quite possible to produce pallor and cold in the rabbit's ear. To do this it is only necessary to irritate the cut end of the sympathetic which remains connected with the vessels. The nerve then becomes excited, so that the muscular fibres of the vessels are thrown into a violent state of contraction, which di- minishes their calibre so much that the blood can hardly make its way through them. Consequently, the ear becomes pale and cold. 25. The practical importance of this local control M.] CONTROL OVER THE HEART 53 exerted by the nervous system is immense. When ex- posure to cold gives a man catarrh, or inflammation of the lungs, or diarrhoea, or some still more serious aliection of the abdominal viscera, the disease is brought about through the nervous system. The impression made by the cold on the skin is conveyed to the nervous centres, and so influences the vaso-motoj' nerves (as the nerves which govern the walls of the vessels are called) of the organ affected as to cause their partial paralysis, and pro- duce that state of congestion (or undue distension of jhe vessels) which so commonly ends in inflammation. (See Lesson XI. ^ 15.) 26. Is the heart, in like manner, under the control of the central nervous system ? As we all know, it is not under the direct influence of the will, but everyone is no less familiar with the fact that the actions of the heart are wonderfully affected by all forms of emotion. Men and women often faint, and have sometimes been killed by sudden and violent joy or sorrow ; and when they faint or die in this way, they do so because the perturbation of the brain gives rise to a SDmething which arrests the heart as dead as you stop a stop-watch with a spring. On the other hand, other emo- tions cause that extreme rapidity and violence of action which we call palpitation. Now there are three sets of nerves in the heart : one set are supplied hy ganglia^ or masses of nerve-cells, in its substance; another set come from the sympathetic nerve ; a third set are branches of a remarkable nerve, which proceeds straight from the brain, and is called the 'pneunwgastric nerve. There is every reason to believe that the regular rhythmical succession of the ordinary contractions of the heart depends upon the ganglia lodged in its substance. At any rate, it is certain that these movements depend neither on the sympathetic, nor on the pneumogastric, since they go on as well when the heart is removed from the bodv. In the next place, there is much reason to believe that the influence which increases the rapidity of the heart's action is exerted through the sympathetic. And lastly, it is quite certain that th einfluence which arreits the heart's action is supplied by the pneumo- 54 ELEMENTA R Y PHYSIOL OGY [LESSi gastric.^ This may be demonstrated in animals, such ai; frogs, with great ease. 27. If a frog be pithed, or its brain destroyed, so as to Vi'.'r. is.^Portion of the web of a frog's foot seen under a low magnifying power, the blood-vessels only being represented except in the corner of the field, where in the portion marked off the pigment spots are also drawn. a. small arteries; 7'. small veins : the minute tubes joining the arteries of the veins are the capillaries. The arrows denote the direction of the circu'a- tion. The larger artery nmning straight up in the middle line breaks up into capillaries at points higher up than can be shown.in the drawing. II.] THE EITDEXCES OF THE CIRCULATIOX. 55 obliterate all sensibility, the animal will continue to live, and its circulation will go on perfectly well for an inde- finite period. The body may be laid open without causing pain or other disturbance, and then the heart will be observed beating with great regularity. It is possible to make the heart move a long index backwards and forwards ; and if frog and index are covered with a glass shade, the air under which is kept moist, the index will vibrate with great steadiness for a couple of days. It is easy to adjust to the frog thus prepared a contri- vance by which electrical shocks may be sent through the pneumogastric nerA-es, so as to irritate them. The moment this is done the index stops dead, and the heart will be found quiescent, with relaxed and distended walls. After a little time the influence of the pneumogastric passes off, the heart recommences its work as vigorously as before, and the index vibrates through the same arc as formerly. With careful management, this experiment may be repeated very many times ; and after every arrest by the irritation of the pneumogastric, the heart resumes its work. 28. The evidence that the blood circulates in man, al- though perfectly conclusive, is almost all indirect. The most important points in the evidence are as follows : — In the first place, the disposition and structure of the organs of circulation, and more especially the arrange- ment of the various valves, will not, as was shown by Harvey, permit the blood to flow in any other direction than in the one described above. Moreover, we can easily with a syringe inject a fluid from the vena cava, for instance, through the right side of the heart, the lungs, the left side of the heart, the arteries, and capillaries, back to the vena cava ; but not the other way. In the second place, we know that in the living body the blood is continually flowing in the arteries towards the capillaries, because when an arter>' is tied, in a living body, it swells up and pulsates on the side of the ligature nearest the heart, whereas on the other side it becomes empty, and the tissues supplied by the artery become pale from the want of a supply of blood to their capillaries. And when we cut an arterj- the blood is pumped cut in jerks from the cut end nearest the heart, whereas little or no blood 56 ELEMENTARY PHYSIOLOGY [less. Fig. i6. — Very small portion of Fig. 15 very highly magnified. A. walls of capillaries ; ^.tissue of web lying between the capillaries; C. cells of epidermis covering web (these are only shown in the right-hand II.] CIRC CLATION IN THE FROG'S WEB. 57 comes from the other end. When, however, we tie a vein the state of things is reversed, the swelhng taking place on the side farthest from the heart, &c. *S:c., showing that in the veins the blood flows from the capillaries to the heart. But certain of the lower animals, the whole, or parts, of the body of which are transparent, readily afford direct proof of the circulation, the blood visibly rushing from the arteries into the capillaries, and from the capillaries into the veins, so long as the animal is alive and its heart is at w^ork. The animal in which the circulation can be most conveniently observed is the frog. The web between its toes is very transparent, and the particles suspended in its blood are so large that they can be readily seen as they shp swiftly along with the stream of blood, when the toes are fastened out, and the intervening web is examined under even a low magnifying powder (Figs. 15 arid 16). and lower part of the field ; in the other parts of the field the focus of the microscope lies below the epidermis) ; D. nuclei of these epidermic cells ; E. pigment cells contracted, not partially expanded as in Fig. 15 ; F. red blood-corpuscle (oval in the frog) passing along capillary — nucleus not visible ; G. another corpuscle squeezing its wa>' through a capillary, the canal of which is smaller than its ewn transverse diameter ; H. another bending as it slides round a corner ; A', corpuscle in capillary seen through the epidermis ; /. white blood-corpuscle. 58 ELEMENTAR V PHYSIOL 0 G V. [less. LESSON III. T//£ BLOOD AND THE LYMPH. 1, In order to become properly acquainted with the characters of the blood it is necessary to examine it with a microscope magnifying at least three or four hundred diameters. Provided with this instrument, a hand lens, and some slips of thick and thin glass, the student will be enabled to follow the present Lesson. The most convenient mode of obtaining small quantities of blood for examination is to twist a piece of string, pretty tightly, round the middle of the last joint of the middle, or ring finger, of the left hand. The end of the finger will immediately swell a little, and become darker coloured, in consequence of the obstruction to the return of the blood in the veins caused by the ligature. When in this condition, if it be slightly pricked with a sharp clean needle (an operation which causes hardly any pain), a good-sized drop of blood will at once exude. Let it be deposited on one of the slips of thick glass, and covered lightly and gently with a piece of the thin glass, so as to spread it out evenly into a thin layer. Let a second slide receive another drop, and, to keep it from drying, let it be put under an inverted watch-glass or wine-glass, with a bit of wet blotting-paper inside. Let a third drop be dealt with in the same way, a few granules of common salt being first added to the drop. 2. To the naked eye the layer of blood upon the first slide will appear of a pale reddish colour, and quite clear and homogeneous. But on viewing it with even a pocket lens its apparent homogeneity will disappear, and it will III.] RED CORPUSCLES OF THE BLOOD. 59 look like a mixture of excessively fine yellowish-red par- ticles, like sand, or dust, with a water)-, almost colourless, fluid. Immediately after the blood is drawn, the particles will appear to be scattered \try evenly through the fluid, but by degrees they aggregate into minute patches, and the layer of blood becomes more or less spotty. The " particles " are what are termed the corpuscles of the blood ; the nearly colourless fluid in which they are suspended is iht plas?>ia. The second slide may now be examined. The drop of blood will be unaltered in form, and may perhaps seem to have undergone no change. But if the slide be inclined, it will be found that the drop no longer flows ; and, indeed, the shde may be inverted without the disturbance of the drop, which has become solidified, and may be removed, with the point of a penknife, as a gelatinous mass. The mass is quite soft and moist, so that this setting, or coagic- lation, of a drop of blood is something very different from its diying. On the third slide, this process of coagulation will be found not to have taken place, the blood remaining as fluid as it was when it left the body. The salt, therefore, has prevented the coagulation of the blood. Thus this ve-r}' simple investigation teaches that blood is composed of a nearly colourless plasma, in which many coloured corpuscles are suspended ; that it has a remarkable power of coagulating ; and that this coagulation may be pre- vented by artificial means, such as the addition of salt. 3. If, instead of using the hand lens, the drop of blood on the first slide be placed under the microscope, the par- ticles, or corpuscles, of the blood will be found to be bodies with very definite characters, and of two kinds, called respectively the red corpuscles and the colourless corpuscles. The former are much more numerous than the latter, and have a yellowish-red tinge ; while the latter, somewhat larger than the red corpuscles, are, as their name injplies, pale and devoid of coloration. 4. The corpuscles differ also in other and more important respects. The r^v/ corpuscles (Fig. 17) are flattened circular disks, on an average ^o^ooth of an inch in diameter, and having about one-fourth of that thickness. It follows that rather more than 10,000,000 of them will lie on a space 6o ELEMENTAR Y PHYSIOLOG Y [less. one inch square, and that the volume of each corpuscle does not exceed TodsroT^ozroo^h of a cubic inch. The broad faces" of the disks are not flat, but somewhat concave, as if they were pushed in towards one another. Hence the corpuscle is thinner in the middle than at the Fig. 17. — Red and White Coritscles of the Bi.ood magnified. A. Moderately magnified. The red corpuscles are seen lying in rouleaux ; at a and a are seen two white corpuscles. B. Red corpuscles much more highly magnified, seen in face ; C. ditto, seen in profile ; D. ditto, in rouleaux, rather more highly magnified ; E. a. r.ed corpuscle swollen into a sphere by imbibition of water. E. A white corpuscle magnified same as/.'.; C. ditto, throwing out some blunt processes ; A', ditto, treated with acetic acid, and showing nucleus magnified same as Z>. H. Red corpuscles puckered or crenate all over. /. Ditto, at the edge only. edges, and when viewed under the microscope, by trans- mitted light, looks clear in the middle and darker at the edges, or dark in the middle and clear at the III.] THE COLOURLESS CORPUSCLES. 6i edges, according to circumstances. When, on the other hand, the disks roll over and present their edges to the eye, they look like rods. All these varieties of appear- ance may be made intelligible by turning a round biscuit or muffin, bodies similar in shape to the red corpuscles, in various ways before the eye. The red corpuscles are very soft, flexible, and elastic bodies, so that they readily squeeze through apertures and passages narrower than their own diameters, and im- mediately resume their proper shapes (Fig. i6, G.H.). The exterior of each corpuscle is denser than its interior, which contains a semi-fluid, or quite fluid matter, of a red colour, called hcBmoglobin. By proper processes this may be resolved into an albuminous substance sometimes called globulin^ and a peculiar colouring matter, which is called hccmatin. The interior substance presents no distinct structure. From the density of the outer as compared with the inner substance of each corpuscle, they are, practically, small flattened bags, or sacs, the form of which may be changed by altering the density of the plasma. Thus, if it be made denser by dissolving saline substances, or sugar, in it, water is drawn from the contents of the cor- puscle to the dense plasma, and the corpuscle becomes still more flattened and very often much wrinkled. On the other hand, if the plasma be diluted with water, the latter forces itself into and dilutes the contents of the corpuscle, causing the latter to swell out, and even be- come spherical ; and, by adding dense and weak solutions alternately, the corpuscles may be made to become suc- cessively spheroidal and discoidal. Exposure to carbonic acid gas seems to cause the corpuscles to swell out ; oxygen gas, on the contrary, appears to flatten them. 5. The colourless corpuscles (Fig. i"], a a, F. G. K.) are larger than the red corpuscles, their average diameter being .).-^ooth of an inch. They are further seen, at a glance, to differ from the red corpuscles by the extreme irregularity of their form, and by their tendency to attach themselves to the glass shde, while the red corpuscles float about and tumble freely over one another. A still more remarkable feature of the colourless 62 ELEMENTARY PHYSIOLOGY. [less icbrpuscles than the irregularity of their form is the /unceasing variation of shape which they exhibit. The (I form of a red corpuscle is changed only by influences ^ from without, such as pressure, or the hke ; that of the colourless corpuscle is undergoing constant alteration, as the result of changes taking place in its own substance. To see these changes well, a microscope with a magni- fying power of five or six hundred diameters is requisite ; and, even then, they are so gradual that the best way to ascertain their existence is to make a drawing of a given colourless corpuscle at intervals of a minute or two. This is what has been done with the corpuscle represented in Fig. i8, in which a represents the form of the corpuscle when first observed ; b, its form a minute afterwards ; c, that at the end of the second ; d, that at the end of the third ; and e, that at the end of the fifth minute. Fig. 1 8. —Successive Forms assumed by Colourless Corpuscles of Human Blood. (M.ngnified about 6oo diameters.) The interval between the forms a,b,c,d, was a minute ; between ^and e two minutes ; so that the whole series of changes from ato e took five minutes. Careful watching of a colourless corpuscle, in fact, shows that every part of its surface is constantly chang- ing— undergoing active contraction, or being passively dilated by the contraction of other parts. It exhibits contractility in its lowest and most primitive form. 6. While they are thus living and active, no correct notion can be formed of the structure of the colourless corpuscles. By diluting the blood with water, or, still better, with w^ater acidulated with acetic acid, the cor- puscles are killed, and become distended, so that their real nature is shown. They are then seen to be sphe- roidal bags, or sacs, with very thin walls ; and to contain in their interior a fluid which is either clear or granular, together with a spheroidal vesicular body, which is called III.] DEVELOPMEXT OE THE CORPUSCLES. (>-^ the nucleus (Fig. 17, A'). It sometimes, though very rarely, happens that the nucleus has a red tint. The sac-like colourless corpuscle, with its nucleus, is what is called a nucleated cell. It will be observed that it lives in a free state in the plasma of the blood, and that it exhibits an independent contractility. In fact, except that it is dependent for the conditions of its exist- ence upon the plasma, it might be compared to one of those simple organisms which are met with in stagnant water, and are called AnicebcB. 7. That the red corpuscles are in some way or other derived from the colourless corpuscles may be regarded as certain : but the steps of the process have not been made out with perfect certainty. There is very great reason, however, for believing that the red corpuscle is simply the nucleus of the colourless corpuscle somewhat enlarged ; flattened from side to side ; changed, by de- velopment within its interior of a red colouring matter ; and set free by the bursting of the sac or wall of the colourless corpuscle. In other words, the red corpuscle is a free nucleus. The origin of the colourless corpuscles themselves is not certainly determined ; but it is highly probable that tliey are constituent cells of particular parts of the solid substance of the body which have been detached and carried into the blood, and that this process is chiefly eftected in what are called the ductless glands (Lesson V. § 27), from w^hence the detached cells pass, as lymph- corpuscles, directly or indirectly, into the blood. The following facts are of importance in their bearing on the relation between the different kinds of cor- puscles : — ia) The invertebrate animals,i which have true blood- corpuscles, possess only such as resemble the colourless corpuscles of man. ih) The lowest vertebrate animal, the Lancelet {Amphi- oxus), possesses only colourless corpuscles ; and the very young embryos- of all vertebrate animals have only colourless and nucleated corpuscles. * lovertebrate animals are animals devoid of backbones, such as insects, snails, sea-anemones, &c. Vertebrate animals are fishes, amphibia, reptiles, birds, and mammals. ^ An embryo is the rudimentary unborn youn^ of any creature. 64 ELEMENTARY PHYSIOLOGY. [less. (6-) All the vertebrated animals, the young of which are born from eggs,i have two kinds of corpuscles — colourless corpuscles, like those of man, and large red-coloured corpuscles, which are generally oval, and further differ from those of man in presenting a nucleus. In fact, they are simply the colourless corpuscles enlarged and coloured. {d) All animals which suckle their yQung (or what are called mammals) have, like man, two kinds of corpuscles ; colourless ones, and small coloured corpuscles — the latter being always flattened, and devoid of any nucleus. They are usually circular, but in the camel tribe they are ellip- tical. And it is worthy of remark that, in these animals, the nuclei of the colourless corpuscles become elliptical. {e) The colourless corpuscles differ much less from one another in size and form, in the vertebrate series, than the coloured. The latter are smallest in the little Musk Deer, in which animal they are about a quarter as large as those of a man. On the other hand, the red corpuscles are largest in the Amphibia (or Frogs and Salamanders), in some of which animals they are ten times as long as in man. 8. As the blood dies, its several constituents, which have now been described, undergo marked changes. The colourless corpuscles lose their contractility, but otherwise undergo little alteration. They tend to cohere neither with one another, nor with the red corpuscles, but adhere to the glass plate on which they are placed. It is quite otherwise with the red corpuscles^ which at first, as has been said, float about and roll, or slide, over each other quite freely. After a short time (the length of which varies in different persons, but usually amounts to two or three minutes), they seem, as it were, to become sticky, and tend to cohere ; and this tendency increases until, at length, the great majority of them become applied face to face, so as to form long series, like rolls of coin. The end of one roll cohering with the sides of another, a network of various degrees of closeness is produced (Fig. 17, A.). The corpuscles remain thus coherent for a certain length of time, but eventually separate and float freely " These are fibhes, amphibia, reptiles, and birds. ^ III.] BLOOD CRYSTALS. 65 again. The addition of a little water, or dilute acids or saline solutions, will at once cause the rolls to break up. It is from this running of the corpuscles together into patches of network that the change noted above in the appearances of the layer of blood, viewed with a lens, arises. So long as the corpuscles are separate, the sandy appearance lasts ; but when they run together, the layer appears patchy or spotted. The red corpuscles rarely, if ever, all run together into rolls, some always remaining free in the meshes of the net. In contact with air, or if subjected to pressure, many of the red corpuscles become covered with little knobs, so as to look like minute mulberries — an appearance which has been mistaken for a breaking up, or spontaneous divi- sion, of the corpuscles (Fig. 17, H.H.). 9. There is a still more remarkable change which the red blood-corpuscles occasionally undergo. Under certain circumstances, the peculiar red substance which forms the chief mass of their contents, and which has been called hcemoglobin (from its readily breaking up into globulin and hsematin, § 6), separates in a crystalHne form. In man, these crystals have the shape of prisms ; in other animals they take other forms. Human blood crystallizes with difficulty, but that of the guinea-pig, rat, or dog much more easily. The best way to see these blood- crystals is to take a little rat's blood, from which the fibrin has been removed, shake it up with a little ether, and let it stand in the cold for some hours. A sediment will form at the bottom, which, when examined with the microscope, will be found to consist of long narrow cr)^stals. Crystallization is much assisted by adding after the ether a small quantity of alcohol. 10. When the layer of blood has been drawn ten or fifteen minutes, the plasma will be seen to be no longer clear. It then exhibits multitudes of extremely delicate filaments of a substance called Fibrin, which have been deposited from it, and which traverse it in all directions, uniting with one another and with the corpuscles, and binding the whole into a semi-solid mass. It is this deposition of fibrin which is the cause of the apparent solidification, or coagulation, of the drop upon the second shde ; but the phenomena of coaguhi- F 66 ELEMENTAR Y PHYSIOLOGY. [less. tion, which are of very great importance, cannot be properly understood until the behaviour of the blood, when drawn in larger quantity than a drop, has been studied. 11. When, by the ordinary process of opening a vein with a lancet, a quantity of blood is collected into a basin, it is at first perfectly fluid : but in a very few minutes it becomes, through coagulation, a jelly-like mass, so solid that the basin may be turned upside down without any of the blood being spilt. At first the clot is a uniform red jelly, but very soon drops of a clear yellowish watery- looking fluid make their appearance on the surface of the clot, and on the sides of the basin. These drops increase in number, and run together, and after a while it has become apparent that the originally uniform jelly has separated into two very different constituents — the one a clear, yellowish liquid ; the other a red, semi-sohd mass, which lies in the liquid, and at the surface is paler in colour and firmer than in its deeper part. The liquid is called Xh^ serum; the semi-solid mass the clot, or crassamoituui. Now the clot obviously contains the corpuscles of the blood, bound together by some other substance ; and this last, if a small part of the clot be examined microscopically, will be found to be that fibrous-looking matter, fibrin^ which has been seen form- ing in the thin layer of blood. Thus the clot is equiva- lent to the corpuscles plus the fibrin of the plasma, while the serum is the plasma minus the fibrinous elements which it contained. 12. The corpuscles of the blood are slightly heavier than the plasma, and therefore, when the blood is drawn, they sink very slowly towards the bottom. Hence the upper part of the clot contains fewer corpuscles, and is lighter in colour, than the lower part — there being fewer corpuscles left in the upper layer of plasma for the fibrin to catch when it sets. And there are some con- ditions of the blood in which the corpuscles run together much more rapidly and in denser masses than usual. Hence they more readily overcome the resistance of the plasma to their falling, just as feathers stuck together in masses fall much more rapidly through the air than the same feathers when loose. When this is the case, the III.] THE COAGULATION OF THE BLOOD. 67 upper stratum of plasma is quite free from red corpuscles before the fibrin forms in it ; and, consequently, the uppermost layer of the clot is nearly ^vhite : it receives the name of the buffy coat. ^''After the clot is formed, the fibrin shrinks and squeezes out much of the serum contained within its meshes ; and, other things being equal, it contracts the more the fewer corpuscles there are in the way of its shrinking. Hence, when the buffy coat is formed, it usually contracts so much as to give the clot a cup-like upper surface. Thus the bufty coat is fibrin naturally separated from the red corpuscles ; the same separation may be effected, artificially, by whipping the blood with twigs as soon as it is drawn, until its coagulation is complete. Under these circumstances the fibrin will collect upon the twigs, and a red fluid will be left behind, consisting of the serum plus the red corpuscles, and many of the colourless ones. 13. The coagulation of the blood is hastened, retarded, or temporarily prevented by many circumstances. {li) Temperature. — A high temperature accelerates the coagulation of the blood ; a low one retards it very greatly ; and some experimenters have stated that, when kept at a sufficiently low temperature, it does not coagu- late at all. (p) The addition of soluble matter to the blood. — Many saline substances, and more especially sulphate of soda and common salt, dissolved in the blood in sufficient quantity, prevent its coagulation ; but coagulation sets in when water is added, so as to dilute the saline solution. iP) Contact with living or not livi/ig matter. — Contact with not living matter promotes the coagulation of the blood. Thus, blood drawn into a basin begins to coagu- late first where it is in contact with the sides of the basin ; and a wire introduced into a living vein will become coated with fibrin, although perfectly fluid blood surrounds it. On the other hand, direct contact with living matter retards, or altogether prevents, the coagulation of the blood. Thus blood remains fluid for a very long time in a portion of a vein which is tied at each end. The heart of a turtle remains alive for a lengthened period (many hours or even days) after it is extracted from F 2 68 ELEMENT A RY PH YSIOL OGY. [less. the body ; and, so long as it remains alive, the blood con- tained in it will not coagulate, though, if a portion of the same blood be removed from the heart, it \\ill coagulate in a few minutes. Blood taken from the body of the turtle, and kept from coagulating by cold for some time, may be poured into the separated, but still living, heart, and then will not coagulate. Freshly deposited fibrin acts somewhat like living matter, coagulable blood remaining fluid for a long time in tubes coated with such fibrin. 14. The coagulation of the blood is an altogether physico-chemical process, dependent upon the properties of certain of the constituents of the plasma, apart from the vitality of that fluid. This is proved by the fact that if blood-plasma be prevented from coagulating by cold, and greatly diluted, a current of carbonic acid passed through it will throw down a white powder>' substance. If this white substance be dissolved in a weak solution of common salt, or in an extremely weak solution of potash or soda, it, after a while, coagulates, and yields a clot of true pure fibrin. It would be absurd to suppose that a substance which has been precipitated from its solution, and redissolved, still remains alive. There are reasons for believing that this white sub- stance consists of two constituents of very similar com- position, which exist separately in living blood, and the union of which is the cause of the act of coagulation. These reasons may be briefly stated thus : — The peri- cardium and other serous cavities in the body contain a clear fluid, which has exuded from the blood-vessels, and contains the elements of the blood without the blood- corpuscles. This fluid sometimes coagulates spon- taneously, as the blood plasma would do, but very often shows no disposition to spontaneous coagulation. When this is the case, it may nevertheless be made to coagulate, and yield a true fibrinous clot, by adding to it a little serum of blood. Now, if serum of blood be largely diluted with water and a current of carbonic acid be gas passed through it, a \vhite powder}- substance will be thrown down ; this, redissolved in a dilute saline, or extremely dilute alkaline, III.] THE COAGULATION OF THE BLOOD. 69 solution will, when added to the pericardial fluid, produce even as good a clot as that obtained with the original serum. This white substance has been called globulin. It exists not only in serum, but also, though in smaller quantities, in connective tissue, in the cornea, in the humours of the eye, and in some other fluids of the body. It possesses the same general chemical properties as the albuminous substance which enters so largely into the composition of the red corpuscles (§ 4), and hence, at present, bears the same name. But when treated with chemical reagents, even with such as do not produce any appreciable eftect on its chemical composition, it very sp;;edily loses its peculiar power of causing serous fluids to coagulate. For instance, this power is destroyed by an excess of alkali, or by the presence of acids. Hence, though there is great reason to believe that the fibrino-plastic globulin (as it has been called) which exists in serum does really come from the red corpuscles, the globulin which is obtained in large quantities from these bodies, by the use of powerful reagents, has no coagu- lating effect at all on pericardial or other serous fluids. Though globulin is so susceptible of change when in solution, it may be dried at a low temperature and kept in the form of powder for many months, without losing its coagulating power. Thus ^/^^////;/, added, under proper conditions, to serous effusion, is a coagulator of that effusion, giving rise to the development of fibrin in it. It does so by its interaction with a substance contained in the serous effusion, which can be extracted by itself, and then plays just the same part towards a solution of globuhn, as 'globuhn does towards its solution. This substance has been called fibiinogen. It is exceedingly like globulin, and may be thrown down from serous exudation by carbonic acid, just as globulin may be precipitated from the serum of the blood. When redis- solved in an alkaline solution, and added to any fluid con- taining globulin, it acts as a coagulator of that fluid, and givee rise to the development of a clot of fibrin in it. In accordance with what has just been stated, serum of blood which has completely coagulated may be kept in one 70 ELEMEXTARY PHYSIOLOGY. [less. vessel, and pericardial fluid in another, for an indefinite period, if spontaneous decomposition be prevented, with- out the coagulation of either. But let them be mixed, and coagulation sets in. ' Thus it seems to be clear, that the coagulation of the blood, and the formation of fibrin, are caused primarily by the interaction of two substances (or two modifications of the same substance), globulin or fibrinoplastin and fibrinogen, the former of which may be obtained from the scrum of the blood, and from some tissues of the body ; while the latter is known, at present, only in the plasma of the blood, of the lymph, and of the chyle, and in fluids derived from them. 15. The proverb that ''blood is thicker than water" is literally true, as the blood is not only " thickened " by the corpuscles, of which it has been calculated that no fewer than 70,000,000,000 (eighty times the number of the human population of the globe) are contained in a cubic inch, but is rendered slightly viscid by the solid matters dis- solved in the plasma. The blood is thus rendered heavier than water, its specific gravity being about 1055. In other words, twenty cubic inches of blood have about the same weight as twenty-one cubic inches of water. The corpuscles are heavier than the plasma, and their volume is usually somewhat less than that of the plasma. Of colourless corpuscles there are usually not more than three or four for every thousand of red corpuscles ; but the number varies very much, increasing shortly after food is taken, and diminishing in the intervals between meals. The blood is hot, its temperature being about loo*^ Fahrenheit. 16. Considered chemically, the blood is an alkaline fluid, consisting of water, of solid and of gaseous matters. The proportions of these several constitutents vary according to age, sex, and condition, but the following statement holds good on the average : — In every 100 parts of the blood there are 79 parts of water and 21 parts of dry solids; in other words, the water and the solids of the blood stand to one another in about the same proportion as the nitrogen and the oxygen of the air. Roughly speaking, one Quarter of the blood III.] GASES OF THE BLOOD. .71 is dry, solid matter; three quarters water. Of the 21 parts of dry sohds, 12 (= fths) belong to the corpuscles. The remaining 9 are about two-thirds (67 parts = fths) albumin (a substance like white of egg, coagulating by heat), and one-third (= fth of the whole solid matter) a mixture of saline, fatty, and saccharine matters, sundry products of the waste of the body, and fibrin. The quantity of the latter constituent is remarkably small in relation to the conspicuous part it plays in the act of coagulation. Healthy blood, in fact, yields in coagulating not more than from two to four parts in a thousand of its weight of fibrin. The total quantity of gaseous matter contained in the blood is equal to rather less than half the volume of the blood ; that is to say, 100 cubic inches of blood will con- tain rather less than 50 cubic inches of gases. These gaseous matters are carbonic acid, oxygen, and nitrogen ; or, in other words, the same gases as those which exist in the atmosphere, but in totally different proportions ; for whereas air contains nearly three-fourths nitrogen, one- fourth oxygen, and a mere trace of carbonic acid, the average composition of the blood gases is nearly two- thirds carbonic acid, rather less than one-third oxygen, and not one-tenth nitrogen. It is important to observe that blood contains much more oxygen gas than could be held in solution by pure water at the same temperature and pressure. This power of holding oxygen appears in some way to depend upon the corpuscles, firstly, because mere serum has no greater power of absorbing oxygen than pure water has ; and secondly, because red corpuscles suspended in water instead of serum absorb oxygen very readily. The oxygen thus held by the red corpuscles is readily given up by them for purposes of oxidation, and indeed can be removed from them by means of a mercurial gas pump. It would appear that the connection between the oxygen and the red corpuscles is of a peculiar nature, being a sort of loose chemical combination with one of their con- stituents, that constituent being the haemoglobin ; for solutions of haemoglobin behave towards oxygen exactly as blood does. The corpuscles differ chemically from the plasma, in 7 2 ELEMENTAR Y PHYSIOL OGY. [less. containing a large proportion of the fats and phosphates, all the iron, and almost all the potash, of the blood ; while the plasma, on the other hand, contains by far the greater part of the chlorine and the soda. 17. The blood of adults contains a larger proportion of solid constituents than that of children, and that of men more than that of women ; but the difference of sex is hardly at all exhibited by persons of flabby, or what is called lymphatic, constitution. Animal diet tends to increase the quantity of the red corpuscles ; a vegetable diet and abstinence to diminish them. Bleeding exercises the same influence in a still more marked degree, the quantity of red corpuscles being diminished thereby in a much greater proportion than thit of the other solid constituents of the blood. 1 8. The total quantity of blood contained in the body varies at different times, and the precise ascertainment of its amount is very difficult. It may probably be esti- mated, on the average, at not less than one-thirteenth of the weight of the body. 19. The function of the blood is to supply nourishment to, and take away waste matters from, all parts of the body. It is absolutely essential to the life of every part of the body that it should be in such relation with a cur- rent of blood, that matters can pass freely from the blood to it, and from it to the blood, by transudation through the walls of the vessels in which the blood is contained. And this vivifying influence depends upon the corpuscles of the blood. The proof of these statements lies in the following experiments : — If the vessels of a limb of a living animal be tied in s.ich a manner as to cut off the supply of blood from the limb, without affecting it in any other way, all the symptoms of death will set in. The limb will grow pale and cold, it will lose its sensibility, and volition will no longer have power over it ; it will stiffen, and eventually mortify and decompose. But, even when the death stiffening has begun to set in, if the ligatures be removed, and the blood be allowed to flow into the limb, the stifl"ening speedily ceases, the tem- perature of ths part rises, the sensibility of the skin re- turns, the will regains power over the muscles, and, in short, the part returns to its normal condition. III.] THE LYMPH. 73 If, instead of simply allowing the blood of the animal operated upon to flow again, such blood, deprived of its fibrin by whipping, but containing its corpuscles, be arti- ficially passed through the vessels, it will be found as effectual a restorative as entire blood ; while, on the other hand, the serum (which is equivalent to whipped blood without its corpuscles) has no such effect It is not necessary that the blood tl as artificially in- jected should be that of the subject of the experiment. Men, or dogs, bled to apparent death, may be at once and effectually revived by fiihng their veins with blood taken from another man, or dog ; an operation which is known by the name of ti-aiisfusion. Nor is it absolutely necessary for the success of this operation that the blood used in transfusion should belong to an animal of the same species. The blood of a horse will permanently revive an ass, and, speaking generally, the blood of one animal may be replaced without injurious effects by that of another closely-allied species ; while that of a very different animal will be more or less in- jurious, and may even cause immediate death. 20. The Lymph, which fills the lymphatic vessels, is, like the blood, an alkaline fluid, consisting of a plasma and corpuscles, and coagulates by the separation of fibrin from the plasma. The lymph differs from the blood in its corpuscles being all of the colourless kind, and in the ver)' small proportion of its solid constituents, which amount to only about 5 per cent, of its weight. Lymph may, in fact, be regarded as blood viiiius its red cor- puscles, and diluted with water, so as to be somewhat less dense than the serum of blood, which contains about 8 per cent, of solid matters. A quantity of fluid equal to that of the blood is pro- bably poured into the blood, daily, from the lymphatic system. This fluid is in great measure the mere overflow of the blood itself — plasma which has exuded from the capillaries into the tissues, and which has not been taken up again into the venous current ; the rest is due to the absorption of chyle from the alimentary canal. 74* ELEMENTARY PHYSIOLOGY. [less. LESSON IV. RESPI RA TIOX. 1. The blood, the general nature and properties of which have been described in the preceding Lesson, is the highly complex product, not of any one organ or con- stituent of the body, but of all. Many of its features are doubtless given to it by its intrinsic and proper structural elements, the corpuscles ; but the general character of the blood is also profoundly affected by the circumstance that eveiy other part of the body takes something from the blood and pours something into it. The blood may be compared to a river, the nature of the contents of which is largely determined by that of the head waters, and by that of the animals which swim in it ; but which is also XQvy much affected by the soil over which it flows, by the water-weeds which cover its banks, and by affluents from distant regions ; by irrigation works which are supplied from it, and by drain-pipes which flow into it. 2. One of the most remarkable and important of the changes effected in the blood is that which results, in most parts of the body, from its simply passing through capillaries, or, in other words, through vessels the walls of which are thin enough to permit a free exchange be- tween the blood and the fluids which permeate the adja- cent tissues (Lesson IL § i). Thus, if blood be taken from the artery which supplies a limb, it will be found to have a bright scarlet colour ; while blood drawn, at the same time, from the vein of the limb, will be of a purplish hue, so dark that it is com- n-.] ARTERIAL AND VEXOUS BLOOD. 75 monly called '' black blood." And as this contrast is met with in the contents of the arteries and veins in general (except the pulmonary artery and veins), the scarlet blood is commonly known as arterial., and the black blood as venous. This conversion of arterial into venous blood takes place in most parts of the body, while life persists. Thus, if a limb be cut off and scarlet blood be forced into its arteries by a syringe, it will issue from the veins as black blood. 3. When specimens of venous and of arterial blood are subjected to chemical examination, the differences pre- sented by their solid and fluid constituents are found to be very small and inconstant. As a rule, there is rather more water in arterial blood, and rather more fatty matter. But the gaseous contents of the two kinds of blood differ widely in the proportion which the carbonic acid gas bears to the oxygen ; there being a smaller cjuantity of oxygen and a greater quantity of carbonic acid, in venous than in arterial blood. And it may be experimentally demonstrated that this difference in their gaseous contents is the only essential difference between venous and arterial blood. For if A'enous blood be shaken up with oxygen, or even with air, it gains oxygen, loses carbonic acid, and takes on the colour and properties of arterial blood. Similarly, if arterial blood be treated with carbonic acid so as to be thoroughly saturated with that gas, it gains carbonic acid, loses oxygen, and acquires the true properties of venous blood ; though, for a reason to be mentioned below, the change is not so complete in this case as in the former. The same result is attained, though more slowly, if the blood, in either case, be received into a bladder, and then placed in the carbonic acid, or oxygen gas ; the thin moist animal membrane allowing the change to be effected with perfect ease, and offering no serious impediment to the passage of either gas. 4. The physico-chemical processes involved in the exchange of carbonic acid for oxygen when venous is converted into arterial blood, or the reverse, in the cases mentioned above, are not thoroughly understood, and are probably somewhat complex. 76 ELEMENTARY PHYSIOLOGY. [l£sS. It is known {a) that gases, mechanically held by a fluid in a given proportion, lend to diffuse into any atmosphere to which they are exposed, until they occupy that atmo- sphere in corresponding proportions ; and (b) that gases separated by a dry porous partition, or simply in contact, diffuse into one another with a rapidity which is inversely proportioned to the square roots of their densities. A knowledge of these physical principles does, in a rough way, lead us to see how the gases contained in the blood may effect an exchange with those in the air, whether the blood be freely exposed, or enclosed in a membrane. liut the application of these principles gives no more than this sort of general insight. For, in the first place, when arterialization takes place through the walls of a bladder, or any other thin animal membrane, the matter is complicated by the circumstance that moisture dissolves carbonic acid far more freely than it will oxygen ; hence a wet bladder has a very different action upon carbonic acid from that which it has upon oxygen. A moist bladder, partially filled with oxygen, and suspended in carbonic acid gas, becomes rapidly distended, in con- sequence of the carbonic acid gas passing into it with much greater rapidity than the oxygen passes out. Secondly, the gases of the blood are not held in a merely mechanical way in it ; the oxygen seems to be loosely combined with the red corpuscles (Lesson III. § 1 6), and there is reason to think that a great part, at least, of the carbonic acid, is chemically connected, in a similarly loose way, with certain saline constituents of the serum. Hence the arterialization of blood in the lungs seems to be a very mixed process, partly physical, and yet, to a certain extent chemical, and consequently very difficult to analyse. The same may also be said of the change from arterial to venous blood in the tissues. Owing to the peculiar relation of oxygen to the red blood-corpuscles, the process which takes place in the tissues is not a simple inter- change by diffusion of the oxygen of the blood for the carbonic acid of the tissues ; on the contrary, the oxygen is given up for purposes of oxidation, the demand being determined by the supply of oxidizable materials in the tissue, while the blood, poor in carbonic acid, takes up, IV.] ARTERIAL A.VD VENOUS BLOOD. 77 apparently by an independent action, a quantity of that gas from the tissues ricli in it. Hence venous blood is characterized not only by the large amount of carbonic acid present, but also by the fact that the red corpuscles have given up a good deal of their oxygen for the purposes, of oxidation, or, as the chemists would say, have become reduced. This is the reason why arterial blood is not so easily converted into venous blood by exposure to carbonic acid as venous blood into arterial by exposure to oxygen. There is, in the former case, a want of some oxidizable substance to carry off the oxygen from and so to reduce the red corpus- cles. When such an oxidizable substance is added (as, for instance, a salt of iron), the blood at once and imme- diately becomes completely venous. Practically we may say that the most important differ- ence between venous and arterial blood is not so much the relative quantities of carbonic acid as that the red cor- puscles of venous blood have lost a good deal of oxygen, are reduced, and ready at once to take up any oxygen /offered to them. 5. The cause of the change of colour of the blood— ot its darkening when exposed to carbonic acid, and its brightening when under the influence of oxygen — is not thoroughly understood. There is reason to think, how- ever, that the red corpuscles are rendered somewhat flatter by oxygen gas, while they are distended by the action of carbonic acid (Lesson III. § 4). Under the former circumstances they may, not improbably, reflect the light more strongly, so as to give a more distinct coloration to the blood ; while, under the latter, they may reflect less light, and, in that way, albw the blood to appear darker and duller. This, however, is not the whole of the matter; for solutions of haemoglobin or of blood-crystals (Lesson IIL § 9), even when perfectly free from actual blood-cor- puscles, change in colour from scarlet to purple, accord- ing as they gain or lose oxygen. It has already been stated (Lesson III. § 16) that oxygen most probably exists in the blood in loose combination with haemo- globin. But, further, there is evidence to show that a solution of haemoglobin, when thus loosely combined with 78 ELEMENTARY PHYSIOLOGY. [less. oxygen^ has a scarlet colour, while a solution of haemo- globin, deprived of oxygen, has a purplish hue. Hence arterial blood, in which the haemoglobin is richly pro- vided with oxygen, would naturally be scarlet, while venous blood, which not only contains an excess of car- bonic acid, but whose haemoglobin also has lost a great deal of its oxygen, would be purple. 6. Whatever may be their explanation, however, the facts are certain (i), that arterial blood, separated by only a thin membrane from carbonic acid, or from a fluid containing a greater amount of carbonic acid than itself, and also carrying certain oxidizable materials, becomes venous ; and (2; that venous blood, separated by only a thin membrane from oxygen, or a fluid containing a greater proportion of free oxygen than itself, becomes arterial. In these facts lies the explanation of the conversion of scarlet blood into dark blood as it passes through the capillaries of the body, for the latter are bathed by the juices of the tissues, which contain carbonic acid, the product of their waste and combustion, in excess, to- gether with highly oxidizable matters. On the other hand, if we seek for the explanation of the conversion of the dark blood in the veins into the scarlet blood of the arteries, we find, ist, that the blood remains dark in the right auricle, the right ventricle, and the pulmonary artery ; 2nd, that it is scarlet not only in the aorta, but in the left ventricle, the left auricle, and the pulmonary veins. Obviously, then, the change from venous to arterial takes place in the pulmonary capillaries, for these are the sole channels of communication between the pulmonary arteries and the pulmonary- veins. 7. But what are the physical conditions to which the blood is exposed in the pulmonary capillaries ? These vessels are very wide, thin walled, and closely set, so as to form a network with very small meshes, which is contained in the substance of an extremely thin mem- brane. This membrane is in contact with the air, so that the blood in each capillary of the lung is separated from the air by only a delicate pellicle formed by its own wall and the lung membrane. Hence an exchange very readily takes place between the blood and the air; the latter IV.] THE AIR-PASSAGES. 79 gaining moisture and carbonic acid, and losing oxygen (Lesson I. §§ 23, 24). ^ This is the essential step in respiration : that it really takes place may be demonstrated very readily, by the experiment described in the first Lesson (§ 3), in which air expired was proved to differ from air inspired, by con- taining more heat, more water, more carbonic acid, and less oxygen ; or, on the other hand, by putting a ligature on the windpipe of a living animal so as to prevent air from passing into, or out of, the lungs, and then examin- ing the contents of the heart and great vessels. The blood on both sides of the heart, and in the pulmonary veins and aorta, will be found to be as completely venous as in the venae cavae and pulmonary artery. But though the passage of carbonic acid gas and hot watery vapour out of the blood and of oxygen into it is the essence of the respirator}^ process — and thus a membrane with blood on one side, and air on the other, is all that is absolutely necessary to effect the purification of the blood — yet the accumulation of carbonic acid is so rapid, and the need for oxygen so incessant, in all parts of the human body, that the former could not be cleared away, nor the latter supplied, with adequate rapidity, without the aid of extensive and complicated accessory machiner}' — the arrangement and working of which must next be carefully studied. 8. The back of the mouth or pharytix communicates by two channels with the external air (see Fig. 40). One of these is formed by the nasal passages, which cannot be closed by any muscular apparatus of their own ; the other is presented by the mouth, which can be shut or opened at will. Immediately behind the tongue, at the lower and front part of the pharynx, is an aperture — the glottis (Fig. ^ 19, 67.) — capable of being closed by a sort of lid — the 'z} <^piglottis — or by the shutting together of its side bound- liWt aries, formed by the so-called vocal choi'ds. The glottis ^ The student must guard himself against the idea that arterial blood con- tains no carbonic acid, and venous blood no oxygen. In passing through the lungs venous blood loses only a part of its carbonic acid ; and arterial blood, in passmg through the tissues, loses only a part of its oxygen. In blood, however venous, there is in health always some oxygen ; and in even the brightest arterial blood there is actually more carbonic acid i.xxw. oxjgen. 8o ELEMENTAR V PHYSIOLOGY. [less. opens into a chamber with cartilaginous walls — the larynx; and leading from the larynx downwards along the front part of the throat, where it may be very readily felt, is the trachea, or windpipe (Fig. 19, Tr.). Fig. 19, — Back View of the Neck and Thorax of a Human Subject FROM WHICH the VeRTEBRAL CoLUMN AND WHOLE POSTERIOR WaLL OF THE Chest are supposed to be removed. M. mouth; Gl. glottis; Tr. trachea ; L.L. left lung; R.L. right lung ; Br. bronchus; P.A. pulmonary artery; P.V. pulmonary veins; Ao. aoTtn ; D. diaphragm ; H. heart ; '/. C.I. vena cava inferior. If the trachea be handled through the skin, it will be found to be firm and resisting. Its walls are, in fact, strengthened by a series of cartilaginous hoops, which hoops are incomplete behind, their ends being united only by muscle and membrane, where the trachea comes into contact with the gullet, or cesophagus. The trachea passes into the thorax, and there divides into two branches, a right and a left, which are termed the bro7ichi (Fig. 19, Br). Each bronchus enters the lung of its own side, ^ IV.] THE AIR-CELLS. 8l and then breaks up into a great number of smaller branches, which are called the bronchial tubes. As these diminish in size, the cartilages, which are continued all through the bronchi and their large ramifications, become smaller and eventually disappear, so that the walls of the smallest bronchial tubes are entirely muscular or mem- branous. Thus while the trachea and bronchi are kept permanently open and per\'ious to air by their cartilages, the smaller bronchial tubes may be almost closed by the ontraction of their muscular walls. The finer bronchial tubes end at length in elongated dilatations, about ^^h of an inch in diameter on the average (Fig. 20, A)., Each of these dilatations is beset with, or perhaps rather is made up of, little sacs, which open irre- gularly into the cavity of the dilatation. These sacs are the air-cells. The very- thin walls ^Fig. 20, B) which separate these air-cells are supported by much delicate and highly elastic tissue, and carry the wide and close-set capillaries into which the ultimate ramifications' of the pulmonary arter\- pour its blood (Fig. 20, D). Thus, the blood contained in these capillaries is exposed on both sid.is to the air — being separated from the air-cell on either hand only by the very delicate pellicle which forms the wall of the cap'illar)-, and the lining of the air-sac. 9. Hence no conditions can be more favourable to a ready exchange between the gaseous contents of the blood and those of the air in the air-cells, than the arrangements which obtain in the pulmonary capillaries ; and, thus far, the structure of the lung fully enables us to understand how it is that the large quantity of blood poured through the pulmonary circulation becomes exposed in very thin streams, over a large surface, to the air. But the only result of this arrangement would be, that the pulmonary air would very speedily lose all its oxygen, and become completely saturated with carbonic acid, if special pro- vision were not made for its being incessantly renewed. 10. If an adult man, breathing calmly in the sitting position, be watched, the respiratory act will be observed to be repeated thirteen to fifteen times every minute. Each act consists of certain components which succeed one another in a regular rhythmical order. First, the bieath is drawn in, or inspired; immediately afterwards G 82 ELEMENTAR Y PHYSIO LOG 3 '. [less. it is driven out, or expired; and these successive acts of inspii-ation and expiration are followed by a brief pause. Thus, just as in the rhythm of the heart the auricular systole, the ventricular systole, and then a pause follow in ■33^'-' Fig. 20. A. Two air-cells {b) with the ultimate bronchial tub8 [a) which opens into them. (IMagnified 20 diameters.) B. Diagrammatic view of an air-cell of A seen in section : a, epithelium ; b, partition between two adjacent ce'.ls, in the thickness of which the capillaries run ; c, fibres of elastic tissue. C. Portion of injected lung magnified : a, the capillaries spread over the walls of two adjacent air-cells ; b, small branches of arteries and veins, D. Portion still more highly magnified. IV.] INSPIRED AND EXPIRED AIR. %2> regular order ; so in the chest, the inspiration, the expi- ration, and then a pause succeed one another. At each inspiration of an adult well-grown man about thirty cubic inches of air are inspired ; and at each expiration the same, or a slightly smaller, volume (allowing for the in- crease of temperature of the air so expired) is given out of the body. 11. The expired air differs from the air inspired in the following particulars : — {a) Whatever the temperature of the external air is, that expired is nearly as hot as the blood, or has a temperature between 98^ and 100''. {b) However dry the external air may be, that expired is quite, or nearly, saturated with watery vapour. {c) Though ordinary air contains nearly 2,100 parts of oxygen, and 7,900 of nitrogen, with not more than 3 parts of carbonic acid, in 10,000 parts, expired air contains about 470 parts of carbonic acid, and only between 1,500 and 1,600 parts of oxygen ; while the quantity of nitrogen suffers little or no change. Speaking roughly, air which has been breathed once has gained five per cent, of carbonic acid, and lost five per cent, of oxygen. The expired air contains, in addition, a greater or less quantity of animal matter of a highly decomposable character. {d) Very close analysis of the expired air shows, firstly, that the quantity of oxygen which disappears is always slightly in excess of the quantity of carbonic acid sup-- plied ; and secondly, that the nitrogen is variable — the expired nitrogen being sometimes slightly in excess of, sometimes slightly less than that inspired, and sometimes remaining stationary. 12. From three hundred and fifty to four hundred cubic feet of air are thus passed through the lungs of an adult man taking little" or no exercise, in the course of twenty- four hours ; and are charged with carbonic acid, and deprived of oxygen, to the extent of nearly five per cent. This amounts to about eighteen cubic feet of the one gas taken in, and of the other given out. Thus, if a man be shut up in a close room, having the form of a cube seven feet in the side, every particle of air in that room will have passed through his lungs in twenty-four hours, and G 2 §4 ELEMENTARY PHYSIOLOGY. [less. a fourth of the oxygen it contained will be replaced by carbonic acid. The quantity of carbon eliminated in the twenty-four hours is pretty nearly represented by a piece of pure char- coal weighing eight ounces. The quantity of water given off from the lungs in the twenty-four hours varies very much, but may be taken on the average as rather less than half a pint, or about nine ounces. It may fall below this amount, or increase to double or treble the quantity. 13. The mechanical arrangements by which the respi- ratory movements, essential to the removal of the great mass of effete matters, and the importation of the large quantity of oxygen indicated, are effected, may be found in — {a) the elasticity of the lungs ; {b) the<*1nobility of the sides and bottom of the thoracic cavity in which the lungs are contained. The thorax may be regarded as a completely shut coni- cal box, with the small end turned upwards, the back of the box being formed by the spinal column, the sides by the ribs, the front by the breast-bone, the bottom by the diaphragm, and the top by the root of the neck (Fig. 19). The two lungs occupy almost all the cavity of this box which is not taken up by the heart. Each is enclosed in its serous membrane, Xhtplcin-a, a double bag (very simi- lar to the pericardium, the chief difference being that the outer bag of each pleura is, over the greater part of its ex- tent, quite firmly adherent to the walls of the chest and the diaphragm (see Fig. 9), while the outer bag of the peri- cardium is for the most part loose), the inner bag closely covering the lung and the outer forming a lining to the cavity of the chest. So long as the walls of the thorax are entire, the cavity of each pleura is practically oblite- rated, that layer of the pleura which covers the lung being in close contact with that w^hich lines the wall of the chest ; but if a small opening be made -into the pleura, the lung at once shrinks to a comparatively small size, and thus develops a great cavity between the two layers of the pleura. If a pipe be now fitted into the bronchus, and air blown through it, the lung is very readily dis- tended to its full size ; but, on being left to itself, it col- lapses, the air being driven out again with some force. >- IT. THE ELASTICITY OE THE LUXGS. 8: The abundant elastic tissues of the walls of the air-cells are, in fact, so disposed as to be greatly stretched when the lungs are full ; and, Vv-hen the cause of the distension is removed, this elasticity comes into play and drives the greater part of the ?,ir out again. The lungs are kept distended in the dead subject, so long as the walls of the chest are entire, by the pressure Fig. -View of Four Ribs of the Dog with the Intercostal ]\IrSCLES. a. The bony rib ; /', the cartilage ; r, the junction of bone and cartilage ; d, unossified, e, ossified, portions of the sternum. A. External intercostal muscle. B. Internal intercostal muscle. In the middle interspace, the external intercoi-tal has been removed to show the internal intercostal beneath it. of the atmosphere. For though the elastic tissue is all the while pulling, as it were, at the layer of pleura which covers the lung, and attempting to separate it from that which lines the chest, it cannot produce such a separation without developing a vacuum between these two layers. 86 ELEMENTARY PHYSIOLOGY. ^ [less. To effect this, the elastic tissue must pull with a force .^:reater than that of the external air (or fifteen pounds to the square inch), an effort far beyond its powers, which do not equal more than one-fourth of a pound on the square inch. But the moment a hole is made in the pleura, the air enters into its cavity, the atmospheric pres- sure inside the lung is equalized by that outside it, and the elastic tissue, freed from its opponent, exerts its full power on the lung. 14. The lungs are elastic, whether alive or dead. During life the air which they contain may be further affected by the contractihty of the muscular walls of the bronchial tubes. If water is poured into the lungs of a recently- killed animal, and a series of electric shocks is then sent through the bronchial tubes, the latter contract, and the water is forced out. Lastly, during life a further source of motion in the bronchial tubes is provided by the cilia — minute filaments attached to the epithelium of the tubes, which incessantly vibrate backwards and forwards, and work in such a manner as to sweep liquid and solid matters outwards, or towards the trachea. 15. The ribs are attached to the spine, so as to be freely moveable upon it ; but when left to themselves they take a position which is inclined obliquely downwards and forwards.^ Two sets of muscles, called i?iiercostals, pass between the successive pairs of ribs on each side. The outer set, c^LWedexiernal intercostals (Fig. 2\^A), run from the rib above, obliquely downwards and forwards, to the rib below. The other set, internal intoxostals (Fig. 21, B)^ cross these in direction, passing from the rib above, down- wards and backwards, to the rib below. The action of these muscles is somewhat puzzling at first, but is readily understood if the fact that when a muscle contracts, it tends to inake the distance betwee7i its two ends as short as possible, be borne in mind. Let a and b in Fig. 23, A, be two parallel bars, moveable by their ends upon the upright c, which may be regarded as at the back of the apparatus, then a line directed from x I I purposely neglect the consideration of the cartilages of the ribs, and some other points, in order not to complicate the question unnecessarily. It may, however, be stated that those fibres of the internal intercostals which are situate between the cartilages act like the external, and raise the ribs. IV.] THE INTERCOSTAL MUSCLES. «7 to y will be inclined downwards and forwards, and one from w\.o z will be directed downwards and backwards. Now It is obvious that there is one position of the rods, and one only, in which the points .r and y are at the shortest possible distance, and one position only in which the points lu and 2 are at the shortest possible distance ; and these are, for x andj the position B, and for -.u and z a w sr I =A Fig. 22. — Diagram of Models illlstrating the Action of the External and Internal Intercostal jNIcscles. B, inspirator^' elevation ; C, expiratory depression. the position C. These positions are respectively such that the points x, j, and ■:<:', 5", are at the ends of straight lines perpendicular to both rods. Thus, to bring x and y into this position, the parallel rods in A must move upwards ; and to bring iu ands' into it, they must move in the opposite way. If the simple apparatus just described be made of wood, hooks being placed at the points .r, j', and ii\ z ; and an elastic band, as long when left to itself as the shortest dis- tance between these points, be provided with eyes which can be readily put on to or taken off these hooks : it will be found that when the bars are in the horizontal position, A, the elasticity of the band, when hooked on to x and y, will bring them up into the position shown in Fig. 22, B ; while, if hooked on to zu and z, it will force them down into the position shown in Fig. 22, C. 88 ELEMENTARY PHYSIOLOGY. [less. Substitute the contractility of the external and internal intercostal muscles for the elasticity of the band, and the latter will precisely exemplify their action ; and it is thus proved that the external intercostals must raise, and the internal intercostals must depress, the bony ribs. i 1 ri Fig. 23— The Diaphragm of a Dog viewed from the Lower or Ardiminal Side. r.C./. the vena cava inferior ; O. the ce^ophagus ; Ao. the aorta; the l)road white tendinous middle {B) is easily distinguished from the radiating muscular fibres {A) which pass down to the ribs and into the pillars (C D) in front of the vertebrae. 16. The diaphragm is a great partition situated between the thorax and the abdomen, and always concave to the latter and convex to the former (Fig. i, D). From its middle, which is tendinous, muscular fibres extend down- wards and outwards to the ribs, and two, especially IV.] IXSP7RATI0X AXD EXr/KATIOX. 89 strong masses, which are called the piUais of the dia- pJiragDi, to the spinal column (Fig. 23). ^^'hen these muscular fibres contract, therefore, they tend to make the diaphragm flatter, and to increase the capacity of the thorax at the expense of that of the abdomen, by pulling down the bottom of the thoracic box (Fig. 24, A). 17, Let us now consider what would be the result of the action of the parts of the respiratory apparatus, which liave been described, if the diaphragm alone should begin to contract at regular intervals. When it contracts it increases the vertical dimensions of the thoracic cavity, and tends to pull away the lining of the bottom of the thoracic box from that which covers the bases of the lungs ; but the air immediately rushing in at the trachea, proportionately increases the distension of the lungs, and prevents the formation of any Aacuum be- tween the two pleural of either lung in this region. When the diaphragm ceases to contract, so much of the elasticity of the lungs as was neutralized by the contraction of the diaphragm, comes into play, and the extra air taken in is driven out again. We ha^e, in short, an Inspiration and an Expiration. Suppose on the Other hand that, the diaphragm being quiescent, the external intercostal muscles contract. The ribs will be raised from their oblique position, the antero- posterior dimensions of the thoracic cavity will be in- creased, and the lungs will be distended as before to balance the enlargement. If now the external intercostals relax, the action of gravity upon the ribs, the elasticity of the cartilages and more especially that of the lungs, will alone suffice to bring back the ribs to their previous posi- tions and to drive out the extra air ; but this expiratory action may be greatly aided by the contraction of the in- ternal intercostals. 18, Thus it appears that we may have either diaphrag- Diatic respiration, or eostal respiratioji. As a general rule, however, not only do the two fonns of respiration coincide and aid one another — the contraction of the diaphragm taking place at the same time with that of the external intercostals, and its relaxation with the contraction of the internal intercostals — but sundr}' other^'ccessory agencies come into play. Thus, the muscles which connect the 90 ELEMENTARY PHYSIOLOGY. [less. ribs with parts of the spine above them, and with the shoulder, may, more or less extensively, assist inspiration ; while those which connect the ribs and breastbone with the pelvis, and form the front and side walls of the abdo- men, are powerful aids to expiration. In fact they assist expiration in two ways : first, directly, by pulling down the ribs ; and next, indirectly, by pressing the viscera of the abdomen upwards against the under surface of the dia- phragm, and so driving the floor of the thorax upwards. It is for this reason that, whenever a violent expiratory effort is made, the walls of the abdomen are obviously flattened and driven towards the spine, the body being at the same time bent forwards. In taking a deep inspiration, on the other hand, the walls of the abdomen are relaxed and become convex, the viscera being driven against them by the descent of the diaphragm — the spine is straightened, the head thrown back, and the shoulders outwards, so as to afford the greatest mechanical advantage to all the muscles which can elevate the ribs. I'-^g. It is a remarkable circumstance that the mechanism Vpf respiration is somewhat different in the two sexes. In Nnen, the diaphragm takes the larger share in the process, the upper ribs moving comparatively little ; in women, the reverse is the case, the respiratory act being more largely the result of the movement of the ribs. Sighing is a deep and prolonged inspiration. " Sniffing " is a more rapid inspiratory act, in which the mouth is kept shut, and the air made to pass through the nose. Coughing is a violent expiratory act. A deep inspira- tion being first taken, the glottis is closed and then burst open by the violent compression of the air contained in the lungs by the contraction of the expiratory muscles, the diaphragm being relaxed and the air driven through the mouth. In sneezing, on the contrary, the cavity of the mouth being shut off from the phar>'nx by the approxima- tion of the soft palate and the base of the tongue, the air is forced through the nasal passages. 20. It thus appears that the thorax, the lungs, and the trachea constitute a sort of bellows without a valve, in which the thorax and the lungs represent the body of the bellows, while the trachea is the pipe ; and the effect of IV.] RESIDUAL AIR. 91 the respiratory movements is just the same as that of the approximation and separation of the handles of the bellows, which drive out and draw in the air through the pipe. There is, however, one difference between the bellows and the respiratory apparatus, of great im- portance in the theory of respiration, though frequently Fig. 24.— Di.agrammatic Sections of the Body in' A. inspiration; B. expiration, yn trachea ; St sternum; /?. diaphragm ; Ab. abdominal walls. The shading roughly indicates the stationary air. overlooked ; and that is, that the sides of the bellows can be brought close together so as to force out all, or nearly all, the air which they contain ; while the walls of the chest, when approximated as much as possible, still enclose a very considerable cavity (Fig. 24, B) ; so that, even after the most violent expiratory effort, a very large quantity of air is left in the lungs. ^-^ I cuh 92 ELEMENTARY PHYSIOLOGY. [less. The amount of this air which cannot be got rid of, and is called Residual air, is, on the average, from 75 to 100 cubic inches. About as much more in addition to this remains in chest after an ordinaiy expiration, and is called Supplemental air. In ordinary breathing, 20 to 30 cubic inches of what is conveniently called Tidal air pass in and out. It follows that, after an ordinary inspiration, 100 + 100 + 30 = 230 cubic inches, may be contained in the lungs. By taking the deepest possible inspiration, another 100 cubic inches, called Complemental air, may be added. 21. It results from these data that the lungs, after an ordinary inspiration, contain about 230 cubic inches of air, and that only about one-seventh to one-eighth of this amount is breathed out and taken in again at the next inspiration. Apart from the circumstance, then, that the fresh air inspired has to fill the cavities of the hinder part of the mouth, and the trachea, and the bronchi, if the lungs were mere bags fixed to the ends of the bronchi, the inspired air would descend so far only as to occupy that one-fourteenth to one-sixteenth part of each bag which was nearest to the bronchi, whence it would be driven out again at the next expiration. But as the bronchi branch out into a prodigious number of bronchial tubes, the inspired air can only penetrate for a certain distance along these, and can never reach the air-cells at all. Thus the residual and supplemental air taken together are, under ordinary circumstances, statiojiajy — that is to say, the air comprehended under these names merely shifts its outer limit in the bronchial tubes, as the chest dilates and contracts, without leaving the lungs ; the tidal air, alone, being that which leaves the lungs and is re- newed in ordinary respiration. It is obvious, therefore, that tbe business of respiration is essentially transacted by the stationary air, which plays the part of a middleman between the two parties — the blood and the fresh tidal air — who desire to exchange their commodities, carbonic acid for oxygen, and oxygen for carbonic acid. Now there is nothing interposed between the fresh tidal air and the stationary air ; they are aeriform fluids, in IV.] THE CONTROL OF RESPIRATIOX. 93 complete contact and continuity, and hence the exchange between them must take place according to the ordinary laws of gaseous diffusion. 22. Thus, the stationary air in the air-cells gives up oxygen to the blood, and takes carbonic acid from it. though the exact mode in which the change is effected is not thoroughly understood. By this process it becomes loaded with carbonic acid, and deticient in oxygen, though to what precise extent is not known. But there must be a very much greater excess of the one, and deficiency of the other, than is exhibited by inspired air, seeing that the latter acquires its composition by diffusion in the short space of time (four to five seconds) during which it is. in contact with the stationary air. In accordance with these facts, it is found that the air expired during the first half of an expiration contains less carbonic acid than that expired 'during the second half. Further, when the frequency of respiration is in- creased without altering the volume of each inspiration, though the percentage of carbonic acid in each inspiration is diminished, it is not diminished in the same ratio as chat in which the number of inspirations increases ; and hence more carbonic acid is got rid of in a given time. Thus, if the number of inspirations per minute is in- creased from fifteen to thirty, the percentage of carbonic acid evolved in the second case remains more than half of what it was in the first case, and hence the total evolution is greater. 23. Of the various mechanical aids to the respiratory pro:ess, the nature and workings of which have now been des:ribed, one, the elasticity of the lungs, is of the nature of a dead, constant force. ' The action of the rest of the apparatus is under the control of the nervous system, and varies from time to time. As the nasal passages cannot be closed by their own action, air has always free access to the pharynx ; but the glottis, or entrance to the windpipe, is completely under the control of the ner\'Ous system — the smallest irritation about the mucous membrane in its neighbour- hood being conveyed, by its nerves, to that part of the cerebro-spinal axis which is called the medulla oblongata Csee Lesson XI. § 16). The medulla obloncrata, thus 94 ELEME.YTARY PHYSIOLOGY. [less. stimulated, gives rise, by a process which will be explained hereafter, termed 7'cjiex action^ to the contraction of the muscles which close the glottis, and commonly, at the same time, to a violent contraction of the expiratory muscles, producing a cough (see § 19). The muscular fibres of the smaller bronchial tubes, no less than the respiratory pump itself, formed by the walls and floor of the thorax, are under the complete control of the nerves which supply the muscles, and which are brought into action in consequence of impressions conveyed to that part of the brain which is called the medulla oblongata, by the pneumogastric and other nerves. 24. From what has been said, it is obvious that there are many analogies between the circulatory and the respiratory apparatus. Each consists, essentially, of a kind of pump which distributes a fluid (aeriform in the one case, liquid in the other) through a series of ramified' distributing tubes to a system of cavities (capillaries qr air-cells), the volume of the contents of which is greater^ than that of the tubes. In each, the pump is the cause of the motion of the fluid, though that motion may be regulated, locally, by the contraction, or relaxation, of the muscular fibres contained in the walls of the distributing tubes. But, while the rhythmic movement of the heart chiefly^4qDends upon a nervous apparatus placed within itself, that of the respiratory apparatus results mainly from the operation ot a nervous centre lodged in the medulla oblongata. 25. As there are certain secondary phenomena which accompany, and are explained by, the action of the heart, so there a:re secondary phenomena which are similarly related to the working of the respiratory apparatus. These are— (<^) the respiratory sounds, and {b) the effect of the inspiratory and expiratory movements upon the circulation. 26. The respiratory sounds or viurmiirs are audible when the ear is applied to any part of the chest which covers one or other of the lungs. They accompany inspiration and expiration, and very much resemble the sounds produced by breathing through the mouth, when the lips are so applied together as to leave a small IV.] EFFECT OF IXSPIRATION ON HEART. 95 interval. Over the bronchi the sounds are louder than over the general surface. It would appear that these sounds are produced by the motion of the air along the air-passages. 27. In consequence of the elasticity of the lungs, a certain foice must be expended in distending them, and this force is found experimentally to become greater and greater the more the lung is distended ; just as, in stretch- ing a piece of india-rubber, more force is required to stretch it a good deal than is needed to stretch it only a little. Hence, when inspiration takes place, and the lungs are distended with air, the heart and the great vessels in the chest are subjected to a less pressure than are the blood-vessels of the rest of the body. For the pressure of the air contained in the lungs is exactly the same as that exerted by the atmosphere upon the surface of the body ; that is to say, fifteen pounds on the square inch. But a certain amount of this pressure exerted by the air in the lungs is coimterbalanced by the elasticity of the distended lungs, ^y^ that in a given condition of inspiration a pound pressure on the square inch is needed to overcome this elasticity, then there will be only fourteen pounds pressure on every square inch of the heart and great vessels. And hence the pressure on the blood in these vessels will bs one pound per square inch less than that on the veins and arteries on the rest of the body. If there were no aortic, or pulmonary, valves, and if the composition of the vessels, and the pressure upon the blood in them, were everywhere the same, the result of this excess of pressure on the surface would be, to drive all the blood from the arteries and veins of the rest of the body into the heart and great vessels contained in the thora;<:. And thus the diminution of the pressure upon the thoracic blood cavities produced by inspiration, would, practically, suck the blood from all parts of the body towards the thorax. But the suction thus exerted, while it hastened the flow of blood to the heart in the veins, would equally oppose the flow from the heart to the arteries, and the two effects would balance one another. As a matter of fact, however, we know — (i.) That the blood in the great arteries is constantly under a very considerable pressure, exerted by their 96 ELEMENTARY PHYSIOLOGY. [less. elastic walls ; while that of the veins is under little pressure. (2.) That the walls of the arteries are strong and re- sisting, while those of the veins are weak and flabby. (3.) That the veins have valves opening towards the heart ; and that, during the diastole, there is no resistance of any moment to the free passage of blood into the heart; while, on the other hand, the cavity of the arteries is shut off from that of the ventricle during the diastole, by the closure of the semilunar valves. Hence it follows that equal pressures applied to the surface of the veins and to that of the arteries must pro- duce very different effects. In the veins the pressure is something which did not exist before ; and, partly from the presence of valves, partly from the absence of re- sistance in the heart, partly from the presence of resistance in the capillaries, it all tends to accelerate the flow of blood toixjards the heart. In the arteries, on the other hand, the pressure is only a fractional addition to that which existed before ; so that, during the systole^ it only makes a com- paratively small addition to the Resistance which has to be overcome by the ventricle ; and during the diastole, it superadds itself to the elasticity of the arterial walls in driving the blood onwards towards the capillaries, inas- much as all progress in the opposite direction is stopped by the semilunar valves. It is, therefore, clear that the inspiratory movement, on the whole, helps the heart, inasmuch as its general result is to drive the blood the way that the heart propels it. 28. In expiration, the difference between the pressure of the atmosphere on the surface, and that which it exerts on the contents of the thorax through the lungs, becomes*- less and less in proportion to the completeness of-the ex- piration. Whenever, by the ascent of the diaphragm and the descent of the ribs, the cavity of the thorax is so fal* diminished that pressure is exerted on the great vessels, the veins, owing to the thinness of their walls, are es- pecially affected, and a check is given to the flow of blood in them, which may become visible as a venous pulse in the great vessels of the neck. In its effect on, the arterial trunks, expiration, like inspiration, is, on the whole, favour- able to the circulation ; the increased resistance 'to the IV. ] .-^ C TIVITY OF KESPIRA TOR Y PROCESS. 97 opening of the valves during the ventricu^ systole being more than balanced by the advanta^^g^ned in the addi- tion of the expiratory pressure to th^^lastic reaction of the arterial walls during the diastole. -| the tubule by the very delicate ';: membrane of which the wall of the vessel is composed. The Malpighian capsule may, in fact, • - be regarded as a funnel, and the Fig. 30.-TRANSVERSE Section membranous walls of the glome- OF TWO Tubules. , . r ,^t r- 1 f, , , J 1 rulus as a piece 01 very delicate n.a. Canals of tubules surrounded ^, . ^ . ■', ^^^'^'^'-'- by their epithelium. filtering- paper, into which the /'. A blood-vessel cut across. blood is pOUred. 11. The blood which supplies the kidneys is brought directly from the aorta by the renal arteries, so that it has but shortly left the heart. The venous blood which enters the heart, and is propelled to the lungs, charged with the nitrogenous, as well as with the other, products of waste, loses only an inappreciable quantity of the former in its course through the lungs ; so that the arterial blood which fills the aorta is pure only as regards carbonaceous waste, while it is impure as regards urea and uric acid. In the healthy condition, the walls of the minute renal arteries and veins are relaxed, so that the passage of the blood is very free ; and but little waste, arising from mus- cular contraction in the walls of these vessels, is thrown into the renal blood. And as the urine which is separated from the renal blood contains proportionately less oxygen and more carbonic acid than the blood itself, any gain of carbonic acid from this source is probably at once counterbalanced. Hence, so long as the kidney is per- forming its functions properly, the blood which leaves the organ by the renal vein is as bright scarlet as that vvhif^h v.] THE RENAL BLOOD. lit enters it by the renal artery. Strictly speaking, it is the purest blood in the body, careful analysis having shown that it contains a sensibly smaller quantity of urea and of water than that of the left side of the heart. This differ- ence is, of course, a necessary result of the excretion of the urinary fluid from the blood as it travels through the kidney. As the renal veins pour their contents directly into the inferior vena cava (see Fig. 25), it follows that the blood in the upper part of this vein is so much the less impure, or venous, than that contained in the inferior vena cava, below the renal veins. 12. Irritation of the nerves which supply the walls of the vessels of the kidney has the immediate effect of stopping the excretion of urine, and rendering the renal blood dark and venous. The tirst effect would appear to be explicable by the diminution of the pressure exerted upon the blood in the Malpighian tufts, in consequence of the diminution in the size of the channels — the small arteries — by which the blood reaches them. And the second effect is probably, in part, a secondary result of the first — the excretion of carbonic acid by the urine ceasing with the suppression of that fluid ; while, to a large extent, it is also the result of a pouring in of carbonic acid into the renal blood, in consequence of the work of the muscles of the sniall vessels, and the waste which results therefrom. 13. That the skin is a source of continual loss to the blood may be proved in various ways. If the whole body of a man, or one of his limbs, be enclosed in a caoutchouc bag, full of air, it will be found that this air undergoes changes which are similar in kind to those which take place in the air which is inspired into the lungs. That is to say, the air loses oxygen and gains carbonic acid ; it also receives a great quantity of watery vapour, which condenses upon the sides of the bag, and may be drawn off by a properly disposed pipe. Under ordinary circumstances no liquid water appears upon the surface of the integument, and the whole process receives the name of the insensible perspiration. But, when violent exercise is taken, or under some kinds of 1 1 2 ELEMENTAR V PHYSIOL OGY. [less . mental emotion, or when the body is exposed to a hot and moist atmosphere, the perspiration becomes sensible ; that is, appears in the form of scattered drops upon the surface. 14. The quantity oi sweaf, or sensible perspiration, and also the total amount of both sensible and insensible per- spiration, vary immensely, according to the temperature and other conditions of the air, and according to the state of the blood and of the nervous system. It is estimated that, as a general rule, the quantity of water excreted by the skin is about double that given out by the lungs in the same time. The quantity of carbonic acid is not above* 3\)th or -^th of that excreted by the lungs ; and it is not certain that in health any appreciable quantity of urea is given off. In its normal state the sweat is acid, and contains fatty matters, even w.ben obtained free from the fatty products of the sebacek^i^^ glands. Ordinarily, perspiration, as it collects upon the skin, is mixed with the fatty secretion of these glands ; and, in addition, contains scales of the external layers of the epidermis, which are constantly being shed. 1— 15. In analysing the process by which the perspiration Is eliminated from the body, it must be recollected, in the first place, that the skin, even if there were no glandular structures connected with it, would be in the position of a moderately thick, permeable rriembrane, interposed be- tween a hot fluid, the blood, and the atmosphere. Even in hot chmates the air is, usually, far from bein^ com- pletely saturated with watery vapour, and in temperate climates it ceases to be so saturated the moment it comes into contact with the skin, the temperature of which is, ordinarily, twenty or thirty degrees above its own. A bladder exhibits no sensible pores, but if filled with water and suspended in the air, the water will gradually ooze through the walls of the bladder, and disappear by evaporation. Now, in its relation to the blood, the skin is such a bladder full of hot fluid. Thus, perspiration to a certain amount must always be going on through the substance of the integument ; but what the amount of this perspiration may be cannot be accurately ascertained, because a second and very impor- V. 1 THE S WE A T- GLANDS. i 1 3 tant source of the perspiration is to be found in what are called the sweat-gUuids. 16. All over the body the integument presents minute apertures, the ends of channels excavated in the epidermis or scarf-skin, and each continuing the direction of a minute tube, usually about ^^^th of an inch in diameter, and a quarter of an inch long, v.hich is imbedded in the A B Fig. 31 A. Section of the skin showing the sweat-glands, a, the epidermis ; b, its deeper layer, the rete Malpighii; e,d, the dermis or true skin ; f, fat cells ; g, the coiled end of a sweat-gland ; h, its duct ; i, its opening on the surface of the epidermis. B. A section of the skin showing the roots of the hairs and the sebaceous glands. /', muscle of c, the hair sheath, on the left hand. dermis. Each tube is lined with an epithelium conti- nuous with the epidermis (Fig. 32, e). The tube sometimes divides, but, whether single or branched, its inner end or ends are blind, and coiled up into a sort of knot, interlaced with a meshwork of capillaries (Fig. 31, A^, and Fig. 33). The blood in these capillaries is therefore separated from the cavity of the sweat-gland only by the thin walls of the capillaries, that of the glandular tube, and its epithelium, which, taken together, coustitute but a very thin pelHcle ; and the arrangement, though different in I iU ELEMEXTA RY PHYS TO I. OGY. LESS 5- I ."/ I'lG. 32. Portion of Fig. 31, A, more highly magnified — somewhat diagrammatic, a, horny epidermis ; i, softer laj'er, rete AInlpighii ; ,, dermis ; d, lowermost vertical layer of epidermic cells ; e, cells liniii;j the sweat duct continuous with epidermic cells ; h, corkscrew canal of sv. eat di'Ct. lo the right of the swea. duct the dermis is raised into a papilla, in which the small artery, ^j breaks up into capiilaries, ultiiuate.'y forminj the veins, i'. v.] THE SWEAT-GLANDS. 15 detail, is similar in principle to that which obtains in the kidney. In the latter, the vessel makes a coil within the Malpighian capsule, which ends a tubule. Here the perspiratory tubule coils about, and among, the vessels. In both cases the same result is arrived at — namely, the exposure of the blood to a large, relatively free, surface, on to which certain of its contents transude. MG. 33 Coiled end of a sweat-gland (Fig. 31, g), epithelium not shown, a, the coil ; /', the duct; c, network of capillaries, inside which the duct gland lies. The number of these glands varies in different parts of the body. They are fewest in the back and neck, where their number is not much more than 400 to a square inch. They are more numerous on the skin of the palm and sole, where their apertures follow the ridges visible on the skin, and amount to between two and three thousand on the square inch. At a rough estimate, the whole integu- ment probably possesses not fewer than from two millions and a quarter to two millions and a half of these tubules, I 2 1 16 ELEMENTAR V PHYSIOL OG V. [less. which therefore must possess a very great aggregate secreting power. 1 7. The sweat-glands are greatly under the influence of the nervous system. This is proved, not merely by the well-known effects of mental emotion in sometimes sup- pressing the perspiration and sometimes causing it to be poured forth in immense abundance, but has been made 3. matter of direct experiment. There are some animals, such as the horse, which perspire ver>^ freely. If the sympathetic nerve of one side, in the neck of a horse, be cut, the same side of the head becomes injected with blood, and its temperature rises (see Lesson II. § 24) ; and, simultaneously, sweat is poured out abundantly over the whole surface thus affected. On irritating that end of the cut nerve which is in connection with the vessels, the muscular walls of the latter, to which the nerve is distri- buted, contract, the congestion ceases, and with it the perspiration. 18. The amount of matter which may be lost by per- spiration, under certain circumstances, is very remarkable. Heat and severe labour, combined, may reduce the weight of a man two or three pounds in an hour, by means of the cutaneous perspiration alone ; and, as there is some rea- son to believe that the quantity of solid matter carried off from the blood does not diminish with the increase of the amount of the perspiration, the total amount of solids which are eliminated by profuse sweating may be con- siderable. The difference between blood which is coming from, and that which is going to, the skin, can only be con- cluded from the nature of the substances given out in the perspiration ; but arterial blood is not rendered venous in the skin, 19. It will now be instructive to compare together in more detail than has been done in the first Lesson (§ 23), the three great organs — lungs, kidneys, and skiri — which have been described. In ultimate anatomical analysis, each of these organs- consists of a moist animal membrane separating the blood from the atmosphere. Water, carbonic acid, and solid matter pass out from the blood through the animal membrane in each organ, v.] irXGS. KIDNEYS, AXD SKLX. 117 and constitute its secretion or excretion ; but the three organs differ in the absolute and relative amounts of the constituents the escape of which they permit. Taken by weight, water is the predominant excretion in all three : most solid matter is given off by the kidneys ; most gaseous matter by the lungs. The skin partakes of the nature of both lungs and kidneys, seeing that it absorbs oxygen and exhales car- bonic acid and water, like the former, while it excretes organic and saline matter in solution, like the latter ; but the skin is more closely related to the kidneys than to the lungs. Hence when the free action of the skin is inter- rupted, its work is usually thrown upon the kidneys, and vice versa. In hot weather, when the excretion by the skin increases, that of the kidneys diminishes, and the reverse is observed in cold weather. This power of mutual substitution, however, only goes a little way ; for if the kidneys be extirpated, or their functions much interfered with, death ensues, however active the skin may be. And, on the other hand, if the skin be covered with an impenetrable varnish, the tem- perature of the body rapidly falls, and death takes place, though the lungs and kidneys remain active. 20. The live?' is a constant source both of loss, and, in a sense, of gain, to the blood which passes through it. It gives rise to loss, because it separates a pecuhar fluid, the dile, from the blood, and throws that fluid into the intes- tine. It is also in another way a source of loss because it elaborates from the blood passing through it a substance called glycogen, which is stored up sometimes in large, sometimes in small, quantities in the cells of the liver. This latter loss, however, is only temporar}', and may be sooner or later converted into a gain, for this glycogen very readily passes into sugar, and either in that form or in some other way is carried off by the blood. In this respect, therefore, there is a gain to the blood of kind or quality though not of quantity of material. Finally, it is very probable that the liver is one source of the colourless corpuscles of the blood. The liver is the largest glandular organ in the body, ordinarily weighing about tifty or sixty ounces. It is a broad, dark, red-coloured organ^ which lies on the right ii8 ELEMENTAR Y PHYSTOLOG Y. [less. side of the body, immediately below the diaphragm, with which its upper surface is in contact, while its lower sur face touches the intestines and the right kidney. Pifj -4— The LiVKK turm-d vv and viewed from P.ki.ow. a vena cava ; /', vena portae : r, bile duct : d, hepatic artery ; /, gall- bladder. The termination of the hepatic vein in the vena cava is not st-en, being covered by the piece of the vena cava. The liver is invested by a coat of peritoneum, which, keeps it in place. It is flattened from above downwards, and convex and smooth above, where it fits into the con- cavity of the lower surface of the diaphragm. Flat and irregular below (Fig. 34), it is thick behind, but ends in a thin edge in front. Viewed from below, as in Fig. 34, the inferior vena cava^ a, is seen to traverse a notch in the hinder edge of the liver as it passes from the abdomen to the thorax. At b the trunk of the vena portcE is observed dividing into the chief branches which enter into, and ramify through, the substance of the organ. At ' reach the surfaces of the lobules by the ultimate ramifications of that vein and artery, become mixed in the capillaries of each lobule, and are carried off by its intralobular vein- let, which pours its contents into one of the ramifications of the hepatic vein. These ramifications, joining together, form larger and larger trunks, which at length reach the hinder margin of the liver, and finally open into the vena cava inferior, where it passes upwards in contact with that part of the organ. Thus the blood with which the liver is supplied is a mixture of arterial and venous blood ; the former brought Fig. 36. a, ultimate branches of the hepatic duct ; h, liver cells. by the hepatic artery directly from the aorta, the latter by the portal vein from the capillaries of the stomach, intestines, pancreas, and spleen. What ultimately becomes of the ramifications of the hepatic duct is not certainly known. Lined by an epithelium, which is continuous with that of the main duct, and thence with that of the intestines, into which the main duct opens, they may be traced to the ver>' surface of the lobules. Their ultimate ramifications are not yet thoroughly determined : but recent investigations v.] STRUCTURE OF THE LIVER. A ^ B Fig. 37. A. Section of partially injected liver magnified. The artificial white line is introduced to mark the limits of a lobule. \'.P. Branches of portal vein breaking up into capillaries, which run towards the centre of the lobule, and join H.V., the intralobular branch of the hepatic vein. The 1 21 ELEMENTAR V PHYSIOL 0 G Y. [less. tend to show that they communicate with minute passages left between the hepatic cells, and traversir.g the lobule in the intervals left by the capillaries (Fig. 37, B.). How- ever this may be, any fluid separated from the blood bv the lobules must really find its way into them. In the lobules themselves all the meshes of the blood- vessels are occupied by the liver cells. These are many- sided, minute bodies, each about io\)oth of an inch in diameter, possessing a nucleus in its interior, and frequently having larger and smaller granules of fatty matter distributed through its substance (Fig. 37, a). It is in the liver cells that the active powers of the liver are supposed to reside. 21. The nature of these active powers, so far as the liver is a source of loss to the blood which traverses it, is determined by ascertaining — a. The character of that fluid, the bile, which in- cessantly flows down the biliary duct, and which, if digestion is not going on, and the passage into the in- testine is closed, flows back into and fills the gall-bladder. d. The difference between the blood which enters the liver and that which leaves it. 22. a. The total quantity of bile secreted in the twenty- four hours varies, but probably amounts to not less than from two to three pounds. It is a golden yellow, slightly alkaline, fluid, of extremely bitter taste, consisting of watci with from 17 per cent, to half that quantity, of solid matter in solution. The solids consist in the first place of a somewhat complex substance which may be separated by crystaflization, and has been called dilin. It is in reality a mixture of two acids, in combination with soda, one called glycocholic, and consisting of carbon, hydrogen, nitrogen, and oxygen, the oih^r ta7n-ocholic,^nd containing in addition to the other elements a considerable quantity of sulphur. Besides the taurocholate and glycocholate of soda, or bile salts as they are sometimes called, the bile contains a remarkable crystalline substance, very fatty- oulline of the liver cells are seen as a fine network of lines throughout the whole lobule. B. Portion of lobule very highly magnified, a, liver cell with n, nucleus (two are often present); ^, capillaries cut across; c, minute biliary passages between the cells, injected with colouring m.-itter. v.] SOURCES OF GALY OF MATTER. \2% looking, but. not really of a fatty nature, called L/ioh'sfcrin, one or more peculiar colouring matters probably related to the ha!;matin of the blood, and certain saline matters. [y. Of these constituents of the bile the water, the choles- terin, and the saline matters, alone, are discoverable in -- ^ the blood ; and, though doubtless some difference obtains--'^-^-^-'*^ between the blood which enters the liver and that which leaves it, in respect of the proportional quantity of these constituents, great practical dithculties lie in the way of the precise ascertainment of the amount of that difference. The blood of the hepatic vein, however, is certainly poorer in water than that of the portal vein. 23. As the essential constituents of bile, the bile acids and the colouring matter, are not discoverable in the blood v.hich enters the liver ; they must be formed at the expense of the tissue of that organ 'itself, or of some constituent of the blood passing through it. 24. We must next consider the chief sources of constant gain to the blood ; and, in the first place, /he sources of g-ain of inatter. The lungs and skin are, as has been seen, two of the principal channels by which the body loses liquid and gaseous matter, but they are also the sole means by which one of the most important of all substances for the main- tenance of life, oxygen, is introduced into the blood. It has already been pointed out that the volume of the oxygen taken into the blood by the lungs is rather greater than that of the carbonic acid given out. The absolute weight of oxygen thus absorbed may be estimated at 10,000 grains (see Lesson VI. § 2). How much is taken in by the skin of man is not cer- tainly known, but in some of the lower animals, such as the frog, the skin plays a very important part in the per- formance of the respiratory function. 25. The blood leaving the liver by the hepatic vein not only contains proportionally less water and fibrin, but pro- portionally more corpuscles, especially colourless cor- puscles, and, what is still more important, under certain circumstances at least, a larger quantity of liver-sugar, or 9'' l^tT 1 24 ELEMENTAR V PHYSIOLOG Y. [li^. glucose, than that brought to it by the portal veins and hepatic artery. That the blood leaving the liver should contain propor- tionally less water and more corpuscles than that entering it, is no more than might be expected from the fact that the formation of the bile, which is separated from this blood, necessarily involves a loss of water and of some solid matters, while it does not abstract any of the cor- puscles. We do not know why less fibrin separates from the blood of the hepatic vein than from the blood brought to the liver. But the reason why there may be more sugar in the blood leaving the liver than in that entering it ; and why, in fact, there may be plenty of sugar in the blood of the hepatic vein even when none whatever is brought to it by the hepatic artery, or portal vein, has been made out by careful and ingenious experimental research. 26. If an animal be fed upon purely animal food, the blood of the portal vein will contain no sugar^one having been absorbed by the walls of the aliments^' canal, nor will that of the hepatic artery contain any^^^, at any rate, more than the merest trace. Nevertheless Jplenty may be found, at the same time, in the blood of the hepatic vein and in that of the vena cava, from the point at Avhich it is joined by the hepatic vein, as far as the heart. Secondly, if, from an animal so fed, the liver be ex- tracted, and a current of cold water forced into the ve7ia portcF, it will flow out by the hepatic vein, carrying with it all the blood of the organ, and will, after a time, come out colourless, and devoid of sugar. Nevertheless, if the organ be left to itself at a moderate temperature, sugar will soon again become abundant in it. Thirdly, from the liver, washed as above described, a substance maybe extracted, by appropriate methods, which resembles starch or dextrine, in chemical composition, consisting as it does of_ cailx>ix_imited with hydrogen and oxygen, the latter being in the same proportions as in water. This " amyloid " substance is the (glycogen spoken of in § 20. It may be dried and kept for long periods without undergoing any change. But, like the vegetable starch and dextrine, this animal amyloid, which must be formed in the liver, since it is cer- k^a^^^l v.] T//E GLYCOGEN OF THE LIVER. 125 tainly not contained either in the blood of the portal vein, or in that of the hepatic artery, is very readily changed by contact with certain matters, which act as ferments, into sugar. Fourthly, it may be demonstrated that a ferment, com- petent to change the " amyloid " glycogen into saccharine ^''glucose^^ exists under ordinary circumstances in the liver. Putting all these circumstances together, the following explanation of the riddle of the appearance of sugar in the blood of the hepatic vein and vena cava, when neither it, nor any compound out of which it is easily formed, exists in the blood brought to the liver, appears to have much probabihty ; though it may possibly require modilication, in some respects, hereafter. The liver forms glycogen out of the blood with which it is supplied. The same blood supplies the ferment which, at the temperature of the body, very speedily converts the comparatively little soluble glycogen into very soluble sugar ; and this sugar is dissolved and carried away by each intralobular vein to the hepatic vein, and thence to the vena cava. Though after death a very considerable quantity of sugar accumulates in the hepatic vein, the amount which, at any given vwnient, can be detected during life is extremely small. This has led some physiologists to suppose that, in health, glycogen is not converted into sugar, but undergoes some other change. A very small quantity of sugar how- ever, so small as to almost escape detection, thrown into the hepatic vein every instant, would amount to a consider- able quantity in the twenty-four hours. This formation of glycogen in the liver goes on in the total absence of starch or sugar from the food. It must, therefore, in such cases be formed at the expense of proteid material (see Lesson VL). It appears, however, that the presence of starch or sugar in the food, though not essen- tial, is very favourable to the production of glycogen in the liver. 27. The lymphatic system has been already mentioned as a feeder of the blood with a fluid which, in general, ap- pears to be merely the superfluous drainage, as it were^of 126 ELEMENTARY PHYSIOLOGY. [less. the blood-vessels ; though at intervals, as we shall see, the lacteals make substantial additions of new matter. It is very probable that the multitudinous lymphatic glands may effect some change in the fluid v\-hich traverses them, or may add to the number of corpuscles in the lymph. Nothing certain is known of the functions of certain bodies which are sometimes called ductless glands, but have quite a different structure from ordinar\' secreting glands ; and indeed do not resemble each other in struc- ture. These are, the thy?-oid gland, which lies in the part of the throat below the larynx, and is that organ which, when enlarged by disease, gives rise to " Derbyshire neck" or "goitre ; " the thymus gland, situated at the base of the heart, largest in infants, and gradually disappearing in adult, or old, persons ; and the i'/(;^;-^-;'^;/^/ capsules, which lie above the kidneys. 28. We are as much in the dark respecting the office of the large viscus called the spleen^ which lies upon the left side of the stomach in the abdominal cavity {Fig. 38). It is an elongated flattened red body, abundantly supplied wdth blood by an artery called the splenic artery^ which proceeds almost directly from the aorta. The blood which has traversed the spleen is collected by the splenic vein., and is carried by it to the z'inia ^orta, and so to the liver. /. I .'.-'- ^ A section of the spleen shows a dark red spongy mass dotted over with minute whitish spots. Each of these last is the section of one of the spheroidal bodies called coi'- puscles of the spleen^ which are scattered through its ';ub- stance, and consist of a solid aggregation of minute bodies, like the white corpuscles of the blood, traversed by a ca- pillary network, which is fed by a small twig of the splenic arter}'. The dark red part of the spleen, in which these corpuscles are embedded, is composed of fibrous and elastic tissue supporting a very spongy vascular network. The elasticity of the splenic tissue allows the organ to be readily distendedJand enables it to return to its former size after distension/Cfclt appears to change its dimensions with the state of the abdominal viscera, attaining its largest size about six hours after a full meal, and falling to its minimum bulk six or seven hours later, if no further supply of food be taken, ^ • THE SPLEEX. 1-7 The biood of the splenic vein is found to contain pro- portionallv fewer red corpuscles, but more colourless cor- puscles and more fibrin, than that in the splenic artery ; and it has been sunjmsed that the spleen is one ot those parts of the econ^Jj^nfT^hich the colourless corpuscles of the blood are especially produced. Dili Fig. 3- The spleen (i>/.) with the splenic artery {SpA.^. Pelow this is seen the solenic vein running to help to form the vena portas {}'.P.,. Ao. the aorta ; D. a pillar of the diaphragm ; P.D. the pancreatic duct expo'^ed by dissection in the substance of the pancreas ; Dsn. the duodenum ; B.D the biliary duct uniting with the pancreatic duct into the common duct, x ; y, the intestinal vessels. 29. It has been seen that heat is being constantly given off from the integument and from the air-passages ; and ever>-thing that passes from the body carries away with it, in like manner, a certain quantity of heat. Furthermore, the surface of the dody is much more exposed to cold than its interior. Nevertheless, the temperature of the body is maintained ver}' evenly, at all times and in all parts, within the range of two degrees on either side of 09° Fahrenheit. This is the result of three conditions : — The first, that heat is constantly being generated in the body ; the second, that it is as constantly being distributed through 128 ELEMENTARY PHYSIOLOGY. [less. the body ; the third, that it is subject to incessant regulation. Heat is generated whenever oxidation takes place ; and hence, whenever proteid substances (see Lesson VI., § 4) or fats, or amyloidal matters, are being converted into the more highly oxidated waste products, — urea, carbonic acid, and water, — heat is necessarily evolved. But these pro- cesses are taking place in all parts of the body by which vital activity is manifested ; and hence every capillary vessel and every extravascular islet of tissue is really a small fireplace in which heat is being evolved, in propor- tion to the activity of the chemical changes which are going on. 30. But as the vital activities of different parts of the body, and of the whole body, at different times, are very different ; and as some parts of the body are so situated as to lose their heat by radiation and conduction much more easily than others, the temperature of the body would be very unequal in its different parts, and at different times, were it not for the arrangements by which the heat is distributed and regulated. Whatever oxidation occurs in any part, raises the tem- perature of the blood which is in that part at the time to a proportional extent. But this blood is swiftly hurried away into other regions of the body, and rapidly gives up its increased temperature to them. On the other hand, the blood which by being carried to the vessels in the skin on the surface of the body begins to have its temperature lowered by evaporation, &c., is hurried away before it has time to get thoroughly cooled into the deeper organs ; and in them it becomes warm by contact, as well as by the oxidating processes in which it takes a part. Thus the blood-vessels and their contents might be compared to a system of hot-water pipes, through which the warm water is kept constantly circulating by a pump ; while it is heated, not by a great central boiler as usual, but by a multitude ot minute gas jets, disposed beneath the pipes, not evenly, but more here and fewer there. It is obvious that, however much greater might be the heat applied to one part of the system of pipes than to another, the general temperature of the water would be even throughout, if it were kept moving with sufficient quickness by the pump. v.] THE REGULATION OF TEMPERATURE. 129 31. If such a system were entirely composed of closed pipes, the temperature of the water might be raised to any extent by the gas jets. On the other hand, it might be kept down to any required degree by causing a larger, or smaller, portion of the pipes to be wetted with water, which should be able to evaporate freely — as, for example, by wrapping them in wet cloths. And the greater the quantity of water thus evaporated, the lower would be the tem- perature of the whole apparatus. Now, the regulation of the temperature of the human body is effected on this principle. The vessels are closed pipes, but a greater number of them are enclosed in the skin and in the mucous membrane of the air-passages, which are, in a physical sense, wet cloths freely exposed to the air. It is the evaporation from these which exercises a more important influence than any other condition upon the regulation of the temperature of the blood, and, conse- quently, of the body. But, as a further nicety of adjustment, the wetness of the regulator is itself determined by the state of the small vessels, inasmuch as exudation from these takes place more readily when the walls of the veins and arteries are relaxed, and the blood distends them and the capillaries. But the condition of the walls of the vessels depends upon the nerves by which they are supplied ; and it so happens that cold so affects these nerves in such a manner as to give rise to contraction of the small vessels, while mode- rate warmth has the reverse effect. Thus the supply of blood to the surface is lessened, and loss of heat is thereby checked, when the external tem- perature is low ; while, when the external temperature is high, the supply of blood to the surface is increased, the fluid exuded from the vessels pours out by the sweat-glands, and the evaporation of this fluid cl^ecks the rise in the temperature of the superficial blood. ^ Hence it is that, so long as the surface of the body per- spires freely, and the air-passages are abundantly moist, a man may remain with impunity, for a considerable time, in an oven in which meat is being cooked. The heat of the air is expended in converting this superabundant per- spiration into vapour, and the temperature of the man's blood is hardly raised. K ELEiMEXTAR Y PH \ 'SIOL 0 G Y. [le^s. y^^yM^J): /==---:-. Ill { Ji'f •i ! I Fig. 39.— a Diagram to illl-strate the Strl-ctire of Glands. A. Typical structure of the mucous membrane, a, an upper, and b, a lower, layer of epithelium cells; c, the dermis with c, a bluoU-vesbel, andy, con- nective libsue corpuscles. \^] STRUCTURE OF GLAXDS. 131 32. The chief iiitermittently active sources of loss to the blood are found among the glands proper, all of which are, in principle, narrow pouches of the mucous membranes, or of the integument of the body, lined by a continuation of the epithelium, or of the epidermis. In Wi^ glaiufs of Lie- berkiihii, which exist in immense numbers in the walls of the small intestines, each gland is nothing more than a simple blind sac of the mucous membrane, shaped like a small test tube, with its closed end outwards, and its open end on the inner surface of the intestine (Fig. 39, i). The sweat-glands of the skin, as we have already seen, are equally simple, blind, tube-like involutions of the integu- ment, the ends of which become coiled up. The sebaceous glands, usually connected with the hair sacs, are shorter, and their blind ends are somewhat subdivided, so that the gland is divided into a narrow neck and a more dilated and sacculated end (Fig. 39, 5). The neck by which the gland communicates with the free surface is called its diici. More complicated glands are produced by the elon- gation of the duct into a long tube, and the division and subdivision of the blind end into multitudes of similar tubes, each of which ends in a dilatation (Fig. 39, 6). These dilatations, attached to their branched ducts, somewhat resemble a bunch of grapes. Glands of this kind are called racemose. The salivary glands and the pancreas are such glands. Now, many of these glands, such as the salivary, and the pancreas (with the perspiratory, or sudoriparous glands, which it has been convenient to consider already), are only active when certain impressions on the nervous system give rise to a particular condition of the gland, or of its vessels, or of both. Thus the sight or smell, or even the thought of food, Avill cause a flow of saliva into the mouth ; the previously B. The same, with only one layer of cells, a and /', the so-called basement membrane between the epithelium, a, and deimis, c. I A simple tubular gland. 2. A tubular gland bifid at its base. In this and succeeding figures the blood-vessels are omitted. 3. A simple saccular gland. 4. A divided saccular gland, with a duct, d. 5. A similar gland still more divided. 6. A racemose gland, fart only leing drawn. K 2 132 ELEMENTARY PHYSIOLOGY. [less. quiescent gland suddenly pouring out its fluid secretion, as a result of a change in the condition of the nervous system. And, in animals, the salivary glands can be made to secrete abundantly, by irritating a nerve which supplies the gland and its vessels. How far this effect is the result of the mechanical influence of the nerve on the state of the circu- lation, by widening the small arteries (see p. 51) and so supplying the gland with more blood, and how far it is the result of a more direct influence of the nerve upon the state of the tissue of the gland itself, making the cells secrete^ just as a nerve when stimulated makes a muscle contract, is not at present finally determined. The liquids poured out by the intermittent glands are always very poor in solid constituents, and consist chiefly of water. Those poured on to the surface of the body are lost, but those w^hich are received by the alimentary canal are doubtless in a great measure re- absorbed. 33. The great intermittent sources of gain of waste pro- ducts to the blood are the muscles, every contraction of which is accompanied by a pouring of certain products into the blood. That much of this waste is carbonic acid is certain from the facts {a) that the blood which leaves a contracting muscle is always highly venous, far more so than that which leaves a quiescent muscle ; {h) that mus cular exertion at once immensely increases the quantity of carbonic acid expired ; but whether the amount of nitro- genous waste is increased under these circumstances, or not, is a point yet under discussion. VI.] FUNCTION OF ALIMENTARY CANAL. 133 LESSON VI. THE FUNCTION OF ALIMENTATION. ^ 1. The great source of gain to the blood, and, except the lungs, the only channel by which altogether new material is introduced into that fluid, putting aside the altogether exceptional case of absorption by the skin, is the aliniciitaiy canal, the totality of the operations of which constitutes the function of aliincntatio>i. It will be useful to consider the general nature and results of the performance of this function before studying its details. 2. A man daily takes into his mouth, and thereby intro- duces into his alimentary canal, a certain quantity of solid and liquid food, in the shape of meat, bread, butter, water, and the like. The amount of chemically dry, solid matter, which must thus be taken in*o the body, if a man of average size and activity is neither to lose, nor to gain, in weight, has been found to be about 8,000 grains. In addition to this, his blood absorbs by the lungs about 10,000 grains of oxygen gas, making a grand total of 18,000 grains (or nearly two pounds and three-quarters avoirdupois) of daily gain of dry, solid, and gaseous matter. 3. The weight of dry solid matter passed out from the alimentary canal does not, on the average, amount 10 more than one-tenth of that which is taken into it, or 800 grains. Now the alimentary canal is the only channel by which any appreciable amount of solid matter leaves the body in an undissolved condition. It follows, there- 134 ELEMENTARY PHYSIOLOGY. [less. fore, that in addition to the 10,000 grains of oxygen, 7,200 grains of dry, solid, matter must pass out of the body by the lungs, skin, or kidneys, either in the form of gas, or dissolved in the liquid excretions of those organs. Further, as the general composition of the body remains constant, it follows either that the elementary constituents of the solids taken into the body must be identical with those of the body itself: or that, in the course of the vital processes, the food alone is destroyed, the substance of the body remaining unchanged : or, finally, that both these alternatives hold good, and that food is, partly, identical with the wasting substance of the body, and replaces it; and, partly, differs from the wasting sub- stance, and is consumed without replacing it. 4. As a matter of fact, all the substances which are used as food come under one of four heads. They are either what may be termed Protcids^ or they are Fats^ or they are Amyloids^ or they are Minerals.) Proteids are composed of the four elements — carbon, hydrogen, oxygen, and nitrogen, sometimes united with sulphur and phosphorus. Under this head come the Gluten of flour ; the Albu- min of white of egg, and blooi serum ; the Fibrin of the blood; the Synfonin, which is the chief constituent of muscle and flesh, and Casein, one of the chief constituents of cheese, and many other similar but less common bodies ; while Gelatin, which is obtained by boiling from connec- tive tissue, and Chondrin, which may be produced in the same way from cartilage, may be considered to be out- lying members of the same group. Fats are composed of carbon, hydrogen, and oxygen only, and contain more hydrogen than is enough to form Avater if united with the oxygen which they possess. All vegetable and animal fatty matters and oils come under this division. Amyloids are substances which also consist of carbon, hydrogen, and oxygen only. But they contain no more hydrogen than is ju t sufficient to produce water with their oxygen. The^e are the matters known as Starch, Dextrine, Sugar, and Gum. It is the peculiarity of the three groups of food-stuffs just mentioned that they can only be obtained (at any VI.] FOOD-STUFFS. 135 rate, at present) by the activity of living beings, whether animals or plants, so that they may be conveniently termed vital food-sti{ffs. Food-stuffs of the fourth class, on the other hand, or Minerals., are to be procured as well from the not-living, as the living world. They are lualer^ and sails of sundr}- alkalies, earths, and metals. To these, in strictness, oxygen ought to be added, though, as it is not taken in by the alimentary canal, it hardly comes within the ordi- nary acceptation of the word food. 5. In ultimate analysis, then, it appears that vital food- stuffs contain either three or four of the elements : carbon, hydrogen, oxygen, and nitrogen; and that mitieral food- stuffs are water and salts. But the human body, in ulti- mate analysis, also proves to be composed of the same four elements, plus water, and the same saline matters as are found in food. More than this, no substance can serve permanently for food— that is to say, can prevent loss of weight and change in the general composition of the body — unless it contains a certain amount of proteid matter in the shape of albumin, fibrin, syntonin, casein, (Sic, while, on the other hand, any substance which contains proteid matter in a readily assimilable shape, is competent to act as a per- manent vital food-stuff. The human body, as we have seen, contains a large quantity of proteid matter in one or other of the forms which have been enumerated ; and, therefore, it turns out to be an indispensable condition, that every sub- stance which is to serve permanently as food, must contain a sufficient quantity of the most important and complex component of the body ready made. It must also contain a sufficient quantity of the mineral ingre- dients which are required. Whether it contains either fats or amyloids, or both, its essential power of supporting the life and maintaining the weight and composition of the body remains unchanged. 6. The necessity of constantly renewing the supply ot proteid matter arises from the circumstance that the secretion of urea from the body (and consequently the loss of nitrogen) goes on continually, whether the body is fed or not: while there is only one form in which V 136 ELEMENTARY PHYSIOLOGY. [less. nitrogen (at any rate, in any considerable quantity) can be taken into the blood, and that is in the form of a solution of proteid matter. If proteid matter be not sup- plied, therefore, the body must needs waste, because there is nothing in the food competent to make good the loss of nitrogen. On the other hand, if proteid matter be supplied, there can be no absolute necessity for any other but the mineral food-stuffs, because proteid matter contains carbon and hydrogen in abundance, and hence is competent to give origin to the other great products of waste, carbonic acid and water. -4n fact, the final results of the oxidation of proteid matters are carbonic acid, water, and ammonia ; and these, as we have seen, are the final shapes of the waste products of the human economy. 7. From what has been said, it becomes readily intel- ligible that, whether an animal be herbivorous or carni- vorous, it iDegins to starve from the moment its vital food-stuffs consist of pure amyloids, or fats, or any mixture of them. It suffers from what may be called nitrogen starvation, and, sooner or later, will die. In this case, and still more in that of an animal de- prived of vital food altogether, the organism, so long as it continues to live, feeds upon itself. In the former case, those excretions which contain nitrogen, in the latter, all its waste products, are necessarily formed at the expense of its own body; whence it has been rightly enough observed that a starving sheep is as much a carnivore as a lion. 8. But though proteid matter is the essential element of food, and under certain circumstances may suffice, by itself, to maintain the body, it is a very disadvantageous and uneconomical food. Albumen, which may be taken as the type of the pro- teids, contains about 53 parts of carbon and 15 of nitrogen in 100 parts. If a man were to be fed on white of ^g%, therefore, he would take in, speaking roughly, 3^ parts of carbon for eveiy part of nitrogen. But it is proved experimentally, that a healthy, full- grown man, keeping up his weight and heat, and taking a fair amount of exercise, eliminates 4,000 grains of VI.] ADVANTAGE OF MIXED DIET. 137 carbon to only 300 grains of nitrogen, or, roughly, only needs one-thirteenth as much nitrogen as carbon. How- ever, if he is to get his 4,000 grains of carbon out of albumen, he must eat 7^547 grains of that substance. But 7,547 grains of albumen contain 1,132 grains of nitrogen, or nearly four times as much as he wants. To put the case in another way, it takes about four pounds of fatless meat (which generally contains about one-fourth its weight of dry solid proteids) to yield 4,000 grains of carbon, whereas one pound will furnish 300 grains of nitrogen. Thus a man confined to a purely proteid diet, must eat a prodigious quantity of it. This not only involves a great amount of physiological labour in comminuting the food, and a great expenditure of power and time in dissolving and absorbing it ; but throws a great quantity of wholly profitless labour upon those excretory organs, which have to get rid of the nitrogenous matter, three-fourths of which, as we have seen, is superfluous. Unproductive labour is as much to be avoided in phy- siological, as in political, economy ; and it is quite possible that an animal fed with perfectly nutritious, proteid matter should die of starvation : the loss of power in various operations required for its assimilation overbalancing the gain ; or the time occupied in their performance being too great to check waste with sufficient rapidity. The body, under these circumstances, falls into the condition of a merchant who has abundant assets, but who cannot get in his debts in time to meet his creditors. 9. These considerations lead us to the physiological justification of the universal practice of mankind in adopt- ing a mixed diet, in which proteids are mixed either with fats, or with amyloids, or with both. Fats may be taken to contain about 80 per cent, of carbon, and amyloids about 40 per cent. Now it has been seen that there is enough nitrogen to supply the waste of that substance per diem, in a healthy man, in a pound of fatless meat ; which also contains 1,000 grains of carbon, leaving a deficit of 3,000 grains of carbon. Rather more than half a pound of fat, or a pound o( sugar, will supply this quantity of carbon. The former, if properly subdivided, the latter, by reason of its 138 ELEMEXTARY PHYSIOLOGY. ri.rss. solubility, passes with great ease into the economy, the digestive labour of which is consequently reduced to a minimum. 10. Several apparently simple articles of food con- stitute a mixed diet in themselves. Thus butcher's meat commonly contains from 30 to 50 per cent, of fat. Bread, on the other hand, contains the proteid, gluten, and the amyloids, starch and sugar, with minute quantities of fat. But, from the proportion in which these proteid and other constituents exist in these substances, they are neither, taken alone, such physiologically economical foods as they are when combined in the proportion of about 200 to 75 ; or two pounds of bread to three-quarters of a pound of meat per diem. 11. It is quite certain that nine-tenths of the dry, solid food which is taken into the body sooner or later leaves it in the shape of carbonic acid, water, and urea (or uric acid) ; and it is also certain that the compounds which leave the body not only are more highly oxidized than those which enter it, but in them is carried away out of the body all the oxygen taken into the blood by the lungs. The intermediate stages of this conversion are, how- ever, by no means so clear. It is highly probable that the amyloids and fats arc very frequently oxidized in the blood, without, properly speaking, ever forming an integral part of the substance of the body ; but whether the proteids may undergo the same changes in the blood, or whether it is necessary for them first to be incorporated with the living tissue, is not positively known. So, again, it is certain that, in becoming oxidized, the elements of the food must give off heat, and it is probable that this heat is sufficient to account for all that is given off by the body ; but it is possible, and indeed 'probable, that there may be other minor sources of heat. 12. Food-stuffs have been divided into heat-prodiicers and tissiie-forjuers — the amyloids and fats constituting the former division, the proteids the latter. But this is a very misleading classification, inasmuch as it implies, on the one hand, that the oxidation of the proteids does not develop heat ; and, on the other, that the amyloids and fats, as they oxidize, subserve only the production of heat. VI.] THE CAI7TV OF THE MOUTH. 139 Proteids are tissite-fonners., inasmuch as no tissue can be produced without them ; but they are also heat- producers^ not only directly, but because, as we have seen (Lesson V. §§ 25, 26), that they are competent to give rise to amyloids by chemical metamorphosis within the body. If it is worth while to make a special classification of the vital food-stuffs at all, it appears desirable to dis- tinguish the essential food-stuffs, or proteids, from the accessory food-stuffs, or fats and amyloids — the former alone being, in the nature of things, necessary to life, while the latter, however important, are not absolutely necessary. 13. All food-stuffs being thus proteids, fats, amyloids, or mineral matters, pure or mixed up with other sub- stances, the whole purpose of the alimentary apparatus is to separate these proteids, iS:c. from the innutritious residue, if there be arn^^aid to reduce them into a con- dition either of solution^ oV of excessively fine subdivision, in order that they may make their way through the delicate structures which form the walls of the vessels ot the alimentary canal. To these ends food is taken into the mouth and masticated, is mixed with saliva, is swallowed, undergoes gastric digestion, passes into the intestine, and is subjected to the action of the secretions of the glands attached to that viscus ; and, finally, after the more or less complete extraction of the nutritive con- stituents, the residue, mixed up with certain secretions of the intestines, leaves the body as Xh^fceces. " The cavity of the mouth is a chamber with a fixed roof, formed by the hard palate (Fig. 40, /), and with a move- able floor, constituted by the lower jaw, and the tongue {k)^ which fills up the space between the two branches of the jaw. Arching round the margins of the upper and the lower jaws are the thirty-two teeth, sixteen above and sixteen below, and, external to these, the closure of the cavity of the mouth is completed by the cheeks at the sides, and, by the lips, in front. When the mouth is shut, the back of the tongue comes into close contact with the palate ; and, where the hard palate ends, the communication between the mouth and the back of the throat is still further impeded by a sort of I40 ELEMENTAR V FHYSIOL OG Y. [less. fleshy curtain — the soft palate or vehim — the middle of which is produced into a prolongation, the uvula (f)^ while its sides, skirting the sides of the passage, or fauces, f IG. 40. A Section' of the Mouth and No-e taken vertically, a little TO THE left of THE MlUDLE LiXE. a, the vertebral column ; b, the gullet ; f, the windpipe ; d, the thyroid cartilage of the larynx ; e, the epiglottis ; /, the uvula ; g, the opening of the left Eustachian tube ; /;, the opening of the left lachrymal duct ; /', the hyoid bone ; /•, the tongue ; /, the hard palate ; ;//, ti, the base of the skull ; o, p, q, the superior, middle, and inferior turbinal bones. The letters g,f, e are placed in the pharynx. form double muscular pillars, which are termed the pillars of the fauces. Between these the tonsils are situated, one on each side. VI.] THE SALIVARY GLANDS. 141 The velum with its uvula comes into contact below with the upper part of the back of the tongue, and with a sort of gristly, hd-like process connected with its base, the epiglottis {e). Behind the partition thus formed lies the cavity of the pharynx^ which may be described as a funnel-shaped bag with muscular walls, the upper margins of the slanting, wide end of which are attached to the base of the skull, while the lateral margins are continuous v/ith the sides, and the lower with the floor, of the mouth. The narrow end of the pharyngeal bag passes into the gullet or oesophagus (^), a muscular tube, which affords a passage into the stomach. There are no fewer than six distinct openings into the front part of the pharynx — four in pairs, and two single ones in the middle line. The two pairs are, in front, the hinder openings of the nasal cavities; and at the sides, close to these, the apertures of the Eustachian tubes {g). The two single apertures are, the hinder opening of the mouth between the soft palate and the epiglottis ; and, behind the epiglottis, the upper aperture of the respira- tory passage, or the glottis, 14. The mucous membrane ^which lines the mouth and the pharynx is beset with minute glands, the buccal glands ; but the great glands from which the cavity of the mouth receives its chief secretion are the three pairs which, as has been already mentioned, are z?i\\tdi parotid,, S!ib>na.villary, sublingual, and v.liich secrete the principal part of the saliva (Fig. 41). Each parotid gland is placed just in front of the ear, and its duct passes forwards along the cheek, until it opens in the interior of the mouth, opposite the second upper grinding tooth. The submaxillary and sublingual glands lie between the loA-er jaw and the floor of the mouth, the submaxillary b^ing situated further back than the sublingual. Their ducts open in the floor of the mouth below the tip of the tongue. The secretion of these salivary glands, mixed with that of the small glands of the mouth, constitutes the saliva — a fluid which, though thin and watery, con- tains a small quantity of animal matter, called Ptyali7ij which has certain very peculiar properties. It does not 142 ELEMENTARY PHYSIOLOGY. [less. act upon proteid food-stuffs, nor upon fats ; but if mixed with starch, and kept at a moderate warm temperature, it turns that starch into grape sugar. The importance of this operation becomes apparent when one reflects that Fig. 41. A dissection of the right site of the face, showing a, the sublingual ; /', the submaxillary glands, with their ducts opening beside the tongue in the floor of the mouth at d ; c, the parotid gland and its duct, which opens on the side of the cheek at e. starch is insoluble, and therefore, as such, useless as nutriment, while sugar is highly soluble, and readily oxidizable. 15. Each of the thirty-two teeth which have been mentioned consists of a citiwn which projects above the gum, and of one or more J^gs, which are embedded in sockets, or what are called alveoli^ in the jaws. The eight teeth on opposite sides of the same jaw are constructed upon exactly similar patterns, while the eight teeth which are opposite to one another, and bite against one another above and below, though similar in kind, differ somewhat in the details of their patterns. The two teeth in each eight which are nearest the middle line in the front of the jaw, have wide but sharp and chisel-like edges. Hence they are called incisors, 'I VI.] THE TEETH. 143 or cutting teeth. The tooth which comes next is a tooth with a more conical and pointed crown. It answers to the great tearing and holding tooth of the dog, and is called the cafiinc or eye-tooth. The next two teeth have broader crowns, with two cusps, or points, on each crown, one on the inside and one on the outside, whence they are termed bicuspid teeth, and sometimes false grinders. All these teeth have usually one fang each, except the bicuspid, the fangs of which may be more or less com- pletely divided into two. The remaining teeth have two or three fangs each, and their crowns are much broader. As they crush and grind the matters which pass between them they are called molars, or true grinders. In the upper jaw their crowns present four points at the four corners, and a diagonal ridge connecting two of them. In the lower jaw the complete pattern is five-pointed, there being two cusps on the inner side and three on the outer. The muscles of the parts which have been described have such a disposition that the lower jaw can be de- pressed, so as to open the mouth and separate the teeth ; or raised, in such a manner as to bring the teeth together ; or move obliquely from side to side, so as to cause the face of the grinding teeth and the edges of the cutting teeth to slide over one another. And the muscles which perform the elevating and sliding movements are of great ■^'strength, and confer a corresponding force upon the - grinding and cutting actions of the teeth. In correspond- ence with the pressure they have to resist, the superficial substance of the crown of the teeth is of great hardness, being formed of eua/ncl, which is the hardest substance in the body, so dense and hard, indeed, that it will strike fire with steel (see Lesson XII.). But notwithstanding its extreme hardness, it becomes worn down in old persons, and, at an earlier age, in savages who live on coarse food. 16. When solid food is taken into the mouth, it is cut and ground by the teeth, the fragments which ooze out upon the outer side of their crowns being pushed beneath them again by the muscular contractions of the cheeks and lips ; while those which escape on the inner side are thrust back by the tongue, until the whole is thoroughly rubbed down. 144 ELEMEXTARY niYSTOLOGY. [less. While mastication is proceeding, the sahvary glands pour out their secretion in great abundance, and the saliva mixes with the food, which thus becomes in- terpenetrated not only with the salivary fluid, but with the air which is entangled in the bubbles of the saliva. When the food is sufficiently ground it is collected, enveloped, in saliva, into a mass or bolus, which rests upon the back of the tongue, and is carried backwards to the aperture which leads into the pharynx. Through this it is thrust, the soft palate being lifted and its pillars being brought together, while the backward movement of the tongue at once propels the mass and causes the epiglottis to incline backwards and downwards over the glottis and so to form a bridge by which the bolus can travel over the opening of the air-passage without any risk of tumbling into it. While the epiglottis directs the course of the mass of food below, and prevents it from passing into the trachea^ the soft palate guides it above, keeps it out of the nasal chamber, and directs it downwards and backwards towards the lower part of the muscular pha- ryngeal funnel. By this the bolus is immediately seized and tightly held, and the muscular fibres contracting above it, while they are comparatively lax below, it is rapidly thrust into the oesophagus. By the muscular walls of this tube it is grasped and propelled onwards, in a similar fashion, until it reaches the stomach. ^ 17. Drink is taken in exactly the same way. It does not fall down the pharynx and gullet, but each gulp is grasped and passed down. Hence it is that jugglers are able to drink standing upon their heads, and that a horse, or ox, drinks with itsthroat lower than its stomach, feats which would be impossilDle if fluid simply fell down the gullet into the gastric cavity. During these processes of mastication, insalivation, and deglutition, what happens to the food is, first, that it is reduced to a coarser or finer pulp ; secondly, that any matters it carries in solution are still more diluted by the water of the saliva ; thirdly, that any starch it may con- tain begins to be changed into sugar by the peculiar con- .stituent (ptyalin) of the saliva. \v-i8x The stomach, like the gullet, consists of a tube "with muscular walls composed of smooth muscular fibres, (1^ VI.] GASTRIC JUICE. ■" 145 and lined by an epithelium ; but it difters from the i;ullet in several circumstances. In the first place, its cavity is greatly larger, and its left end is produced into an enlarge- ment which, because it is on the heart side of the body, is called the cardiac dilatation (Fig. 42, b). The opening of the gullet into the stomach, termed the cardiac aperture, is consequently nearly in the middle of the whole length of the organ, which presents a long, convex, greater curva- ture., along its front or under edge, and a short concave, lesser curvature., on its back or upper contour. Towards its right extremity the stomach narrows, and, where it passes into the intestine, the muscular fibres are so dis- posed as to form a sort of sphincter around the aperture of communication. This is called the pylorus (Fig. 42, d). ,^-^he mucous membrane lining the wall of the stomach &' very delicate, and multitudes of small glands open upon "ts surface. Some of these are simple, but others (Fig. 3) possess a somewhat more complicated structure, their blind ends being subdivided. It is these glands, and more especially the more complicated ones, the so- called peptic glands, which, when food passes into the stomach, throw out a thin acid fluid, the gastric Juice. When the stomach is empty, its mucous membrane is pale and hardly more than moist. Its small arteries are then in a state of contraction, and comparatively little blood is sent through it. On the entrance of food a nervous action is set up, which causes these small arteries to dilate ; the mucous membrane consequently receives a much larger quantity of blood, it becomes very red, little drops of fluid gather at the mouth of the glands, and finally run down as gastric juice. The process is very similar to the combined blushing and sweating which takes place when the sympathetic in the neck is divided. Pure gastric juice appears to consist of I'ttle more than water, containing a few saline matters in solution, and its acidity is due to the presence of free hydrochloric acid ; it possesses, however, in addition a small quantity of a peculiar substance called pepsin, which seems to be not altogether dissimilar in chemical composition to, though very different in its effects irom, ptyalin (§ 14). Thus, when the food passes into the stomach, the con- L 146 ELEMEXTARY PIIYSJOLOGY. [less. tractions of that organ roll it about and mix it thoroughly with the gastric juice. 19. It is easy to ascertain the properties of gastric juice experimentally, by putting a small portion of that part of the mucous membrane which contains the peptic glands into acidulated water containing small pieces of meat, Fig. -The Stomach laid oien behind. a, the oesophagus ; l>, the cardiac dilatation ; c, the lesser curvature ; d, the pylorus ; e, the biliary duct ; /, the gall-bladder ; g, the pancreatic duct, opening in coniTion with the cj-stic duct opposite //; //, /, the duodenum. hard-boiled Qgg, or other proteids, and keeping the mixture at a temperature of about 100^. After a few hours it will be found that the white of egg, if not in too great quantity, has become dissolved ; while all that remains of the meat is a pulp, consisting chiefly of the connective tissue and fatty matters which it contained. This is artificial digestion, and it has been proved by ex- periment that precisely the same operation takes place when food undergoes natural digestion within the stomach of a living animal. VI.] PEPTOXE. 147 The proteid solution thus effected is called a peptone, and has pretty much the same characters, whatever the nature of the proteid which has been digested. Peptone differs from all other proteids in its extreme solubility, and in the readiness with which it passes through animal membranes. :Many proteids. as fibrin, Fig. 43. One f f the glands which secrete the gastric juice, magnified about 350 diameters. , are naturally insoluble in water, and others, such as white of egg, though apparently soluble, are not completely so, and can be rendered quite solid or coagulated by being simply heated, as when an egg is boiled. A solution of pep- tone however is perfectly fluid, does not become solid, and L 2 1 48 ELEMENTA R Y PH\ 'SIOL OGY. [less. is not at all coagulated by boiling. Again, if a quantity of white of ^^g be tied up in a bladder, and the bladder immersed in water, very little of the proteid will pass through the bladder into the water, provided that there are no holes. If, however, peptone be used instead of albumin, a very large quantity will speedily pass through into the water, and a quantity of water will pass from the outside into the bladder, causing it to swell up. This process is called osmosis^ and is evidently of great import- ance in the economy ; and the purpose of the conversion of the various proteids by digestion into peptone seems to be, in part at least, to enable this class of food-stuff to pass readily into the blood through the thin partition formed by the walls of the mucous membrane of the intes- tine and the coats of the capillaries. Similarly, starch, even when boiled, and so partially dissolved, will not pass through membranes, whereas sugar does so with the greatest ease. Hence the reason of the conversion of starch, by digestion, into sugar. It takes a very long time (some days) for the dilute acid alone to dissolve proteid matters, and hence the solvent power of gastric juice must be chiefly attributed to the pepsin. As far as we know gastric juice has no direct action on fats ; by breaking up, however, the proteid framework in which animal and vegetable fats are imbedded, it sets these free, and so helps their digestion by exposing them to the action of other agents. It appears,' too. that gastric juice has no direct action on amyloids ; on the contrary, the conversion of the starch into sugar begun in the mouth appears to be wholly or partially arrested by the acidity of the contents of the stomach, ptyalin being active only in an alkaline or neutral mixturer"^"^ 20. By continual rolling about, with constalit additions of gastric juice, the food becomes reduced to the con- sistence of pea-soup, and is called (J->^j/;/t'. In this state it is, in part, allowed to escape through the pylorus and to enter the duodenum ; but a great deal of the fluid (consisting of peptone together with any saccharine fluids re^3ulting from the partial conversion of starch, or other- wise) is at once absorbed, making its way, by imbibition, through the walls of the delicate and numerous vessels of VI.] ABDOMT.WAL VISCERA. if Fir,. 44. — The Viscera of a Rabbit as skhx iimx simply otexing the Cavities of the Tiiokax" and Ar,i)n^:i;N' without any fukthf.k Dissix I ion. A. Casity of the thorax, pleural cavitj^ of either side; B. Diaphragm ; 150 ELEMENTARY PHYSIOLOGY. [less. the stomach into the current of the blood, which is rush- ing though the gastric veins to the vena porta!. 21. The intestijies form one long tube, with mucous and muscular coats, like the stomach ; and, like it, they are enveloped in peritoneum. They are divided into two por- tions— the small intestines and the large intestines j the latter having a much greater diameter than the former. The small intestines again are subdivided into the duodenum., the jejunum, and the ileum^ but there is no natural line of demarcation between these. The duodenum, however, is distinguishable as that part of the small intestine which immediately succeeds the stomach, and is bent upon itself and fastened by the peritoneum against the back wall of the abdomen, in the loop shown in Fig. 42. It is in this loop that the head of the pancreas lies (Fig. 38). The ileum (Fig. 45, a) is no wider than the jejunum or duodenum, so that the transition from the small intestine to the large {e) is quite sudden. The opening of the small intestine into the large is provided with prominent lips which project into the cavity of the latter, and oppose the passage of matters from it into the small intestine, while they readily allow of a passage the other way. This is the ileo-co'cal valve (Fig. 45, d). The large intestine forms a blind dilatation beyond the ileo-Ccecal valve, which is called the ccpcumj and from this an elongated, blind process is given off, which, from its shape, is called the vermiform appendix of the caecum (Fig. 45,^). The csecum lies in the lower part of the right side of the abdominal cavity. The colon, or first part of the large intestine, passes upwards from it as the ascending colon; then making a sudden turn at a right angle, it passes across to the left side of the body, being called the C, ventricles of the heart ; D, auricles ; E, pulmonary artery ; F, aorta ; G, lungs, collapsed, and occupying only back part of chest ; //, lateral portions of pleural membranes ; /, cartilage at the end of sternum (ensiform cartilage) ; A', portion of the vail of body left between thorax and abdomen ; a, cut ends of the ribs ; L, the liver, in this case lying more to the left than the right of the body ; M, the stomach, a large part of the greater curvature being shown ; ^V, duodenum ; O, small intestine ; P, the caecum, so largely developed in this and other herbivorous animals ; Q, the large intestine. VI.] THE LARGE LXTESTINE. 151 transverse colon in this part of its course ; and next, suddenly bending backwards along the left side of the Fk;. 45. The termination of the ileum, a, in the caecum, and the continuation of the latter into the colon, c ; d. the ileo-caecal valve ; e, the aperture of the a/>/>endix vcrmifortnis (/-) into the cjecum. (iV"^' /'''/^^ / / %^ J? Fig. 46. — Semi-diagram.matic View of Two Villi of the Small Intestines. (Magnified about 50 diameters.) , subiytance of the villus ; h, its epithelium, of which some cells are seen detached at /-' ; c d, the artery and vein, with th;ir connecting capillary network, which envelopes and hides e, the lacteal radicle which occupies the centre of the villus and opens into a network of lacteal vessels at its base. 152 ELEMEXTARY niySIOLOGY. [i.r.ss. abdomen, it becomes the descending colon. This reaches the middle hne and becomes the recUun, which is that part of the large intestine which opens externally. 22, The mucous membrane of the whole intestine is provided with numerous small and simple glands (named after Lieberkiihn), which pour into it a secretion, the intestinal juice., the precise functions of which are unknown, though it appears in some creatures at least to possess the power of converting starch into sugar, and proteids into peptone. At the commencement of the duodenum are certain racemose glands, called the glands of Brunner, whose function is wholly unknown. Structures peculiar to the small intestine are the 7'alvulce conniventes., transverse folds of the mucous membrane, which increase the surface ; and the villi, which are minute thread-like processes of the mucous membrane on the valvulce connivcfi^s and elsewhere, set side by side, like the pile of velvet. Each villus is coated by epithelium, and contains in its interior the radicle, or commencement, of a lacteal) vessel (Lesson II. § 6), between which and the epithelium lies a capillary net- work with its afferent artery and efferent vein. The intestines receive their blood almost directly from the aorta. Their veins carry the blood which has tra- versed the intestinal capillaries to the vena portce. The fibres of the muscular coat of the intestines (which lies between the mucous membrane and the serous, or peritoneal, investment) are disposed longitudinally and circularly ; the longitudinal coat being much thinner, and ^ ' placed outside the circular coat. Now the circular fibres -y of any part contract, successively, in such a manner that the lower fibres, or those on the side of the anus, contract after the upper ones, or those on the side of the pylorus. It follows from this so-called peristaltic contraction., that the contents of the intestines are constantly being pro- pelled, by successive and progressive narrowing of their calibre, from their upper towards their lower parts. The large intestine presents noteworthy peculiarities in the arrangement of the longitudinal muscular fibres of the colon into three bands, which are shorter than the walls of the intestine itself, so that the latter is thrown into puckers and pouches ; and in the disposition of VI.] PA XCREATIC JUICE. 15J muscular fibres around the tennination of the rectum imo a ring-Hke sphincter muscle, which keeps the aperture firmly closed, except when defecation takes place. 23. The only secretions, besides those of the proper intestinal glands, which enter the intestine, are those of the liver and the pancreas — the bile and the pancreatic in ice. The ducts of these organs ha^'e a common opening in the middle of the bend of the duodenum; and, since the common duct passes obliquely through the coats of the intestine, its walls serve as a kind of valve, obstructing the flow of the contents of the duodenum into the duct, but readily permitting the passage of bile and pancreatic juice into the duodenum (Figs. 36, 38, 42). Pancreatic juice is an alkaline fluid not unlike saliva in many respects ; it differs, however, in containing a con- siderable quantity of proteid material. Bile we have already studied. After gastric digestion has been going on some time, /and the semi-digested food begins to pass on into tlie /duodenum, the pancreas comes into activity, its blood- vessels dilate, it becomes red and full of blood, its cells secrete rapidly, and a copious How of pancreatic juice takes place along its duct into the intestine. ' The secretion of bile by the liver is much more con- tinuous than that of the pancreas, and is not so markedly increased by the presence of food in the stomach. There is, however, a store of bile laid up in the gall-bladder; and as the acid chyme passes into the duodenum, and flows over the common aperture of the gall and pancreatic ducts, a quantity of bile from this reservoir in the gall-bladder is ejected into the intestine. The bile and pancreatic juice together here mix with the rhyme and convert it into what is called chyle. 3lb^. Chyle differs from chyme in two respects. In the firsTplace, the alkali of the bile neutralizes the acid of the chyme ; in the second place, both the bile and the pancreatic juice appear to exercise an influence over the fatty matters contained in the chyme, which facilitates the subdivision of these fats into very minute separate particles. The chyme, in fact, which results from the digestion of fatty food, is a mere mixture of watery fluid with oily matters, which are ready to separate from it 154 ELEMENTARY PHYSIOLOGY. [less. and unite with one another. In the chyle, on the other hand, the fatty matters are suspended in the fluid, just as oil may be evenly diffused through water by gradually rubbing it up with white of Qji These fibres are bound up by fibrous (or connective)"'' tissue with blood-vessels, &c. into small bundles ; and these bundles are again similarly bound up together in various ways so as to form muscles of various shapes and sizes. Every fibre has the power, under certain condi- tions, of shortening in length, while it increases its other dimensions, so that the absolute volume of the fibre remains unchanged. This power is called muscular contractility J and whenever, in virtue of this power, a muscular fibre contracts^ it tends to bring its two ends, with whatever may be fastened to them, together. The condition which ordinarily determines the con- traction of a muscular fibre is a change of state in a V vir.] RIGOR MORTIS. IS9 nerve fibre, which is in close anatomical connection with the muscular fibre. The nerve fibre is thence called a violor fibre, because, by its influence on a muscle, it becomes the indirect means of producing motion (Lesson XI. § 6.). Muscle is a highly elastic substance. It contams a large amount of water (about as much as the blood), and during life has a clear and semi-transparent aspect. When subjected to pressure in the perfectly fresh state, and after due precautions have been taken to remove all the contained hXood, striated muscle (Lesson XII. § 15) yields a fluid which undergoes spontaneous coagulation at ordinary temperatures. At a longer or shorter time after death this coagulation takes place within the muscles themselves. They become more or less opaque, and, losing their previous elasticity, set into hard rigid masses, which retain the form which they possess when the coagu- lation commences. Hence the limbs become fixed in the position in which death found them, and the body passes into the condition of what is termed the "death-stiffening," or rigor mortis. After the lapse of a certain time the coagulated matter liquefies, and the muscles pass into a loose and flaccid condition, which marks the commencement of putrefaction. It has been observed that the sooner rigor mortis sets in, the sooner it is over ; and the later it commences, the longer it lasts. The greater the amount of muscular exertion and consequent exhaustion before death, the sooner rigor mortis sets in. Rigor mortis evideatly presents some analogies with the coagulation of the blood, and the substance which thus coagulates within the fibre {myosin or muscle-clot as it is sometimes called) is in many respects not unlike fibrin. It forms at least the greater part of the substance which may be extracted from muscle by dilute acids, and is called syntonin (see Lesson VI. § 4). Besides myosin, muscle contains other varieties of proteid material about which we at present know little ; a variable quantity of fat ; certain inorganic saline matters, phosphates and potash being, as is the case in the red blood-corpuscles, in excess ; and a large number of substances existing in small quantities, and often classed together as " cxtrac- i6d elementary physiology. [less. Lives." Some of these extractives contain nitrogen ; the most important of this class is krcatin^ a crystalHne body which is supposed to be the chief form in which nitrogenous waste matter leaves the muscle on its way to become urea. The other class of extractives contains bodies free from nitrogen. Perhaps the most important of these is lactic acid, which seems always to be formed when a muscle contracts or when it enters into rigor mortis. For it is a curious fact that a muscle when at rest has a neutral or alkaline reaction as shown by testing it with litmus, but becomes acid when it has been contracting for some time or become rigid by death. Most muscles are of a deep, red colour ; this is due in part to the blood remaining in their vessels ; but only in part, for each fibre (into which no capillary enters) has a reddish colour of its own, like a blood-corpuscle but fainter. And this colour is probably due to the fibre possessing a small quantity of that same haemoglobin in which the blood-corpuscles are so rich. Muscles may be conveniently divided into two groups, according to the manner in which the ends of their fibres are fastened ; into muscles not attached to solid levers, and muscles attached to solid levers. 5. Muscles not attached to solid levers. — Under this head come the muscles which are appropriately called hollo'u muscles, inasmuch as they enclose a cavity or surround a space ; and their contraction lessens the' capacity of that cavity, or the extent of that space. The muscular fibres of the heart, of the blood-vessels, of the lymphatic vessels, of the alimentary canal, of the urinary bladder, of the ducts of the glands, of the iris ol the eye, are so arranged as to form hollow muscles. In the heart the muscular fibres are of the striated kind, and their disposition is exceedingly complex. The cavities which they enclose are those of the auricles and ventricles ; and, as we have seen, the fibres, when they contract, do so suddenly, and together. The iris of the eye is like a curtain, in the middle of which is a circular hole. The muscular fibres are of the smooth or unstriated kind (see Lesson XII.), and they are disposed in two sets : one set radiating from the edges of the hole to the circumference of the curtain ; and the VII.] MUSCL ES A TTA CHED TO LE FEA'S. 1 6 1 other set arranged in circles, concentrically with the aper- ture. The muscular fibres of each set contract suddenly and together, the radiating fibres necessarily enlarging the hole, the circular fibres diminishing it. In the alimentary canal the muscular fibres are also of the unstriated kind, and they are disposed in two layers ; one set of fibres being arranged parallel with the length of the intestines, while the others are disposed circularly, or at right angles to the former. As has been stated above (Lesson VI. § 22), the contrac- tion of these muscular fibres is successive ; that is to say, all the muscular fibres, in a given length of the intestines, do not contract at once, but those at one end contract first, and the others follow them until the whole series have contracted. As the order of contraction is, naturally, always the same, from the upper towards the lower end, the effect of this peristaltic contraction is, as we have seen, to force any matter contained in the alimentar}^ canal, from its upper towards its lower extremity. The muscles of the walls of the ducts of the glands have a substantially similar arrangement. In these cases the contraction of each fibre is less sudden and lasts longer than in the -xidLSQ of the heart. L I 6. Muscles attached to definite levo's. — The great ma- jority of the muscles in the body are attached to distinct levers, formed by the bones, the minute structure of which is explained in Lesson Xll. § 11. In such bones as are ordinarily employed as levers, the osseous tissue is arranged in the form of a shaft (Fig. 47, ^), formed of a very dense and compact osseous matter, but often contain- ing a great central cavity [b) which is filled with a very delicate vascular and fibrous tissue loaded with fat called ina7'j'ow. Towards the two ends of the bone, the compact matter of the shaft thins out, and is replaced by a much thicker but looser sponge-work of bony plates and fibres, which is termed the cancellous tissue of the bone. The surface even of this part, however, is still formed by a thin sheet of denser bone. At least one end of each of these bony levers is fashioned into a smooth, articular surface, covered with cartilage, which enables the relatively fixed end of the bone to play upon the corresponding surface of some other bone with M 1 62 ELEMENTAR Y PH YSIOL OGY. [less. which it is said to be artiailated (see § 1 1), or, contrariwise, allows that other bone to move upon it. VII.] KAVDS OF LEVERS. 163 It is one or other of these extremities which plays the part of fulcrum when the bone is in use as a lever. Thus, in the accompanying figure (Fig. 48) of the bones of the upper extremity, with the attachments of the biceps muscle to the shoulder-blade and to one of the two bones of the fore-arm called the radius; P indicates the point of action of the power (the contracting muscle) upon the radius. Fig. 48.— The Bones of the Upper Extrs^uty with the Biceps Muscle. The two tendons by which this muscle is attached to the scapula are seen at a. P indicates the attachment of the muscle to the radius, and hence the point of action of the power ; F, the fulcrum, the lower end of the humerus on which the upper end of the radius (together with the ulna) moves ; W, the weight (of the hand}. But to understand the action of the bones, as levers, properly, it is necessary to possess a knowledge of the different kinds of levers, and be able to refer the various combinations of the bones to their appropriate lever- cl^^es. -^ A lever is a rigid bar, one part of which is absolutely or relatively fixed, while the rest is free to move. Some one point of the moveable part of the lever is set in motion by a force, in order to communicate more or less of that motion to another point of the moveable part, which pre- sents a resistance to motion in the shape of a weight or other obstacle. M 2 164 ELEMENTARY PHYSIOLOGY. [less. Three kinds of levers are enumerated by mechanicians, the definition of each kind depending upon the relative positions of the point of support, orfidcrwnj of the pomt which bears the resistance, weight, or other obstacle to be overcome by the force ; and of the point to which the force, or power employed to overcome the obstacle, is applied. If the fulcrum be placed between the power and the weight, so that, when the power sets the lever in motion, the weight and the power describe arcs, the concavities of which are turned towards one another, the lever is said to be of the/rj-/ order. (Fig. 49, 1.) ■^^ f Fig. IIL The upper three figures represent the three kinds of levers ; the lower, the foot, when it takes the character of each kind. — W, weight or resist- ance ; F, fulcrum ; P, power. If the fulcrum be at one end, and the weight be between it and the power, so that weight and power describe con- centric arcs, the weight moving through the less space when the lever moves, the lever is said to be of the second order. (Fig. 49, II.) And if, the fulcrum being still at one end, the power be between the weight and it, so that, as in the former case, the power and weight describe concentric arcs, but the power moves through the less space, the lever is of the third order. (Fig. 49, III.) 7. In the human body, the following parts present ex- amples of levers of the first order. VII.] EXAMPLES OF LEVERS. 165 {a) The skull in its movements upon the atlas, as fiil- cruin. {b) The pelvis in its movements upon the heads of the thigh-bones, 2i'i fulcrum. u"- - {c) The loot, when it is raised, and the toe tapped on the [:;> ground, the ankle-joint \i€\wsr Here the fulcnmi lies where the rib is articulated with 1 This muscle, called rectus, is attached above to the haunch-bone and below to the knee-cap (Fig. 2, 2, p. 12). The latter bone is connected by a strong ligament with the tibia. 2 This muscle lies in the front abdominal wall on each side of the middle line. It is attached to the sternum above and to the front of the pelvis below (Fig. 2, 3). l66 ELEMENTARY PHYSIOLOGY. [less. the spine ; the power is at the sternum — virtually the opposite end of the rib ; and the resistance to be over- come lies between the two. {c) The raising of the body upon the toes, in standing on tiptoe, and in the first stage of making a step forwards, (Fig. 49, II.) Here the fulcrum is the ground on which the toes rest ; the power is applied by the muscles of the calf to the heel (Fig. 2, I.) ; the resistance is so much of the weight of the body as is borne by the ankle-joint of the foot, which of course lies between the heel and the toes. 9. Three examples of levers of the third order are — \a) The spine, head, and pelvis, considered as a rigid bar, which has to be kept erect upon the hip-joints. Here the fulcrum lies in the hip-joints ; the weight is at the centre of gravity of the head and trunk, high above the fulcrum ; the power is supplied by the extensor, or flexor, muscles of the thigh, and acts upon points comparatively close to the fulcrum. (Figs. 2, 2, and II.) ib) Flexion of the forearm upon the arm by the biceps muscle, when a weight is held in the hand. In this case, the weight being in the hand and the ful-. crum at the elbow-joint, the power is applied at the point of attachment of the tendon of the biceps, close to the latter. (Fig. 48.) (<;) Extension of the leg on the thigh at the knee-joint. Here the fulcrum is the knee-joint ; the weight is at the centre of gravity of the leg and foot, somewhere between the knee and the foot ; the power is applied by the muscles in front of the thigh (Fig. 2, 2) through the ligament of the knee-cap, or patella, to the tibia, close to the knee- joint. 10. In studying the mechanism of the bodv, it is verv important to recollect that one and the same 'part of the body may represent each of the three kinds of levers, according to circumstances. Thus it has been seen that the foot may, under some circumstances, represent a lever of the first, in others, of the second order. But it may become a lever of the third order, as when one dances a weight resting upon the toes, up and down, by moving only the foot. In this case, the fulcrum is at the ankle- VII.] yoiNTS. 167 joint, the weight is at the toes, and the power is furnished by the extensor muscles at the front of the leg (Fig. 2, i), which are inserted between the fulcrum and the weight. (Fig. 49, ni.) 11. It is very important that the levers of the body should not slip, or work unevenly, when their movements are extensive, and to this end they are connected together in such a manner as to form strong and definitely ar- ranged y^/;//jr or articulations. Joints may be classified into imperfect and perfect. {a) bnperfect joints are those in which the conjoined levers (bones or cartilages) present no smooth surfaces, capable of rotatory motion, to one another, but are con- nected by continuous cartilages, or ligaments, and have only so much mobility as is permitted by the flexibility of the joining substance. Examples of such joints as these are to be met with in the vertebral column— the flat surfaces of the bodies of the vertebrae, being connected together by thick plates of very elastic fibro-cartilage, which confer upon the whole column considerable play and springiness, and yet prevent any great amount of motion between the several vertebrae. In the pelvis (see Plate, Fig. VI.), the pubic bones are united to each other in front, and the ihac bones to the sacrum behind, by fibrous or cartilaginous tissue, which allows of only a slight play, and so gives the pelvis a little more elasticity than it would have if it were all one bone. {b) In all perfect joints, the opposed bony surfaces which move upon one another are covered with cartilage, and between them is placed a sort of sac, which lines these cartilages, and, to a certain extent, forms the side walls of the joint ; and which, secreting a small quantity of viscid, lubricating fluid — the synovia — is called a syno- vial viembrayie. 12. The opposed surfaces of these articular cartilages, as they are called, may be spheroidal, cylindrical, or pulley-shaped ; and the convexities of the one answer, more or less completely, to the concavities of the other. Sometimes, the two articular cartilages do not come directly into contact, but are separated by independent plates of cartilage, which are termed inter-articular. The i68 ELEMENTAR V PHYSIO LOG V. [les?;. opposite faces of these inter-articular cartilages are fitted to receive the faces of the proper articular cartilages. While these co-adapted surfaces and synovial mem- branes provide for the free mobility of the bones entering into a joint, the nature and extent of their motion is Fig. 50. — A Section of the Hip Joint taken THRorcH the Aceta- bulum OR Articular Cup of the Pelvis and the Middle ok the Head and Neck of the Thigh-bone. L.T. Ligamentum teres, or round ligament. The spaces marked with an interrupted line (----) represent the articular cartilages. The cavity of the synovial membrane is indicated by the dark line between these, and as is shown, extends along the neck of the femur beyond the limits of the cartilage. The peculiar shape of the peMs causes the section to have the remarkable outline shown in the cut. This will be intelligible if compared with Fig. VI. in the Plate. defined, partly by the forms of the articular surfaces, and partly by the disposition of the ligaiue?its, or firm, fibrous cords which pass from one bone to the other. 13. As respects the nature of the articular surfaces, joints may be what are called ball and socket joints, when VII.] HIXGE yOlNlS. 169 the spheroidal surface furnished by one bone plays in a cup furnished by another. In this case the motion of the former bone may take place in any direction, but the extent of the motion depends upon the shape of the cup— being- very great when the cup is shallow, and small in propor- tion as it is deep. The shoulder is an example of a ball and socket joint with a shallow cup ; the hip of such a joint with a deep cup (Fi g- D o\ Fig. 51.— LoN-GlTfDIXAL AND VERTICAL SecTION THROfGH THE Elbow-joint. H. humerus ; Ul. ulna ; Tr. the triceps muscle which extends the arm ; Bi. the biceps muscle which flexes it. 14. Hinge-joints are single or double. In the former case, the nearly cylin^ical head of one bone fits into a corresponding socket of the other. In this form of hinge- joint the only motion possible is in the direction of a plane perpendicular to the axis of the cylinder, just as a door can f. 1 70 ELEMENTAR Y PHYSIOL OGY. [less. only be made to move round an axis passing through its hinges. The elbow is the best example of this joint in the human body, but the movement here is limited, because the olecranon, or part of the ulna which rises up behind the humerus, prevents the arm being carried back behind the straight line ; the arm can thus be bent to, or straight- ened, but not bent back (Fig. 51). The knee and ankle present less perfect specimens of a single hinge-joint. A double hinge-joint is one in which the articular surface of each bone is concave in one direction, and convex in another, at right angles to the former. A man seated in a saddle is " articulated " with the saddle by such a joint. For the saddle is concave from before backwards, and convex from side to side, while the man presents to it the concavity of his legs astride, from side to side, and the convexity of his seat, from before backwards. The metacarpal bone of the thumb is articulated with the bone of the wrist, called trapeziu?n^ by a double hinge- joint. 15. A pivot-join i is one in which one bone furnishes an axis, or pivot, on which another turns ; or itself turns on its own axis, resting on another bone. A remarkable example of the former arrangement is afforded by the atlas and axis, or two uppermost vertebrae of the neck (Fig. 52). The axis possesses a vertical peg, the so-called odontoia process (b), and at the base of the peg are two, obliquely placed, articular surfaces {a) The atlas is a ring-like bone, with a massive thickening on each side. The inner side of the front of the ring plays round the neck of the odon- toid peg, and the under surfaces of the lateral masses glide over the articular faces on each side of the base of the peg. A strong ligament passes between the inner sides of the two lateral masses of the atlas, and keeps the hinder side of the neck of the odontoid peg in its place (Fig. 52, A). By this arrangement, the atlas is enabled to rotate through a considerable angle either way upon the axis, without any danger of falling forwards or backwards — accidents which would immediately destroy life by crushing the spinal marrow. The lateral masses of the atlas have, on their upper faces, concavities (Fig. 52, A, a) into which the two convex, occipital condyles of the skull fit, and in which they play VII.] PIVOT yoiXTs. 171 upward and downward. Thus the nodding of the head is effected by the movement of the skull upon the atlas ; while, in turning the head from side to side, the skull does not move upon the atlas, but the atlas slides round the odontoid peg of the axis vertebra. Fig. 52. A. The Atlas \'iewed from above : a a, upper articular surfaces of its lateral masses for the condyles of the skull ; b, the peg of the axis vertebra. B. Side view of the axis vertebra : a, articular surface for the lateral mass of the atlas ; b, peg or odontoid process. The second kind of pivot-joint is seen in the forearm. If the elbow and forearm, as far as the wrist, are made to rest upon a table, and the elbow is kept firmly fixed, the hand can nevertheless be freely rotated so that either the palm, or the back, is turned directly upwards. When the palm is turned upwards, the attitude is called sicpination (Fig. 53, A') ; when the \)^z\ pro7iation (Fig. 53, B). The forearm is composed of two bones ; one, the tilna^ which articulates with the humerus at the elbow by the hinge-joint already described, in such a manner that it can move only in flexion and extension (see § 17), and has no power of rotation. Hence, when the elbow and wrist are rested on a table, this bone remains unmoved. But the other bone of the forearm, the radius, has its small upper end shaped like a very shallow cup with thick edges. The hollow of the cup articulates with a sphe- roidal surface furnished by the humerus ; the lip of the cup, with a concave depression on the side of the ulna. The large lower end of the radius bears the hand, and has, on the side next the ulna, a concave surface, which articulates with the convex side of the small lower end of that bone. 172 ELEMEIVTAR V PHYSIOL OGV [less. Thus the upper end of the radius turns on the double surface, furnished to it by the pivot-hke ball of the humerus, and the partial cup of the ulna : while the lower end of the radius can rotate round the surface furnished to it by the lower end of the ulna. In supi?iation,\he radius lies parallel with the ulna, with its lower end to the outer side of the ulna (Fig. 53, A). In Fig. S3- The bones of the tight forearm in supination (A) and pronation (B). H. humerus ; R. radius ; U. ulna. pro?iati09i, it is made to turn on its own axis above, and round the ulna below, until its lower half crosses the ulna, and its lower end lies on the inner side of the ulna (Fig. 53, B). 16. The ligaments which keep the mobile surfaces of bones together are, in the case of ball and socket joints. VII.] LIG.iMEXrS. 173 strong fibrous capsules which surround the joint on all sides. In hinge-joints, on the other hand, the ligamentous tissue is chiefly accumulated, in the form of lateral liga- 7ne?its, at the sides of the joints. In some cases ligaments are placed within the joints^ as in the knee, where the bundles of fibres which cross obhquely between the femur and the tibia are called a-ucial ligaments ; or, as in the hip, where the round ligament passes from the bottom of the socket or acetabulum of the pelvis to the ball furnished by the head of the femur (Fig. 50). Again, two ligaments pass from the apex of the odon- toid peg to either side of the margins of the occipital foramen, i.e. the large hole in the base of the skull, through which the spinal cord passes to join the brain ; these, from their function in helping to stop excessive rotation of the skull, are called check ligaments (Fig. 54, a). Fig. 54. -" The vertebral column in the upper part of the neck laid open, to show a, the check ligaments of the axis ; h, the broad ligament which extends from the front margin of the occipital foramen along the hinder faces of the bodies of the vertebrae : it is cut through, and the cut ends turned hack to show, c, the special ligament which connects the point of the " odon- toid " peg with the front margin of the occipital foramen ; /. the atlas ; //. the axis. In one joint of the body, the hip, the socket or aceta- bulum (Fig. 50) fits so closely to the head of the femur, and the capsular ligament so completely closes its cavity on 1 74 ELEMENTAR V PHYSI OLOGY. [less. all sides, that the pressure of the air must be reckoned among the causes which prevent dislocation. This has deen proved experimentally by boring a hole through the floor of the acetabulum, so as to admit air into its cavity, when the thigh-bone at once falls as far as the round and capsular ligaments will permit it to do, showing that it was previously pushed close up by the pressure of the external air. 17. The different kinds of movement which the levers thus connected are capable of performing, are called flexion and extension j abdiiction and adduction j rotation and circumduction. A limb is flexed, when it is bent ; extended, when it is straightened out. It is abducted, when it is drawn away from the middle line ; adducted, when it is brought to the middle line. It is rotated, when it is made to turn on its own axis ; circumducted, when it is made to describe a conical surface by rotation round an imaginary axis. No part of the body is capable of perfect rotation like a wheel, for the simple reason that such motion would necessarily tear all the vessels, nerves, muscles, (Sec. which unite it with other parts. 18. Any two bones united by a joint may be moved one upon another in, at fewest, two different directions. In the case of a pure hinge-joint, these directions must be opposite and in the same plane ; but, in all other joints, the movements may be in several directions and in various planes. In the case of a pure hinge-joint, the two practicable movements— viz. flexion and extension — may be effected by means of two muscles, one for either movement, and running from one bone to the other, but on opposite sides of the joint. When either of these muscles contracts, it will pull its attached ends together, and bend or straighten, as the case may be, the joint towards the side on which it is placed. Thus the biceps muscle is attached, at one end, to the shoulder blade, while, at the other end, its tendon passes in front of the elbow-joint to the radius (Figs. 48 and 51) ; when this muscle contracts, therefore, it bends, or flexes, the forearm on the arm. At the back of the joint there is the triceps (7>. Fig. 51) ; when this con- tracts, it straightens, or extends, the forearm on the arm. VII. ] ORIGIN AND INSER TION OF MUSCLES. 1 75 In the other extreme form of articulation — the ball and '.iocket joint — movement in any number of planes may be effected, by attaching muscles in corresponding number and direction, on the one hand, to the bone which affords the socket, and on the other to that which furnishes the head. Circumduction will be effected by the combined and successive contraction of these muscles. 19. It usually happens that the bone to which one end of a muscle is attached is absolutely or relatively sta- tionary, while that to which the other is fixed is moveable. In this case, the attachment to the stationary bone is termed the origin^ that to the moveable bone the instTiiou, of the muscle. The fibres of muscles are sometimes fixed directly into the parts which serve as their origins and insertions : but, more commonly, strong cords or bands of fibrous tissue, called tendons, are interposed between the muscle proper and its place of origin or insertion. When the tendons play over hard surfaces, it is usual for them to be separated from these surfaces by sacs containing fluid, which are called biirscEj or even to be invested by synovial sheaths, i.e, quite covered for some distance by a synovial bag forming a double sheath very much in the same way that the bag of the pleura covers the lung and the chest wall. Usually, the direction of the axis of a muscle is that of a straight line joining its origin and its insertion. Btrt in some muscles, as the superior oblique nuiscle of the eye, the tendon passes over a pulley formed by ligament, and completely changes its direction before reaching its inser- tion. (See Lesson iX.) Again, there are muscles which are fleshy at each end, and have a tendon in the middle. Such muscles are called digastric, or two-bellied. In the curious muscle which pulls down the lower jaw, and specially receives this name of digastric, the middle tendon runs through a pulley connected with the hyoid bone ; and the muscle, which passes downwards and forwards from the skull to this pulley, after traversing it, runs upwards and forwards, to the lower jaw (Fig. 55). 20. We may now pass from the consideration of the mechanism of mere motion to that of locomotion. When a man who is standing erect on both feet pro- 176 ELEMENTAR Y PHYSIOL 0 G Y. [less. ceeds to lualk^ beginning with the right leg, the body is inchned so as to throw the centre of gravity forward ; and, the right foot being raised, the right leg is advanced for the length of a step, and the foot is put down again. In the meanwhile, the left heel is raised, but the toes of the left foot have not left the ground when the right foot has reached it, so that there is no moment at which both Fig. 55.— Thk Course of the Digastric Muscle. D, its posterior belly ; D', its anterior belly ; between the two is the tendon passing through its pulley connected with Hy. the hyoid bone. feet are off the ground. For an instant, the legs form two sides of an equilateral triangle, and the centre of the body is consequently lower than it was when the legs were parallel and close together. The left foot, however, has not been merely dragged away from its first position, but the muscles of the calf, having come into play, act upon the foot as a lever of the second order, and thrust the body, the weight of which rests largely on the left astragalus, upwards, forwards, and to the right side. The momentum thus communicated to the body causes it, with the whole right leg, to describe an arc over the right astragalus, on which that leg rests below. The centre of the body consequently rises to its former height as the right leg becomes vertical, and descends again as the right leg, in its turn, inclines forward. When the left foot has left the ground, the body is supported on the right leg, and is well in advance of the left foot ; so that, without any further muscular exertion, VII.] WALKING AND RUNNING. 177 the left foot swings forward like a pendulum, and is carried by its own momentum beyond the right foot, to the position in which it completes the second step. When the intervals of the steps are so timed that each swinging leg comes forward into position for a new step without any exertion on the part of the walker, walking is effected with the greatest possible economy of force. And, as the swinging leg is a true pendulum, — the time of vibration of which depends, other things being alike, upon its length (short pendulums vibrating more quickly than long ones), — it follows that, on the average, the natural step of short-legged people is quicker than that of long- legged ones. In runni^ig, there is a period when both legs are off the ground. The legs are advanced by muscular contraction, and the lever action of each foot is swift and violent. Indeed, the action of each leg resembles, in violent running, that which, when both legs act together, consti- stute a Jump, the sudden extension of the legs adding to the impetus, which, in slow walking, is given only by the feet. 21. Perhaps the most singular motor apparatus in the body is the larynx, by the agency of which voice is produced. The essential conditions of the production of the human voice are : — a. The existence of the so-called vocal cJiords. b. The parallelism of the edges of these chords, without which they will not vibrate in such a manner as to give out sound. c. A certain degree of tightness of the vocal chords, without which they will not vibrate quickly enough to produce sound. d. The passage of a current of air between the parallel edges of the vocal chords of sufficient power to set the chords vibrating, 22. The larynx is a short tubular box opening above into the bottom of the phar)'nx and below into the top of the trachea. Its framework is supplied by certain carti- lages more or less moveable on each other, and these are connected together by joints, membranes and muscles. Across the middle of the larynx is a transverse partition, N 178 ELEMENTAR V PHYSIO LOG V. [less. formed by two folds of the lining mucous membrane, stretching from either side, but not quite meeting in the middle line. They thus leave, in the middle line, a chink or slit, running from the front to the back, called the glottis. The two edges of this slit are not round and flabby, but sharp and, so to speak, clean cut ; they are al'io strengthened by a quantity of elastic tissue, the fibres of which are disposed lengthways in them. These sharp Fig. 56. Diagram of the larynx, the thyroid cartilage [Th.) being supposed to be transparent, and allowing the right arytenoid cartilage (^r.), vocal chords (K) and thyro-aryteooid muscle {TliA.), the upper part of the cricoid carti- lage (Cr.)) and the attachment of the epiglottis {Ep.), to be seen. C.th. the right crico-thyroid muscle ; 7>. the trachea ; Hy. the hyoid bone. free edges of the glottis are the so-called 7'ocal chords or vocal ligaments. 23. The thyroid cartilage (Fig. 56, Th) is a broad plate of gristle bent upon itself into a V shape, and so disposed that the point of the V is turned forwards, and constitutes what is commonly called "Adam's apple." Above, the thyroid cartilage is attached by ligament and membrane to the hyoid bone (Fig. 56, Hy.). Below and behind, its broad sides are produced into little elongations or horns, which are articulated by ligaments w-ith the outside of a VII.] THE LARYXX. 179 great ring of cartilage, the cricoid (Fig. 56, Cr.), which forms, as it were, the top of the windpipe. The cricoid ring is much higher behind than in front, and a gap, filled up by membrane only, is left between its Fig. 57. — A Vertical and Transverse Section through the Larynx, THE hinder Half of which is removed. E^. Epiglottis ; Th. thyroid cartilage ; «, cavities called the ventricles of larynx above the vocal ligaments {V); x the right thyro-arytenoid muscle cut across ; Cr. the cricoid cartilage. upper edge and the lower edge of the front part of the thyroid, when the latter is horizontal. Consequently, the thyroid cartilage, turning upon the articulations of its horns with the hinder part of the cricoid, as upon hinges, can be moved up and down through the space occupied by this membrane. When it moves downwards, the distance between the front part of the thyroid cartilage and the back of the cricoid is necessarily increased ; and when it moves back again to the horizontal position, diminished. There is, on each side, a large muscle, the crico-thyj'oid, which passes from the outer side of the cricoid cartilage N 2 [8o ELEMENTAR V PHYSIOL OGY [less. obliquely upwards and backwards to the thyroid, and pulls the latter down (Fig. 56, C.th). 24. Perched side by side, upon the upper edge of the back part of the cricoid cartilage are two small irregularly- shaped but, roughly speaking, pyramidal cartilages, the arytenoid cartilages (Fig. 58, Ary.). Each of these is articulated by its base with the cricoid cartilage by means of a shallow joint which permits of very varied movements, y ^ ! Ary. I \ ; Fig. 58. — The Parts surrounding the Glottis partially dissected and viewed from above Th. The thyroid cartilage; Cr. the cricoid cartilage ; V. the edges of thi vocal ligaments bounding the glottis ; A ry. the arj'tenoid cartilages ; Th.A. thyro-arytenoid ; C.rt./. lateral crico-arytenoid ; C.a.p. posterior crico-arytenoid ; Ar.p. posterior arytenoid muscles. and especially allows the front portions of the two arytenoid cartilages to approach, or to recede from, each other. It is to the fore part of one of these arytenoid cartilages that the hinder end of each of the two vocal ligaments is fastened ; and they stretch from these points horizontally across the cavity of the larynx, to be attached, close to- gether, in the re-entering angle of the thyroid cartilage rather lower than half-way between its top and bottom. Now when the arytenoid cartilages diverge, as they do when the larynx is in a state of rest, it is evident that the Vil.i TH£ LARYNX. i8i aperture of the glottis will be V-shaped, the point of the V being fonvards, and the base behind. For, in front, or in the angle of the th>Toid, the two vocal ligaments are fastened permanently close together, whereas, behind, their extremities will be separated as far as the ar}-tenoids, to which they are attached, are separated Fig 59. i^View of the human larynx from above as actually seen'by the aid of the instrument called the lar^-ngoscope : A, in the condition when voice is being produced : B, at rest, when no voice is produced. e. Epiglottis (foreshortened). c.v. The vocal chords. c.v.s. The so-called false vocal chords, folds of mucous membrane lying above the real vocal chords. a. Elevation caused by the ar\'tenoid cartilages. sr.v. Elevations caused by small cartilages connected with the arj'tenoids. /. Root of the tongue. II. Diagram of the same. from each other. Under these circumstances a current of air passing through the glottis produces no sound, the parallelism of the vocal chords being wanting ; whence it is that, ordinarily, expiration and inspiration take place quietly. Passing from one arytenoid cartilage to the other, at their posterior surfaces are certain muscles called the posterior aiytenoid (Fig. 58, Ar.p.). There are i82 ELEMENTARY PHYSIOLOGY. [i.es!^. also two sets of muscles connecting each arytenoid with the cricoid, and called from their positions respectively the posterior and lateral crico-arytenoid (Fig. 58, C.a.p., Ca.l.). By the more or less separate or combined action of these muscles, the arytenoid cartilages and, conse- quently, the hinder ends of the vocal chords attached to them, may be made to approach or recede from each other, and thus the vocal chords rendered parallel or the reverse. We have seen that the crico-thyroid muscle pulls the thyroid cartilage down, and thus increases the distance between the front of the thyroid and the back of the cricoid, on which the arytenoids are seated. This move- ment, the arytenoids being fixed, must tend to pull out the vocal chords lengthways, or in other words to tighten them. Running from the re-entering angle in the front part of the thyroid, backward, to the ar}'tenoids, alongside the vocal chords (and indeed imbedded in the transverse folds, of which the chords are the free edges) are two strong muscles, one on each side (Fig. 58, Th.A.), called thyro- arytenoid. The effect of the contraction of these muscles is to pull up the thyroid cartilage after it has been de- pressed by the crico-thyroid muscles, and consequently to slacken the vocal chords. Thus the parallelism {b) of the vocal chords is deter- mined chiefly by the relative distance from each other of the arytenoid cartilages ; the tension {c) of the vocal cords is determined chiefly by the upward or downward move- ment of the thyroid cartilage ; and both these conditions are dependent on the action of certain muscles. The current of air {d) whose passage sets the chords vibrating is supplied by the movements of expiration, which, when the chords are sufficiently parallel and tense, produce that musical note which constitutes the voice, but otherwise give rise to no audible sound at all. 25. Other things being alike, the musical note will be low or high, according as the vocal chords are relaxed or tightened ; and this again depends upon the relative pre- dominance of the contraction of the crico-thyroid and thyro-arytenoid muscles. For when the thyro-ar}^tenoid muscles are fully contracted, the thyroid cartilage will be pulled up as far as it can go, and the vocal chords will be rendered relatively lax ; while, when the crico-thyroid vii.i THE VOICE. \%z muscles are fully contracted, the thyroid cartilage will be depressed as much as possible, and the vocal chords will be made more tense. The range of any voice depends upon the difference of tension which can be given to the vocal chords, in these two positions of the thyroid cartilage. Accuracy of singing depends upon the precision with which the singer can voluntarily adjust the contractions of the thyro- arytenoid and crico-thyroid muscles — so as to give his Diagfirtl of a model illustrating the action of the levefs and muscles of the larynx. The stand and vertical pillar represent the cricoid and ar>-tenoid cartilages, while the rod {b c\, moving on a pivot at c, takes the place of the thyroid cartilage ; a b '\s zxi elastic band representing the vocal ligament. Parallel with this runs a cord fastened at one end to the rod b c, and, at the other, passing over a pulley to the weight B. This represents the thyro- arytenoid muscle. A cord attached to the middle of b c, and passing overa second pulley to the weight A, represents the crico-th\Toid muscle. It is obvious that when the bar ,b c) is pulled down to the position c d, the elastic band {a b) is put on the stretch. vocal chords the exact tension at which their vibration will yield the notes required. The quality of a voice — treble, bass, tenor, &c. — on the other hand, depends upon the make of the particular larv'nx, the primitive length of its vocal chords, their elasticity, the amount of resonance of the surrounding parts, and so on. Thus, men have deeper notes than boys and women, because their larynxes are larger and their vocal chords 1 84 ELEMENTARY PHYSIOLOGY. [less. longer-n^ence, though equally elastic, they vibrate less swiftly.^f^ 26, Speech is voice modulated by the throat, tongue, and lips. Thus, voice may exist without speech ; and it is commonly said that speech may exist without voice, as in whispering. This is only true, however, if the title of voice be restricted to the sound produced by the vibration of the vocal chords ; for, in whispering, there is a sort of voice produced by the vibration of the muscular walls of the lips which thus replace the vocal chords. A whisper is. in fact, a very low whistle. The modulatio7i of the voice into speech is effected by changing the form of the cavity of the mouth and nose, by the action of the muscles which move the walls of those parts. Thus, if the pure vowel sounds — E (as in he), A (as in hay), A' (as in ah), O (as in o?-), O' (as in oh), 00 (as in cool), are pronounced successively, it will be found that they may be all formed out of the sound produced by a con- tinuous expiration, the mouth being kept open, but the form of its aperture, and the extent to w^hich the lips are thrust out or drawn in so as to lengthen or shorten the distance of the orifice from the larynx, being changed for each vowel. It will be narrowest, with the lips most drawn back, in E, widest in A', and roundest, with the lips most protruded, in 00. Certain co7isona7its also may be pronounced without interrupting the current of expired air, by modification of the form of the throat and mouth. Thus the aspirate, H, is the result of a little extra ex- piratory force — a sort of incipient cough. .S and Z, Sh and J (as m jugular = G soft, as mge?itry), Th, L, R, E, V, may likewise all be produced by continuous currents of air forced through the mouth, the shape of the cavity of which is pecuHarly modified by the tongue and lips. 27. All the vocal sounds hitherto noted so far resemble one another, that their production does not involve the stoppage of the current of air which traverses either of the modulating passai?es. VII. i SPEEClf. iSS But the sounds of M and N can only be formed by- blocking the current of air which passes through the mouth, while free passage is left through the nose. For J/, the mouth is shut by the lips ; for iV, by the application of the tongue to the palate. 28. The other consonantal sounds of the English language are produced by shutting the passage through both nose and mouth ; and, as it were, forcing the expira- tory vocal current through the obstacle furnished by the latter, the character of which obstacle gives each consonant its peculiarity. Thus, in producing the consonants B and P^ the mouth is shut by the lips, which are then forced open in this explosive manner. In 7" and D, the mouth passage is suddenly barred by the application of the point of the tongue to the teeth, or to the front part of the palate ; while in K and G (hard, as in go) the middle and back of the tongue are similarly forced against the back part of the palate. 29. An artificial larynx may be constructed by properly adjusting elastic bands, which take the place of the vocal chords ; and, when a current of air is forced through these, due regulation of the tension of the bands will give rise to all the notes of the human voice. As each vowel and consonantal sound is produced by the modification of the length and form of the cavities, which lie over the natural larynx, so, by placing over the artificial larynx chambers to which any requisite shape can be given, the various letters may be sounded. It is by attending to these facts and principles that various speaking machines have been constructed. _ 30. Although the tongue is credited with the respon- sibility of speech, as the "unruly member," and undoubtedly takes a very important share in its production, it is not absolutely indispensable. Hence, the apparently fabulous stories of people who have been enabled to speak, after their tongues had been cut out by the cruelty of a tyrant, or persecutor, may be quite true. Some years ago I had the opportunity of examining a person, whom I will call Mr. R., whose tongue had been removed as completely as a skilful surgeon could perform the operation. When the mouth was widely opened, the truncated face of the stump of the tongue, apparently 1 86 ELEMENTAR Y PHYSIOL 0 G V. [less. covered with new mucous membrane, was to be seen, occupying a position as far back as the level of the an- terior pillars of the fauces. The dorsum of the tongue was visible with difficulty ; but I beheve I could discern some of the circumvallate papillae upon it. None of these were visible upon the amputated part of the tongue, which had been preserved in spirit ; and which, so far as I could judge, was about 2.} inches long. When his mouth was open, Mr. R. could advance his tongue no further than the position in which I saw it ; but he informed me that, when his mouth was shut, the stump of the tongue could be brought much more forward. Mr. R.'s conversation was perfectly intelligible ; and such words as t/imk, t/ie, cow, kill, were well and clearly pronounced. But tin became ^«y t^iok, fack or pack; toll, pool J dog, thog; dine, vine; dew, thew j cat, catf; mad, madf; goose, goothj big, pig, bich, pick, with a guttural ch. In fact, only the pronunciation of those letters the formation of which requires the use of the tongue was affected ; and, of these, only the two which involve the employment of its tip were absolutely beyond Mr. R.^s power. He converted all t^s, and d^s, into f^s,p^s, v^s, or th's, Th was fairly given in all cases ; s and sh, I and r, with more or less of a lisp. Initial g^s and t^s were good; but final ^'j were all more or less guttural. In the former case, the imperfect stoppage of the current of air by the root of the tongue was of no moment, as the sound ran on into that of the following vowel ; w^hile, when the letter was terminal, the defect at once became apparent. viii.1 SENSATIONS. t^y LESSON VIII. SENSATIONS AND SENSORY ORGANS. 1. The agent by which all the motor organs (except the cilia) described in the preceding Lesson are set at work, is muscular fibre. But, in the living body, muscular fibre is made to contract only by a change which takes place in the motor or efferent Jierve, which is distributed to it. This change again is effected only by the activity of the central nervous organ, with which the motor nerve is connected. The central organ is thrown into activity immediately, or ultimately, only by the influence of changes which take place in the molecular condition of nerves, called se?isory or afferent, which are connected, on the one hand, with the central organ, and, on the other hand, with some other part of the body. Finally, the alteration of the afferent nerve is itself produced only by changes m the condition of the part of the body with w^hich it is connected ; wiiich changes usually result from external impressions. 2. Thus the great majority (if not the whole) of the movements of the body an^ of its parts, are the effect of an influence (technically termed a stimulus or irritation) applied directly, or indirectly, to the ends of aff^erent nerves, and giving rise to a molecular change, which is propagated along their substance to the cent?'al 7iervous organ with which they are connected, "^t^he molecular activity of the afferent ner\'e communicates itself to the central organ, and is then transmitted along the motor nerves, which pass from the central organ to the muscles affected. And, when the disturbance in the molecular 1 88 ELEMENTAR V PH YSJOLOGY. [less. condition of the efferent nerves reaches their extremities, it is communicated to the muscular fibres, and causes their particles to take up a new position, so that each fibre shortens and becomes thicker. 3. Such a series of molecular changes as that just described is called a reflex action — the disturbance caused by the irritation being as it were refiected back, along the motor nerves, to the muscles. A reflex action, strictly so called, takes place without our knowing anything about it, and hundreds of such actions are going on continually in our bodies without our being aware of them. But it very frequently happens that we learn that something is going on, when a stimulus affects our afferent nerves, by having what we call a feeling or sensation. We class sensations along with emotions^ and volitions^ and thoughts^ under the common head of states of consciousness. But what consciousness is, we know not ; and how it is that anything so remark- able as a state of consciousness comes about as the result of irritating nervous tissue, is just as unaccountable as any other ultimate fact of nature. 4. Sensations are of very various degrees of definiteness. Some arise within ourselves, we know not how or where, and remain vague and undefinable. Such are the sensa- tions of unconifortableness, or faintness, o{ fatigue, or of restlessness. We cannot assign any particular" place to these sensations, which are very probably the result of affections of the afferent nerves in general brought about by the state of the blood, or that of the tissues in which they are distributed. And however real these sensations may be, and however largely they enter into the sum of our pleasures and pains, they tell us absolutely nothing of the external world. They are not only diffuse, but they are also subjective sensations. 5. What is termed the 7nuscular sense is less vaguely localized than the preceding, though its place is still inca- pable of being very accurately defined. This muscular sensation is the feehng of resistance which arises when any kind of obstacle is opposed to the movement of the body, or of any part of it ; and it is something quite different from the feeling of contact or even of pressure. VIII.] MUSCULAR SENSE. 189 Lay one hand flat on its back upon a table, and rest a disc of cardboard a couple of inches in diameter upon the ends of the outstretched fingers ; the only result will be a sensation of contact — the pressure of so light a body being inappreciable. But put a two-pound weight upon the card- board, and the sensation of contact will be accompanied, or even obscured, by the very difterent feeling oi pressure. Up to this moment the fingers and arm have rested upon the table ; but now let the hand be raised from the table, and another new feeling will make its appearance — that of 7'esistance to effort. This feeling comes into existence with the exertion of the muscles which raise the arm, and is the consciousness of that exertion given to us by the muscular sense. Anyone who raises or carries a weight, knows well enough that he has this sensation ; but he may be greatly puzzled to say where he has it. Nevertheless, the sense itself is very delicate, and enables us to form tolerably accurate judgments of the relative intensity of resistances. Persons who deal in articles sold by weight, are constantly enabled to form very precise estimates of the weight of such articles by balancing them in their hands ; and in this case, they depend in a great measure upon the mus- cular sense. 6. In a third group of sensations, each feeling, as it arises, is assigned to a definite part of the body, and is produced by a stimulus applied to that part of the body ; but the bodies, or forces, which are competent to act as stimuli, are very various in character. Such are the sensa- tions of touch, which is restricted to the integument covering the surface, and to some portions of the mem- branes lining the internal cavities of the body ; and of taste and smell, which are similarly confined to certain regions of the mucous membrane of the mouth and nasal cavities. Any portion of the body to which a sensation is thus restricted is called a sensor}^ organ. And lastly, in a fourth group of sensations, each feeling requires for its production the application of a single kind of stimulus to a very specially modified part of the integu- ment. The latter sers-es as an intermediator between the physical agent of the sensation and the sensor}- nerve, I90 ELEMENTARY PHYSIOLOGY. [less. which is to convey to the brain the impulse necessary to awake in it that state of consciousness which we call the sensation. Such are the sensations of sight and hearitig. The physical agents which can alone awaken these sensa- tions (under natural circumstances) are light and sound. The modified parts of the integument, which alone are competent to intermediate between these agents and the nerves of sight and hearing, are the eye and the ear. 7. In every sensory organ it is necessary to distinguish the terminal expansion of the afferent or sensory nerve, and the structures which intermediate between this ex- pansion and the physical agent which gives rise to the sensation. And in each group of special sensations there are cer- tain phenomena which arise out of the structure of the organ, and others which result from the operation of the central apparatus of the nervous system upon the materials supplied to it by the sensory organ. 8. The sense of Touch (including that of heat and cold) is possessed, more or less acutely, by all parts of the free surface of the body, and by the walls of the mouth and nasal passages. Whatever part possesses this sense consists of a mem- brane (integumentary or mucous) composed of a deep layer made up of fibrous tissue, containing a capillary network and the ultimate terminations of the sensory nerves ; and of a superficial layer consisting of epi- thelial or epidermic cells, among which are no vessels. Wherever the sense of touch is delicate, the deep layer is not a mere flat expansion, but is raised up into multi- tudes of small, close-set, conical elevations (see Fig. 32), which are called papilla:. In the skin, the coat of epi- thelial or epidermic cells does not follow the contour of these papillae, but dips down between them and forms a tolerably even coat over them. Thus, the points of the papillse are much nearer the surface than the general plane of the deep layer whence these papillae proceed. Loops of vessels enter the papilla?, and the fine ultimate terminations of the sensory nerve-fibres distributed to the skin terminate in them, but in what way has not been thoroughly made out. VIII.] TOUCH. 191 In certain cases, the delicate fibrous sheath, or neuri- lemma, of the nerv^e which enters the papilla, enlarges in the papilla into an oval swelling, which is called a tactile corpuscle (see Lesson XII.). These corpuscles are found in the papillse of those localities which are endowed with a very delicate sense ot touch, as in the tips of the fingers, the point of the tongue, &c. 9. It is obvious, from what has been said, that no direct contact takes place between a body which is touched and the sensory nerve.— a thicker or thinner layer of epithelium, or epidermis, being situated between the two. In fact, if this layer is removed, as when a surface of the skin has been blistered, contact with the raw surface gives rise to a sense of pain, not to one of touch properly so called. Thus, in touch, it is the epidermis, or epithelium, which is the intermediator between the nerve and the physical agent, the external pressure being transmitted through the horny cells to the subjacent ends of the nerves, and the kind of impulse thus transmitted must be modified by the thickness and character of the cellular layer, no less than by the forms and number of the papillae. 10. Certain very curious phenomena appertaining to the sense of touch, are probably due to these varying anatomical arrangements. Not only is tactile sensibility to a single impression much duller in some parts than in others — a circumstance which might be readily ac- counted for by the different thickness of the epidermic layer — but the power of distinguishing double simul- taneous impressions is very different. Thus, if the ends of a pair of compasses (which should be blunted with pointed pieces of cork) are separated by only one-tenth or one-twelfth of an inch, they will be distinctly felt as two, if applied to the tips of the fingers ; whereas, if applied to the back of the hand in the same way, only one impression will be felt ; and, on the arm, they may be separated for a quarter of an inch, and still only one impression will be perceived. Accurate experiments have been made in different parts of the body, and it has been found that two points can be distinguished by the tongue, if only one-twenty- fourth of an inch apart ; by the tips of the fingers if 192 ELEMENTARY PHYSIOLOGY. [less. one-twelfth of an inch distant ; while they may be one inch distant on the cheek, and even three inches on the back, and still give rise to only one sensation. II. The feeling of warmth, or cold, is the result of an excitation of sensory nerves distributed to the skin, which are probably distinct from those which give rise to the sense of touch. And it would appear that the heat must be transmitted through the epidermic or epithe- lial layer, to give rise to this sensation ; for, just as touch- ing a naked nerve, or the trunk of a nerve, gives rise only to pain, so heating or cooling an exposed nerve, or the trunk of a nerve, gives rise not to a sensation of heat or cold, but simply to pain. Again, the sensation of heat, or cold, is relative rather than absolute. Suppose three basins be prepared, one filled with ice-cold water, one with water as hot as can be borne, and the third with a mixture of the two. If the hand be put into the hot-water basin, and then transferred to the mixture, the latter will feel cold; but if the hand be kept awhile in the ice-cold water, and then transferred to the very same mixture, it will feel warm. Like the sense of touch, the sense of warmth varies in delicacy in different parts of the body. The cheeks are very sensitive, more so than the lips ; the palms of the hands are more sensitive to heat than their backs. Hence a washerwoman holds her flat-iron to her cheek to test the temperature, and one who is cold spreads the palms of his hands to the fire. 12. The organ of the sense of Taste is the mucous membrane which covers the tongue, especially its back part, and the hinder part of the palate. Like that of the skin, the deep, or vascular, layer of the mucous membrane of the tongue is raised up into papillae, but these are large, separate, and have separate coats of epithelium. Towards the tip of the tongue they are for the most part elongated and pointed, and are called filifor7nj over the rest of the surface of the tongue, these are mixed with other larger papillae, with broad ends and narrow bases, Q'aXio.d. fungiforjnj but towards its root there are a number of large papillae, arranged in the figure of a V with its point backwards, each of which' VIII.] THE TONGUE. 193 is like a fungiform papilla surrounded by a wall. These are the circumvallate papilla; (Fig. 61, C.p). The larger of these papilla have subordinate small ones upon their surfaces. They are very vascular, and they receive nervous filaments from two sources, the one the nerve Fig. 61. — The Molth widely opened to show the Tongue and Palate. Uv. the uvula ; Tn. the tonsil between the anterior and posterior pillars of the fauces; C/. circumvallate pap. Use ; Fp. fungiform papillae. The mi- nute filiform pap'.llae cover the interspaces between these. On the right side the tongue is partially dissected to show the course of the filaments -of the glossopharyngeal nerve, VI II. called glossopharyngeal., the other xho. gustatory, which is a branch oiihQ Jifth nerve. (See Lesson XI. § 18.) The latter chiefly supplies the front of the tongue, the former its back and the adjacent part of the palate : and there O 194 ELEMENTARY PHYSIOLOGY. [less. is reason to believe that it is the latter region which is more especially the seat of the sense of taste. The great majority of the sensations we call taste, how- ever, are in reality complex sensations, into which smell and even touch largely enter. When the sense of smell is interfered with, as when the nose is held tightly pinched, it is very difficult to distinguish the taste of various ob- jects. An onion, for instance, the eyes being shut, may then easily be confounded with an apple. 13. The organ of the sense of Smell is the delicate mucous membrane which lines a part of the nasal cavities, and is distinguished from the rest of the mucous mem- brane of these cavities — firstly, by possessing no cilia ; secondly, by receiving its nervous supply from the olfac- tory, or first, pair of cerebral nerves, and not, like the rest of the mucous membrane, from the fifth pair. Each nostril leads into a spacious nasal chamber, sepa- rated, in the middle line, from its fellow of the other side, by a partition, or septum, formed partly by cartilage and partly by bone, and continuous with that partition Avhich separates the two nostrils one from the other. Below, each nasal chamber is separated from the cavity of the mouth by a floor, the bony palate (Figs. 62 and 63) ; and when this bony palate comes to an end, the partition is continued down to the root of the tongue by a fleshy curtain, the soft palate, which has been already described. The soft palate and the root of the tongue together, constitute, under ordinary circumstances, a moveable partition between the mouth and the pharynx, and it will be observed that the opening of the larynx, the ^ioitis, lies behind the partition ; so that when the root of the tongue is apphed close to the soft palate no passage of air can take place between the mouth and the pharynx. But in the upper part of the pharynx above the partition are the two hinder openings of the nasal caA-ities (which are called the posterio7' naves) separated by the termination of the septum ; and through these wide openings the air passes, with great readiness, from the nostrils along the lower part of each nasal chamber to the glottis, or in the opposite direction. It is by means of the passages thus freely open to the air that Ave breathe, as we ordinarily do, with the mouth shut. Each nasal chamber rises, as a high vault, far above the THE XASAL CAVITY [95 %-. Fig. 62.— Vertical Longitldixal Sections of the Xasal Cavity. The upper figure represents the outer wall of the left nasal cavity ; the lower figure the right side of the middle partition, or septum {SpA.al the nose, which forms the inner wall of :he right nasal cavity. /, the olfactory nerve and its branches ; V, branches of the fifth nerve ; Pa. the palate, which separates the nasal cavity from that of the mouth ; S. T. the superior turbinal bone ; M. T. the middle turbinal ; I.T. the inferior turbinal. The letter / is placed in the cerebral cavity ; and the partition on which the olfactory lobe rests, and through which the filaments of the olfaccory nerves pr.s£;, is the cribriform plate. O 2 1 96 ELEMENTAR Y PHYSIO LOG Y [less. level of the arch of the posterior nares — in fact, about as high as the depression of the root of the nose. The upper- most and front part of its roof, between the eyes, is formed by a dehcate horizontal plate of bone, perforated like a sieve by a great many small holes, and thence called the cribriform plate (Fig. 63, 6V.). It is this plate (with the membranous structures which line its two faces) alone which, in this region, separates the cavity of the nose from that which contains the brain. The olfactor}' lobes which are directly connected with, and form indeed a part of, the brain, enlarge at their ends, and their broad extremities rest upon the upper side of the cribriform plate ; sending immense numbers of delicate filaments,the olfactory nerves, through it to the olfactory mucous membrane (Fig. 62). On each wall of the septum this mucous membrane forms a flat expansion, but on the side walls of each nasal cavity it follows the elevations and depressions of the inner surfaces of what are called the upper and middle turbinal, or spongy bones. These bones are called spongy because the interior of each is occupied by air cavities separated from each other by ver>- delicate partitions only, and communicating with the nasal cavities. Hence the bones, though massive-looking, are really exceedingly light and delicate, and fully deserve the appellation of spongy (Fig. 63). There is a third light scroll-like bone distinct from these "wo, and attached to the maxillaiy bone, which is called the inferior turbinal, as it lies lower than the other two, and imperfectly separates the air passages from the proper olfactory chamber (Fig. 62). It is covered by the ordinary ciliated mucous membrane of the nasal passage, and receives no filaments from the olfactory nerve (Fig. 62). 14. From the arrangements which have been described, it is clear that, under ordinary circumstances, the gentle inspiratory and expiratory currents will flow along the comparatively wide, direct passages afforded by so much of the nasal chamber as lies below the middle turbinal ; and that they will hardly move the air enclosed in the , narrow interspace between the septum and the upper and middle spongy bones, which is the proper olfactory chamber. If the air currents are laden with particlc3 cf odorous VIII.] SMELLIXG. 197 matter, they can only reach the olfactory inembrane by diffusing themselves into this narrow interspace ; and, if there be but few of these panicles, they will run the risk ol not reaching the olfactory mucous membrane at all, unltss the air in contact with it be exchanged for some of the odoriferous air. Hence it is that, when we wish to Cr J.T. S/i. PL. Fig. 63. — A Transverse and Vertical Section of the Osseous Walls OF the Nasal Cavity taken nearly through the Letter / in THE FOREGOING FiGURE. C>\ the cribriform plate ; S.T., M.T. the chambered superior and middle tur- binai bones on which and on the sepium {,Sp.) the filam.ents of the olfactory nerve are distributed ; /. T. the inferior turbinal bone : PL the palate ; A 11. ihQ A >itfum or chamber which occupies the greater part of the maxillarj- bone and opens into the nasal caWty. perceive a faint odour more distinctly, we sniff, or snuff up the air. Each sniff is a sudden inspiration, the eftect of which must reach the air in the olfactory chamber at the same time as, or even before, it aftects that at the nos- trils ; and thus must tend to draw a little air out of that chamber from behind. At the same time, or immediately afterwards, the air sucked in at the nostrils entering with a sudden vertical rush, part of it must tend to flow directly into the olfactory chamber, and replace that thus drawn otit. The loss of smell which takes place in the course of a severe cold may, in part, be due to the swollen state of 1 9S ELEMENTAR V PHYSIOLOGY. [les;;. the mucous membrane v.hich covers the inferior turbinal bones, which thus impedes the passage of odoriferous air to the olfactory chamber. 15. The Ear, or organ of the sense of Hearing, is very much more complex than either of the sensory organs yet described. It will be useful to distinguish the essential parts of this complicated apparatus from certain other parts, vhich, though of great assistance to the sense, are not absolutely necessary, and therefore may be called accessory. The essential parts, on either side of the head, consist, substantially, of two peculiarly formed membranous bags, called, respectively, the membranous labyrinth and the scala media of the cochlea. Both these bags are lodged in cavities which they do not completely fill, situated in the midst of a dense and solid mass of bone (from its hardness called petrosal), which forms a part of the temporal bone, and enters into the base of the skull. Each bag is filled with a fluid, and is also supported in a fluid which fills the cavity in which it is lodged. In the interior of each bag, certain small, mobile, hard bodies are contained ; and the ultimate filaments of the auditory nerves are so distributed upon the walls of the bags that their terminations must be knocked by the vibrations of these small hard bodies, should anything set them in motion. It is also quite possible that the vibrations of the fluid contents of the sacs may themselves suffice to affect the filaments of the auditory nerve ; but, however this may be, any such effect must be greatly intensified by the co-operation of the solid particles. In bathing in a tolerably smooth sea, on a rocky shore, the movement of the little waves as they run backwards and forwards is hardly felt by anyone lying down ; but in bathing on a sandy and gravelly beach, the pelting of the showers of little stones and sand, which are raised and let fall by each wavelet, makes a very definite impres- sion on the nerves of the skin. Now, the membrane on which the ends of the auditory nerves are spread out is virtually a sensitive beach, and waves, which by themselves would not be felt, are readily perceived when they raise and let fall hard particles. Both thes^ membranous bags are lined by an epithe- lium. VI r I.] THE ORGAN OF HEARING. 199 ^ The auditory nerve after passing through the dense bone of the skull is distributed to certain regions of each bag, where its ultimate hlaments come into peculiar con- nection with the epithelial lining. The epithelium itself too at these spots becomes specially modified. In certain parts of the membranous labyrinth, for instance, the epi- thelium connected with the terminations of the auditory nerve is produced into long, stiff, slender, hair-like pro- FlG. 64. — DtAGRAM TO ILLUSTRATE THE TERMINATION OF THE AUDI- TORY Nerve in an Ampulla. I. The epithelium of the ampulla. II. The membranous wall of the am- pulla on which the epithelium rests. a, a filament of the auditory nerve running through the wall of the am- pulla and breaking up into a fine network (<^) in the epithelium ; c, epithe- lium cell with long stiff hair-like filament, d (this cell is supposed by some to be directly continuous with the nerve network) ; e, cells, not bear- ing filaments, placed by the side of, and supporting the filament-bearing cells ; f, a deeper layer of smal er cells. cesses (Fig. 64, d)^ which project into the fluid filling the bag, and which therefore are readily aftected by any vibra- tion of that fluid, and communicate the impulse to the ends of the nerve. In certain o her parts of the same labyrinth these hairs are scanty or absent, but their place is supplied by minute angular particles of "calcareous sand (called otoconia or otolif/ics), lying free in the fluid of the bng. 200 ELEMENTARY PHYSIOLOGY. [less. These, driven by the vibrations of that fluid, strike the epithehum and so affect the auditory nerve. In the scala media of the cochlea, minute, rod-hke bodies, called \kiQ fibres of Corti, and which are peculiarly modifi'ed cells of the epithelial lining of the scala, appear to serve the same object. 1 6. For simplicity's sake, the membranous labyrinth and the scala media, have hitherto been spoken of as if they were simple bags ; but this is not the case, each bag having a very curious and somewhat complicated form. (Figs. 65 and 66.) Fig. 65.— The Membranous Labyrinth, twice the Natiral Size. Ut. the Utriculus, or part of the vestibular sac, into which the semicircular canals open; A, A, A, the ampullje ; P. A. anterior vertical semicircular canal ; P. V. posterior vertical semicircular canal ; H. horizontal semicir- cular canal. The sacculus i.> not seen, as in the position in which the labyrinth is drawn the sacculas lies behind the utriculus. The white circles on the ampulla; of the posterior vertical and horizontal cana's indi- cate the cut ends of the branches of the auditor^' nerve ending in those ampullae ; the branches to the ampulla of the anterior vertical canal are seen in the space embraced by the canal, as is also the branch to the utriculus. This form is also followed to a certain extent by the bony casing of the cavity in which each is lodged. Thus the membranous labyrinth is surrounded by a bony laby- 7'inth, and the scala media is only a part of an intricate structure called the cochlea. The bony labyrinth and cochlea with all the parts inside each constitute together what is called the infernal ea?'. ^^ The membranous labyrinth (Fig. 65) has the figure of an oval vestibular sac, consisting of two parts, the one called utriculus, the other sacculus hemisphericus. The hoop-like semicircular canals open into the utriculus. They are three in number, and, two being vertical, are called the VIII.] THE COCHLEA, 201 anterior {P. A) and posterior (P.V.) vertical semicircular cinalsj while the third, lying outside, and horizontally, is t ermed the ext^^riial horizontal semicircular canal (//). One end of each of these canals is dilated into what is called an ampulla (A). It is upon the walls of these ampuUce and those of the vestibular sac that the branches of the auditory nerve are distributed. In each ampulla the nervous filaments may be traced to a transverse ridge caused by a thickening of the con- nective tissue which forms the walls of the canal (as well as of all other parts of the membranous labyrinth), and also by a thickening of the epithelium. Some of the epithelium cells are here prolonged into the fine hair-like processes described above. It is probable that these cells are specially connected with the terminations of the nerve filaments. In the vestibule are similar but less marked ridges, or patches ; here, however, the hair-like prolongations of the epithelium cells are absent or scanty, but, instead, otohthes are found in the fluid. The fluid which fills the cavities of the semicircular canals and utriculus is termed endolymph. That which separates these delicate structures from the bony chambers in which they are contained is the perilymph. Each of these fluids is little mo^ than water. 17. In the scala media'^ of the cochlea the primitive bag is drawn out into a long tube, which is coiled two and a half times on itself into a conical spiral, and lies in a much wider chamber of corresponding form, excavated in the petrous bone in such a way as to leave a central column of bony matter called the modiolus. The scala media has a triangular transverse section (Fig. 66\ being bounded above and below by the membranous walls which converge internally and diverge externally. At their con- vergence, the walls are fastened to the edge of a thin plate of bone, the lamina spiralis (L.S. Fig. 66), which winds round the modiolus. At their divergence they are ^ I employ this term as the equivalent of cavnlis cochlearis. The true nature and connections of these parts have only recently been properly worked o'lt, and the account now given will be found to be somewhat dif- ferent from that in the first edition of this work. See particularly the explanation nf Fis;. 67. ELEMENT A RY PH YSIOL OGY. [LESS. fixed to the wall of the containing bony chamber, which thus becomes divided into two passages, communicating at the summit of the spire, but elsewhere separate. These two passages are called respectively the scala tympajii and scala vestibuli^ and are filled with perilymph. The scala media, w^hich thus lies between the other two scalae, opens below, or at the broad end of the cochlea, by a narrow duct into the sacculus hemisphericus, but at its opposite end terminates blindly. (Fig. 70.) -^o.}/ S0.1 CiV Fig. 66. — A Section through the Axis of the Cochlea, magnified THREE Diameters. Sc.M. scala media ; Sc.V. scala vestibuli ; Sc.T. scala tympani ; L.S. lamina spiralis ; ]\Id. bony axis, or modiolus, round which the scalse are wound ; C.N. cochlear nerve. That branch of the auditory nerve which goes to supply the cochlea, enters the broad base of the central column or modiolus, and there divides into branches, which, spreading out in a spiral fashion in channels excavated in the bony tissue, are distributed to the lamina spiralis throughout its whole length. They do not end here ; but in any section of the lamina spiralis (Fig, 66, L.S .) they may be found running outwards from the central column across the lamina towards the angle of the scala media, in w^hich indeed they become finally lost. The upper wall of the scala media, that which separates it from the scala vestibuli, is called the vienibra^ie of Reissner. The opposite or lower wall, which separates it from the scala tympani, is the basilar viejubrane. The latter is very elastic, and on it rest the fibres of Corti {C C, Fig. 67), each of which is composed of two filaments VIII.] THE COCHLEA. 201 Sect M Fig. 67. — A Section through that Wall of the " Scai.a Media OF the Cochlea wiiicH lies next to the Scala Tympani. a. That end of the lam'na spiralis which passes into the inner wall, pillar, or modiolus of the bony cochlea ; c, the outer wall of the bony cochlea ; Sea. T. the cavity of the scala tympani ; Sea. M the cavity of the scala media ; d, the elastic basilar membrane which separates the scala media from the scala lyiupani ; / '. a vessel which lies in this, cut through ; c, the so-called mem- 204 ELEMENTARY PHYSIOLOGY. [less. joined at an angle. An immense number of these filaments are set side by side, with great regularity, throughout the whole length of the scala media, so that this organ pre- sents almost the appearance of a key-board, if viewed from either the scala vestibuli or the scala tympani. These fibres of Ccrti lie among a number of epithelium cells forming the lining of the scala media at this part, and those cells which are close to the fibres of Corti have a peculiarly modified form. The ends of the nerves have not yet been distinctly traced, but they probably come into close relation either with these fibres or with the modified epithelium cells lying close to them, which are capable of being agitated by the slightest impulse. 1 8. These essential parts of the organ of hearing are, we have seen, lodged in chambers of the petrous part of the temporal bone. Thus the membranous labyrmth is contained in a bony labyrinth of corresponding form, of which that part which lodges the sac is termed the ves- tibule., and those portions which contain the semicircular canals, the bony semicircular canals. And the scala media is contained in a spirally-coiled chamber, the cochlea, which it divides into two passages. Of these, one, the scala 7>estibuli, is so called because at the broad end or base of the cochlea it opens directly by a wide aperture into the vestibule ; by this opening the perilymph which fills the vestibule and bony semicircular canals and surrounds the membranous labyrinth, is put in free communication with the perilymph which fills the scala vestibuli of the cochlea, and, by means of the com- munication which exists between the two scalar at the summit of the spire, with that of the scala tympani also. In the fresh state, this collection of chambers in the petrous bone is perfectly closed ; but in the dry skull there are two wide openings, termed y^//^j-/r^?, or windows, on its outer wall ; i.e. on the side nearest the outside of the skull. Of these fenestras, one, termed ovalis (the brane of Corti ; C C, the fibres of Corti : VTI. the filaments of the auditor^' nerve, li is doubtful whether the membrane of Corti really has the extent and connections given to it in this figure, which must not be taken for more than a general representation rf the disposition of the parts. The membrane of Reissner, which separates the scala media from the scala vestibuli, is not represented. VIII.] THE TYMPAXUM. 205 oval window), is situated in the wall of the vestibular cavity ; the other, rotiDida (the round window), behini and below this, is the open end of the scala tyfnpani at the base of the spire of the cochlea. In the fresh state, each of these windows or fenestrte is closed by a tibrous membrane, continuous with the periosteum of the bone. F;g. 6S.— Transverse Section through the Side Walls of thh Sklll TO SHOW THE PaRTS OF EaR. Co. Concha or external ear; E.M. external auditory meatus ; Ty !\r. tym- panic membrane; Inc. Mall, incus and malleus; A.S.C., P.S.C., E.S.C. anterior, posterior, and external semicircular canals ; Coc. cochlea ; E i. Eustachian tube ; I.M. internal auditory meatus, through which the audi- tory nerve passes to the urgan of hearing. 'Y\iQ. fe7iesira rotunda is closed only by membrane ; but fastened to the centre of the membrane of i\\Q fenestra avails., so as to leave only a narrow margin, is an oval plate of bone, part of one of the little bones to be described short'y. 19. The outer wall of the internal ear is still far away from the exterior of the skull. Between it and the visible opening of the ear, in fact, are placed in a straight line, 2o6 ELEMENTAR Y PHYSIOLOGY. LESS. first, the drum of the ear, or tympamimj secondly, the long external passage, or meatus (Fig. 68). The drum of the ear and the external meatus, which together constitute the middle ear, would form one cavity, were it not that a delicate membrane, the tympanic mem- brane {Ty.M. Fig. 68), is tightly stretched in an oblique direction across the passage, so as to divide the compara- tively small cavity of the drum from the meatus. Fig. 69.— The Membrane of the Drlm of the Ear seen from the INNER Side, with the small Bonks of the Ear: and the Walls of the Tympanum, with the Air-cells in the Mastoid Part of THE Temporal Bone. M.C. mastoid cells ; Mall, malleus; Inc. incus ; .St. stapes ; a h, lines drawn through the horizontal axis on which the malleus and incus turn. The membrane of the tympaiium thus prevents any communication by means of the meatus, between the drum and the external air, but such a communication is pro- vided, though in a roundabout way, by the Eustachian tube {Eu. Fig. 68), which leads directly from the fore part of the drum inwards to the roof of the pharynx, where it opens. 20. Three small bones, the auditor}' ossicles, lie in the cavity of the tympanum. One of these is the stapes., a small bone shaped like a stirrup. It is the foot-plate ot this bone which, as already mentioned, is firmly fastened to the membrane of the fenestra ovalis, while its hoop projects outwards into the tympanic cavity (Fig. 69). ■III.] THE AUDITORY OSSICLES. 207 Another of these bones is the vialleus {Mall. Figs. 68, 69, 70), or hammer-bone, a long process of which is simi- larly fastened to the inner side of the tympmic membrane (Fig. 70), and a very much smaller process, the slender process, is fastened, as is also the body of the malleus, to the bony wall of the tympanum by ligaments. The Fig. 7:;.— a Diagram ii-lustrative of the rflative Positions ok the vARiois Parts of the Ear. E.M. external auditory meatus ; Ty.M. tympanic membrane ; Ty. tym- panum ; Mall, malleus : hic. incus ; Stp. stapes ; F.o. fenestra ovalis ; F.r fenestra rotunda; Eu. Eustachian tube; M.L. membranous labv- rinth, only one semicircular canal with its ampulla being represented : Sea. v.. Sea. T.. Sca.M., the scalae of the cochlea, which is supposed to be unrolled. rounded surface of the head of the malleus fits into a cor- responding pit in the end of a third bone, the incus or anvil bone, which has two processes — one, horizontal, which rests upon a support afforded to it by the walls of the tympanum ; while the other, vertical, descends almost parallel with the long process of the malleus, and articu- lates with the stapes, or rather- unites with a little bone, 2o8 ELEMENTARY PHYSIOLOGY. [le-s. the OS orbiculare, which articulates with the stapes (Figs, 69 and 70). The three bones thus form a chain between the fenestra ovahs and the tympanic membrane ; and the whole series turns upon a horizontal axis, the two ends of which, formed by the horizontal process of the incus and the slender process of the malleus, rest in the walls of the tympanum. The general direction of this axis is represented by the line a b in Fig. 69, or by a line perpendicular to the plane of the paper, passing through the head of the malleus in Fig. ']o. It follows, therefore, that whatever causes the membrane of the drum to vibrate backwards and for- wards, must force the handle of the malleus to travel in the same Avay. This must cause a corresponding motion of the long process of the incus, the end of which must drag the stapes backwards and forwards. And, as this is fastened to the membrane of. the fenestra ovalis, which is in contact with the perilymph, it must set this fluid vibrating throughout its whole extent, the thrustings in of the membrane of the fenestra ovalis being compensated by corresponding thrustings out of the membrane of the fenestra rotunda, and vice vosd. The vibrations of the perilymph thus produced will affect the endolymph, and this the otolithes, hairs, or fibres ; by which, finally, the auditory nerves will be excited. 21. The membrane of the fenestra ovalis and the tym- panic membrane will necessarily vibrate the more freely the looser they are, and the reverse. But there are two muscles — one, called the stapedius., which passes from the floor of the tympanum to the orbicular bone, and the other, the tensof^ tyinpani, from the front wall of the drum to the malleus. Each of the muscles when it contracts tightens the membranes in question, and restricts their vibrations or, in other words, tends to check the effect of any cause which sets these membranes vibrating 22. The outer extremity of the external meatus is sur- rounded by the concha or ext'-rnal ear {Co. Fig. 68*, a broad, peculiarly-shaped, and for the most part caitila- ginous p'atc, the general plane of v.-hich is at rii:ht angles with that of the axis of the auditory opening. The concha can be moved bv most animals and bv some human beinrs ^•I 1 1. ] FUXC TION OF A I'D I TOR V OSSICLES. 209 in various directions by means of muscles, which pass to it from the side of the head. 23. The manner in which the complex apparatus now described intermediates between the physical agent, which is the condition of the sensation of sound, and the nervous expansion, the affection of which alone can excite that sensation, must next be considered. All bodies which produce sound are in a state of vibra- tion, and they communicate the vibrations of their own substance tq the air with which they are in contact, and thus throw that air into waves, just as a stick waved back- wards and forwards in water throws the water into waves. The aerial waves, produced by the vibrations of sono- rous bodies, in part enter the external auditory passage, and in part strike upon the concha of the external ear and the outer surface of the head. It may be that some of the latter impulses are transmitted through the solid struc- ture of the skull to the organ of hearing ; but before they reach it they must, under ordinary circumstances, have become so scanty and weak, that they may be left out of consideration. The aerial waves which enter the meatus all impinge upon the membrane of the drum and set it vibrating, stretched membranes taking up vibrations from the air with great readiness, 24. The vibrations thus set up in the membrane of the tympanum are communicated, in part, to the air contained in the drum of the ear, and, in part, to the malleus, and thence to the other auditory ossicles. The vibrations communicated to the air of the drum impinge upon the inner wall of the tympanum, on the greater part of which, from its density, they can produce very little effect. Where this wall is formed by the mem- brane of the/e/icsfra rotunda., however, the communication of motion must necessarily be greater. The vibrations which are communicated to the malleus and the chain of ossicles may be of two kinds : vibrations of the particles of the bones, and vibrations of the bones as a w^hole. If a beam of wood, freely suspended, be very gently scratched with a pin, its particles will be thrown into a state of vibration, as will be evidenced by the sounri P 2 1 o ULEMEiVTAR Y PHYSIOL 0 G Y. [less. given out, but the beam itself will not be moved. Again, if a strong wind blow against the beam, it will swing visibly, without any vibrations of its particles among them- selves. On the other hand, if the beam be sharply struck with a hammer, it will not only give out a sound, showing that its p irticles are vibrating, but it will also swing from the impulse given to its whole mass. Under the last-mentioned circumstances, a blind man standing near the beam would be conscious of nothing but the sound, the product of molecular vibration, or invisible oscillation of the particles of the beam ; while a deaf man in the same position, would be aware of nothing but the visible oscillation of the beam as a whole. 25. Th'js, to return to the chain of auditory ossicles, while it seems hardly to be doubted that, when the mem- brane of the drum vibrates, they may be set vibrating both as a whole and in their panicles, it depends upon sub sidiary arrangements u^h ether the large vibrations, or the minute ones, shall make themselves obvious to the audi- tory nerve, which is in the position of our deaf, or blind, man. The evidence at present is in favour of the conclusion, that it is the vibrations of the bones, as a whole, which are the chief agents in transmitting the impulses of the aerial waves. For, in the first place, the disposition of the bones and the mode of their articulation are very much against the transmission of molecular vibrations through their sub- stance, while, on the other hand, they are extremely favour- able to their vibration en masse. The long processes of the malleus and incus swing, like a pendulum, upon the axis furnished by the short processes of these bones ; while the mode of connection of the incus with the stapes, and of the latter with the edges of the fenestra ovaliSj allows that bone free play, inwards and outwards. In the second place the total length of the chain of ossicles is very small compared with tiie length of the waves of audible sounds, and physical considerations teach us that in a like small rod, similarly capable of swinging en masse, the minute molecular vibrations would be inappreciable. 'Ihirdly, it is affirmed, as the result of experiments, that the bone called columella, which, in birds, takes the place of the VI 11.] FUXCTIOX OF THE COCHLEA. 211 chain of ossicles in man, does actually vibrate as a whole, and at the same rate as the membrane of the drum, when aerial vibrations strike upon the latter. 26. Thus, there is reason to believe that when the tym- panic membrane is set vibrating, it causes the process of the malleus, which is fixed to it, to swing at the same rate ; the head of the malleus consequently turns through a small arc on its pivot, the slender process. But the turning of the head of the malleus involves that of the head of the incus upon its pivot, the short process. In consequence the long process of the incus swings through an arc which has been estimated as being equal to about two-thirds of that described by the handle of the malleus. The extent of the push is thereby somewhat diminished, but the force of the push is proportionately increased ; in so confined a space this change is advantageous. The long process, however, is so fixed to the stapes that it cannot vibrate without, to a corresponding extent and at the same rate, pulling this out of, and pushing it into, the fenestra ovalis. But every pull and push imparts a corresponding set of shakes to the perilymph, which fills the bony labyrinth and cochlea, external to the membranous labyrinth and scala media. These shakes are communicated to the en- dolymph and fluid of the scala media, and, by the help of the otolithes and the fibres of Corti, are finally converted into impulses, which act as irritants of the ends of the vestibular and cochlear divisions of the auditory nerve. 27. The difference between the functions of the mem- branous labyrinth (to which the vestibular ner\-e is distri- buted) and those of the cochlea are not quite certainly made out, but the following views have been suggested : — The membranous labyrinth may be regarded as an ap- paratus whereby sounds are appreciated and distinguished according to their intensity or quantity ; but which does not afford any means of discriminating their qualities. The vestibular nerve tells us that sounds are weak or loud, but gives us no impression of tone, or melody, or harmony. The cochlea, on the other hand, it is supposed, enables the mind to discriminate the quality rather than the quantity or intensity of sound. It is suggested that the excitement of any single filament of the cochlear nerve p 2 212 ELEMEXTARY FIIYSIOLOGY. [lcss, gives rise, in the mind, to a distinct musical impression ; and that every fraction of a tone which a well-trained ear is capable of distinguishing is represented by its separate ncrve-fibre. Under this view the scala media resembles a key-board, in function, as well as in appearance, the fibres of Corti being the keys, and the ends of the nerves repre- senting the strings which the keys strike. If it were p-jssible to irritate each of these nerve-fibres experi- mentally, we should be able to produce any musical tone, at will, in the senscrium of the person experimented upon, just as any note on a piano is produced by striking the appropriate key. 28. A tuning-fork may be set vibrating, if its own par- ticular note, or one harmonic with it, be sounded in its neighbourhood. In other words, it will vibrate under the influence of a particular set of vibrations, and no others. If the vibrating ends of the tuning-fork were so arranged as to impinge upon a nerve, their repeated minute blows would at once excite this nerve. Suppose that of a set of tuning-forks, tuned to every note and distinguishing fractions of a note in the scale, one were thus connected with the end of every fibre of the cochlear nerve ; then any vibration communicated to the perilymph would affect the tuning-fork which could vibrate with it, while the rest would be absolutely, or relatively, indifferent to that vibration. In other words, the vibra- tion would give rise to the sensation of one particular tone, and no other, and every musical interval would be repre- sented by a distinct impression on the sensorium. 29. It is suggested that the fibres of Corti are competent to perform the function of such tuning-forks ; that each of ihem is set vibrating to its full strength by a particular ];ind of wave sent through the perilymph, and by no other ; and that each affects a particular fibre of the cochlear nerve only. But it must be remembered that the view here given is a suggestion only wdiich, however probable, has not yet been proved. Indeed recent inquiries have rather diminished than increased its probability. The fibres of the cochlear nerve may be excited by in- ternal causes, such as the varying pressure of the blood and the like : and in some persons such internal influences do give rise to veritable musical spectra, sometimes of a VIII.] FUXCTIOy OF EUSTACHIAN TUBE. 213 ver}' intense character. But, for the appreciation of music produced external to us, we depend upon the intermedia- tion of the scala media and its Cortian fibres. 30. It has aheady been explained that the stapedius and te Jisor tynipaiii mnscl&s are competent to tighten the mem- brane of the fenestra ovahs and tliat of the tympanum, and it is probable that they come into action when the sonorous impulses are too violent, and would produce too extensive vibrations of these membranes. They therefore tend to moderate the effect of intense sound, in much the same way that, as we shall find, the contraction of the circular fibres of the iris tends to moderate the effect of intense light in the eye. The function of the Eustachian tube is, probably, to keep the air in the tympanum, or on the inner side of the tympanic membrane, of about the same tension as that on the outer side, which could not always be the case if the tympanum were a closed cavity. 2 14 ELEMENTAR } ' PHYSIOL 0 G Y, [less. LESSON TX. r//r <9A'6:.4-^' O/' SIGHT. 1. In studying the organ of the sense of sight, the eye, it is needful to become acquainted, firstly, with the struc- ture and properties of the sensor}- expansion in which the optic nerve, or nerve of sight, terminates ; secondly, with the physical agent of the sensation ; thirdly, with the intermediate apparatus by which the physical agent is assisted in acting upon the nervous expansion. The ball, or globe, of the eye is a globular body, mov- ing freely in a chamber, the orhit^ which is furnished to it by the skull. The optic nerve, the root of which is in the brain, leaves the skull by a hole at the back of the orbit, and enters the back of the globe of the eye, not in the middle, but on the inner, or nasal, side of the centre. Having pierced the wall of the globe, it spreads out into a very delicate membrane, varying in thickness from <^th of an inch to less than half that amount, which lines the hinder two-thirds of the globe, and is termed the retina. This retina is the only organ connected with sensory nervous fibres v.-hich can be affected, by any agent, in such a manner as to give rise to the sensation of light. 2. If the globe of the eye be cut in two, transversely, so as to divide it into an anterior and a posterior half, the retina will be seen lining the whole of the concave wall ot the posterior half as a membrane of great delicacy, and, for the most part, of even texture and smooth surface. But, exactly opposite the middle of the posterior wall, it presents a slight circular depression of a yellowish hue, IX.] :^ERVOUS ELEMENTS OE EETIXA, fTOipT Fig. 71. Ijiagrammatic views of the nervous (A) and the connective fB) elements of thte retina, supposed to be separated from one another. A, the ner\-oi.3 structures-^, the rods ; c, the cones; U c\ the granules of the outer layer, with which these are connected ; d d, interwoven very delicate nervous fibres, from which fine nervous filaments, bearing the inner granules, ff, proceed towards the front surface \ g g , the continuation of these fine nerves, which become convoluted and intervvoven with the processes of the ganglionic corpuscles, hli! ; ii, the expansion of the fibres of the optic nerve. B, the conneciive tissue— ««, external or posterior limiting membrane : ee, radiaJ fibres passing to the internal or anterior limiting membrane ; e' e , nuclei ; (id, the intergranular layer ; gg, the molecular layer ; /, the anterior limitinrj membrane. (Magnified about 250 diameters.) 2l6 ELEMENT AR V PHYSIOLOGY. [less. the macula littea, or yellow spot (Fig. 72, 7n.l.\ Fig. 75, 8"),— not easily seen, however, unless the eye be perfectly fresh, — and, at some distance from this, towards the inner, or nasal, side of the ball, is a radiating appearance, pro- duced by the entrance of the optic nerve and the spreading out of its fibres into the retina. Fig. 72.— The Eye-ball divided transversely in the Middle Line, AND VIEWED FROM THE FrONT. s, sclerotic ; ch, choroid, seen in section only. r, the cut edge of the retina ; v.v. vessels of the retina, springing from o, the optic nerve or blind spot; in.l., the yellow spot, the dai-ker spot in its middle being the fovea centralis 3. A very thin vertical slice of the retina, in any region except the yellow spot and the entrance of the optic nerve, may be resolved into the structures represented separately in Fig. 71. The one of these (A) occupies the whole thickness of the section, and comprises its essential, or nervous, elements. The outer (or posterior) fourth, or rather less, of the thickness of these consists of a vast multitude of minute, either rod-like, or conical bodies, ranged side by side, perpendicularly to the plane of the retina. This is the layer o/rods and cones {b c). From the front ends or bases of the rods and cones very delicate fibres pass, and in each is developed a granule-like body {b' c'), which forms a part of what has been termed the IX] THE RETIXA. 2i7 outer layer of granules. It is probable that these fibres next pass into and indeed form the close meshwork of very delicate nervous fibres which is seen at d d' (Fig. 71, A).' From the anterior surface of this meshwork other fibres proceed, containing a second set of granules, which forms the inner granular layer iff) In front of this laver is a stratum of convoluted fine nervous fibres {gg') — and anterior to this again numerous ganglionic corpuscles {hh'). Processes of these ganglionic cor- puscles extend, on the one hand, into the layer of con- voluted nerve-fibres ; and on the other are probably continuous with the stratum of fibres of the optic nerve (/). These delicate nervous structures are supported by a sort of framework of connective tissue of a peculiar kind (B), which extends from an in?ier or anterior limiting membrane {I), which bounds the retina and is in contact with the vitreous humour, to an outer or posterior limiting membrane, which lies at the anterior ends, or bases, of the rods and cones near the level of b' c' in A. Thus the framework is thinner than the nervous substance of the retina, and the rods and cones lie altogether outside of it, and wholly unsupported by any connective tissue. They are, however, as we shall see, imbedded in the layer of pigment on which the retina rests (§ 16). The fibres of the optic nerve spread out between the limiting membrane (/) and the ganglionic corpuscles (//), and the vessels which enter along with the optic nerve ramify between the limiting membrane and the inner granules (//';• Thus, not only the ner\-ous fibres, but the vessels, are placed altogether in front of the rods and cones. At the entrance of the optic nerve itself, the ner\-ou3 fibres predominate, and the rods and cones are absent. In the yellow spot, on the contrary, the cones are abun- dant and close set, becoming at the same time longer and more slender, while rods are scanty, and are found only towards its margin. The layer of fibres of the optic nerve disappears, and all the other layers, except that of the cones, become extremely thin in the centre of the macula lutea (Fig. 73). 4. The most notable property of the retina is its power 2i8 ELEMENTARY PHYSIOLOGY. [less. Fig. 73. — A Diagrammatic Sec /ion of the Macula Ll'tea, or Yellow Spot. a a, the pigment of the choroid ; b, c, rods and cones : dd, outer granular layer : / f, inner granular layer ; / g, molecular layer ; // h, layer of gan- glionic cells ; i i, fibres of the optic nerve. (Magnified about 60 diameters.) IX. 3 THE YELLOW SPOT. 219 of converting the vibrations of ether, which constitute the physical basis of hght, into a stimuhis to the fibres of the optic nerve — which fibres, when excited, have the power of awakening the sensation of hght in, or by means of, the brain. The sensation of hght, it must be understood, is the work of the brain, not of the retina ; for, if an eye be destroyed, pinching, gah-anizing, or otherwise irritating the optic ner\-e, will still excite the sensation of light, be- cause it throws the fibres of the optic nerve into activity ; and their activity, however produced, brings about in the brain certain changes which give rise to the sensation of light. Light, fiilling directly on the optic nen'e, does not excite it ; the fibres of the optic ner.-e, in themselves, are as blind as any other part of the body. But just as the delicate filaments of the ampulla?, or the otoconia of the vestibular sac, or the Cortian fibres of the cochlea, are contrivances for converting the delicate vibrations of the perilymph and endolymph into impulses which can excite the auditory nerves, so the structures in the retina appear to be adapted to convert the infinitely more deli- cate pulses of the luminiferous ether into stimuli of the fibres of the optic nerve. 5. The sensibility of the different parts of the retina to light varies very greatly. The point of entrance of the optic nerve is absolutely blind, as may be proved by a very simple experiment. Close the left eye, and look steadily with the right at the cross on the page, held at ten or twelve inches' distance. ►P • The black dot will be seen quite plainly, as well as the cross. Now, move the book slowly towards the eye, which must be kept steadily fixed upon the cross ; at a certain point the dot will disappear, but, as the book is brought still closer, it will come into view again. It results from optical principles that, in the first position of the book, the figure of the dot falls between that of the cross (which throughout lies upon the yellow spot) and the entrance of the optic nerve : while, in the second position, it falls on the entrance of the optic nerve itself; and, in the third, inside that point. So long as the image 2 :o ELEMENTAR Y PHYSIO LOG Y [less. of the spot rests upon the entrance of the optic nerve, it is not perceived, and hence this region of the retina is called the d///ic/ spot. 6. The impression made by light upon the retina not only remains during the whole period of the direct action of the light, but has a certain duration of its own, how- ever short the time during which the light itself lasts. A flash of lightning is, practically, instantaneous, but the Fig. 74. — PiCMF.NT Cells from the Choroid Coat. A. Branched pigment cells from the deep layer. B. Pigment epithelium, a, seen in face ; b, seen in profile ; c, pigment granules. sensation of light produced by that flash endures for an appreciable period. It is found, in fact, that a luminous impression lasts for about one-eighth of a second ; whence it follows, that if any two luminous impressions are separated by a less interval, they are not distin- guished from one another. For this reason a " Catherine-wheel," or a lighted stick turned round very rapidly by the hand, appears as a circle of fire ; and the spokes of a coach wheel at speed are not separately visible, but only appear as a sort of opacity, or film, within the tire of the wheel. 7. The excitability of the retina is readily exhausted. Thus, looking at a bright light rapidly renders the part of the retina on which the light falls, insensible ; and on looking from the bright light towards a moderately-lighted surface, a dark spot, arising from a temporary blindness IX ] COLOUR BLIXDNESS. 221 of the retina in this part, appears in the field of view. If the bright hght be of one colour, the part of the retina on which it falls becomes insensible to rays of that colour, but not to the other rays of the spectrum. This is the explanation of the appearance of what are called coniplc- luentiDy colours. For example, if a bright red wafer be stuck upon a sheet of white paper, and steadily looked at for some time with one eye, when the eye is turned aside to the white paper a greenish spot will appear, of about the size and shape of the wafer. The red image has. in fact, fatigued the part of the retina on which it fell for red light, but has left it sensitive to the remaining coloured rays of which white light is composed. But we know that if from the variously coloured rays which make up the spectrum of white light Ave take away all the red rays, the remaining rays together make up a sort of green. So that, when white light falls upon this part, the red rays in the white light having no effect, the result of the ope- ration of the others is a greenish hue. If the wafer be greoi, the compleinentary image, as it is called, is red. 8. In some persons, the retina appears to be affected in one and the same way by rays of light of various colours, or even of all colours. Such colou7'-bli)id persons are unable to distinguish between the leaves of a cherry-tree and its fruit by the colour of the two, and see no differ- ence between blue and yellow cloth. This peculiarity is simply unfortunate for most people, but it may be dangerous if unknowingly possessed by railway guards or sailors. It probably arises either from a defect in the retina, which renders that organ unable to respond to different kinds of luminous vibrations, and consequently insensible to red rays or yellow rays, &c., as the case may be, or it may proceed from some unusual absorptive power of the humours of the eye which pre- vents particular rays from reaching the retina; or the fault may lie in the brain itself 9. The sensation of light may be excited by other causes than the impact of the vibrations of the lumi- niferous ether upon the retina. Thus, an electric shock sent through the eye, gives rise to the appearance of a flash of light : and pressure on any part of the retina produces a luminous image, which lasts as long as the ■222 ELEMENTARY PHYSIOLOGY. [less. pressure, and is called 2. phosphene. If the point of the finger be pressed upon the outer side of the ball of the eye, the eyes being shut, a luminous image — which, in my own case, is dark in the centre, with a bright ring at the circumference (or, as Newton described it, like the "eye " in a peacock's tail) — is seen ; and this image lasts as long as the pressure is continued. Most persons, again, have experienced the remarkable display of subjective fireworks which follows a heavy blow upon the eyes, pro- duced by a fall from a horse, or by other methods well known to English youth. It is doubtful, however, whether these effects of pressure, or shock, really arise from the excitation of the retina proper, or whether they are not rather the result of the violence done to the fibres of the optic nerve apart from the retina. 10. The last paragraph raises a distinction between the " fibres of the optic nerve" and the "retina" which may not have been anticipated, but which is of much importance. We have seen that the fibres of the optic ner\e ramify in the inner or anterior fourth of the thickness of the retina, while the layer of rods and cones forms its outer or posterior fourth. The light, therefore, must fall first upon the fibres of the optic nerve, and, only after tra- versing them, can it reach the rods and cones. Conse- quently, if the fibrillas of the optic nerve themselves are capable of being affected by light, the rods and cones can only be some sort of supplementary optical appa- ratus. But, in fact, it is the rods and cones which are affected by light, while the fibres of the optic nerve are themselves insensible to it. The evidence on ^vhich th's statement rests is — a. The blind spot is full of nervous fibres, but has no cones or rods. b. The yellow spot, where the most acute vision is situated, is full of close-set cones, but has no nerve fibres. c. If you go into a dark room with a single small bright candle, and, looking towards a dark wall, move the light up and down, close to the outer side of one eye, so as to allow the light to fall very obliquely into the eye. IX.] fUXCTlOX OF THE RODS AXD CONES. ii^ one of what are called Purkinjes figures is seen. This is a vision of a series of diverging, branched, red lines on a dark held, and in the interspace of two of these lines is a sort of cup-shaped disk. The red lines are the retinal blood-vessels, and the disk is the yehow spot. As the candle is moved up and down, the red lines saift their position, as shadows do when the light which throws them changes its place. Now, as the light falls on the inner face of the retina, and the images of the vessels to which it gives rise shift their position as it moves, whatever perceives these images must needs lie on the other, or outer, side of the vessels. But the fibres of the optic nerve lie among the vessels, and the only retinal structures which lie outside them are the granular layers and the rods and cones. d. Just as, in the skin, there is a limit of distance within which two points give only one impression, so there is a minimum distance by which two points of light falling on the retina must be separated in order to appear as two. And this distance corresponds pretty well with the dia- meter of the cones. II. The impact of the ethereal vibrations upon the sensory expansion, or essential part of the visual appa- ratus alone, is sufficient to give rise to all those feelings, which we term sensations of light and of colour, and to that feeling of outness which accompanies all visual sen- sation. But, if the retina had a simple transparent covering, the vibrations radiating from any number of distinct points in the external world would affect all parts of it equally, and therefore the feeling aroused would be that of a generally diffused luminosity. There would be no separate feehng of light for each separate radiating point, and hence no correspondence between the visual sensations and the radiating points which aroused them. It is obvious that, in order to produce this correspond- ence, or, in other words, to have distinct vision, the essential condition is, that distinct luminous points in the external world shall hi represented by distinct feelings of light. And since, in order to produce these distinct feelings, vibrations must impinge on separate rods or cones, it follows that, for the production of distinct vision, some apparatus must be interposed between the retina and the external world, by the action of which, distinct luminous 224 ELEMENTAR V PII YSIOL OGY. [less. points in the latter shall be represented by corresponding points of light on the retina. In the e}e of man and of the higher animals, this acces- sory apparatus of vision is represented by structures which, taken together, act as a biconvex lens, composed of sub- stances which have a much greater refracti\e power than the air by which the eye is surrounded ; and which throw upon the retina luminous points, which correspond in number, and, in one sense, in position, \s ith those luminous points in the external world from which ethereal vibrations proceed towards the eye. The luminous points thus thrown upon the retina form a picture of the external world — a pic- ture being nothing but lights and shadows, or colours, ar- ranged in such a way as to correspond with the disposition of the luminous or coloured parts of the object represented. 12. That a biconvex lens is competent to produce a picture of the external world on a properly arranged screen is a fact of which everyone can assure himself by simple experiments. An ordinary spectacle glass is a trans- parent body denser than the air, and convex on both sides. If this lois be held at a certain distance from a screen or wall in a dark room, and a lighted candle be placed on the opposite side of it, it w-ill be easy to adjust the distances of candle, lens, and wall, so that an image of the flame of the candle, upside down, shall be thrown upon the wall. The spot on which the image is formed is called a focus. If the candle be now brought nearer to the lens, the image on the wall wall enlarge, and grow blurred and dim, but maybe restored to brightness and definition by moving the lens further from the wall. But if, when the new adjustment has taken place, the candle be moved away from the lens, the image will again become confused, and, to restore its clearness, the lens will have to be brought nearer the wall. Thus a convex lens forms a distinct picture of luminous objects, but only at the focus on the side of the lens oppo- site to the object ; and that focus is nearer when the object is distant, and further off when it is near. 13. Suppose, however, that, leaving the candle unmoved, a lens with more convex surfaces is substituted for the first, the image will be blurred, and the lens will have to be moved nearer the wall to give it definition. If, on the other hand, a lens with less convex surfaces is sub- IX.] • THE EYEBALL. 225 stituted for the first, it must be moved further from the wall to attain the same end. In other words, other things being alike, the more con- vex the lens the nearer its focus ; the less convex, the further off its focus. If the lens were elastic, pulling it at the circumference would render it flatter, and thereby lengthen its focus ; while, when let go again, it would become more convex, and of shorter focus. Any material more refractive than the medium in which it is placed, if it have a convex surface, causes the rays of light which pass through the less refractive medium to that surface to converge towards a focus. If a watch-glass be fitted into one side of a box, and the box be then filled v.ith water, a candle may be placed at such a distance outside the v\-atch-glass that an image of its flame shall fall on the opposite wall of the box. If, under these cir- cumstances, a doubly convex lens of glass were introduced into the water in the path of the rays, it would act (though less powerfully than if it were in air) in bringing the rays more quickly to a focus, because glass refracts light more strongly than water does. A camera obscura is a box, into one side of which a lens is fitted, so as to be able to slide backwards and forwards, and thus throw on the screen at the back of the box dis- tinct images of bodies at various distances oft'. Hence the arrangement just described might be tenned a water camera. 14. The intermediate organs, by means cf which the physical agent of vision, light, is enabled to act upon the expansion of the optic nerve, comprise three kinds of apparatus : Ui) a " water camera,"' the eyeball ; ib) muscles for mo\ing the eyeball ; {c) organs for protecting the eveball, viz. the eyelids, with their lashes, glands, and muscles ; the conjunctiva ; and the lachrymal gland and its ducts. The eyeball is composed, in the first place, of a tough, firm, spheroidal case consisting of fibrous or connective tissue, the greater part of which is white and opaque, and is called the sclerotic (Fig. "j^., 2\ In front, however, ihis fibrous capsule of the eye, though it does not change its essential character, becomes transparent, and receives Q 226 ELEiMENTAR \ ' PHYSIOLOGY. [less. the name of the cornea (Fig. 75, i). The corneal por- tion of the case of the eyeball is more convex than the sclerotic portion, so that the ^vhole form of the ball is such as would be produced by cutting off a segment from the Fig. 75. — Horizontal Section' of the Eyeball. I, cornea ; i', conjunctiva ; 2, sclerotic ; 2', F.heat]i of optic nerve ; 3, choroid ; 3", rods and cones of the retina ; 4, ciliary muscle ; 4', circular portion of ciliary muscle ; 5, ciliary process ; 6, posterior chamber between 7, the iris and the suspensory ligament ; 7', anterior chamber ; 8, artery of retina in the centre of the optic nerve ; 8', centre of b'.ind spot ; 8'', niacu'a lutea ; Q, ora serrata (this is of course not seen in a section such as thi>, but is introduced to show its position) ; to, space behind the suspensory ligament (canal of Petit); t2, crystalline lens ; 13, vitreous humour ; 14 marks the position of the ciliary ligament ; a, optic axis 'in the actual eye of which this is an exact copy, the yellow spot happened, curiously enough, not to be in the optic axis) ; d, line of equator of the eyeball. front of a spheroid of the diameter of the sclerotic, and replacing this by a segment cut from a smaller, and con- sequently more convex, spheroid.. IX.] STRICTURE OT THE EYEBALL. 227 15. The corneo-sclerotic case of the eye is kept in shape by what are termed the humours — watery or semi-fluid substances, one of which, the aqueous humour (Fig. 75, 7'), which is hardly more than water holding a few organic and saline substances in solution, distends the corneal chamber of the eye, while the other, the vitreous (Fig. 75, 13), which is' rather a delicate jelly than a regular fluid, keeps the sclerotic chamber full. The two humours are separated by the very beautiful, transparent, doubly-convex crystalline lens (Fig. 75, 12), denser, and capable of refracting light more strongly than either of the humours. The crystalline lens is composed of fibres having a somewhat complex arrangement, and is highly clastic. It is more convex behind than in front, and it is kept in place by a delicate, but at the same time strong and elastic, membranous frame or suspensoiy ligament^ which extends from the edges of the lens to what are termed the ciliary processes of the choroid coat (Figs. 75, 5, and 76, c). In the ordinary condition of the eye this ligament is kept tense, i.e. is stretched pretty tight, and the front part of the lens is consequently flattened. 16. This cJwroid coat (Fig. 75, 3) is a highly vascular membrane, in close contact with the sclerotic externally, and lined, internally, by a layer of small polygonal bodies containing much pigmentary matter, called //^w^'/// cells (Fig. 74). These pigment cells are separated from the vitreous humour by the retina only. The rods and cones of the latter are in immediate contact with them. The choroid lines every part of the sclerotic, except just where the optic nerve enters it at a point below, and to the inner side of the centre of the back of the eye ; but VN-hen it reaches the front part of the sclerotic, its inner surface becomes raised up into a number of longitudinal ridges, with intervening depressions, like the crimped frills of a lady's dress, terminating within and in front by rounded ends, but passing, externally, into the iris. These ridges, which when viewed from behind seem to radiate on all sides from the lens (Figs. 76, r, and 75, 5), are the above- mentioned ciliary processes. 17. The iris itself (Figs. 75, 7, and 76, a, b) is, as has been already said, a curtain with a round hole in the Q 2 ELEMEXTARY PHYSIOLOGY. [less. middle, provided with circular and radiating unstriped muscular fibres, and capable of having its central aperture enlarged or diminished by the action of these fibres, the contraction of which, unlike that of other unstriped mus- cular fibres, is extremely rapid. The edges of the iris are firmly connected with the capsule of the eye, at the junc- tion of the cornea and sclerotic, by the connective tissue which enters into the composition of the so-called ciliary ligament. Unstriped muscular fibres, having the same attachment in front, spread backwards on to the outer surface of the choroid, constituting the ciliary muscle (Fig. 75, 4). If these fibres contract, it is obvious that they will pull the choroid forwards ; and as the frame, or suspensory ligament of the lens, is connected with the ^ - .^-. '- "^^m^' Fig. 76. — ViKW ok Fkont Halt of the Eyebali- sken kuom uehind. a, circular fibres ; l\ radiating fibres of the iris ; c, ciliary processes ; (i, choroid. The crystalline lens has been removed. ciliary processes which simply form the anterior termina- tion of the choroid), this pulling forward of the choroid comes to the same thing as a relaxation of the tension of that suspensory ligament, which, as I have just said, like the lens itself, is highly elastic. The iris does not hang down perpendicularly into the space between the front face of the crystalline lens and the posterior surface of the cornea, which is filled by IX.] ADjrSTMRXT OF TIIK EVE. 229 the aqueous humour, but applies itself veiy closely to the anterior face of the lens, so that hardly any interval is left between the two (Figs, 75 and 77;. The retina, as we have seen, lines the interior of the eye. being placed between the choroid and vitreous humour, its rods and cones being imbedded in the former, and its anterior limiting membrane touching the latter. About a third of the distance back from the front of the eye the retina seems to end in a wavy border called the or a serrata (Fig. 75, 91, and in reality the nervous ele- ments of the retina do end here, having become consider- ably reduced before this line, is reached. Some of the connective tissue elements however pass on as a delicate kind of membrane at the back of the ciliaiy processes towards the crystalline lens. 18. The eyeball, the most important constituents of which have now been described, is, in principle, a camera of the kind described above — a water camera. That is to say, the sclerotic answers to the box, the cornea to the watch-glass, the aqueous and vitreous humours to the water filling the box, the crystalline to the glass lens, the introduction of which was imagined. The back of the box corresponds v.ith the retina. But further, in an ordinaiy camera obscura, it is found desirable to have what is termed a diaphragm (that is, an opaque plate with a hole in its centre) in the path of the rays, for the purpose of moderating the light and cutting ott the marginal rays which, owing to certain optical pro- perties of spheroidal surfaces, give rise to defects in the image formed at the focus. In the eye, the place of this diaphragm is taken by the iris, which has the peculiar advantage of being self-regu- lating : dilating its aperture, and admitting more light when the light is weak ; but contracting its aperture and admitting less light when the illumination is strong. 19. In the water camera, constructed according to the description given above, there is the defect that no provi- sion exists for adjusting the focus to the varying distances of objects. If the box were so made that its back, on which the image is supposed to be thrown, received distinct images of very distant objects, all near ones would be 230 ELEMENTARY PHYSIOLOGY. [less. indistinct. And if, on tlie other hand, it were fitted to receive the image of near objects, at a given distance, those of either nearer, or more distant, bodies would be bkirred and indistinct. In the ordinary camera this diffi- cuUy is overcome by shding the lenses in and out, a pro- cess which is not compatible with the construction of our water camera. But there is clearly one way among many, in which this adjustment might be effected — namely, by changing the glass lens ; putting in a less convex one when more distant objects had to be pictured, and a more convex one when the images of nearer objects were to be thrown upon the back of the box. But it would come to the same thing, and be much more convenient, if, without changing the lens, one and the same lens could be made to alter its convexity. This is what actually is done in the adjustment of the eye to distances. 20. The simplest way of experimenting on the adjust- ?ncnt of the eye is to stick two stout needles upright into a straight piece of wood, not exactly, but nearly in the same straight line, so that, on applying the eye to one end of the piece of wood, one needle {a) shall be seen about six inches off, and the other {b) just on one side of it at twelve inches' distance. If the observer look at the needle b, he will find that he sees it very distinctly, and without the least sense of effort ; but the image of rt: is blurred and more or less double. Now let him try to make this blurred image of the needle a distinct. He will find he can do so readily enough, but that the act is accompanied by a sense of effort somewhere in the eye. And in proportion as a becomes distinct, b will become blurred. Nor will any effort enable him to see a and b distinctly at the same time. Z::-^\. Multitudes of explanations have been given of this remarkable power of adjustment, but it is only within the last few years that the problem has been solved, by the accurate determination of the nature of the changes in the eye which accompany the act. When the flame of a taper is held near, and a little on one side of, a person's eye, anyone looking into the eye from a proper point of view, will see three images of the flame, two upright and IX.] ADJUSTMENT OF THE EYE, one inverted. One upright figure is reflected from the front of the cornea, which acts as a convex mirror. The second proceeds from the front of the crystalhne lens, which has the same effect ; while the inverted image pro- ceeds from the posterior face of the lens, which, being convex backwards, is, of course, concave forwards, and acts as a concave mirror. Suppose the eye to be steadily fixed on a distant object, and then adjusted to a near one in the same line of vision, the position of the eyeball remaining unchanged. Then the upright image reflected from the surface of the cornea, and the inverted image from the back of the lens, will remain unchanged, though ir is demonstrable that their size or apparent jjosition must change if either the cor- nea, or the back of the lens, alter either their form or Illustrates the change ii Fig. 77 the form of the lens when adjusted- D to near objects. -A to di.lant. their position. But the second upright image, that re- flected by the front face of the lens, does change both its size and "its position ; it comes forward and grows smaller, proving that the front face of the lens has become more convex. The change of form of the lens is, in fact, that represented in Fig. 'j'j. These may be regarded as \\\q^ facts of adjustment with which all explanations of that process must accord. They at once exclude the hypotheses (i) that adjustment^ is the result of the compression of the ball of the eye by its muscles, which would cause a change in the form of the cornea ; (2) that adjustment results from a shifting of the lens bodily, for its hinder face does not move ; (3) that it results from the pressure of the iris upon the front face 232 ELEMENTARY PHYSIOLOGY. [lkss. of the lens, for under these circumstances the hinder face of the lens would not remain stationary. This last hypo- thesis is further negatived by the fact that adjustment takes place equally well when the iris is absent. One other explanation remains, which is, in all proba- bility, the true one, though not altogether devoid of diffi- culties. The lens, which is very elastic, is kept habitually in a state of tension by the elasticity of its suspensory ligament, and consequently has a flatter form than it would take if left to itself. If the ciliar)- muscle contracts, it must, as has been seen, relax that ligament, and thereby diminish its elastic tension upon the lens. The lens, consequently, will become more convex, returning to its former shape when the ciliary muscle ceases to contract, and allows the choroid to return to its ordinary place. If this be the true explanation of adjustment, the sense of effort we feel must arise from the contraction of the ciliary muscle. 22. Adjustment can take place only within a certain range, ^\hich admits of great individual variations. As a rule, no object which is brought within less than about ten inches of the eye can be seen distinctly without effort. But many persons are born with the surface of the cornea more convex than usual, or with the refractive power of the eye increased in some other way ; while, very generally, as age draws on, the cornea flattens. In the former case, objects at ordinary distances are seen indistinctly, because these images fall not on the retina, but in front of it ; while, in the latter, the same indis- tinctness is the result of t'\:^rays of light striking upon the retina before they have been brought to a focus. The defect of the former, or short-sighted people, is amended by wearing concave glasses, which cause the rays to diverge ; of the latter, or long-sighted people, by wearing convex glasses, which make the rays converge. In the water camera the image brought to a focus on the screen at the back is inverted; the image of a tree for instance is seen with the roots upwards and the leaves and branches hanging downwards. The right of the image also corresponds with the left of the object and 'ix.J THE MUSCLES OE THE EYEBALL. vice versa. Exactly the same thing takes place in the eye with the image focussed on the retina. It too is inverted. (See Lesson X. § ii.) _ 23. The muscles which move the eyeball are altogether six in number— four straight muscles, or recti, and two oblique muscles, the obliqui (Fig. 78;. The straight mus- cles are attached to the back of the orbit, round the edges of the liole through which the optic nerve passes, and run straight forward to their insertions into the sclerotic- one, the superior rectus, in the middle line above : one. C.k. Fig. 78. A, the muscles of the right eyeball viewed from above, and B of the left eyeball viewed from the outer side ; S.R. the superior rectus ; hif. R. the inferior rectus ; E.R., Iii.R. the external rectus ; S.Ob, the superior oblique ; Inf.Ob. the inferior oblique ; CJi. the chiasma of the optic nerves //.) ; ///. t!ie third nerve which supplies all the nuiscles except the superior oblique and the external rectus. the inferior, opposite it below ; and one half-way on each side, the external and internal recti. The eyeball is completely imbedded in fat behind and laterally ; and these muscles turn it as on a cushion ; the superior rectus inclining the axis of the eye upwards, the inferior down- wards, the external outwards, the internal inwards. The two oblique muscles are both attached on the outer side of the ball, and rather behind its centre ; and they both pull in a direction from the point of attachment towc^rds the inner side of the orbjt— the lower, because 234 ELEMENTARY PHYSIOLOGY. [lfsS. it arises here ; the upper, because, though it arises along with the recti from the back of the orbit, yet, after passing forwards and becoming tendinous at the upper and inner corner of the orbit, it traverses a pulley -hke loop of ligament, and then turns downwards and outwards to its insertion. The action of the oblique muscles is somewhat complicated, but their general tendency is to roll the eyeball on its axis, and pull it a little forward and inward. 24. The eyelids are folds of skin containing thin plates of cartilage, and fringed at their edges with hairs, the eye- lashes, and with a series of small glands called Meibomian. Fig. 7.9. The front view of the right eye dissected to show, Orb., the orbicular muscle of the eyelids ; the pulley and insertion of the superior oblique, S.Ob., and the inferior oblique, Inf. Ob. ; L.G. the lachrymal gland. Circularly disposed fibres of striped muscle lie beneath the integuments of the eyelids, and constitute the orbi- cularis muscle which shuts them. The upper eyelid is raised by a special muscle, the levator of the upper lid, which arises at ihc back of the orbit and runs forwards to end in the lid. The lower lid has no special depressor. 25. At the edge of the eyelids the integument becomes continuous with a delicate, vascular, and highly nervous mucous membrane, the conjunctiva., which lines the interior of the lids and the front of the eyeball, its epithelial layer being even continued over the cornea. IX.] THE LACHRYMAL APPARATUS. The numerous small ducts of a gland which is lodged in the orbit, on the outer side of the ball (Fig. 79, Z.c;.), the lacJuymal gland, constantly pour its water)- secretion into the interspace between the conjunctiva lining the upper eyelid and that covering the ball. On the inner side of the eye is a reddish fold, the canincula lacJrrynialis, a sort of rudiment of that third eyelid which is to be found in many animals. Above and below, close to the caruncula, the edge of each eyelid presents a minute aperture (the punctum lacJuymalc), the opening of a -L.C. Fig. 80. A front view of the left eye, with the eyelids partially dissected to show lachrymal gland, L.G., and lachrymal duct, L.D. small canal. The canals from. abo\e and below con- verge and open into the lacJuymal sac ; the upper blind end of a duct {L.D., Fig. 80) which passes down from the orbit to the nose, opening below the inferior tur- binal bone (Fig. 40, //)• It is through this system of canals that the conjunctival mucous membrane is con- tinuous with that of the nose ; and it is by them that the secretion of the lachrymal canal is ordinarily carried away as fast as it forms. But, under certain circumstances, as when the con- junctiva is irritated by pungent vapours, or when painful emotions arise in the mind, the secretion of the lachrymal gland exceeds the drainage power of the lachrymal duct, and the fluid, accumulating between the lids, at length overflows in the form of tears. 2;/) ELEMENTARY PHYSIOLOGY. [less. LESSON X. THE COALESCENCE OF SEXSATIOXS WITH OXE AXOTHER AXD WITH OTHER STATES OF COX- SCIOUSXESS. 1. In explaining the functions of the sensory organs, I have hitherto confined myself to describing the means by which the physical agent of a sensation is enabled to irritate a given sensory nerve ; and to giving some account of the simple sensations which are thus evolved. Simple sensations of this kind are such as might be produced by the irritation of a single nerve-fibre, or of several nerve-fibres by the same agent. Such are the sensations of contact, of warmth, of sweetness, of an odour, of a musical note, of whiteness, or redness. But very few of our sensations are thus simple. IVIost of even those which v.e are in the habit of regarding as simple, are really compounds of different sensations, or of sensations with ideas, or with judgments. For example, in the preceding cases, it is very difficult to separate the sensation of contact from the judgment that something is touching us ; of sweetness, from the idea of something in the mouth ; of sound or light, from the judgment that something outside us is shining, or sounding. 2. The sensations of smell are those which are least complicated by accessories of this sort. Thus, particles of musk diffuse themselves with great rapidity through the nasal passages, and give rise to the sensation of a powerful odour. But beyond a broad notion that the odour is in the nose, this sensation is unaccompanied by any ideas of locality and direction. Still less does it give rise to any conception of form, or size, or force, x.j ycDGMEXTS AXn SEXSATiOXS. t- or of succession, or contemporaneity. If a man had no other sense than that of smell, and musk were the only odorous body, he could have no sense of outness— no power of distinguishing between the external world and himself. 3. Contrast this with what may seem to be the equally simple sensation obtained by drawing the linger along the table, the eyes being shut. This act gives one the sensation of a flat, hard surface outside oneself, which appears to be just as simple as the odour of musk, but is really a complex state of feeling compounded of— {(i) Pure sensations of contact. ib) Pure muscular sensations of two kinds, — the one arising from the resistance of the table, the other from the actions of those muscles which draw the finger along. (/) Ideas of the order in which these pure sensations succeed one another. {d) Comparisons of these sensations and their order, with the recollection of like sensations similarly arranged, which have been obtained on previous occasions. [c) Recollections of the impressions of extension, flat- ness, &c. niade on the organ of vision when these pre- vious tactile and muscular sensations were obtained. Thus, in this case, the only pure sensations are those of contact and muscular action. The greater part of what we call the sensation is a complex mass of present and recollected ideas and judgments. 4. Should any doubt remain that we do thus mix up our sensat'ons with our judgments into one indistinguish- able whole, shut the eyes as before, and, instead of touching the table with the finger, take a round lead pencil between the fingers, and draw that along the table. The '*' sensation " of a flat hard surface will be just as clear as before ; and yet all that we touch is the round surface of the pencil, and the only pure sensations we owe to the table are those afforded by the muscular sense. In fact, in this case, our "sensation" of a flat hard surface is entirely a judgment based upon what the muscular sense tells us is going on in certain muscles. A still more striking case of the tenacity with which we adhere to complex judgments, ^\hich we conceive to be pure sensations, and are unable to analyse otherwise 238 ELEMENTARY PHYSIOLOGY. [less. than by a process of abstract reasoning, is afforded by our sense of roundness. Anyone taking a marble between tAvo fingers will say that he feels it to be a single round body ; and he will probably be as much at a loss to answer the cjuestion how he knows that it is round, as he would be if he were asked how he knows that a scent is a scent. Nevertheless, this notion of the roundness of the marble is really a very complex judgment, and that it is so may be shown by a simple experiment. If the index and middle fingers be crossed, and the marble placed between them, so as to be in contact with both, it is utterly impossible to avoid the belief that there are two marbles instead of one. Even looking at the marble, and seeing that there is only one, does not weaken the apparent proof derived from touch that there are two.^ The fact is, that our notions of singleness and round- ness are, really, highly complex judgments based upon a few simple sensations ; and when the ordinary con- ditions of those judgments are reversed, the judgment is also reversed. With the index and the middle fingers in their ordinary position, it is of course impossible that the outer sides of each should touch opposite surfaces of one spheroidal body. If, in the natural and usual position of the fingers, their outer surfaces simultaneously give us the impression of a spheroid (which itself is a complex judgment), it is in the nature of things that there must be two spheroids. But, when the fingers are crossed over the marble, the outer side of each finger is really in contact with a spheroid ; and the mind, taking no cognizance of the crossing, judges in accordance with its universal experience, that two spheroids, and not one, give rise to the sensations which are perceived. 5. Phenomena of this kind are not uncommonly called dchisiofis of the senses ; but there is no such thing as a fictitious, or delusive, sensation. A sensation must * A ludicrous form of this experiment is to apply the crossed fingers to the end of the nose, when it at once appears double ; and, in spite of the absurdity of the conviction, the mind cannot expel it, so long as the sensa- tions last. X.] AUDITORY SPECTRA. 239 exist to be a sensation, and, if it exists, it is real and not delusive. But the judgments we form respecting the causes and conditions of the sensations of which Nvc are aware, are very often erroneous and delusive enough ; and such judgments may be brought about in the domain of every sense, either by artificial combinations of sensations, or by the influence of unusual conditions of the body itself. The latter give rise to what are called subjective sensations, Mankind would be subject to fewer delusions than they are, if they constantly bore in mind their liability to false judgments due to unusual combinations, either artificial or natural, of true sensations. Men say, '' I felt,"' " I heard," " I saw " such and such a thing, when, in ninety- nine cases out of a hundred, what they really mean is, that they judge that certain sensations of touch, hearing, or sight, of which they were conscious, were caused by such and such things. 6. Among subjective sensations within the domain of touch, are the feelings of creeping and prickling of the skin, which are not uncommon in certain states of the circulation. The subjective evil smells and bad tastes which accompany some diseases are very probably due to similar disturbances in the circulation of the sensory organs of smell and taste. Many persons are liable to what maybe called aiiditory spectra — music of various degrees of complexity sounding in their ears, without any external cause, while they are wide awake. I know not if other persons are similarly troubled, but in reading books written by persons with whom I am acquainted, I am sometimes tormented by hearing the words pronounced in the exact way in which these persons would utter them, any trick or peculiarity of voice, or gesture, being, also, very accurately reproduced. And I suppose that everyone must have been startled, at times, by the extreme distinctness with which his thoughts have embodied themselves in apparent voices. The most wonderful exemplifications of subjective sen- sation, however, are afforded by the organ of sight. Anyone who has witnessed the sufferings of a man labouring under delirium tremens (a disease produced by excessive drinking), from the marvellous distinctness of - p ELEMENTAR Y Pll YSIOLOGY. [less. his visions, which sometimes take the forms of devils, sometimes of creeping animals, but almost always of something fearful or loathsome, will not doubt the inten- sity of subjective sensations in the domain of vision. 7. But that illusive visions of great distinctness should appear, it is not necessar)- for the nervous system to be thus obviously deranged. People in the full possession of their faculties, and of high intelligence, may be subject to such appearances, for which no distinct cause can be iissigned. An excellent illustration of this is the famous case of Mrs. A. given by Sir David Brewster, in his *• Natural Magic," (See Appendix.) It should be mentioned that Mrs. A. was naturally a person of very vivid imagination, and that, at the time the most notable of these ilkisions appeared, her health was .veak from bronchitis and enfeebled digestion. It is obvious that nothing but the singular courage and clear intellect of Mrs. A. prevented her from becoming a mine of ghost stories of the most excellently authenticated kind. And the particular value of her history lies in its showing, that the clearest testimony of the most un- impeachable witness may be quite inconclusive as to the objective reality of something which the witness has seen. Mrs. A. undoubtedly saw what she said she saw. The evidence of her eyes as to the existence of the apparitions, and of her ears to those of the voices, was, in itself, as perfectly trustworthy as their evidence would have been had the objects really existed. For there can be no doubt that exactly those parts of her retina which would have been affected by the image of a cat, and those parts of h.er auditory organ which would have been set vibrating 1)y her husband's voice, or the portions of the sensorium with which those organs of sense are connected, were thrown into a corresponding state of activity by some internal cause. What the senses testify is neither more nor less than the fact of their own affection. As to the cause of that affec- tion they really say nothing, but leave the mind to form its own judgment on the matter. A hasty or superstitious person in Mrs. A.'s place would have formed a wrong judgment, and would have stood by it on the plea that " she must believe her senses " X] DELUSIOXS OF THE JUDGMEXT. 241 8. The delusions of the judgment, produced not by abnornieil conditions of the body, but by unusual or artificial combinations of sensations, or by suggestions of ideas, are exceedingly numerous, and, occasionally are not a little remarkable. Some of those which arise out of the sensation of touch have already been noted. I do not know of any produced through smell or taste, but hearing is a fertile source of such errors. What is called ventriloquism (speaking from the belly), and is not uncommonly ascribed to a mysterious power of producing voice somewhere else than' in the larynx, depends entirely upon the accuracy with which the per- former can simulate sounds of a particular character, and upon the skill with which he can suggest a belief in the existence of the causes of these sounds. Thus, if the ventriloquist desire to create the belief that a voice issues from the bowels of the earth, he imitates with great accuracy the tones of such a half-stifled voice, and sug- gests the existence of some one uttering it by directing his answers and gestures towards the ground. These gestures and tones are such as would be produced by a given cause ; and no other cause being apparent, the mind of the bystander insensibly judges the suggested cause to exist. 9. The delusions of the judgment through the sense of sight — optical delusions^ as they are called — are more numerous than any others, because such a great number of what we think to be simple visual sensations are really very complex aggregates of visual sensations, tactile sensations, judgments, and recollections of former sensa- tions and judgments. It will be instructive to analyse some of these judgments into their principles, and to explain the delusions by the application of these principles. 10. When an external body is felt by the touch to be in a given place, the image of that body falls on a point of the retina which lies at one end of a straight line joining the body and the 7-etina, and traversing a particular j-egion of the centre of the eye. This straight line is called the OPTIC AXIS. Conversely, t^dien any part of the surface of the retina 242 ELEMEXTARY PHYSIOLOGY. [less. h excited^ the luminous sensation is referred by the mind to some point outside the body, in the direction of the optic axis. It is for this reason that when a phosphene is created by pressure, say on the outer and lower side of the eye- ball, the luminous image appears to lie above, and to the inner side of, the eye. Any external object which could produce the sense of light in the part of the retina pressed upon must, owing to the inversion of the retinal images (see Lesson IX. § 23), in fact occupy this position ; and hence the mind refers the light seen to an object in that position, 11. The same kind of explanation is applicable to the apparent paradox that, w-hile all the pictures of external objects are certainly inverted on the retina by the refract- ing media of the eye, we nevertheless see them upright. It is difficult to understand this, until one reflects that the retina has, in itself, no means of indicating to the mind which of its parts lies at the top, and which at the bottom; and that the mind learns to call an impression on the retina high or low, right or left, simply on account of the association of such an impression with certain coincident tactile impressions. In other words, when one part of the retina is affected, the object causing the affection is found to be near the right hand ; when another, the left ; when another, the hand has to be raised to reach the object ; when yet another, it has to be depressed to reach it. And thus the several impressions on the retina are called right, left, upper, lower, quite irrespectively of their real positions, of which the mind has, and can have, no cognizance. 1 2. When an external body is ascertained by touch to be simple, it fo7-ms but one image on the retina of a single eye; and luhcn two or 7nore images fall on the j-etina of a single eye, they ordinarily proceed from a corresponding number of bodies which are distinct to the touch. Conversely, the sensation of two or more images is judged by the mind to proceed from two or more objects. If two pin-holes be made in a piece of cardboard at a distance less than the diameter of the pupil, and a small object like the head of a pin be held pretty close to the eye, and viewed through these holes, two images of the X.] VISUAL SIZE AXD FORM i^3 head of the pin will be seen. The reason of this is, that the rays of hght from the head of the pin are spht by the card into two minute pencils, which pass into the eye on either side of its centre, and cannot be united again and brought to one focus on account of the nearness of the pin to the eye. Hence they fall on different parts of the retina, and each pencil of rays, being very small, makes a tolerably distinct image of its own of the pins head on the retina. Each of these images is now referred outward (§ lo) in the direction of the appropriate optic axis, and two pins are apparently seen instead of one. A like expla- nation applies to i)iultiplyi)ig glasses and doubly refract- ing crystals, both of which, in their own ways, split the pencils of light proceeding from a single object into two or more separate bundles. These give rise to as many images, each of which is referred by the mind to a distinct external object. 13. Certain visual plietiomena ordinarily accompany those products of tactile sensation to 'u.'hich lue give the name of sizc^ distance^ and form. Thus, other things being alike, the space of the retina covered by the image of a large object is larger than that covered by a small object j while that covered by a near object is larger than that covered by a distinct object ; and, other conditions being alike, a near object is more bri'liant than a distant one. Furthermore, the shadows of objects differ with the forms of their surfaces, as determined by touch. Conversely, if these visual sensations can be produced, they inevitably suggest a belief in the existence of objects competent to produce the corresponding tactile sensations. What is called perspective, whether solid or aerial, in drawing, or painting, depends on the application of these principles. It is a kind of visual ventriloquism — the painter putting upon his canvas all, the conditions requisite for the production of images on the retina, having the size, relati\"e form, and intensity of colour of those which would actually be produced by the objects themselves in nature. And the success of his picture, as an imitation, depends upon the closeness of the resemblance between the images it produces on the retina, and those which would be pro- duced by the objects represented. 14. To most persons the image of a pin, at live or six R 2 244 ELEMENTAR V PHYSIOL 0 G Y. [less. inches from, the eye, appears blurred and indistinct — the eye not being capable of adjustment to so short a focus. If a small hole be made in a piece of card, the circumferential rays which cause the blur are cut off, and the image becomes distinct. But at the same time it is magnified, or looks bigger, because the image of the pin, in spite of the loss of the circumferential rays, occupies a much larger extent of the retina when close than when distant. All convex glasses produce the same effect — while concave lenses diminish the apparent size of an object, because they diminish the size of its image on the retina. 1 5. The moon, or the sun, when near the horizon appear ver}^ much larger than they are when high in the sky. When in the latter position, in fact, we have nothing to compare them with, and the small extent of the retina which their images occupy suggests small absolute size. But as they set, we see them passing behind great trees and buildings which we know to be very large and very distant, and yet occupying a larger space on the retina than the latter do. Hence the vague suggestion of their larger size. 16. If a convex surface be lighted from one side, the side towards the light is bright — that turned from the light, dark, or in shadow ; while a concavity is shaded on the side towards the light, bright on the opposite side. If a new half-crown, or a medal with a well-raised head upon its face, be lighted sideways by a candle, we at once know the head to be raised (or a canieo) by the disposition of the light and shade ; and if an intaglio, or medal on which the head is hollowed out, be lighted in the same way, its nature is as readily judged by the eye. But now, if either of the objects thus lighted be viewed with a convex lens, which inverts its position, the light and dark sides will be reversed. With the reversal the judg- ment of the mind will change, so that the cameo will be regarded as an intaglio, and the intaglio as a cameo ; for the light still comes from where it did, but the cameo appears to have the shadows of an intaglio, and vice ve7'sd. So completely, however, is this interpretation of the facts as a matter of judgment, that if a pin be stuck beside the medal so as to throw a shadow, the pin and its shadow, being -reversed by the lens, will suggest that the direction X.] VISIOX IVITII TWO EYES. 245 of the liglit is also reversed, and the medals will seem to be what they really are. 17. Whenever an external object is watched rapidly changing its form, a continuous series of different pictiwes of the object is inipressed tipon the same spot of the retina. Cojiversely, if a continnons series of different pictures of one object is inipressed upon one part of the retina, tJie mind judges that they are due to a single external object^ undergoing changes of form. This is the principle of the curious toy called the thati- matrope., or " zootrope/' or " wheel of life," by the help of which, on looking through a hole, one sees images of jugglers throwing up and catching balls, or boys playing at leapfrog over one another's backs. This is managed by painting at intervals, on a disk of card, figures and jugglers in the attitudes of throwing, waiting to catch, and catching ; or boys " giving a back," leaping, and coming into position after leaping. The disk is then made to rotate before an opening, so that each image shall be pre- sented for an instant, and follow its predecessor before the impression of the latter has died away. The result is that the succession of different pictures irresistibly suggests one or more objects undergoing successive changes — the juggler seems to throw the balls, and the boys appear to jump over one another's backs. 1 8. When a?i external object is ascertained by touch to he single., the centres of its retinal images in the two eyes fall upon the centres of the yellow spots of the two eyes, when both eyes are directed towa7'ds it; but if there be two external objects, the centres of both their linages cannot fall, at the same ti?ne, upon the centres of the yellow spots. Conversely, when the centres of two images, forjned simultaneously in the two eyes, fall upon the centres of the yellow spots, the mind judges the images to be caused by a single external object; but if not, by two. This seems to be the only admissible explanation of the facts, that an object which appears single to the touch and when viewed with one eye, also appears single when it is viewed with both eyes, though two images of it are neces- sarily formed ; and on the other hand, that when the centres of the two images of one object do not fall on the 246 ELF.MF.XTA R Y rUYSTOT OGV. [i,es=t. centres of the yellow spots, both images are seen sepa- rately, and we have double vision. In squinting, the axes of the two eyes do not converge equally towards the object viewed. In consequence of this, when the centre of the image formed by one eye falls on the centre of the yellow spot, the corresponding part of that formed by the other eye does not, and double vision is the result. For simplicity's sake we have supposed the images to fall on the centre of the yellow spot. But though vision is distinct only in the yellow spot, it is not absolutely limited to it ; and it is quite possible for an object to be seen as a single object with two eyes, though its images fall on the two retinas outside the yellow spots. All that is neces- sary is that the two spots of the retinas on which the images fall should be similarly disposed towards the centres of their respective yellow spots. Any two points of the two retinas thus similarly disposed towards their respective yellov/ spots (or more exactly to the points in which the optic axes end), are spoken of as corresponding^ points; and any two images covering two corresponding areas are conceived of as coming from a single object. It is obvious that the inner (or nasal) side of one retina corre- sponds to the outer (or cheek^i side of the other. 19. /;/ single 7'ision witJi i^^o eyes, the axes of the two eyes, of the movements of which the muscular sense gives an indication, cut ojie another at a greater angle when the object appi'oaches, at a less angle when it goes further off. Conve?'sely, if without changing the position of an object, the axes of the tivo eyes which view it can be made to converge or diverge, the object will seem to approach or go further off. In the instrument called the pseudoscope, mirrors or prisms are disposed in such a manner that the angle at which rays of light from an object enter the two eyes, can be altered without any change in the object itself; and consequenththe axes of these eyes are made to converge or diverge. In the former case the object seems to approach ; in the latter, to rec'^'V. 20. Wheti a body of moderate size, ascertained by touch to be solid, is viewed with both eyes, the images of it, formed by the two eyes, are necessarily different {one showing more of its right side, the other of its left side). X.] THE STEREOSCOPE. 247 Nevertheless, they coalesce into a co7nmon i?nage, which gives the ifnpression of solidity. Conversely, if the two images of the right and left aspects of a solid body be made to fall npon the retinas of the two eyes in such a way as to coalesce into a common linage, they a7'e judged by the mind to proceed from the single solid body which alone, -under ordinary circ?nn- stanccs, is competent to produce them. The stereoscope is constructed upon this principle. Whatever its form, it is so contrived as to throw the images of two pictures of a sohd bod}', such as Avould be obtamed by the right and left eye of a spectator, on to such parts of the retinas of the person who uses the stereoscope as would receive these images, if they really proceeded from one solid body. The mind immediately judges them to arise from a single external solid body, and sees such a solid body in place of the two pictures. The operation of the mind upon the sensations presented to it by the two eyes is exactly comparable to that which takes place when, on holding a marble between the finger and thumb, we at once declare it to be a single sphere (§ 4). That which is absolutely presented to the mind by the sense of touch in this case is by no means the sensa- tion of one spheroidal body, but two distinct sensations of two convex surfaces. That these two distinct convexities belong to one sphere, is an act of judgment, or process of unconscious reasoning, based upon many particulars of past and present experience, of which we have, at the moment, no distinct consciousness. 248 ELEMENTARY PHYSIOLOGY. [less. LESSON XI. THE NERVOUS SYSTEM AND INNERVATION. 1. The sensory organs are, as we have seen, the chan- nels through which particular physical agents are enabled to excite the sensory nerves with which these organs are connected ; and the activity of these nerves is evidenced by that of the central organ of the nervous system, which activity becomes manifest as a state of consciousness — the sensation. We have also seen that the muscles are instruments by which a motor nerve, excited by the central organ with which it is connected, is able to produce motion. The sensory nerves, the motor nerves, and the central organ, constitute the greater part of the nervous system., which, with its function of iiinervaiion., we must now study somewhat more closely, and as a whole. 2. The nervous apparatus consists of two sets of nerves and nerve-centres, which are intimately connected together and yet may be conveniently studied apart. These are the cerebro-sphial system and the sympathetic system. The former consists of the cerebro-spiiial axis (composed of the brain and spinal cord) and the cerebral and spinal nerves., which are connected with this axis. The latter comprises the chain of sympathetic gan^ii^lia, the nerves which they give otT, and the nervous cords by which they are connected with one another and with the cerebro- spinal nerves. Nerves are made up entirely of nerve-fibres, the struc- ture of which is somewhat different in the cerebro-spinal and in the sympathetic systems. (See Lesson XII., § i6.) XI.] THE SPLVAL CORD. 249 Nerve-centres, on the other hand, are composed of fief've- cells or ganglionic corpuscles, mingled with nerve-fibres (Lesson XII., § 16). Such cells, or corpuscles, are found in various parts, of the brain and spinal cord, in the S}'mpa- thetic ganglia, and also in the ganglia belonging to spinal nerves as well as in certain sensory organs, such as the retina and the internal ear. 3. The cerebrospinal axis lies in the ca^•ity of the skull and spinal column, the bony walls of which cavity are lined by a very tough fibrous membrane, serving as the periosteum of the component bones of this region, and called the dura niafer. The brain*and spinal cord them- selves are closely invested by a very vascular fibrous tissue, called pia niafer. The numerous blood-vessels supplying these organs run for some distance in the pia mater, and where they pass into the substance of the brain or cord, the fibrous tissue of the pia mater accom- panies them to a greater or less depth. The outer surface of Ihepia 7naler, and the inner surface of the dura niafer, pass into a delicate fibrous tissue, lined by an epithelium, which is called the cznz' impulses. It is obvious, there- fore, that in the dorsal part of the cord, nervous impulses from the brain are sent through the anterior part of the white matter. The posterior half of the white matter may be cut XI.] THE CORD AS A COXDUCTOR. 259 through at one point, and the anterior hah" at a point a httle higher up, so that all the white fibres shall be divided transversely by the one cut or the other, without any in- terference Avith the material continuity of the cord, or damage to the grey matter. When this has been done, irritation of those sensory nerves which are connected with parts below the section excites the sensation of pain as strongly as ever. Hence it follows, that the afferent impulses, which excite pain when they reach the brain, pass through, and are conveyed by, the grey matter. And it has been found, by experi- ment, that, so long as even a small portion of the grey matter remains entire, these afferent impulses are efficiently transmitted. Singularly enough, however, irritation of the grey matter itself is said not to cause pain.^ If one-half of the cord, say the right, be cut through, transversely, down to its very middle, so as to interrupt all continuity of both white and grey matter between its upper and lower parts, irritation of the skin of the right side of the body, below the line of section, will give rise to as much pain as before, but all voluntary power will be lost in those muscles of that side, which are supplied by nerves coming off from the lower portion of the cord. Hence it follows, that the channels by which the afferent impulses are conveyed must cross over from the side of the cord which they enter to the opposite side ; while the efferent impulses, sent down from the brain, must travel along that side of the cord by which they pass out. If this be true, it is clear that a longitudinal section, taken through the exact middle of the cord, will greatly impair, if not destroy, the sensibility of both sides of the body below the section, but will leave the muscles under the control of the will. And it is found experimentally that such is very largely the case. 15. Such are the functions of the spinal cord, taken as a whole. The spinal nerves arc, as we have said, chiefly distributed to the muscles and to the skin. The nerves of the blood-vessels, for instance, the so-called 7'rt'j-6'-w^/^rnerves (Lesson II. § 23), belong not to the spinal, * This is why, in the experiment described at end of § ii., it is better for testing the presence of sensations to irritate afferent nerves connected with the cord rather than the cut end of the cord itself. S 2 26o ELEMENTAR \ ' PHYSIOLOG \ ' [less. Fig. 83. — The Base of the Brains A . frontal lobe : B. temporal lobe of the cerebral hemispheres ; Ci>. cerebel- lum ; /. the olfactory nerve ; //. the optic nerve ; ///. /F. VI. the nerves of the muscles of the eye ; V. the trigeminal nerve ; VII. the portio dura; VIII. the auditory nerve ; IX. the glossopharj'ngeal ; JC. the pneumo- gastric ; XI. the spinal accessory ; XII. the hypoglossal, or motor nerve of the tongue. The number VI. is placed upon t\\Q pons Varolii. The crura cerebri are the broad bundles of fibres which lie between the third and the fourth ner^ es on each side. The medulla oblongata {M) is seen to be really a continuation of the spinal cord ; on the lower end are seen the two crescents of grey matter ; the section, in fact, has been carried through the spinal cord, a little below the proper medulla oblongata. From the sides of the medulla oblongata are seen coming off the X., XI., and XII. nerves ; and just where the medulla is covered, so to .speak, by the trans- versely disposed pons Varolii, are seen coming off the VII. nerve, and more towards the middle line the VI. Out of the substance of the pons springs the V. nerve. In front of that is seen the well-defined anterior xr.l VASO-MOTOJi XERVES. 261 but to the sympathetic system. Along the spinal column, however, the spinal nerves give off branches which run in and join the sympathetic system. And it appears that many at least of the fibres which run along in the s>tti- pathetic nerves going to blood-vessels, do really spring from the spinal cord, tinding their way into the sym- pathetic system through these communicating or commis- sural branches. Experiments moreover go to show that the nervous in- fluence which keeps up the tone of the blood-vessels, that is, which keeps them in the usual condition of moderate contraction, proceeds from the spinal cord. The cord is, therefore, spoken of as containing centres for the vaso-motor nerves or, more shortly, vaso-motor centres. For example, the muscular walls of the blood-vessels supplying the ear and the skin of the head generally, are made to contract, as has been already mentioned, by nervous fibres derived immediately from the sympathetic. These fibres, however, do not arise from the sympathetic ganglia, but simply pass through them on their way from the spinal cord, to the upper dorsal region of which they can all be traced. At least, this is the conclusion drawn from the facts, that irritation of this region of the cord produces the same effect as irritation of the vaso-motor nerves themselves, and that destruction of this part of the cord paralyses them. Recent researches, however, have shown that the ner- vous influence does not originate here, but proceeds from higher up, from the medulla oblongata in fact, and simply passes down through this part of the spinal cord on its way to join the sympathetic ganglia. 16. The brain (Fig. 83) is a complex organ, consisting of several parts, the hindermost of which, termed medulla border of the pons ; and coming forward in front of that line, between the /K. and ///. nerves, on either side, are seen the crura cerebri. The two round bodies in the angle between the diverging crura are the so-called corpora albicantia, and in front of thena is Z', the pituitary body. _ This rests on the chiasma, or junction, of the optic ner\'es ; the continuation of each nerve is seen sweeping round the crura cerebri on either side. Im- mediately in front, between the separated frontal lobes of the cerebral hemispheres, is seen the corpus callosum, CC. The fissure of Sylvius, about on a level with I. on the left and //. nn the right side, marks the di\-ision between frontal and temporal lobes. 26i ELEMENTARY PfiYSlOLOGV. [less. oblongata^ passes insensibly into, and in its lower part has the same structure as, the spinal cord. Above, however, it widens out, and the central canal, spreading with it, becomes a broad cavity, which (leaving certain anatomical minutiae aside) may be said to be widely open above. This cavity is termed the fourth ventricle. Overhanging the fourth ventricle is a great laminated mass, tho. cerebellum {Cb. Figs. 83, 84, 85). On each side, this organ sends down several layers of trans- verse fibres, which sweep across the brain and meet in the middle line of its base, forming a kind of bridge (called p07is Varolii, Fig. 83) in front of the medulla oblongata. The longitudinal nerve-fibres of the medulla oblojigata pass forwards, among, and between these layers of trans- verse fibres, and become visible, in front of the pons, as two broad diverging bundles, called crura cerebri (Fig. 83). Above the crura cerebri lies a mass of nervous matter raised up into four hemispherical elevations, called corpora quadrigemina (C.Q. Fig. 85). Between these and the crura cerebri is a narrow passage, which leads from the fourth ventricle into what is termed the third ventricle of the brain. The third ventricle is a narrow cavity lodged between two great masses of nervous matter, called optic thalajui, into which the crura cerebri pass. The roof of the third ventricle is merely membra- nous ; and a peculiar body of unknown function, the pineal body, is connected with it. The floor of the third ventricle is produced into a sort of funnel, which ends in another anomalous organ, \\\t pituitary body {Pt. Fig. 85 ; P. Fig. 83). The third ventricle is closed, in front, by a thin layer of nervous matter ; but, beyond this, on each side, there is an aperture in the boundary wall of the third ventricle which leads into a large cavity. The latter occupies the centre of the cerebral hemisphere, and is called the lateral ve7ttricle. Each hemisphere is enlarged backwards, down- wards, and forwards into as many lobes ; and the lateral ventricle presents corresponding prolongations, or cornua. The floor of the lateral ventricle is formed by a mass of nervous matter, called the corpus striatum, into which the fibres that have traversed the optic thalamus enter (Fig. 85, C.S). XI.1 THE CEREBRAL HEMISPHERES. 26^ The hemispheres are so large that they overlap all the other parts of the brain, and, in the upper view, hide them. Fig. 84. A side view of the brain and upper part of the spinal cord in place — the parts which cover the cerebro-spinal centres being removed. C. C. the con- voluted surface of the right cerebral hemisphere ; Cl<. the cerebellum ; M. Ob. the medulla oblongata ; B. the bodies of the cervical vertebras ; Sp. their spines ; N. the spinal cord with the spinal nerves. Their applied faces are separated by a median fissure for the greater part of their extent ; but, inferiorly, are 264 ELEMENTARY PHYSIOLOGY. [r.F.s?. joined by a thick mass of transverse fibres, the corpus callosiim (Fig. 83, CC). The outer surfaces of the hemispheres are marked out into convolutiojis^ or gyri^ by numerous deep Jissia-es (or sulci), into which the pia mater enters. One large and deep fissure which separates the anterior from the middle division of the hemisphere is called \\\q fissure of Sylvius (Fig. 83). 17. In the medulla obloiigala the arrangement of the white and grey matter is substantially similar to that which obtains in the spinal cord ; that is to say, the white matter is external and the grey internal. But in the cere- bellum and cerebral hemisphei'cs, the grey matter is external and the white internal ; while, in the optic thalami and corpora striata, grey matter and white matter are variously intermixed. 18. Nerves are given off from the brain in pairs, which succeed one another from before backwards, to the num- ber of twelve (Fig. 85). ThQ first pair, counting from before backwards, are the olfactory nerves^ and the second are the optic nerves. The functions of these have already been described. The third pair are called motores oculi (movers of the eye), because they are distributed to all the muscles of the eye except two. The nerves of the fourth pair and of the sixth pair supply, each, one of the muscles of the eye, on each side ; the fourth going to the superior oblique muscle, and the sixth to the external rectus. Thus the muscles of the eye, small and close together as they are, receive their nervous stimulus by three distinct nerves. Each nerve of the fifth pair is very large. It has two roots, a motor and a sensory, and further resembles a spinal nerve in having a ganglion on its sensory root. It is the nerve which supplies the skin of the face and the muscles of the jaws, and, having three chief divisions, is often called trigeminal. One branch containing sensory fibres, supplies the front of the tongue and is often spoken of as the gustatory. The seventh pair furnish with motor nerves the muscles of the face, and some other muscles, and are ciSit^ facial. xi.l TirK CRANIAL NERVES. 265 The eighth pair are the auditoy nerves. As the seventh and eighth pairs of nerves leave the cavity of the skull together, they are often, and especially by English writers on anatomy, reckoned as one, divided into /^r//(? dura^ or hard part (the facial) ; 2ind por/io moil is, or soft part (the auditory) of the " seventh "' pair. The 7iinth pair in order, the glossopharyngeal^ are mixed nerves ; each being, partly, a nerve of taste, and supplying the back of the tongue, and, partly, a motor nerve for the pharyngeal muscles. KiG. 85. -A Diagram iLLrSTRATts-G the Arrangement of the Parts OF THE ErAIN and THE OrIGIN OF THE NeRVES. //. the cerebral hemispheres; C.S. corpus striatum; Th. optic thalamus; P. pineal body; Pi. pituitary body ; C.Q. corpora quadrigemina ; Cb. cere- bellum ; M. medulla oblongata ; /. — A'll. the pairs of cerebral ner>'es; S/>. I, Sp. 2, the first and second pairs of spinal nerves. The tenth pair is formed by the two pneumogastric nerves, often called the par vagjun. These ver>' impor- tant nerv^es, and the next pair, are the only cerebral nerves which are distributed to regions of the body remote from the head. The pneumogastric supplies the larynx, the lungs, the liver, and the stomach, and branches of it are connected with the heart. 266 KLRMENTARY PIiysTOLOCrV. {i.vs?,. The eleventh pair again, called spinal accessory, differ widely from all the rest, in arising from the sides of the spinal marrow, between the anterior and posterior roots of the dorsal nerves. They run up, gathering fibres as they go, to the medulla oblongata, and then leave the skull by the same aperture as the pneumogastric and glossopha- ryngeal. They are purely motor nerves, supplying certain muscles of the neck, while the pneumogastric is mainly sensory, or at least afferent. As, on each side, the glosso- pharyngeal, pneumogastric, and spinal accessory nerves leave the skull together, they are frequently reckoned as one pair, which is then counted as the eighth. The last two nerves, by this method of counting, become the 7iinth pair, but they are really the twelfth. They are the motor nerves which supply the muscles of the tongue. 19. Of these nerves, the two foremost pair do not pro- perly deserve that name, but are really processes of the brain. The olfactory pair are prolongations of the cere- bral hemispheres ; the optic pair, of the walls of the third ventricle .; and it is worthy of remark, that it is only these two pair of what may be called false nerves which arise from any part of the brain but the medulla oblongata — all the other true nerves being indirectly, or directly, traceable to that part of the brain, while the olfactory and optic nerves are not so traceable. 20. As might be expected from this circumstance alone, the medulla oblongata is an extremely important part of the cerebro-spinal axis, injury to it giving rise to immediate evil consequences of the most serious kind. Simple puncture of one side of the floor of the fourth ventricle produces for a while an increase of the quantity of sugar in the blood, beyond that which can be destroyed in the organism. The sugar passes off by the kidneys, and thus this slight injury to the medulla produces a temporary disorder closely resembling the disease called diabetes. More extensive injury arrests the respirator)^ processes, the medulla oblongata being the nervous centre which gives rise to the contractions of the respiratory muscles and keeps the respirator}- pump at work. The motor nerves engaged in ordinary respiration are certain spinal nerves, viz. the intercostal nerves supplying XI.] DECUSSATION OF THE PYRAMIDS. 267 the intercostal muscles and the phrenic nerve supplying the diaphragm. These motor nerv-es are undoubtedly brought into action by impulses proceeding at interv^als from the medulla oblongata. But how these rhythmic impulses originate in the medulla oblongata is not very clear. There are reasons for thinking that the presence of venous blood in the lungs acts as a stimulus to the endings of the pneumogastric nerves, and sets going impulses which, travelling up along those nerves to the medulla oblongata, there produce respirator}^ movements by reflex action. But this is not all, for respiration, though profoundly modified, is not arrested by division or destruction of the pneumogastric ner^-es. Probably the medulla oblongata contains a nervous mechanism which acts as an independent centre in a manner some- what similar to the ganglia of the heart ; and so goes on of itself, though extremely sensitive to, and thus continually influenced by, the condition of the blood not only in the lungs but all over the body. If the injuries to the medulla oblongata be of such a kind as to irritate the roots of the pneumogastric nerve violently, death supervenes by the stoppage of the heart's action. in the manner already described (See Lesson II.) 21. The afferent impulses, which are transmitted through the cord to the brain and awake sensation there, cross, as we have seen, from one-half of the cord to the other, immediately after they enter it by the posterior roots of the spinal nerves ; while the efferent, or volitional, impulses from the brain remain, throughout the cord, in that half of it from which they will eventually pass by the anterior roots. But at the lower and front part of the medulla oblongata, these also cross over ; and the white fibres which convey them are seen passing obliquely from left to right and from right to left in what is called the decussation of the anterior pyramids (Fig. 83\ Hence, any injur}-, at a point higher up than the decussation, to the nerve-fibres which convey motor impulses from the brain, paralyses the muscles of the body and limbs of the opposite side. Division, therefore, of one of the crura cerebri, say the right, gives rise to paralysis of the left side of the body and limbs, and the animal operated upon falls over to the 2(A ELEME.VTARY PHYSIOLOGY. [lf.ss. left side, because the limbs of that side are no longer able to support the weight. But, as the motor nerves given off from the brain itself and arising from the medulla above the decussation of the pyramids do not cross over in this way, it follows, that disease or injury at a given point, on one side of the medulla oblongata, involving at once the course of the volitional motor channels to the spinal marrow, and the origins of the cranial motor nerves, will affect the same side of the head as that of the injury, but the opposite side of the body. If the origin of the left facial nerv^e, for example, be injured, and the volitional motor fibres going to the cord destroyed, in the upper part of the medulla oblongata, the muscles of the face of the left side will be paralysed, and the features will be drawn over to the opposite side, the muscles of the right side having nothing to counteract their action. But it is the right arm, and the right leg and side of the body, which will be powerless. 22. The functions of most of the parts of the brain which lie in front of the medulla oblongata are, at present, very ill understood ; but it is certain that extensive injury, or removal, of the cerebral hemispheres puts an end to intelligence and voluntary movement, and leaves the animal in the condition of a machine, working by the reflex action of the remainder of the cerebro-spinal axis. We have seen that in the frog the movements of the body which the spinal cord alone, in the absence of the whole of the brain including the medulla oblongata, is capable of executing, are of themselves strikingly complex and varied. But none of these movements are voluntary or spontaneous ; they never occur unless the animal be stimula ted. Removal of the cerebral hemispheres is alone sufficient to deprive the frog of all spontaneous or voluntary movements ; but the presence of the medulla oblongata and other parts of the brain (such as the corpora quadri- gemina, or what corresponds to them in the frog, and the cerebellum) renders the animal master of movements of a far higher nature than when the spinal cord only is left. In the latter case the animal does not breathe when left to itself, lies fiat on the table with its fore limbs XI.] I-C'XCl'IONS OF THE CEREBRUM. 269 beneath it in an unnatural position ; Avhen irritated kicks out its legs, and may be thrown into actual convul- sions, but never jumps from place to place ; when thrown into a basin of water falls to the bottom like a lump of lead, and when placed on its back will remain so, without making any effort to turn over. In the former case the animal sits on the table, resting on its front limbs, in the position natural to a frog ; breathes quite naturally ; when pricked behind jumps away, often getting over quite a considerable distance ; when thrown into water begins at once to swim, and continues swimming until it finds some object on which it can rest ; and when placed on its back immediately turns over and resumes its natural position. Not only so, but the following very striking experiment may be performed with it. Placed on a small board it remains perfectly motionless so long as the board is horizontal ; if, however, the board be gradually tilted up so as to raise the animal's head, directly the board becomes inclined at such an angle as to throw the frog's centre of gravity too much backwards, the creature begins slowly to creep up the board, and, if the board continues to be inclined, will at last reach the edge, upon which when the board becomes vertical he will seat himself with apparent great content. Nevertheless, though his movements when they do occur are extremely well combined and appa- rently identical with those of a frog possessing the whole of his brain, he never moves spontaneously, and never stirs unless irritated. There can be no doubt that the cerebral hemispheres are the seat of powers, essential to the production of those phenomena which we term intelligence and will ; but there is no satisfactory proof, at present, that the manifesta- tion of any particular kind of mental faculty is especially allotted to, or connected with, the activity of any parti- cular region of the cerebral hemispheres. 23. Even while the cerebral hemispheres are entire, and in full possession of their powers, the brain gives rise to actions which are as completely reflex as those of the spinal cord. When the eyelids wink at a flash of light, or a threatened i)low, a reflex action takes place, in which the afferent nerves are the optic, the efferent the facial. When a bad 270 ELEMENTARY PHYSIOLOGY. [less. cmell causes a grimace, there is a reflex action through the same motor nerve, while the olfactory nerves constitute the afferent channels. In these cases, therefore, reflex action must be effected through the brain, all the nerves involved being cerebral. When the whole body starts at a loud noise, the afferent auditory nerve gives rise to an impulse which passes to the medulla oblongata, and thence affects the great majority of the motor nerves of the body. 24. It may be said that these are mere mechanical ac- tions, and have nothing to do with the operations which we associate with intelligence. But let us consider what takes place in such an act as reading aloud. In this case, the whole attention of the mind is, or ought to be, bent upon the subject-matter of the book ; while a multitude of most delicate muscular actions are going on, of which the reader is not in the slightest degree aware. Thus the book is held in the hand, at the right distance from the eyes ; the eyes are moved from side to side, over the lines and up and down the pages. Further, the most delicately adjusted and rapid movements of the muscles of the lips, tongue, and throat, of the laryngeal and respiratory muscles, are involved in the production of speech. Perhaps the reader is standing up and accompanying the lecture with appropriate gestures. And yet every one of these muscular acts may be performed with utter unconsciousness, on his part, of anything but the sense of the words in the book. In other words, they are reflex acts. 25. The reflex actions proper to the spinal cord itself are 7tatural, and are involved in the structure of the cord and the properties of its constituents. By the help of the brain we may acquire an infinity of artificial reflex actions, that is to say, an action may require all our attention and all our volition for its first, or second, or third performance, but by frequent repetition it becomes, in a manner, part of our organization, and is performed without volition, or even consciousness. As ever>'one knows, it takes a soldier a long time to learn his drill — for instance, to put himself into the attitude of "attention" at the instant the word of command is heard. But, after a time, the sound of the word gives rise to the act, whether the soldier be thinking of it, or not. There XI.] THE SYMPATHETIC SYSTEM. r;! is a story, which is credible enough, though it may not be true, of a practical joker, who, seeing a discharged veteran carrying home his dinner, suddenly called out "Atten- tion !" whereupon the man instantly brought his hands down, and lost his mutton and potatoes in the gutter. The drill had been thorough, and its effects had become embodied in the man's nervous structure. The possibility of all education (of which military drill is only one particular form) is based upon the existence of this power which the nervous system possesses, of organ- izing conscious actions into more or less unconscious, or reflex, operations. It may be laid down as a rule, that if any two mental states be called up together, or in succes- sion, with due frequency and vividness, the subsequent production of the one of them will suffice to call up the other, and that whether we desire it or not. The object of intellectual education is to create such indissoluble associations of our ideas of things, in the order and relation in which they occur in nature ; that of a moral education is to unite as fixedly, the ideas of evil deeds with those of pain and degradation, and of good actions with those of pleasure and nobleness. 26. The sympathetic system consists chiefly of a double chain of ganglia, lying at the sides and in front of the spinal column, and connected with one another, and with the spinal nerves, by commissural cords. From these ganglia, nerves are given off which for the most part follow the distribution of the vessels, but which, in the thorax and abdomen, form great networks, ox plexuses .,\y^ovi the heart and about the stomach. It is probable that a great pro- portion of the fibres of the sympathetic system is derived from the spinal cord ; but others also, in all probability, originate in the ganglia of the sympathetic itself. The sympathetic nerves influence the muscles of the vessels generally, and those of the heart, of the intestines, and of some other viscera : and it is probable that their ganglia are centres of reflex action to afferent ner^-es from these organs. But many of the motor nerves of the vessels are, as we have seen, under the influence of particular parts of the spinal cord, though they pass through sympathetic ELEMKXTAKY PHYSIOLOGY. [less. LESSON XII. HISTOLOGY I OR, THE MINUTE STRUCTURE OF THE TISSUES. 1. The various organs and parts of the body, the work- ing of which has now been described, are not merely separable by the eye and the knife of the anatomist into membranes, nerves, muscles, bones, cartilages, and so forth ; but each of them is, by the help of the microscope, susceptible of a finer analysis, into certain minute con- stituents which, for the present, may be considered the ultimate structural elements of the body. 2. There is a time when the human body, or rather its rudiment, is of one structure throughout, consisting of a more or less transparent matrix^ very similar in nature to the substance of which the white blood-corpuscles are composed, and often called protoplasm^ through which are scattered minute rounded particles of a different optical aspect. These particles are called nuclei; and as the matrix, or matter in which these nuclei are im- bedded, readily breaks up into spheroidal masses, one for each nucleus, and these investing masses easily take on the form of vesicles or cells^ this primitive structure is called cellular, and each cell is said to be nucleated. The material of the body when in this stage of growth is often spoken of as indiffcreiit tissue. A very fair idea of its nature may be formed by supposing a multitude of white blood-corpuscles to be collected together into a soft but yet semi-solid mass. In the present use of the term any distinct mass of pro- toplasm or living material may be called a cell. In the vast majority of cases, however, the cell contains a nucleus^ CELLS. 273 distinguished as has just been said from the cell-substance in which it hes. Very frequently, but by no means always, the outer layer of the cell-substance is hardened into a distinct casing or envelope, the cell-wall, the cell then becoming an undeniable vesicle, and the cell-substance being often spoken of as the cell contents. The cell- substance may remain as soft semi-solid protoplasm, or may be hardened in various ways, or may be wholly or partially liquefied ; in the latter case a cell-wall is naturally always present. A, vertical section of a layer of epidermis, or epithelium, from its free to its deep surface. B, lateral views of the cells of which this layer is composed at different heights : a, cell in the deepest layer, and therefore most recently formed and least altered ; b, cell higher up, and therefore somewhat changed ; c, d, cells still more changed, and much flattened. C, scales such as d viewed from their flat sides. (Magnified about 250 diameters.) As development goes on, the nuclei simply increase in number by division and subdivision, without undergoing any marked change;^ but the substance in which they are » Each nucleus divides into two, and each half soon grows up into the size of the parent nucleus. While this is going on the matrix round the nuclei also divides, each new nucleus having a quantity of matrix allotted to it, so as to form a new cell exactly like the old one, from which it sprang. T 274 ELEiMENTAR 1 ' PHYSIO LOG } '. [ less. imbedded, becomes very variously modified, both chemi- cally and structurally, and gives rise to those peculiarities by which completely formed tissues are distinguished from one another. 3. In the adult body the simplest forms of tissue, i.e. those in which the matrix has been least changed, are perhaps the various kinds of epithelium (including the epidermis). These are distinctly cellular in nature, that is, the portion of the matrix belonging to each nucleus can, with a little 13ains, be recognized as distinct from the portions belonging to the other nuclei. In fact they differ from white blood cor- puscles chiefly in two points : firstly, the matrix of each cell becomes more or less chemically changed so as to lose its soft protoplasmic nature (and at the same time its power of executing amoeboid movements) ; and, secondly, takes on a rigid definite form, which may or may not be globular. These epithelial tissues are constantly growing in their deepest parts, and are, as constantly, being shed at their surfaces. The deep part consists of a layer of such globular, nucleated cells as have been mentioned, the number of which is constantly increasing by the spontaneous division of the nuclei and cells. The increase in number thus effected causes a thrusting of the excess of cell population towards the surface ; on their way to which they become flattened, and their walls acquire a horny texture. Arrived at the surface, they are mere dead horny scales, and arc thrown off (Fig. 86). Epithelium of the kind just described is called squamous. It is found in the mouth, and its scales may always be obtained in abundance by scraping the inside of the lip. Epidermis consists of exactly similar cells, except that the conversion of the topmost cells into horny scales is still more complete. The nucleus, too, is eventually lost. The deep layers of epidermis, consisting of softer cells not yet flattened or made horny, often form quite a distinct part, and these are often spoken of as the rete Diucosum. (See Fig. 32, ^y Fig. 88, C, ^.) 1 - In other parts of the alimentary tract, as in the intes- tines, the full-grown epithelial cells are placed side by side with one another, and perpendicular to the surface of XII.] EPITHELIUM. the membrane. Such epithelium is called cylindrical (Fig. 46, b,B)^ or coluinnar. In some places, such as in the gastric glands, m some parts of the kidney, in the ureters and elsewhere, the epithelial cells remain globular or spheroidal. Squamous epithelium generally consists of many layers of cells, one over the other ; in other forms of epithelium there are few, in some cases apparently only one layer. Fig. 87.— Ciliated Epithelium. a, the submucous vascular tissue ; b, the deep layer of young epithelii cells : c, the cylindrical full-grown cells, with {d) the cilia. (Magnifi about 350 diameters.) Ciliated epithelium is usually of the cylindrical kind, and differs from other epithelium only in the circumstance that one or more incessantly vibrating filaments are developed from the free surface of each cell. (See Lesson VII. §3.) 4. In certain regions of the integument, the epidermis becomes metamorphosed into nails and hairs. Underneath each nail the deep or dermic layer of the integument is peculiarly modified to form the bed of the nail. It is very vascular, and raised up into numerous parallel ridges, like elongated papillae (Fig. 88, B, C). The surfaces of all these arc covered with growing epi- dermic cells, which, as they flatten and become converted into horn, coalesce into a solid continuous plate, the nail. At the hinder part of the bed of the nail, the integument forms a deep fold, from the bottom of which, in like manner, T 2 276 ELEMENTARY PHYSIOLOGY. [less. new epidermic cells are added to the base of the iiailj which is thus constrained to move forward. A B A, a longitudinal and vertical section of a nail : a, the fold at the base of the nail ; b, the nail ; c, the bed of the nail. The figure B is a transverse section of the same — a, a small lateral fold of the integument ; by nail ; c, bed of the nail, with its ridges. The figure C is a highly-magnified view of a part of the foregoing — c, the ridges ; d, the deep layers of epidermis ; e, the homy scales coalesced into nail substance. (Figs. A and B magnified about 4 diameters; Fig. C magnified about 200 diameters.] The nail, thus constantly receiving additions from belov. and from behind, slides forwards over its bed, and project: XII.] NAILS AND HAIRS. 277 beyond the end of the finger, where it is Worn aAvay, or cut off. vU -7; Fig. 89.— a Hair in its Hair-sac. a, shaft of hair above the skin; h, cortical substance of the shaft, the medulla not being visible ; c, newest portion of hair growing on the papilla 0 ; d, cuticle of hair : e, cavity of hairsac ; /, epidermis (and root-sheaths) of the hair-sac corresponding to that of the integument {vi) ; g, division between dermis and epidermis ; h, dermis of hair-sac corresponding to der- mis of integument (/) ; k, mouths of sebaceous glands ; w, horny epidermis of integument. 27 ELEMRNTAR \ ' riTYS/OL OCV. [I.RSS. 5. A hair, like a nail, is composed of coalesced horny cells ; but instead of being only partially sunk in a fold of the integument, it is at first wholly enclosed in a kind of bag, the hair-sac, from the bottom of which a papilla (Fig. 89, i), which answers to a single ridge of the nail, arises. The hair is developed by the conversion into horn, and coalescence into a shaft, of the superficial epidermic cells coating the papilla. These coalesced and cornified cells being continually replaced by new growths from below, which undergo the same metamorphosis, the shaft of the hair is thrust out until it attains the full length natural to it. Its base then ceases to grow, and the old papilla and sac die away, but not before a new sac and papilla have been formed by budding from the sides of the old one. These d^J- Part of the shaft of a hair enclosed within its root-sheaths and treated with caustic soda, which has caused the shaft to become distorted. — a, medulla ; f', cortical substance ; c, cuticle of the shaft ; from d to /, the root-sheaths, in section. (Magnified about 200 diameters,) give rise to a new hair. The shaft of a hair of the head consists of a central pith, or incduftary matter, of a loose and open texture, which sometimes contains air ; of a cortical substance surrounding this, made up of coalesced elongated horny cells ; and of an outer cuticle, composed of flat horny plates, arranged transversely round the shaft, so as to overlap one another by their outer edges, like closelv-packed tiles. The superficial epidermic cells of the hair-sac also coalesce by their edges, and become converted into root-sheaths, which embrace the root of the hair, and usually come away with it, when it is plucked out. Xir.l ,. CARTILAGR. 279 Two sebaceous glands commonly open into the hair- sac near its opening, and supply the hair with a kind of natural pomatum ;, and delicate unstriped muscular fibres are so connected with the hair-sac as to cause it to pass from its ordinary oblique position into one perpendicular to the skin, when they contract (Fig. 31, B). They are made to contract by the intiuence of cold and terror, which thus give rise to ''' horripilation" or "goose- skin," and the " standing of the hair on end." 6. The crysfalline lens is composed of fibres, which are the modified cells of the epidermis of that inverted portion of the integument, from which the whole anterior chamber of the eye and the lens are primitively formed. 7. Cartilage,-'\\\AQ epithelium and epidermis are found only on the free surfaces of the organs, gristle, or cartilage, is a deep-seated structure (see Lesson VII.). Like them it is essentially cellular in nature, but differs from them widely in appearance on account of the development of a large quantity of the so-called ijitercelhdar substance. That is to say, the several cells do not lie closely packed together and touching each other, but are separated from each other' by a quantity of material of a>different nature from themselves. Just as in indifferent tissue each nu- cleus is imbedded in a matrix of protoplasm, so in carti- lage, each cell, i.e. each nucleus with its allotted quantity of protoplasm.) is imbedded in a matrix of intercellular substance. Inasmuch as during the growth of cartilage the cells remain soft and protoplasmic, while the intercellular sub* stance is converted into a solid semi-transparent hard matter, it comes to pass that the soft nucleated cells appear to lie in cavities in the harder intercellular sub* stance or matrix. In epithelium it is only the deepest lying cells which undergo division, and so carry on the growth of the tissue. In cartilage, cell-division is much more general ; a cell lying in its cavity divides first into two, then into four, and so on, the intercellular substance meanwhile growing in between the young cells and thrusting them apart. It is by means of the repeated divisions of the cells in this way, and subsequent development of intercellular matrix in between the young cells, that cartilage grows. Con- 28o ELEMENTAR V PHYSIOLOGY ' [less. Fig. 91. A'section of cartilage, showing the matnx {a), with the groups ot cells (-5) containing nuclei (c) and fat globules {d). (Magnified about 350 diameters. ) ^ //#,H'„ Fig. -Connective Tissi'e. A, unchanged : a, connective tissue ; l>, fat cells. B, acted upon by acetic acid, and showing {a) the swollen and transparent gelatine-yielding matter, and {b) the elastic fibres. (Magnified about 300 diameters.) XII.] CONNECTIVE TISSUE. i%i sequently, the cells are frequently seen arranged in groups with more or less matrix between, according to their age. The cells remain during life soft and protoplasmic, but often contain a number of large oil globules. It is to the hard matrix which yields, on boiling, the substance chon- drnie, that the physical features of cartilage, its solidity and elasticity, are due. Cartilage contains no vessels, or only such as extend a little way into it from adjacent parts. 8. Connective tissue (also called Jibj'ous, or areolary or sometimes cellular tissue), the most extensively diffused Fig. 93. Connective tissue corpuscles {a, nucleus, h, cell substances), of various shapes, those to the right hand branching, and the branches joining. of all in the body, at first sight seems to differ -wholly from the preceding tissues. Viewed under the microscope, it is seen to consist of bands or cords, or sheets of -whitish substance, having a -wavy, fibrous appearance, and capable ^of being split up mechanically into innumerable fine fila- ments oxjibrillce. The addition of acetic acid causes it to swell up and become transparent, entirely losing its fibrous aspect ; and, further, reveals the presence of two elements which acetic acid does not affect, viz. nuclei and certain 2^2 ELRMENTARV rilVSTOLOaY. [l.F.SS. sharply defined fibres of difierent degrees of fineness, which are called elastic fibres. If the acid be now very care- fully neutralized by a weak alkali, the connective tissue assumes its former partial opacity and fibrillated aspect. The nuclei thus brought to light by acetic acid are worthy of attention because careful examination shows that they belong to certain cells which exist in all connective tissue in greater or less number, though never in abundance. These cells, generally called co7i7ieciive tissue corpuscles, consist of a nucleus and protoplasmic cell-substance, and in fact are not unlike cartilage cells except that they are very often very irregular in form, and as a general rule very /" m ■a Fig. 94.— Fat Cells. A, having their natural aspect. B,collapsed, the fat being exhausted. C, with fatty crystals. The nuclei are not seen in this case. (Magnified about 350 diameters.) small. Indeed we may very justly compare connective tissue with cartilage, much as they seem to differ in general appearance. The connective tissue corpuscles correspond to the cartilage-cells ; both are imbedded in a matrix which, in the case of cartilage, remains structureless, but becomes solid and dense, while it, in the case of con- nective tissue, is altered or metamorphosed, as it is said, into a substance composed of excessively fine filaments, mingled with which are elastic fibres. The fine fibrillated substance is not very elastic, and when boiled swells up and yields gelatine. The elastic fibres do not yield gelatine, and, as their name indicates, are highly elastic. The proportion of elastic fibre to the Xll.j /:-/ T CELLS. :S3 g-c!atine-yielding constituents of connective tissue varies in different parts of the body. Sometimes it is so great that elasticity is the most marked character of the resuhing tissue. LigiDiients and tendons are simply cords, or bands, while fascicB are sheets, of very dense connective tissue. In some parts of the body, the connective tissue is more or less mixed with, or passes into, cartilage, and such tissues are csXled Jibro-ca?'tilages (see Lesson VII.), or, in other words, the matrix of the cartilage becomes more or less fibrillated, thus indicating the analogies of the two tissues. The name cellular applied to this tissue is apt to lead to confusion. When first used it referred to the cavities left in the meshes of the network of fibres ; it has nothing whatever to do with cells technically so called. 9. Fat cells are scattered through the connective tissue, in which they sometimes accumulate in great quantities. Fig. 95.— Capillaries of Fat. A, network round a group of fat cells, a, the artery ; /', the vein. B, the loops of capillaries round three individual fat cells. They are spheroidal sacs, composed of a delicate mem- brane, on one side of which is a nucleus, and distended by fatty matter, from which the more solid fats sometimes crystallize out after death. Ether will dissolve out the fat, and leave the sacs empty and collapsed (B, Fig. 94). 284 ELEMENTAR V PHYSIOL OGY. [less. They are, in fact, cells with a distinct cell wall, the cell contents, or cell substance, of which have been wholly, or all but wholly, converted into fat. Considerable aggregations of fat cells are constantly present in some parts of the body, as in the orbit, and about the kidneys and heart : but elsewhere their presence, in any quantity, depends ver)' much on the state of nutri- tion. Indeed, they may be regarded simply as a reserve, formed from the nutriment which has been taken into the body in excess of its average consumption. 10. Pigment cells are either epidermic, or epithelial, cells, in which coloured granules are deposited ; or they are connective tissue corpuscles of the deeper parts of the body, in which a like deposit occurs. Thus the colour of the choroid arises partly from the presence of a layer of epithelial cells (see Fig. 74), placed close to the retina, con- taining pigment granules, and partly from a large num- ber of irregularly-shaped, connective tissue corpuscles crammed with pigment, which belong to the deeper connective tissue layer of the choroid. The pigment cells of the frog's web are, for the most part, connective .'tissue corpuscles, containing colouring matter. ;^ II. Jyone is essentially composed of an animal basis impregnated with salts of carbonate and phosphate of lime, through the substance of which are scattered minute cavities — the lacinice, which send out multitudinous rami- lications, called canaliculi. The canaliculi of different lacunae unite together, and thus establish a communica- tion between the different lacunae. If the earthy matter be extracted by dilute acids, a nucleus may be found in each lacuna ; and if young, fresh bone be carefully examined, a certain amount of cell substance will be found filling up the lacuna round the nucleus ; and, not unfrequently, the intermediate substance appears minutely fibrillated. In fact bone, if we lay on one side the earthy matters, presents very close analogies in its fundamental structure with both cartilage and connective tissue. The corpuscles lodged in the lacunas correspond to the cor- puscles of connective tissue and to the cells of cartilage, while the matrix in which the earthy matter is deposited corresponds to the matrix of cartilage, and to the fibril- lated material of connective tissue. (These three tissues XII.] BO.VE. 2S5 indeed are often classed together as " the connective tissue group.") In a dry bone the lacunae are usually filled with air. When a thin section of such a bone is, as usual, covered with water and a thin glass, and placed under the microscope, the air in the lacunae refracts the hght which passes through them in such a manner as to prevent its reaching the eye, and they appear black. Hence the lacunae were, at one time, supposed to be solid bodies, containing the lime salts of the bone, and were called bone corpuscles (Fig. 96, C). All bones, except the smallest, are traversed by small ',anals, converted by side branches into a network, and containing vessels supported by more or less connective tissue and fatty matter. These are called Haversitvi canals (Fig. 96, A, B). They always open, in the long run, upon the surface of the bone, and there the vessels which they contain become connected with those of a sheet of tough connective tissue, which invests the bone, and is called Periosteum. In many long bones, such as the thigh bone, the centre of the bone is hollowed out into a considerable cavity, containing great quantities of fat, supported by a delicate connective tissue, rich in blood-vessels, and called the marrow^ or medulla. The inner ends of the Haversian canals communicate with this cavity, and their vessels are continuous with those of the marrow. When a section of a bone containing Haversian canals is made, it is found that the lacunae are dispersed in con- centric zones around each Haversian canal, so that the substance of the bone appears laminated ; and, where a medullary cavity exists, more or fewer of these concentric lamellae of osseous substance surround it. This structure arises from the mode of growth of bones. In the place of every bone there exists, at first, either car- tilage, or connective tissue hardly altered from its primitive condition of indifferent tissue. When ossification com- mences, the vessels from the adjacent parts extend into the ossifying tissue, and the calcareous salts are thrown down around them. These calcareous salts invade all the ossifying tissue, except the immediate neighbourhood of its nuclei, around each of which a space, the lacuna, is left. The lacunae and canalicuU are thuSj substantially, 286 ELEMEN I 'A K V FH \ 'SIOL OGY [less. ^,:^- " ■ ^ ;ffl^ ***• ^t^ '^ \ i B Fig. 96. A transverse section of bone in the neighbourhood of two Haversian canals, a a; h, lacunae. (Magnified about 250 diameters.) A longitudinal section of bone with Haversian canals, a a, and lacuna:, b. (Magnified about 100 diameters.) Lacunse, f, and canaliculi, d. (Magnified about 600 diameters.) X I T . ] OSSIFICA 77 OX. 2 S 7 gaps left in the ossific matter around each nucleus, whence it is that nuclei are found in the lacunae of fully-formed bone.i Bone, once formed, does not remain during life, but is constantly disappearing and being replaced in all its parts. Nevertheless, the growth of a bone, as a general rule, takes place only by addition to its free ends and surfaces. Thus the bones of the skull grow in thickness, on their surfaces, and in breadth at their edges, where they unite by sutures; and when the sutures are once closed, they cease to in- crease in breadth. The bones of the limbs, which are preceded by complete small cartilaginous models, grow in two ways. The car- tilage of which they consist grows and enlarges at its extremities until the bones have attained their full size, and remains to the end of life as articula?' cartilage. But in the middle, or shaft, of the bone, the cartilage does not grow with the increase in the dimensions of the bone, but the small primary bone which results from the ossification of the cartilaginous model becomes coated by successive layers of bone, produced by the ossification of that part of the periosteum which lies nearest to it, and which really consists of indifferent tissue, that is of nuclei imbedded in a matrix. The shaft of the bone thus formed is gradually hollowed out in its interior to form the medullary cavity, so that, at last, the primitive cartilage totally disappears. When ossification sets in, the salts of lime are not diffused uniformly through the whole mass of the pre- existing cartilage, or connective tissue, but begin to be deposited at particular points called centres of ossification., and spread from them through the bone. Thus, a long bone has usually, at fewest, three centres of ossification — one for the middle or shaft, and one for each end ; and it is only in adult life that the three bony masses thus formed unite into one bone. 12. Teeth partake more of the nature of bones than of any other organ, and are, in fact, partially composed of true bony matter, here called ce??ient; but their chief con- stituents are two other tissues, called dentine and enamel. Each tooth presents a crown, which is exposed to wear, I For the sake of simplicity I purposely omit all mention of the complex secondary processes in the ossification of cartilage. 2S3 ELEMENT A RY PH YSIOL OGY. [less. and one or more fangs, which are buried in a socket furnished by the jawbone and the dense mucous mem- brane of the mouth, which constitutes the giun. The Hne of junction between the crown and the fang is the neck of the tooth. In the interior of the tooth is a cavity, which communicates with the exterior by canals, which traverse the fangs and open at their points. This cavity is the pulp cavity. It is occupied by a highly vascular and nervous tissue, the deiital pulp., which is continuous below, through the openings of the fangs, with the mucous membrane of the gum. Fig. 97. A, vertical, E, horizontal section of a tooth. — a, enamel of the crov.n; h, pulp cavity ; f, cement of the fangs; d, dentine. (Magnified about ihrec diameters.) The chief constituent of a tooth is dentine— 7\. dense calcified substance containing less animal matter than bone, and further differing from it in possessing no lacuna?, or proper canaliculi. Instead of these it presents innumerable, minute, parallel, wavy tubules, which give off lateral branches. The wider ends of these tubules open into the pulp cavity, while the narrower ultimate terminations ramify at the surface of the dentine, and may even extend into the enamel or cement (Fig. 98, C). XII.] TEETH. 289 The enamel consists of very small six-sided fibres, set closely, side by side, nearly at right angles to the surface of the dentine, and covering the crown of the tooth as far Fig. 98. Enamel fibres viewed in Iransverse section. Enamel fibres separated and viewed laterally. A section of a tooth at the junction of the dentine {a) with the cement (fe) • /', c, irregular cavities in which the tubules of the dentine end ; d, fiae tubules continued from them \f, g, lacuna and canalicuH of the cement. (Maenihcd about 400 diameters.) U 290 ELEMEXTARY PHYSIOLOGY. [i^Ess. as the neck, towards which the enamel thins off and joins the cement (Fig. 98, A, B). Enamel is the hardest tissue of the body, and contams not more than 2 per cent, of animal matter. The cement coats the fangs, and has the structure of true bone ; but as it exists only in a thin layer, it is devoid of Haversian canals (Fig. 98, C). 13. The development of the teeth commences long before birth. A groove appears in the gum of each side of each jaw ; and, at the bottom of this groove of the gum, five vascular and nervous papiUce arise, making twenty in all. The walls of the groove grow together, between and over each of the papillae, and thus these become enclosed in what are called the dental sacs. Each papilla gradually assumes the form of the future tooth. Next, a deposit of calcific matter takes place at the summit of the papillae, and extends thence downwards, towards its base. In the crown the deposit takes on the form of enamel and dentine ; in the root, of dentine and cement. As it increases it encroaches upon the substance of the papilla, which remains as the tooth pulp. The fully formed teeth press upon the upper walls of the sacs in which they are enclosed, and, causing a more or less complete absorption of these walls, force their way through. The teeth are then, as it is called, cut. The cutting of this first set of teeth, called deciduous, or Diilk teeth, commences at about six months, and ends with the second year. They are altogether twenty in number — eight being cutting teeth, or incisors ; four, eye teeth, or canines ; and eight, grinders, or molars. Each dental sac of the milk teeth, as it is formed, gives off a little prolongation, which becomes lodged in the jaw, enlarges, and develops a papilla from which a new tooth is formed. As the latter increases in size, it presses upon the root of the milk tooth which preceded it, and thereby causes the absorption of the root and the final falling out, or shedding, of the milk tooth, whose place it takes. Thus every milk tooth is replaced by a tooth of what is termed the permanent dentition. The permanent incisors and canines are larger than the milk teeth of the same name, but otherwise differ little from them. The permanent molars, which replace the milk molars, are MCSCLE. 291 small, and their crowns have only two points, whence they are called bicuspid. They never have more than two fangs. 14. We have thus accounted for twenty of the teeth of the adult. But there are thirty-two teeth in the complete adult dentition, twelve grinders being added to the twenty teeth which correspond with, and replace, those of the milk set. When the fifth, or hindermost, dental sac of the milk teeth is formed, the part of the groove which lies behind it also becomes covered over, extends into the back part of the jaw, and becomes divided into three dental sacs. In these, papillse are formed and give rise to the great permanent back grinders, or molars, which have four, or live, points upon their square crowns, and, in the upper jaw, commonly possess three fangs. The first of these teeth, the anterior molar of each side, is the earliest cut of all the permanent set, and appears at six years of age. The last, or hindermost, molar is the last of all to be cut, usually not appearing till twenty-one or twenty-two years of age. Hence it goes by the name of the " wisdom tooth." 15. Muscle is of two kinds, striated or striped, and smooth, plain, or uustriated. Striated muscle, of which all the ordinar}' muscles of the trunk and limbs consist, is composed of a number of long parallel cylindrical fibres, called elementary or ultimate muscular fibres, which are bound together by connective tissue into small bundles. These small bundles again are united into larger bundles, and these into one aggregate, by con- nective tissue, which supports the vessels and nerves of the muscle, and usually forms at one or both ends of the muscle a tendon (see Lesson VII.), and sometimes gives rise to a dense sheath or fascia on its exterior. Into the ultimate muscular fibre neither vessels, nor connective tissue, enter. Each fibre is, however, en- veloped in a sheath formed by a tough, elastic, transparent structureless membrane, the sarcolemma (Fig. 99, D, b). The sarcolemma is not contractile, but its elasticity allows it to adjust itself, pretty accurately, to the changes of form of the contractile substance which it contains. This contractile substance, when uninjured, presents a very strongly-marked transverse striation, its substance U 2 2^1 ELEMENTARY PHYSIOLOGY. [less. appearing to be composed of extremely minute disks of a partially opaque substance, imbedded at regular intervals in a more transparent matter. A more faint striation, separating these disks into longitudinal series, is also observable. When the sarcolemma is torn, the contractile substance of dead muscle may, under some circumstances, be either divided into disks '(Fig. 99, C), but it may be A. a muscular fiijre, devoid cf sarcolemma, and breaking up at one end into \x.9,Jibrill(e ; B, separate fibrilla; ; C, a muscular fibre breaking up into disks ; I), a muscular fibre, the contractile substance of which C^) is torn, while the sarcolemma (b) has not given way. (Magnified about 350 diameters.) more readily broken up into minute fibrillcB (Fig. 99, A, B), each of Avhich, viewed by transmitted light, pre- sents dark and light parts, which alternate at intervals corresponding with the distances of the transverse striae in the entire fibre. Nuclei are observed here and there in the contractile substance within the sarco- lemma. NERVOUS TISSUE, 293 In the heart, the muscular fibres are striated, and have the same essential structure as that just described, but they possess no sarcolemma. f -' 0 mm / %\ ' \ Fig. 100.— Cai'ILLARIes of Striated Muscle. A. Seen longitudinally. The width of the meshes corresponds to that of an ultimate fibre, a. small artery ; i', small vein. 1>. Transverse section of striated muscle, a, the cut ends of the ultimate fibres ; />, capillaries filled with injection material ; <:, parts where the capil- laries are absent ornot filled. Smooth muscle consists of elongated band-like fibres, devoid of striation, each of \vhich bears a rod-like nucleus. These fibres do not break up into tibrilke, and have no sarcolemma (Fig. loi). 16. Nervous tissue contains two elements, ncrve-Jibrcs and ganglionic corpuscles. (3rdinary nerve-fibres, such as constitute the essential constituents of all the cerebro- spinal nerves except the olfactory, are during life, or when perfectly fresh, subcylindrical filaments of a clear, some- what oily, look. But shortly after death, a sort of coagu- lation sets up within the fibre, and it is then found to be composed of a very delicate, structureless, outer mem- brane (which is not to be confounded with the ncuri- 294 ELEMENTAR \ ' PH\ 'SIOL 0 G Y. [less. Icinina . forming a tube, through the centre of which runs the axis cylinder^ which is probably composed of an aggregation of very fine filaments. Between the axis cylinder and the tube is a fluid, rich in fatty matters, from which a solid strongly refracting substance has been thrown down and lines the tube. Such is the structure of all the larger nerve fibres, which lie, side by side, in the trunks of the nerves, bound together by delicate connective tissue, and en- closed in a sheath of the same substance, called the n:urilem7na. In the trunks of the nerves, the fibres remain perfectly distinct from one another, and rarely, if ever, divide. But when the nerves enter the central organs, and when they approach their peripheral terminations, the nerve-fibres frequently divide into branches. In any case they become gradually finer and finer ; until at length, axis-cylinder, sheath, and contents arc no longer separable, and the nerve fibre is reduced to a delicate filament, the ultimate termination of which, in the sensory organs and in the muscles, is not yet thoroughly made out. 17. In Lesson VIII. mention is made of peculiar bodies called tactile cor- ated muscular fibres fmscles, which are oval masses of spe- from the. miHfilero'jf • n ' i-/~ j ^- ^- • cially modified connective tissue in re- lation with the ends of the nerves in the papillse of the skin. In Fig. 103 four such papillae, which have been rendered transparent and stripped of their epidermis, are seen, and the largest contains a tactile corpuscle (.] THE CASE OF MRS, A. 305 closely pinned, as is usual with corpses, round the head and under the chin ; and, though the eyes were open, the features were solemn and rigid. The dress was evidently a shroud, as Mrs. A. remarked even the punctured pattern usually worked in a peculiar manner round the edges of that garment. Mrs. A. described herself as, at the time, sensible of a feeling like what we conceive of fascination, compelling her, for the time, to gaze upon this melancholy apparition, which was as distinct and vivid as any re- flected reality could be, the light of the candle upon the dressing-table appearing to shine fully upon its face. After a few minutes she turned round to look for the reality of the form over her shoulder, but it was not visible, and it had also disappeared from the glass when she looked again in that direction. ****** (7) On the 17th March, Mrs. A. was preparing for bed. She had dismissed her maid, and was sitting with her feet in hot water. Having an excellent memory, she had been thinking upon and repeating to herself a striking passage in the Ediiiburgh Reviciu, when, on raising her eyes, she saw seated in a large easy-chair before her the figure of a deceased friend, the sister of Mr. A. The figure was dressed, as had been usual with her, with great neatness, but in a gown of a peculiar kind, such as Mrs. A. had never seen her wear, but exactly such as had been described to her by a common friend as having been worn by Mr. A.'s sister during her last visit to England. Mrs. A. paid particular attention to the dress, air, and appearance of the figure, which sat in an easy attitude in the chair, holding a handkerchief in one hand. Mrs. A. tried to speak to it, but experienced a difficulty in doing so, and in about three minutes the figure disappeared. About a minute afterwards, Mr. A. came into the room, and found Mrs. A. slightly nervous, but fully aware of the delusive nature of the apparition. She described it as having all the vivid colouring and apparent reality of life ; and for some hours preceding this and other visions, she experienced a peculiar sensation in her eyes, which seemed to be relieved ^vhen the vision had ceased. ***** * 3o6 ELEMENTARY PHYSIOLOGY. [app. B. (9) Un the nth October, when sitting in the drawing- room, on one side of the fire-place, she saw the figure of another deceased friend moving towards her from the win- dow at the farther end of the room. It approached the fire-place, and sat down in the chair opposite. As there were several persons in the room at the time, she describes the idea uppermost in her mind to have been a fear lest they should be alarmed at her staring, in the way she was conscious of doing, at vacancy, and should fancy her intellect disordered. Under the influence of this f^ar, and recollecting a story of a similar effect in your ^ work on Demonology, which she had lately read, she summoned up the requisite resolution to enable her to cross the space before the fire-place, and seat herself in the same chair with the figure. The apparition remained perfectly dis- tinct till she sat down, as it were, in its lap, when it vanished. J Sir Walter Scott ; to whom Sir David Brewster's Letters on Natural Magic were addresbed. INDEX X 2 INDEX. Abdomen, 5 Abdominal aorta, loi Abduction and adduction, 174 Absorption, 154 Accessory food stuffs, 139 Acetabulum, 173 "Adam's apple," 178 Adduction, 174 Afferent nerves, 187, 254 Air, 19, 78, 82, 83, 91, 92, 95, 100 its necessity to life, 3, 4 stationary and tidal, 92 Air-cells, 81, 82 Albumin, 71, 134, 136 Alimentary canal, 6, 133, 161 Alimentation, 15, 133-155 Amoeboid movements, 157 Ammonia, 20 Ampullx, 200, 201, 207 Amyloids, 134- 139, I54, 298 Anatomy, outlines of, 5 Aorta, 29 Apex of the heart, 31 Aqueous humour, 227 Arachnoid membrane and fluid, 249 Areolar tissue, 9, 281 A^ms, the, 5 Arms, bones of the, ii, 172 Arteries, 15, 22, 23, 29, 43 Artery, hepatic, 31, 50, iiS renal, 107 splenic, 126 Aiticular cartilages, 1 1 Articulations (i't't- Joints) Arytenoid muscles and carti- lages, 180, 181 Asphyxia, 98-100 Atlas and axis, 170, 173 Auditory nerves, 265 ossicles, 206, 209 spectra, 239 Auricles, 34, 42 Auriculo-ventricular ring, 35 valves, 36, 39 Ball and socket joints, i6c 175 Base of the heart, 31 Beating of the heart, 44 Biceps muscle, 10, 163 Bicuspid teeth, 143 Bile, 117, 122, 153 Biliary duct, 146 Bilin, 122 Bladder, 104 310 IXDEX. Blind spot of the retina, 2i6, 219, 222 Wood, 15, 17 circulation of the, 15, 19, 21-57, 299 its characters, 58, 61, 65, 70, 74 . arterial and venous, 74 red and white corpus- cles, 59 coagulation, 59, 65 effect of saline matter on, 67 transfusion of, 73 crystallization, 65, 77 chemical constituents, 70 specific gravity, 70 its function, 72 quantity of, 72 its sources of loss and gain, loi, 103, 123, 131 of the liver, 122 its temperature, 70, 128 Blushing, 51 Body, its structure and func- tions, I Bone, 284-287 Bones, 10, 11, 161 their dissolution, 20 • of the ear, 210 Brain, 6, 14, 19, 158, 261 Bread, 138, 298, 299 Brewster, Sir 1)., on illusive visions, 240, 302 Bronchi, 80 Bronchial tubes, 81, 82 Buccal glands, 141 Bufify coat of the blood, 67 Bursae, 175 C. CtCUM, 150 Cancellous tissue, 161 Canine teeth, 143, 290 Capillaries, 15, 21, 78, 301 Carbon, 3, 134, 137, 298 Carbonate of lime, 2, 10 Carbonic acid, 2, 3, 16, 97, 98, 106, 132 Carbonic acid gas, 71, 75, 79, 83 Carbonic oxide gas, 99 Cardiac aperture of the sto- mach, 145 Cartilage, 10, 11, 18, 85, 167, 279 Caruncula lachrymalis, 235 Casein, 134 Cavity of the mouth, 139 Cellular structure, 272 Cement of the teeth, 290 Central nervous organ, 187 Cerebellum, 260, 262, 268 Cerebral hemispheres, 263 Cerebral nerves, 248 Cerebro-spinal axis, 7, 93, 248, 266, 268 nervous cen- tres, 14 system, 248 Cheeks, 192 Chest, 5 Cholesterin, 123 Chondrin, 134, 281 Chordae tendmene, 35, 38 Chords, vocal, 79, 177, 180- 182 Choroid, 216, 218, 220, 227, 232 Chyle, 22, 28, 153 Chyme, 148 Cilia, 86, 157, 301 Ciliary ligament, muscle, and processes, 226, 228, 232 Ciliated epithelium, 275 Circulation, organs of, 15, 19, .21-57; 299 Circulation and respiration, analogies of, 94 nVDEX. Circumduction and rotation, 174 Clavicle, 1 1 Clot, 66 Coagulation of the blood, 59, 65-70 Coccyx, II Cochlea, 198, 200, 204, 211 Cold, 127 Cold and heat, feeling of, 192 Colon, 150 Colour blindness, 221 Colour of the blood, 74, 77 Columnje carneae, 38 Combustive process {see Oxy- gen) Complementary colours, 221 Concha (external ear), 205, 208 Cones of the retina, 216, 223, 301 Congestion of blood-vessels, 53 Conjunctiva, 234 Connective tissue, 9, 158, 201, 215, 280, 301 Consciousness, 188 Consonants and vowels, how sounded, 184 Contraction, muscular, ro, 158, 161 of blood-vessels, 23, 5 1 ■ of the heart, 40 of blood-corpuscles, 62 of the bronchial tulies, 81, 86 of the stomach, 146 of cilia, 158 Cornea, 226 Coronary arteries, 29, 50 veins, 29 Corpus callosum, 261, 264 Corpuscles of lymph, 73 of the blood, 56, 59 65, 301 of the spleen, 126 Corpus striatum, 262 Cortian membrane and fibres, 200, 204, 211, 212 Coughing, 90 Cranial nerves, 265 Crassamentum, 66 Cribriform plate, 196 Cricoid cartilage, 179, 180 Crico-thyroid muscle, 179 Crystalline lens, 226, 231, 279 D. Death, local, 18 general, 18-20 Deciduous teeth, 290 Decussation of the pyramids, 267 Deglutition, 144 Delusions, 241 of the senses, 238, 302 Dentine, 288 Dermis, 8, 113 Dextrine, 134 Diaphragm, 6, 33, ^?> Diastole, auricular and ven- tricular, 41, 96 respiratory, 82, 96 Diet, mixed, its importance, }Z1 Digastric muscle, 175, 176 Digestion, 146, 153 Digits, 5 Double vision, 246 Drink, its necessity to life, 3 Drinking, how perfnmed, 144 Drum of the ear, 205, 209 Ductless glands, 63 Ducts of glands, 131 Duodenum, 127, 146, 150 Dura mater, 249 p:. Ear, 198-213 Efferent nerves, 187, 254 IXDEX. Emotion, i88 Enamel, 143, 289 Endocardimn, 36 Endolymph, 2or, 20S, 211 Epidermis, 8, 113, 191, 273, 274 Epiglottis, 79, 141, 144, 179, 181 Epithelium, 9, 109, 191, 198, 273-275» 301 Erect position of man, 12 Essential food-stuffs, 139 Ether, 224 Eustachian tubes, 140, 205, 207, 213 Excrenientitious matter, 3, 133. Excretion, organs of, 3, 15, lOI Expiration and inspiration, 81, S9, 91, 93, 95 Extension and flexion, 174 Extra-vascular parts, 21 Eye, the, 214 235 muscles of the, 175 humours of the, 227 adjustment of the, 229 Eyeball, 225 Eye-lashes, 234 Eyelids, 234 F. Facial nerves, 264, 268 Faeces, 15, 139, 298, 299 Fainting, 53 Faintness, 188 Fat-cells, 283 Fatigue, 188 Fats, 134-139. 154, 298, 299 Fauces, 140 Feeling, sense of, 188 Fenestr?e, 204, 206, 207, 209, 211 Fibres and membrane of Corti, 200, 203, 204, 211, 212 Fibrillce, 281, 292 Fibrin, 65, 66, 124, 134 Fibrinogen, 69 Fibrous tissue, 9, 281 Fissure of SvLviu^ 261. 264 Flexion and extension, 174 Food, 98, 298, 299 its necessity to life, 3 its operation in maintain- ing weight, 4 daily quantity required, 133 Food-stuffs, 134-139, 154 Foot, 165 Frog, circulation in foot of, 54, 56 blood -corpuscles of, 64 reflex action of its spinal cord, 257 Functions of the body, 2 of the blood, 72 G. Gains and losses of the body, 117, 298 of the blood, loi, 103, 123,131 Gall-bladder, 118, 122, 146 Galvanip shocks, effect of, 14 (^aiiglia, 53, 250 (Ganglionic corpuscles, 215, 217, 249, 293, 296 Gaseous elements in food, ^34 . . Gases in inspired and expired air, 83, 93 Gases of the blood, 71, 75 , Gastric juice, 145, 148 Gelatine, 9, 134, 282 General death, 18 Glands, structure of, 130 ^■,v JXDEX. zn 0 lands, mesenteric, 28 salivary, 131 racemose, 131 -sebaceous, 112, 131 perspirator}-, 112, 131 saccular, 1 3 1 of Lieberkuhn, 131 buccal, 141 parotid, 141 submaxillary, 141 sublingual, 141 — peptic, 145 lachr)-mal, 234 Meibomian, 234 Glasses, convex and concave, 225, 244 Globulin, 61, 65, 69 Glomerulus, loS Glossopharyngeal nerves, 193, 260, 265 Glottis, 79, 93, 141,. 178, 180, 194 Glucose, 124 Gluten, 134 Glycocholic acid, 122 Glycogen, 117, 124 Gullet, 8, 140 Gum as food. 134 Gums, the, 288 Gustatory nerve, 193, 264 H. H.^MATIN, 61, 65 Haemoglobin, 61, 65, 71, 77, 99, 160 Hairs, 277, 278 Hand, 1 71, 192 Haversian canals, 285, 286 Head, 5, 166 Hearing, 190, 198-213 Heart, its structure and action, 6, 15, 19. 30-57 its muscles, 160 sounds of the, 45 Heat, generation of, 3, 4, 127 of the blood, 1 7, 70 of inspired and expired air, 83 — — - regulation of, 126-129 feeling of, 192 Heat-producers, 13S Hepatic arteiy, 31, 50, 118 duct, 118 — — vein, 31, 50, 119 Hilus, 104 Hinge-joints, 170, 175 Ilistolog)', 272-301 Humerus, 171 Humours of the eye, 227 Hunger, 3 Hydrogen, 3, 134, 135, 298 I. Ice-chamber, experimental, 2 Ileo-c^ecal valve, 150 Ileum, 150 Iliac arteries, loi Ilium, II Illusions, spectral, 240, 302 Incisors, 142, 290 Incus, 205, 206, 211 Inferior vena cava, 29, 104, 120 Innervation, 248-271 Insensible perspiration , 1 1 1 Insertion and origin of muscles, 175 . . o Inspiration and expiration, 81, 89, 91, 93> 95 Integument, 8 Inter-articular cartilages, 167 Intercellular substance, 279 Intercostal muscles, 86, 89 Intestines, 15, 150 Intra-lobular veinlet, 120 Invertebrate animals, 63 Iris, 160, 227, 229 Irritation, 1S7 Ischium, 1 1 314 AVDEX. J. Lime-water, 2 Lips, 192 Jejunum, 150 Liver, 6, 117 Joints, 162, 167-172 its structure, I18-122, 29S Judgments and sensations, 237 cells, 117, 120, 122 Jumping, 177 circulation, 118 Lobes of the brain, 260 Lobules of the liver, 119, 121 K. Local death, 18 Locomotion, 157, 175 Kidneys, 6, 16, loi, 104- "7, Luminous impressions, 220 298 Lungs, 6, 16, 17, 19, 81, 84- compared with '. lungs and 100, loi, 117 skin, 116 their arteries and veins. Knee-pan, ii. 29 Kreatin, 160 kidneys and skin com- pared, 116 Lymph, 22, 73 L. Lymphatic system, 51, 125 capillaries, 63 Labyrinths of the ear, 19S, corpuscles, 63 200, 204, 207, 211 glands, 26, 126 Lachrymal ducts, 140 Lymphatics, 8, 26 glands 234 sac, 235 Lacteals, 28, 151, 154 Lactic acid, 160 Lancelot, the, 63 Laryngoscope, the, 181 Lar)'nx, 80, 177, 181 Legs, 5 bones of the, 11 Lens, crystalline, 226, 231, 279 Levator muscle of the evelid, 234 Levers, action of the bones, 163 Life, 18, 19 Ligaments, 10, ii, 172, 2S3 Light, 224 sensation of, 214, 219 Limbs, 5 Lime {str Carbonate of Lime, Phosphate of Lime). M. Macula lutea (yellow spot), 216, 217, 219, 222, 226 Malleus, 205, 207, 209 Malpighian capsules, 108 Marrow, 161, 285 Mastication, 144 Mastoid cells, 206 Meat, 138 Meatus, internal and external, 205-208 Mechanical force, 3, 4 Medulla oblongata, 19, 93, 260, 261, 266, 270 Mental emotion, 13, 53 Mesenteric glands, 28 Mesentery, 28 Minerals as food, 134, 29S, 299 Mitral valve, 36 INDEX. 315 Molar teeth, 143, 290 Mortification, 18 Motion and locomotion, 156- 186 Motor fibre, 159 Motor nerves, 187 Mouth, 8, 15 Mucous membrane, 9, 141, 145, 152 Mucus, 9 Muscles, 10, 132, 157, 1 58 of the ear, 208, 213 of the eyeball, 233 of the heart, 36 — — of the mouth, 143 of motion and locomo- tion, 156-186 of the vessels, 23 of the voice, iSi striated and smooth, 1 58, 291 not attached to solid levers, 160 attached to levers, 161 minute structure of, 291 Muscular contraction, 12, 14, 158 sense, 188 tissue, 301 Musk deer, blood-corpuscles of, 64 Myosin, 159 Xerves, facial, 264 action on glands, 132 glossopharyngeal, 193 gustatory, 193, 264 of the heart, 53 action on the kidneys, III motor or efferent, and sensory or afferent, 187, 253 action on muscles, 159 olfactor}', 194, 196 action on respiration, 93 on the stomach. 145 116 on sweat-glands. of the tongue, 266 Nervous action, its rapiditv, 301 Nervous system and innerva- tion, 17, 248-271 Nervous tissue, 293-296 Neurilemma, 191, 294 Nitrogen, 3, 83, 132, 154, 135, 298 Nitrogen starvation, 136 Nose {see Smell) Nuclei of blood-corpuscles, 60, 63 Nuclei and nucleoli, 272, 2S7, 292, 294, 295, 296 Nutrition, 15, 133-155 N. Nails, 276 Nasal passages, 93, 158 Nerve fibres, 293, 295, 301 Nerves, auditory, 198, 202, 208, 211, 265 of the brain, 264 action on blood-vessels, 129 of the eye, 214, 216, 264 O. (JESOPHAGUS, 8, 80, 140, 144 Olfactor)' nerves, 264, 295 Optic axis, 241 — — nerve, 214, 217, 218. 222, 226, 233, 264 thalami, 262 Optical delusions, 241 Orbicularis muscle, 234 3i6 IXDEX. Origin and insertion of muscles, 175 Orra serrata, 226, 229 Osmosis, 148 Ossa innominata, 1 1 Ossification, 285 Otoconia or otoliths, 199, 201, 208, 211 Oxygen, 3, 17, 19, 79, 83, 97, 98, 123, 133, 298 Palate, 139, 192 194 Pallor, 52 I'alpitation, 53 Pancreas, 6, 127, 131 Pancreatic duct, 146 juice, 153 Papillai-y muscles, 35, 38 Papillae, 190, 192 Paralysis, 253, 255 Patella, 11, 166 Pelvis, I r of the kidney, 107 Pepsin, 145 Peptic glands, 145 Peptone, 147 Pericardium, 31 Perilymph, 201, 202, 208, 211 Periosteum, 285, 287 Peristaltic contraction, 152, 161 Peritoneum, 105, 118, 150 Perspective, 243 Perspiration, 1 1 i-i 1 7, 300 Petrosal, 198 Phalanges, 5 Pharynx, 8, 15, 79, 93, 140 Phosphate of lime in bone, 10 Phosphenes, 222, 242 Physiolog)', human, 2 Pia mater, 249 Pigment cells, 227, 284 Pillars of the diaphragm, SS Pivot-joints, 170 Plasma, 59, 66, 72 Pleura, 84 Plexuses, 271 Pneumogastric nerves, 53, 254, 260, 265, 267 Pons Varolii, 262 Portal circulation, 50 Pronation, 171 Proteids, 125, 134 139. I54. 298, 299 Protein, 3 Protoplasm, 272, 274 Pseudoscope, 246 Ptyalin, 141, 144 Pubis, II Pulmonary capillaries, 78, 81 Pulse, 45 Punctum lachiymale, 235 I'urkinje's figures, 223 Putrefaction, 159 Pylorus, 145, 146 Pyramids, 267 R. Racemose glands, 131 Radius, 171 Receptacle of the chyle, 28 Rectum, 152, 155 Reflex action, 94, 188, 256, 268, 269 Reissner's membrane, 202, 204 Renal artery, 107 excretion, 301 vein, 109 Respiration, 74-100, 299 and circulation, analogies of, 94 Respiratory sounds, 94 Restlessness, 188 Retina, 214, 220, 222, 226, 229 RibSj II, 85, 90 INDEX. M Rigor mortis, 159 Rods and cones of the retina, 216, 223, 301 Rotation and circumduction, 174 Sacrum, ii Salamanders, blood-corpuscles of, 64 Saliva, 141, 144 Salivary glands, 131 Salts in food, 135 Sarcolemma, 367, 291, 294 Scala media of the cochlt-a, 198, 200, 201, 203, 207, 211, 212 Scala tympani, 204 Scala vestibuli, 204 Scapula, 1 1 Sclerotic, 216, 225 Sebaceous glands, 112, 131, 279 Secretory glands, 102 Semilunar valves, 35j 38 Sensation, a state of conscious- ness, 188 Sensations and judgments, 237 coalescence of, 236 and sensory organs, 14, 187-213 Senses, delusions of the, 23S, 302 Sensory nerves, 187 organs, 14 Septum, 194 Serous membranes, ^^t^ Serum, 33 of blood, 66 Sighing, 90 Sight, 190 the organ of, 214-235 Singing, 183 Skeleton, 10 Skin, 8, 16, 102, 111-117, 29S lungs and kidneys, com- pared, 116 Skull, 5, 6, ir, 165 Smell, 194- 1 98 Sneezing, 90 Sniffing, 90, 197 Sound, 181, 184, 209 Sounds of the heart, 45 Speaking machines, 185 Spectral illusions, 240, 302 Speech, 177, 184 Sphmcter, 105 Spinal canal, 6 cord, 6, 249, 255-259, 263 nerves, 248-255, 263 Spine, 166 Spleen, 6, 126 Splenic artery and vein, 126 Squinting, 246 Stapedius muscle, 208, 213 Stapes, 206, 210 Starch, 117, 124, 125, 134, 14:, 144, 148, 154 Stereoscope, 247 Stimulus or irritation, 19, 187 Stomach, 15, 144 Subjective sensations, 239 Sublingual glands, 141 Submaxillary glands, 141 Sugar, 117, 124, 125, 134, 142, 144, 148, 154 Sulphuretted hydrogen, 99 .Superior vena cava,, 29 Supination. 171 Supra-i-enal capsules, 126 Suspensory ligament, 227 Sweat-glands, 112, 113, 131 Sweet-bread {sec Pancreas) Symmetry of the body, 6 Sympathetic ganglia, 6, 7, 24S, 261, 271 — nerve, 52, 53 Synovia, 11, 167 3i8 INDEX. Synovial membrane, ii, 167 Syntonin, 134, 159 Systole, auricular and ventri- cular, 41, 96 respiratory, 82, 96 T. Tactile corpuscles, 191, 294, 296 Taste, 192-194 Taurocholic acid, 122 Tears, 235 Teeth, 142, 287-291 Temperature of the blood, 70, 128 Tendons, 10, 175, 2S3 Tensor tynipani, 208, 213 Thaumatrope, 245 Thickness of the blood, 70 Tliirst, 3 Thoracic duct, 28, 155 Thorax, 5, 84 Thought, 188 Thymus gland, 126 Thyro-arytenoid muscles, 179, 182 Thyroid cartilage, 178, 180 gland, 126 Tibia, 166 Tissues, 9, 78, 85, 103 dissolution of, 20 minute structure of, 272- 296 Tissue-formers, 138 Tongue, 140, 143, 192, 193 not indispensable for speech, 185 Tonsils, 140 Touch, 190-192 Trachea, 80 Transfusion of blood, 73 Transmigration, 20 Trapezium, 170 Triceps muscle, 169, 174 Tricuspid valve, 35, 36 Trigeminal nerve, 264 Tripod of life, 19 Trunk, 5 Tubules of the kidneys, 108, no Turbinal bones, 140, 196 Tympanum, 205, 209 U. Ulna, 171. Urea, 3, 16, 105, 106, 135, 138 Ureter, 104 Urethra, 105 Uric acid, 105, 106 Urine, 105, 301 Uvula, 140 V. Valves of aorta and pulmo- nary artery, 26 of the heart, 36 of lymphatic vessels, 28 of veins, 24 Valvulce conniventes, 152 Vascular system, 21-57 Vaso-motor centres and nerves, 53, 259, 261 Vegetable diet, 136, 137 Veins, 15, 24, 29 Velum (soft palate), 140 Vena cavre, superior and in- ferior, 29, 104, 120 Vena portce, 31, 50, 118, 121, 127, 152 Venous pulse, 96 Ventilation, 100 Ventricles of the heart, 35, 42 of the larynx, 179 INDEX. Z^9 Ventricles of the brain, 261 Ventriloquism, 241 Vermiform appendix, 150 Vertebrate animals, 63 Vertebrae, 6, 167, 173 Vestibular sac, 200 Vibrations of ether, 219 Villi, 28 of the intestines, 151, 154 \ ision, single and double, 242 \'ision, single with two eyes, ..245 \ isual size and form, 243 Vital actions, 18 Vital food-stuffs, 135 \'itreous humour, 226 Vocal chords, 79, 177, 180- 182 Voice, 177, 183 Volition, 188 Vowels and consonants, how sounded, 1S4 W. Walking and running, 176, 177 Warmth and coldness, feeling of, 192 Water, 16, 135, 138, 298, 299 elimination of, 3, 84 Weight lost in action, 3, 4 of constituents of the body, 297 Whispering, 184 Whistling, 184 Windpipe, 80, qo Work and waste, 4, 298 Vellow-spot, 216, 217, 219, 222, 226, 301 ZOOTROPE, 245 THE END. T.ONDON : K. CLAY, SONS, AND TAYLOR, PRINTERS, BREAD STREET HILL, QUEEN VICTORIA STREET. May 1879. A CATALOGUE OF EDUCATIONAL BOOKS, PUBLISHED BY MACMILLAN AND CO, BEDFORD STREET, STRAND, LONDON. 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