^mmW Hmvmitg ^ito^tg BOUGHT WITH THE INCOME FROM THE SAGE ENDOWMENT FUND THE GIFT OF 1891 ..^,.. .2..a...a.. J., in. iif.Alc.f; ^^"njan Physiolog olin,an? ^^24 031 278 967 Cornell University Library The original of tliis bool< is in tine Cornell University Library. There are no known copyright restrictions in the United States on the use of the text. http://www.archive.org/details/cu31924031278967 HUMAN PHYSIOLOGY ADVANCED SCIENCE MANUALS. MAGNETISM AND ELECTRICITY. By Arthur William Poyher, M.A. With 317 Diagrams. Crown Svo. $1.50 PHYSIOGRAPHY. By John Thornton, M.A. With 11 maps, 255 IDustrations, and Coloured Map of Ocean Deposits. Crown Svo. ®1.40 HUMAN PHYSIOLOGY. By John Thornton, M.A. 284 Illustrations, some coloured. Crown Svo $1.50 HEAT. By Mark R. Wright, M.A. With 136 Illustrations and numerous Examples and Examination Papers. Crown Svo. $1.50 LIGHT. By W. T. A. Emtage, M.A. With 232 Illustrations. Crown 8vo. $1.50 BUILDING CONSTRUCTION. 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New York, London, Bombay, and Calcutta. HUMAN PHYSIOLOGY JOHN THORNTON, M.A. AUTHOR OF 'elementary PHYSIOGRAPHY' 'ADVANCED PHYSIOGRAPHY' ETC. HEAD MASTER OF THE MUNICIPAL SECONDARY SCHOOL, BOLTON WITH 284 ILLUSTRATIONS, SOME COLOURED NEW EDITION, REVISED (7906) REISSUE LONGMANS, GREEN, AND CO. 91 AND 93 FIFTH AVENUE, NE-W YORK LONDON, BOMBAY, AND CALCUTTA 1908 .4// rights reserved D CONTENTS CHAPTER PAGE I. HISTOLOGY, OR THE MICROSCOPICAL STRUC- TURAL ELEMENTS OF THE BODY . I II. MUSCULAR AND NERVOUS TISSUE . . . . 34 III. BLOOD .... ... 67 IV. THE HEART AND THE CIRCULATION . . . 8 1 V. BLOOD-VESSELS AND CIRCULATION . . 98 VI. RESPIRATION 120 VII. FOOD 153 viii. digestion and absorption . ... 166 ix. the lymphatic system ..... 20^ x. the liver and the ductless glands . . . 2 1 7 xi. excretion — the kidneys and the skin . 229 xii. animal heat . . . . . . 253 xiii. the larynx and voice 260 xiv. the spinal cord . .... 267 xv. the brain 282 xvi. . touch, temperature, muscular sensations, taste and smell 327 xvii. the eye and the sense of sight . 343 xviii. the ear and the sense of hearing . . . 383 Appendix : — Measurements, French and English . ■ 4ii Note on Microscope . . . .412 The Chemistry of the Body . .... 412 Supporting Tissue of the Nerve-centres . . . 419 viii Human Physiology Appendix — continued. pace The Heart of the Frog ,42° Artificial Model or Schema of Circulation . 4^3 The Stethoscope . . . . . .425 The Distribution of the Spinal Nerves • 42° The Ophthalmoscope .... 42? Accommodation and Scheirrer's Experiment . • 43° Restoration from Suffocation after Drowning . 432 Syllabus . 433 Progressive Questions , . 436 Glossary ... . 45 ^ Index . . . . , . . .461 HUMAN PHYSIOLOGY CHAPTER I HISTOLOGY, OR THE MICROSCOPICAL STRUCTURAL ELEMENTS OF THE BODY I. The Microscopic Elements of the Body. — A simple examination and dissection of the human body, or that of any other higher animal, soon shows that it is made up of easily separated portions having distinct forms, such as the heart, lungs, stomach, brain, &c. Such parts are called organs, and each organ is found to have its special work or function in the animal economy. A department of study called descriptive anatomy furnishes an account of the several organs, members, and regions of the body, pointing out their form, structure, and mutual connections. Much of this knowledge may be obtained from the dead body, and the student is supposed to have already acquired by this means and from the earlier chapters of an elementary work on animal physiology a rudimentary acquaintance with the build of the human body and with the position, form, and function of its chief organs. Dissection further teaches that the body of any of the higher animals is made up of different kinds of materials, such as muscle, fat, cartilage, nervous matter, &c. These various constituent materials are spoken of as textures or tissues, and while some of the organs are formed mainly of one elementary texture or tissue, other organs or parts of the body are compounded of several tissues. The portion of anatomy that treats of the minute structure of the tissues is called Histology (Gk. histos, a web ; logos, a discourse). Histology requires the use of the microscope, and by its aid we quickly learn that the separable structural 2 Human Physiology elements of the various tissues are for the most part either minute corpuscles named ' cells ' or elongated threads named 'fibres.' But since many of the iibres, if not all, are either modified cells or are derived directly or indirectly from cells, and since all the tissues contain cellular elements and originate as collections of cells, the cell may be regarded as the histo- logical unit of the body. To the minute anatomy of the tissues we shall devote our first chapter, as a sound knowledge of this is essential to the proper understanding of much that follows. Human physiology, in fact, may, from one point, be regarded as the knowledge of the structure, properties, and functions of cells and their derivatives. As we shall presently see, the cells of the body diflfer widely in form, appearance, and structure in the different tissues enter- ing into the composition of the organs, while corresponding to these differences of structure and constitution will be found differences of function or properties. In other words, morpho- logical differentiation (Gk. morphe, form) is accompanied by physiological differentiation. Thus the cells of muscle, or motor cells, possess the power of contraction, and do the mechanical work of the body ; the cells of the alimentary canal either secrete fluids from the blood and pour it out on the food to digest it and render it soluble, or they are specially adapted for absorbing into the blood-stream the food so pre- pared. The liver cells, among other functions, further elaborate the food-stuffs absorbed into the blood so that they may serve the more easily as nutriment for the cells of the various tissues. For every cell needs to assimilate food in order to live, and every cell must be supplied with oxygen for the oxidation of the matter by which its energy is produced. The cells must also be able to get rid of the waste products of their activity. The lungs are the organs of the body where a thin layer of cells in the cavities allows the passage of oxygen into the blood to be carried to all the tissues, and where also a discharge of carbon dioxide, CO.,, one of the products of cell activity, takes place ; while the cells of the kidneys perform the special duty of freeing the blood from the nitrogenous waste passed into it. There iSj therefore, a physiological division of labour in the Histology 3 human body, and the parts work together harmoniously because the organs and tissues are co-ordinated and regulated by a complex system of cells and fibres known as the Nervous System. Although the modern conception of a cell differs from that which prevailed some few years ago, the doctrine that the bodies of all animals and plants consist either of a single cell or of a number of cells and their products, and that all cells proceed from pre-existing cells (pmnis cellula e celluld), is now regarded as the basis of biological science. It is often spoken of as the cell theory, and may be regarded as ' the greatest discovery in the natural sciences in modern times.' To the study of the animal cell we now therefore proceed. 2. The Animal Cell. — An ani- mal cell is a minute corpuscle of living substance, or protoplasm, those of the human body seldom exceeding ^iTjth of an inch in dia- ^ „. , ^ „ , „ ° " " " Fig. I.— Diagram of a Cell, the Proto- meter, while many are but one- plasm of which is composed of ... . T. • . e . Spongioplasm and Hyaloplasm, tenth this size. It consists of two Highly magnified. distinct parts, the main substance a pro'op'asm ;«, nucleus ; ^ , ,, ,,1 7 - «', nucleolus. of the cell, called protoplasm, and a tiny body embedded in the protoplasm, called the nucleus. Seen under a low power of the microscope, the protoplasm appears either to be a homogeneous substance or to contain granular matter and vacuoles (globular cavities containing a watery fluid) ; but viewed with very high powers, part of the protoplasm is seen to be fibrillated and to form a fine network. This network is known as the reticulum or spongioplasm, and the rest of the protoplasm occupying the meshes of the network is a clear substance termed hyaloplasm. The arrangement of the network and the size of the meshes vary in different cells, the nodes or junctions of the fibrils probably giving rise to some of the granular appearances before mentioned. It must, how- ever, be remembered that the protoplasm often includes actual granules of nutritive material or of matter stored up for nutrition, B 2 4 Human Physiology and that the vacuoles may contain glycogen or other substances in solution. The external layer of protoplasm in some cells becomes altered so as to form a cell-membrane or cell-wall, but this is more frequently found in vegetable than in animal cells. Within the protoplasm, and generally near the middle of the cell, is the minute body of somewhat firmer consistence called the nucleus. It is probably a portion of the cell-proto- plasm somewhat altered in chemical nature, and its chief office is to preside over the nutritive activity and reproduction or division of the cell. It is believed by many biologists that the nucleus is also the conveyer of hereditary properties. The cletxf arecu Fig. 2. — A dividing Cell. Formation of Chromatin Network in Daughter-nuclei. (Rahl.) The upper daughter-nucleus is at an earlier stage than the lower one. The cell-protoplasm is now completely divided. nucleus is spherical or elongated in shape, often bounded by a membrane, and consists of a clear substance known as the nuclear matrix or nucleoplasm, pervaded by a network of fibres differing in nature from those of the cell-substance. The intranuclear network may show enlargements called nucleoli (fig. i). Nuclei are often recognised by treating a cell with acetic acid, which destroys the cell-protoplasm and thus renders . the nucleus more visible. The intranuclear network stains with certain dyes, and is hence called chromoplasm (Gk. chroma, colour), while the matrix or nucleoplasm is achromatic. His- tologists distinguish between a resting or non-dividing nucleus Histology S and a dividing nucleus. When cells are about to divide, the nucleus undergoes certain definite changes, the protoplasm outside the nucleus taking part in the process by means of the so-called attraction spheres. These consist of a wheel-like arrangement of fine fibrils starting from a central particle called the attraction particle, and the twin spheres are connected by the delicate system of fibrils denominated an achromatic spindle. The fibrillar network of the nucleus becomes arranged into definite patterns (skeins, stars, rosettes, &c.), finally separating into two groups of chromatic fibres and giving origin to daughter-nuclei, around which the protoplasm of the cell ultimately arranges itself and divides to form two separate cells. This indirect process of cell- division is spoken of as karyokinesis (Gk. karuon, kernel, nucleus; andkinesis, movement). A cell may therefore be described as a minute mass of protoplasm con- taining a specially for- med constituent known as the nucleus, the pro- toplasm consisting of two substances, a network called spongio- plasm and a clear fluid called hyaloplasm, and the nucleus being formed of a fluid called nucleoplasm, and a system of fibres called chromoplasm. Protoplasm is a living substance, and forms the ' physical basis of life.' 3. Composition and Properties of Protoplasm. — Proto- plasm is a semi-fluid transparent substance that swells up in but does not mix with water. It usually contains granular matter and vacuoles ; but these are not essential to its nature. It consists of 80 to 85 per cent, of water with 15 to 20 per cent, of solids, chiefly proteids, the chief proteid being plasten. Proteids are substances containing carbon, hydrogen, nitrogen, oxygen, and sulphur; and they are distinguished by certain Fig. 3. — An Amoeboid Pale Corpuscle of the Newt, showing a Double Nucleus with Reticulum of Chromoplasm, and the Protoplasm showing Spongioplasm and Hyaloplasm. 6 Human Physiology reactions (see Appendix). Carbohydrates like glycogen, as well as minute quantities of inorganic substances like calcium, are also present. The vital properties of protoplasm may be studied either in the amoeba, a microscopic unicellular animal found in stagnant water and damp earth, or in the white blood- corpuscle, which is an example of a cell retaining its simple primitive form in an adult higher animal. The chief of these physiological characters are : — ( I. ) Power of Movement. — On examining an amoeba with a high power of the microscope, it is seen to consist of an irregular mass of protoplasm with a nucleus (or nuclei), the protoplasm usually containing granular matter and vacuoles. Its shape soon changes ; processes called pseudo- podia are extended and retracted ; at times the whole mass seems to follow a process and the animal changes its position. Colourless blood- corpuscles may be noticed to exhibit similar movements, which are hence termed anmboid. (2. ) Power of Response to Excitation, or Irritability. — Besides the apparent spontaneous movements just described, protoplasm can be excited by external agencies called stimuli. Such stimuli are heat, Fig. 4.— Human Colourless Blood-corpuscle, showing its successive changes of outline within ten minutes when kept moist on a warm stage. (Schofield.) electricity, foreign particles, &c., and the protoplasm exhibits its excita- bility by movement or contraction of its mass when a suitable stimulus is applied. ( 3. ) Power of Assimilation and Growth. — The amoeba and other similar protoplasmic cells possess the power of taking in material and using it as food. Such matter becomes modified by chemical changes in the pro- toplasm of the cell, so that part of it becomes converted into protoplasm itself, while the part that is not assimilated is rejected. The power of digestion and nutrition possessed by protoplasm is accompanied with a power of respiration. Oxygen is absorbed and drawn into chemical com- bination, but it is afterwards exhaled, for the most part united with carbon as carbon dioxide. (4.) Power of Secretion — Protoplasmic cells can elaborate new sub- stances from the food supplied, some of which are stored for future use and some of which are excreted. In higher animals special cells in organs called secreting glands are set apart to prepare material known as enzymes or ferments. Living protoplasmic cells are thus the seat of con- tinuous chemical changes, the cell either building up its own substance from the food supplied, or forming materials like glycogen, fat, ferments, &c. , within its substance ; while, on the other hand, these constructive pro- cesses are always accompanied by destructive changes, of which oxidation forms the chief. The whole series of chemical changes within a cell, or within a body composed of cells, beginning with assimilation and ending with excretion, is termed metabolism. The process of building up living J.'l iSbOu(/Pv y material, or constructive metabolism, is called anabolism ; the breaking down of material into simple products in the protoplasm, or destructive metabolism, is called katabolism, (5.) Power of Repiodnction.— When an amceba, by the growth of its protoplasm, has reached a certain size, its power of reproduction is mani- fested by fission, a simple division of the cells preceded by a division of the nucleus taking place. Two independent amcebse are thus produced, each of which may afterwards undergo similar direct division. A more complex process, called karyokinesis or indirect division, is more common both in vegetable and animal cells. This has already been referred to. The body of man, as well as that of every higher animal, is developed from a single cell, termed the ovum, which after fertilisation first divides into two, each of which again divides. By further division, eight, sixteen, thirty-two, and so on, cells are formed. _ After cell- multiplication has proceeded for some time, the cells become arranged in three layers ; the outermost layer is termed the epiblast, the innermost the hypoblast, and the middle layer the meso- blast. From these three layers all the tissues and organs of the adult are formed. From the epiblast are formed the epider- _ mis and nervous system ; the hypoblast Fig. 5.- Ovum. Magnified will form the digestive and absorbent 35° times, membranes of the digestive and respira- a, vitelline membrane, zona pellucida tnvi7 ovctpms • tVio m pcnbl a et will irivp or zoKa rarfM^a. *, external border tory s> stems , tne raesoblast will give ^^ ^^^ ,^ ^^^ internal border of origm to the blood-corpuscles, the con- the vitelline membrane. <:, germinal nective tissues, the bones and muscles of vesicle and germinal spot, the body. In this process of development from the three primary embryonic layers there is not merely cell-multipli- cation, but the cells become modified and metamorphosed from their primitive condition. Some become thin and flat, and cohering at their edges form the blood-vessels ; others become elongated and threadUke to form the fibres of muscular tissue ; others become separated by intercellular substance derived from the cells themselves, and in this intercellular sub- stance fibres such as are found in connective tissue may arise, or calcareous matter be deposited, as in bone. To the study of the varieties of tissue chus produced in the adult we now proceed. 4. Classification of Tissues. — Having described the struc- ture and properties of the protoplasmic units called animal cells, we now proceed to study the different kinds of material, or tissues, which are formed of or arise from these cells. The number of distinct tissues or textures forming the various organs of the human body has been reduced to five ; (i) epithelial or surface-limiting tissues ; (2) connective or support- ing tissues ; (3) muscular or contractile tissues ; (4) nervous or sensory tissues; (5) blood and lymph particles, or nutritive tissues. Each of these classes, as will be seen, may be sub- 8 Human Physiology divided ; but the various kinds are not always sharply dis- tinguished, forms of transition between them being often recognisable. 5. Epithelial Tissues. — Epithelium is a tissue covering the external and internal free surfaces of the body, and composed of cells placed side by side, with a small amount of cementing intercellular substance. It thus forms— (i) the outer surface of the skin, where it is known as epidermis ; (2) the covering of mucous membranes, i.e. those membranes that line the passages and cavities of the body that communicate with the exterior, including the ducts and tubes of glands opening into these cavities ; (3) the terminal parts of the organs of special sense ; (4) the inner surface of serous membranes, i.e. the membranes forming what appear closed sacs in the thorax and. abdomen ; (5) the inner surface of the heart, blood-vessels, and lym- phatics ; ' (6) the inner lining of the ventricles of the brain and the central canal of the spinal cord. Epithelial cells consist of protoplasm and a nucleus, and when multiplying undergo division by karyokinesis in the way already described. Having different functions, as protective, secreting, &c., and being exposed to diverse conditions, they present varieties of structure and form. As regards arrange- ment, epithelial cells may be stratified, forming several super- posed layers ; or simple, forming a tissue of one layer. Where there are but two or three layers fitting into one another the term transitional may be used. As regards shape, epithelial cells are classified as squamous or flat, columnar or cylindrical, and cubical. No blood-vessels pass into epithelial tissue, the cells deriving their nourishment by imbibition of the plasma exuded into the subjacent tissue, but in many parts nerve fibrils, as fine filaments, exist among the epithelial cells. 6. Stratified Epithelium. —This epithelium forms the external surface of the body, or epidermis, and is also found lining the cavity of the mouth, pharynx, oesophagus, and the anterior surface of the cornea. The epidermis consists of (a) the superficial horny layer of flattened cells ; (b) the stratum lucidum, formed of dense horny scales, showing traces ' The term endothelium is applied by some authors to the single layer of flattened cells which line the internal free surface of the serous sacs, blood-vessels, and lymphatics (4 and 5 above). mstoiogy 9 of a nucleus ; (c) the rete mucosum or Malpighian layer, consisting of several layers of nucleated cells, the deeper ones of which become columnar as they rest on the underlying corium or dermis. It is in the deepest part of a stratified epithelium that new cells are formed by cell-division. Cells are pushed outwards from below, and as they near the surface they not only become compressed and changed in shape, but they undergo a change in chemical composition, their protoplasm becoming converted into a horny substance, keratin, the nucleus also being often involved. Before this conversion occurs there is usually a deposit of granular material within Fig. 6. — Section of Epidermis from the Human Hand. Highly magnified. (Ranvier.) //, horny layer, consisting of j, superficial horny scales ; sw, swollen-out horny cells : J. /, stratum lucidum ; M, rete mucosum or Malpighian layer, consist- ing of, j>, prickle-cells, several rows deep, and c, elongated cells forming a single stratum near the corium ; «, part of a plexus of nerve-fibres in the superficial layer of the cutis vera. From this plexus fine varicose nerve- fibrils may be traced passing up between the cells of the Malpighian layer. the cells, and the upper layer of cells of the rete mucosum has been termed the stratum gramtlosuin. In the deeper layers of a stratified epithelium the surfaces of the cells are not close together, but a radiating system of fibrils connects the protoplasm of adjacent cells, and between these lie very fine channels. The exact purpose of these intercellular bridges and canals is not fijlly understood. lO Human Physiology When such cells, known as prickle-cells, are isolated, the broken fibrils give them a spiked or dentated appearance. In coloured races the colour is due to pigment-granules, contained chiefly in the cells of the deeper layer of the rete mucosum. Pig- ment epithelial cells also occur on the internal surface of the choroid coat and iris of the eyeball. The older superficial cells of the epi- dermis are continually being re- moved by wasting and friction, a constant renewal taking place from below. 7. Transitional Epithelium, — This variety is found in several places. A squamous transitional epithelium lines the bladder and ureters, and consists of three or four layers of cells with distinct nuclei. Fig. 7. — Two * Prickle-cells from the deeper part of the Epidermis. (Ranvier. ) d, space around the nucleus, prohably caused by shrinking of the latter. fc.'".: ■^ J,.- - • 1 Fig. 8.— Section of the Transitional Epithelium lining the Bladder. (E. A. S.) a, superficial, b, intermediate, and c, deep layer of cells. The inner or most super- ficial layer consists of some- what flattened cells, each of which overlies two or three pear-shaped cells, which form the second layer. A third layer fills up the intervals be- tween the cells of the second layer. A columnar transitional epithelium of three or four layers of cells is found on the lining membrane of the larynx, trachea, and large bronchi, but these will be described with the corresponding simple epi- -. , - thelium below. 8. Simple Squamous or Pavement Epithelium._A single layer of Fig. g.— Pavement Epithelium, scraped from a Serous Membrane. a, cell-body ; b, nucleus; c, nucleoli. (Henle.) Histology II simple scaly epithelium, consisting of flattened polygonal cells fitting together at their edges, lines (a) the air-cells of the lungs, certain looped tubules of the kidney, and the inner surface of the iris and choroid ; [b] the free surfaces of serous membranes — as the pleura, pericardium, peritoneum, arachnoid — and the interior of the heart, blood-vessels, and lymphatics. It is to the epithelial tissue of the latter division [b) that the term endothelium is sometimes applied. The outline of these' simple squamous epithelial cells is readily brought out by treatment with silver nitrate, as this salt darkens the small amount of intercellular substance. 9. Simple Columnar Epithelium. — This kind of epithelium consists of cells of cylindrical or prismatic form set upright on a surface, and, except in the cases mentioned above, the cells form but one layer side by side. Owing to mutual compression Fig. 10. — Columnar Epithelium Cells of the Rabbit's Intestine. (E. A. S.) The cells have been isolated after maceration in very weak chromic acid. The proto- Silasm is reticular and vacuo- ated ; the striated border {sty^ is well seen, and the bright disc separating this from the cell-protoplasm ; «, nucleus with intranuclear network ; a, a thinned out wing-like projection of the cell, which probably fitted between two adjacent cells. Fig. II. — A row of Columnar Cells from an In- testinal Villus of the Rabbit. (E. A. S.) sir^ striated border ; w, wander-cells between the epithelium cells. and the frequent presence of lym- phoid or wander-cells, their shape is often very irregular. Columnar epi- thelium is found Hning the ali- mentary canal from the lower end of the oesophagus to the anus, as well as on the free surface of the trachea and largest air-tubes. In some cells of columnar epithelium, as those of the small intestine, the free edge is finely striated. Columnar cells often undergo a modification of shape owing to the secretion in their interior of mucin, the chief organic constituent of mucus. The mucus distends the upper part of the cell until a rupture occurs and the mucus is discharged on the surface. Such mucus-secreting cells are called goblet or chalice cells, and the columnar epithelial cells of the intestinal and respiratory tracts may at any time undergo this transfer- 12 Human Physiology mation into goblet-cells temporarily, while the epithelial cells of mucus-secreting glands have this function permanently. 10. Ciliated Epithelium.— This variety of epithelium consists of cells, usually columnar, having at their free ends Fig. 12. — Simple Columnar Epithelium from the Mucous_ Membrane oi the Intestine, with goblet-cells pouring out their contents. (Klein and Noble Smith.) fine hair-like processes called cilia. Each cell bears a brush of cilia, about of an inch long in the windpipe, and during life or for a short time after removal from the body the cilia exhibit a rapid whip-like movement, the cells of the surface of a membrane moving simultaneously or in quick suc- cession in one direction. Bend- ing swiftly in one direction and returning to the upright position more slowly, these cilia set in motion in a definite direction the fluid which bathes the cili- ated surface. It is thus that mucus is moved along the bronchial tubes and trachea to the pharynx. Ciliated epithe- lium is found in the respiratory region of the nose, on the upper half of the pharynx, along the Eustachian tubes, in the lower part of the larynx, except over the vocal cords, in the trachea and bronchial tubes, lining the ventricles of the brain and the central canal of the spinal cord, and on the mucous lining of the uterus and Fallopian tubes. As to the nature and cause pf ciliary movement little is definitely nn m' Fig. 13. — Ciliated Epithelium Cells from the Trachea of the Rabbit. Highly magnified. (E. A. S.) ;«', wi'^, w/^, mucus-secreting cells, lying between the ciliated cells, and seen in various stages of mucin-formation. Histology 1 3 known, except that the source of motion is contained in the ciliated cell itself and is independent of any connection with nerve -fibres. Ciliary movement may be readily studied by taking a piece of the gill of a salt-water mussel, mounting it in sea water, and examining it with a power of about 200 diameters. ' 1 1. Sensory Epithelium. — Various forms of modified epi- thelial cells are connected with the terminations of certain sen- sory nerves to form the receptive end-organ for different kinds of vibrations. Examples of such sensory epithelium are the rods and cones of the retina, the auditory hair-cells, &c. These will be best studied in connection with the different senses. 12. Functions of Epithelium. — Putting aside the varieties of sensory epithelium, it will be seen that the functions of epithelium are either protective or secretive. Thus the layers of epithelium forming the epidermis, the epithelium lining the air-passages and that lining the eyelids, serve mainly as a pro- tective covering. But the epithelial cells of the salivary glands, those of ttie gastric glands which secrete gastric juice, those of the liver, of the sweat-glands, &c., are composed of protoplasm which is the seat of active chemical operations. New substances are formed from the blood and poured out as secretions to fulfil important functions, or discharged from the body as excretions. ' A secreting epithelium may be regarded as a partition between the blood, or, more properly speaking, between the lymph on the one side and the lumen of the secreting gland on the other. From the lymph the materials are taken by the secreting cells and then worked up into the components of the secretion, and finally discharged on the other side into the lumen, and thence by the ducts of the secreting gland to their destination. The amount of secretion is in some cases, as in that of the kidney, very largely influenced by the amount of blood reaching the organ, and by the blood pressure ; this again is dependent on the size of the blood- vessels, which is regulated by the vaso-motor nerves that supply their muscular tissue.'— Halliburton. Ciliated epithelium, besides being protective, also aids by its movements in pro- pelling fluids and minute particles from the body. ' See note on Microscope in Appendix. 14 Human Physiology 13. Basement Membrane. — Beneath the epithelium of certain parts appears a fine homogeneous membrane spoken of as basement membrane or membrana propria. That at the base of most mucous membranes and beneath the epithehum of secreting glands, however, is found to consist of a very thin layer of flattened cells belonging to the variety of tissue called connective tissue, now to be treated of. 14. Connective Tissues.— The term connective tissues in- cludes a number of tissues which at first sight appear to differ greatly, but which are, nevertheless, properly grouped together owing to their common function, origin, histological and chemical similarities. Their common function is that of connecting and supporting the other tissues, and they have a common origin, as they are all derived from the mesoblast in the developing body. The histological similarities or points of structure in common are three microscopic elements which exist in varying proportion in each, viz. a ground-substance or matrix, cells, and fibres. All varieties of connective tissue that contain white fibres yield gelatine on boiling. Moreover, these tissues may under certain circumstances replace each other or merge into one another. There are three chief varieties of connective tissue, the first variety being separated into six kinds. The following table gives a list of these tissues, with a few preliminary remarks about each : — I. Connective tissues proper. (I.) Areolar Tissue — This is distributed as an irreg'ilar meshwork or loose connective tissue through all parts of the body. It is found beneath the skin and mucous membranes, it forms a sheath for the muscles and the organs generally, binds the parts of organs together and different organs one to another. It contains cells and fibres (both white and yellow) em- bedded in a ground-substance. (2.) White Fibrous Tissue. — This forms the chief part of tendons and ligaments, is found in the true skin and the denser fascise binding down the muscles. It consists of bundles of white parallel fibres, the other elements being relatively unimportant. The fibres run for the most part in one or two directions, instead of interlacing in every direction as in areolar tissue. (3.) Yellow Elastic Tissue. — This is the variety of connective tissue in which elastic fibres preponderate. It is found in the ligamentum nuchae of quadrupeds, in the ligamenta subflava between the arches of adjacent vertebra, in the walls of the trachea and its branches, and with other textures in the walls of the blood-vessels. (4.) Retiform or Adenoid Tissue. — This is found chiefly in lymphatic glands and allied structures. Histology 1 5 (5.) Adipose Tissue or Fat.— This is developed from areolar tissue, the protoplasm of the cells being for the most part replaced by fat, the fibrous element almost disappearing. (6. ) Mucous or jelly like connective tissue, found in the vitreous humour of the eye and in the body during its early development. The above varieties of connective tissue are often referred lo as the fibrous connective tissues par excellence. II.— Cartilage or gristle, of which there arc three varieties : — (I.) Hyaline cartilage. (2. ) White fibro-cartilage. (3.) Yellow fibro-cartilage. III. — Bone and deniine. In all the forms of connective tissue it will be noticed that the intercellu- lar substance is greatly developed and the cells are much less numerous than in epithelium, epithelium having but little intercellular material. 15. Areolar Tissue. — The most widely distributed kind of connective tissue has a more or less open texture, and appears to the naked eye to consist of fine transparent threads and films intercrossing in every direction, and leaving, especially when stretched, open spaces or areolae between them. Hence the name given to it, areolar tissue. Its universal distribution in and around the other tissues and organs of the body is thus described in Quain's ' Anatomy ' : — 'Areolar Tissue. — It we make a cut through the skin and proceed to raise it from the subjacent parts, we observe that it is loosely connected to them by a soft filamentous substance of considerable tenacity and elasticity, and having, when free from fat, a white fleecy aspect ; this is the substance known as areolar tissue. In like manner the areolar tissue is found underneath the serous and mucous membranes which are spread over various internal surfaces, and serves to attach these membranes to the parts which they line or invest ; and as under the skin it is named ' ' subcutaneous, " so in the last-mentioned situations it is called ' ' subserous " and " submucous " areolar tissue. But on proceeding further we find this substance lying between the muscles, the blood-vessels, and other deep- seated parts, occupying, in short, the intervals between the different organs of the body where they are not otherwise insulated, and thence named "intermediate ; " very generally, also, it becomes more consistent and membranous immediately around these organs, and under the name of the " investing " areolar tissue, affords each of them a special sheath. It thus forms inclosing sheaths for the muscles, the nerves, the blood- vessels, and other parts. Whilst the areolar tissue might thus be said in some sense both to connect and to insulate entire organs, it also performs the same office in regard to the finer parts of which these organs are made up ; for this end it enters between the fibres of the muscles, uniting them into bundles ; it connects the several membranous layers of the hollow viscera, and binds together the lobes and lobules of compound glands ; it also accompanies the vessels and nerves within these organs, following their branches nearly to their finest divisions, and affording them support i6 Human Physiology and protection. This portion of the areolar tissue has been named the "penetrating," "constituent," or "parenchymal." ' It thus appears that the areolar is one of the most general and most extensively distributed of the tissues. It is, moreover, continuous through- out the body, and from one region it may be traced without interruption into any other, however distant ; a fact not without interest in practical medicine, seeing that in this way dropsical waters, air, blood, and urine, effused into the areolar tissues, and even the matter of suppuration, when not confined in an abscess, may spread far from the spot where they were first introduced or deposited. Fib. 14.— Subcutaneous Areolar Tissue from a young Rabbit. Hisrhlv magnified. (E. A. S.> The figure shows the appearance of the tissue examined perfectly fresh. The white fibres are in wavy bundles, the elastic fibres form an open network i i vacuolated cells (plasma-cells) ; g, granular cell ; c, c, branching lamellar cells ; c' a flattened cell of which only the nucleus and some scattered granules are visible •' f fibriUated cell. ' •'' ' On stretching out a portion of areolar tissue by drawing gently asunder the parts between which it lies, it presents an appearance to the naked eye of a multitude of fine, soft, and somewhat elastic threads, quite trans- parent and colourless, like spun glass ; these are intermixed with fine transparent films, or delicate membranous laminee, and both threads and laminae cross one another irregularly and in all imaginable directions leaving open interstices or areolse between them. These meshes are, of course, more apparent when the tissue is thus stretched out ; it is plain Histology 17 also that they are not closed cells, as the term " cellular tissue,'' which was formerly used to denote the areolar tissue, might seem to imply, but merely interspaces, which open freely into one another : many of them are occupied by the fat, which, however, does not lie loose in the areolar spaces, but is enclosed in its own vesicles. A small quantity of colourless transparent fluid of the nature of lymph is also present in the areolar tissue, bnt, in health, not more than is sufficient to moisten it. ' On comparing the areolar tissue of different parts, it is observed in some to be more loose and open in texture, in others more dense and close, according as free movement or firm connection between parts is to be provided for. ' Examined under the microscope, the transparent threads are seen to consist of wavy bundles of very fine parallel fibres {white fibres) running in various directions, with a few single branching fibres of another kind, known as elastic fibres. Besides these fibres several varieties of connective-tissue cells may be seen; (\) flattened connective-tissue corpuscles, with an oval nucleus, and often showing branching processes, c, c ; (2) plasma-cells, with the protoplasm markedly vacuolated, /,/ ; (3) granular cells, or cells containing distinct granules, g. In addition to the fixed connective-tissue cells, leucocytes iden- tical with white blood-corpuscles and lymph-corpuscles are often plainly visible (wander-cells). Cementing the white fibres together and forming the matrix or basis of the tissue between the bundles, is a clear homogeneous material contain- ing mucin, called the ground-substance. It is difficult to see in the fresh tissue, but, like the intercellular cement of epithelium, it is stained brown by silver nitrate. It is in depressions of the ground-substance on the surface of the white fibres or in the spaces — cell-spaces — of the ground-substance uniting the bundles that the fixed connective-tissue cells are found. Both the cells and the spaces in which they lie may intercommuni- cate by their branches, and thus bring into connection the superficial and deeper parts of the tissue. Blood-vessels, lymphatics, and nerves are conveyed in areolar connective tissue to the parts in which they are to be distributed. A few capillaries are distributed to the tissue itself, and numerous lymphatic networks are found in it. It is doubtful whether any nerves terminate in it, for it may be cut in a living animal without giving any pain, unless the instrument meets nerves passing through it to other parts. C 1 8 Human Physiolog)' i6. White Fibrous Tissue.— Besides forming the wavy bundles in areolar tissue, white fibres arranged in parallel bundles form compact bands or cords, the ligaments by which the bones are connected together at the joints, and tendons by which muscular fibres are affixed to the bones. They also form fibrous membranes, such as the periosteum covering the bones, the perichondrium covering cartilages, the dura mater lining the skull, and the fasciae or aponeuroses enveloping and binding together the muscles. In the true skin and mucous Fig. is.— Bundle of White Fibres of Areolar Tissue, partly unravelled. membrane the bundles of interlacing white fibres form a close felt work. Compact white fibrous tissue has a shining pearly aspect. It is exceedingly strong and pliant, but quite inelastic. Exa- mined with a high power, a small bundle of white fibres, when teased with a needle, is seen to consist of very fine transparent fibres or filaments from ^o-othj to -ss^-sts of an inch in thickness. These fine filaments do not occur singly, but are cemented into small bundles by a small quantity of the mucin ground-sub- stance. Each filament of a bundle seems parallel with its neighbouring filaments, neither branching nor uniting with others. Though tr3.nsparent when seen with transmitted light, Histology 19 the filaments in mass appear white. Acetic acid causes them to swell and become almost invisible. Boiled with water they are converted into gelatine. In fibrous tissue (tendon and ligament) the cells, termed Fig. 16. — Tendon of Mouse's Tail, stained with logwood, showing chains of cells between the tendon-bundles. 175 diameters. (E. A. S.) • tendon-cells,' are all flattened and arranged in rows, parallel to the bundles of fibres. The ends of these nucleated tendon- cells lie close together, the nuclei of adjacent cells often being in close proximity, as may be noticed in the figure. Fibrous tissue receives blood-vessels that run in tendons and ligaments between the longitudinal fibrous bundles, sending communicating branches across to form an open net- work. Lymphatics are abundant both in the enveloping sheaths of tendons and in the penetrating areolar tissue. Many tendons, ligaments, and fibrous sheaths possess nerve fibres, those of tendon often terminating in a special manner (par. 49). 17. Yellow Elastic Tissue. — Elastic tissue is that variety of connective tissue in which elastic fibres abound. These elastic fibres, already referred to as occurring in areolar tissue, are also found in small and variable propor- tions among the white fibrous bands and sheaths already mentioned, and between the bundles of white fibres of the dermis and mucous mem- branes. They form a fenestrated membrane in the middle coat of large blood-vessels, and are found in large numbers in the walls of the pulmo- nary alveoli. They also occur closely massed together in the elastic C2 Fig. 17.— Elastic Fibres of Areolar Tissue. 20 Human Physiology ligaments which extend between the arches of the vertebrje and in the ligamentum nuchas of the neck. When seen in quantity the elastic fibres have a yellowish appearance. They axe. distinguished from the white fibres not only by their elasticity and colour, but also by their sharper out- line, by joining or anastomgsing in their course so as to form a network, and by a tendency to curl up at the ends when broken across. Elastic fibres are best seen after treating a tissue with acetic acid. This causes the white fibres to swell and become indistinct, but the elastic fibres remain unaffected. Elastic fibres do not yield gelatine on boiling, but they yield or are composed of a different substance, elastin. Their size varies from gJgj to 2J505 of an inch, large fibres occurring in the ligamenta sub- flava of the vertebrae and very fine ones being found in the vocal cords. 1 8. Retiform or Adenoid Tissue. —This is a variety of areolar or connective tissue found in the spleen, in the tohsils, in lymphatic glands, and in many mucous membranes. It is com- FlG. i8.- Reticulum from the Medullary part of a Lymphatic Gland. (E. A. S.) tr, end of a trabecula of fibrous tissue ; r, r, open reticulum of the lymoh-path, con- tniuous with the fibrils of the trabecula ; r- , ^, denser reticulum of the medullary lymphoid cords._ The cells of the tissue are not represented, the figure being taken from a preparation in which only the connective tissue fibrils and the reticulum are posed of a very fine network or reticulum of fibrils continuous with white fibres of ordinary connective tissue, but having few or no elastic fibres. Around the fibrils of the network are wrapped the fixed cells of the tissue, so that it seems to be formed, when these cells are not cleared away, of stellate cells and their anastomosing processes. The meshes of the network are occupied with lymph and by numerous corpuscles which closely resemble lymph-corpuscles. These are known as lymphoid cells, and the tissue containing them is known as lymphoid or adenoid (gland-like) tissue. In the spleen the Histology 2 1 interstices of the retiform tissue are mostly occupied by blood instead of lymph. Retiform tissue thus forms the stroma or framework of lymphoid tissue. (See fig. i8.) 19. Adipose Tissue, or Fat. — Fatty tissue is distributed almost generally through the body, though collected more abundantly in certain positions. It forms in the healthy subject a layer beneath the skin, is gathered in quantity around the kidneys, fills up the furrows on the surface of the heart, exists in abundance in the marrow of the bones, but it is absent from the brain and lungs. Examined by the microscope, fat is seen to consist of cells or vesicles collected into lobules, Fig. ig. — A small Fat-lobule from the Subcutaneous Tissue of the Guinea-pig, Magnified about 20 diameters (£. A. S.) u^ small artery distributed to the lobule ; z/, small vein. The capillaries within the lobule are not visible. these lobules being collected into clusters, which appear like grains to the naked eye, the cells and lobules being held to- gether by a small amount of areolar tissue in which the blood- vessels ramify. Each lobule has its afferent artery, capillaries, and efferent vein, but no nerves. The cells in well- nourished bodies are round or oval, ^^th to ^-J^j^th inch in diameter. They are ordinary connective-tissue cells in which oily matter has been secreted from the blood, leaving the original proto- plasm of the cell as a mere envelope. The nucleus of the protoplasm may often be detected flattened out in this envelope. Fat consists of stearic, oleic and palmitic acids united with glycerine. The fat is fluid during life, but becomes 22 Human Physiology solid after death, often forming needle-shaped crystals. During starvation the fat may be absorbed and used up in the body, and then the cells become ordinary connective-tissue cells again. Adipose tissue serves as a protective packing material, prevents the heat of the body from passing away too rapidly, as it is a bad conductor, and furnishes a store of substance rich in carbon and hydrogen for use in the body. Fig. 20. — A few Cells from the margin of the Fat-lobule represented in the preceding figure. Highly magnified. (E. k, S.) J.g, fat-globule distending a fat-cell ; », nucleus ; w, membranous envelope of the fat- cell ; cr^ bunch of crystals within a fat-cell ; c, capillary vessel ; w, venule ; ci^ con- nective-tissue cell : the fibres of the connective tissue are not represented. 20. Cartilage. — Cartilage or gristle is a tough bluish-white substance, opaque in mass, but translucent in thin slices. Though of firm consist- ence it is very elastic, yielding readily to pressure or torsion, but recover- ing its shape when the constraining force is removed. On prolonged boiling it yields an albuminoid substance named chondritis which is closely allied to gelatine. No nerves have been found in cartilage and it is devoid of sensibility. It is also non-vascular, and derives its nourishment by imbibition of lymph which exudes from the neighbouring capillary vessels, i.e. from the vessels of the perichondrium or from those of the bone and synovial membrane in articular cartilage. All cartilage, except articular cartilage, is surrounded by a vascular membrane consisting chiefly of white fibrous tissue, and called the perichondrium. When a thin slice is examined under the microscope cartilage is seen Histology 23 to consist of nucleated cells and a firm ground-substance or matrix. The matrix is either without distinct structure, i.e, homogeneous like ground glass, or fibrous. Two varieties of fibrous are found, one in which the fibres are white like those of white fibrous tissue, and the other in which the fibres are yellow like those in yellow elastic tissue. Cartilage may therefore be divided thus : — ( Costal (I.) Hyaline . . ■ ■ \ Articular L Temporary (2.) Fibro-cartilage . f White 1 Y< ellow. ( I. ) Hyaline cartilage (Gk. hualinos, of glass) occurs in the adult, form- ing the costal cartilages, the nasal cartilages, investing the ends of bones ct '3jdii^ OtB* OK- -rte. -J^ ^»-a» ■gi^j^ oa™- *'- -30- ■30 -? -=!&)>. ^S-5-> ^I- ^fe> ^•™ -^ DC ^g - J. •»r ? Sa 0® ffi& '" #" & % a® ft® c 5) ft i 'e £-3 4 - a 1- *. i. 1 ■ ■a ? ■ C3 "^ H ■^ "5 8 3 Fig. 21. — Vertical Section of Articular^ Cartilage covering the lower end of the Tibia, Human. Magnified about 30 diameters, ff, cells and cell-groups flattened conformably^ with the surface ; ^, cell-groups irregularly arranged ; c, cell-groups disposed perpendicularly to the surface ; d^ layer of calcified cartilage ; ^, bone. at the joints, entering into the structure of the larynx, trachea, and bronchi. A temporary form exists in the foetus and young animals which is destined tp be replaced with bone by the deposition of lime salts. Examined under the microscope, hyaline cartilage is seen to consist of rounded, oval, or irregular cells, lying in what appears a homogeneous matrix resembling ground glass. In articular cartilage the cells are scattered in groups of two, four, or eight through the matrix, the arrangement of the cell groups being vertical near the surface of the bone, bat horizontal near the outer surface of the cartilage where it joins the synovial membrane (see fig. 21). The protoplasm of the cells under high powers shows fibrils and granules, and 24 Human Physiology its nucleus has the usual intranuclear network. The part of the matrix next the cartilage-cell forms a capsule, and as the cells multiply by- indirect division a new capsule soon forms around each, the primary capsule disappearing. It is thought by some observers that fine channels in the matrix pass from one cartilage cell to another. In the costal carti- lages the cells are often larger and contain a little fat, while the matrix is more distinctly fibrillated. (2.) White fibro-cartilage is found where great strength and a certain amount of rigidity are required. It occurs [a) connecting cartilage form- ing part ot the intervertebral discs ; {b) as interarticular discs between the ordinary articular cartilages of certain joints, e.g. the knee-joint ; (c) as marginal cartilages round the rim of the shoulder and hip-joints ; {d) as lining the groove in which certain tendons of muscle glide. Under the microscope white fibro-cartilage sjows a matrix composed of many fibres, in which cartilage-cells appear. (3.) Yellow fibro-cartilage, or elastic cartilage, is found in the epiglottis, the cornicula of the larynx, in the outer ear, and in the Eustachian tube. It is more flexible and tough than the hyaline cartilage, and the matrix in which the cells lie is composed mainly of fine elastic interlacing fibres. Cartilage has several important uses. It assists in binding bones together and yet allowing a certain degree of movement ; it acts as a buffer to deaden shocks ; it reduces friction at joints ; it serves to keep open and maintain the shape of tubes, as in the trachea ; it forms attachments for muscles and ligaments ; and it serves to deepen joint cavities. The costal or rib cartilages form an important part of the framework of the thorax and impart elasticity to its walls. 21. Bone— Physical Properties and General Structure. — With the exception of enamel, bone is the hardest structure of the body, and yet it possesses considerable toughness and elasticity. In its fresh state it is pinkish-white externally, though redder within. On cutting through a bone the eye easily sees two kinds of bony tissue, a hard and compact tissue like ivory forming the outside shell, and a looser tissue internally, with fibres and thin plates, having the appearance of lattice-work and hence called cancellous tissue, a gradual transi- tion from one kind to the other being apparent. In a long bone the large round end consists of the cancellous tissue with a thin coating of compact tissue ; in the hollow shaft of the long bone the sides are almost entirely of compact tissue. In tabular or flat bones we find two layers or plates of compact tissue at the surface and a spongy texture known as diploe between. A fresh or living bone is covered by an outer tough fibrous membrane called the periosteum. From a close network of blood-vessels in the periosteum branches find their way through small openings on the surface of the bone, mainly to Histology 2 5 nourish the compact tissue, running in the bone through tiny channels called Haversian canals. By stripping the periosteum from the surface of living bone, small bleeding-points indicating the entrance of the periosteal vessels may be seen, and a longi- tudinal section of a long bone not only shows that the substance of the bone must be well supplied with blood, as it is seen to exude in all parts, but the interior of such a bone is seen to be Fig. 22. — A. Transverse Section of a Bone (Ulna) deprived of its earth by acid. (Sharpey.) The openings of the Haversian canals are seen. Natural size. A small portion is shaded to indicate the part magnified in Fig. B. B. Part of the Section A, magnified 20 diameters. The lines indicating the concentric lamellee are seen, and among them the lacunee appear as little dark specks. occupied by a cylindrical cavity filled with marrow, the marrow being surrounded by a vascular areolar envelope lining the medullary cavity, and termed the endosteum. Besides the blood-vessels that pass from the periosteum, long bones have 26 Human Physiology a nutrient artery entering at some part of the shaft which, passing into the medullary canal, breaks up in the marrow. Other small vessels reach the articular extremities to supply the cancellous tissue, for during life the spaces of the cancellous tissue contain marrow and blood-vessels. 2 2. The Periosteum. — The periosteum adheres very firmly to the bone and invests every part except at the joints where it is covered with cartilage. Its chief uses are to support the Fig. 23. — Transverse Section of Compact Tissue (of Humerus). Magnified about 150 diameters. (Sharpey.) Three of tlie Haversian canals are seen, with their concentric rings ; also the lacunse, with the canaliculi extending from them across the direction of the lamellee. The Haversian apertures had_ become filled with air and debris (from the grinding), and therefore appear black in the figure, which represents the object as viewed with transmitted light. (In a longitudinal section the Haversian canals would be seen uniting with one another. ) vessels going to the bone and to afford attachment for the tendons and ligaments where they are fixed to the bone. It consists of two layers : {a) an outer layer, chiefly of white fibres, in which the blood-vessels ramify before entering the compact bony tissue ; {b) an inner layer consisting largely of elastic fibres and granular cells called osteoblasts or bone-forming cells. Fine nerves and lymphatics are found in the periosteum and accomuany the arteries into the bone. Since bony tissue is Histology 27 nourished in great part by vessels that leave the periosteum, it is apt to die if this membrane be removed. 2?.— Minute Structure of Bone.— Besides the spaces in the cancellous or spongy tissue of bone formed by the reticulated spicules of bone, com- pact bony tissue is permeated by anastomosing channels called Haversian canals. Fig. 22, A, represents a cross section of a long bone near the middle, and on examining this with a hand-glass the openings of these longitudinal passages may be seen. Their average diameter is about jjoth of an inch, though wider near the medullary cavity and less near the outside of the bone. Fig. 22, B, shows a small part of A magnified. Here the Haversian canals are seen to be surrounded by an appearance of concentric rings. This appearance is caused by the trans- verse section of thin tubes or cylin- ders of bony tissue (spoken of as concentric plates or lamella) fitting inside one another and surrounding the Haversian canal. Besides the concentric lamelte there are other thin layers of bony tissue arranged parallel to the surface as at a, while others run between the Haversian sets. Compact bone is in fact made up of these bony lamellae closely applied and lying in various direc- tions. The Haversian canals are minute channels through the com- pact tissue, the bony lamellae around them having a concentric arrange- ment. In a longitudinal section the Haversian canals are seen to be but short, as they soon unite with others, and thus form a network of tubes in the compact bone. All over the section little dark specks are seen among the lamellae. These are in reality minute cavities named lacunce. With a higher power these lacunse are more dis- tinctly seen, while extending from them are minute tubes named canaliculi. The canaliculi pass across the lamellse and connect neighbouring lacunse with one another and with the Haversian canal. Most of the Haversian canals contain two small blood-vessels, nerve- fibres, and a lymphatic vessel, and they communicate externally with the periosteum and internally with the marrow. Each Haversian canal, with its concentric lamellze, lacunae, and canaliculi, form what is known as a Haversian system. The lacunae are situated between the lamellae, and each lacuna is occupied by a bone-cell, a flattened nucleated cell that sends processes into the canaliculi. The plasma of the blood that passes through the capillary walls passes into the dense bony substance by means ot the . Haversian Canal, Highly magnified. Fig. 24. — Section of e showing its contents. (E. A. S.) rf, small arterial capillary vessel ; w, large venous capillary ; w, pale nerve-fibres cut across ; /, cleft-like lymphatic vessel : one of the cells forming its wall communicates by fine branches with the branches of a bone-corpuscle. 1 he substance in which the vessels run is connective tissue with ramified cells ; its finely granular appear- ance is probably due to the cross section of fine fibrils. 28 Human Physiology lacunae and canaliculi, the bone-cells playing, no doubt, an important part in the nutritive process. The lamellEe of which we have spoken are formed of fine fibres decus- sating'so as to form a close network. By peeHng off thin films from the surface of a bone that has been softened in acid and examining it under the microscope, this fibro-reticular structure can be made out. In many places the various lamellae may be seen to be bolted or held together by larger tapering fibres known as perforating fibres. Bone, in fact, is a variety of connective tissue, and consists of (i) cells or branched bone-corpuscles, (2) interlacing and decussating fibres, (3) a ground-substance in which calcium salts are deposited. The lamellae are made up of the fine fibres just mentioned lying in calcified ground-substance, while the cells occupy the lacunae between the lamells, and send processes into the canaliculi. Fig 25 —Lamella: torn off from a Decalcified Human Parietal Bone at some depth from the surface. (Sharpey.) a, lamelte, showing decussating fibres ; b, b, thicker part, where several lamella; are nn^tJl","., '/' ^' P='f°ff"."g P"-es. Apertures through which perforating fibres had passed are seen especially m the lower part, a., a, of the figure. Magnifude as seen under a power of 200 but not drawn to a scale. (From a drSwing by Allen Thomson.) 24. Chemical Composition of Bone.— Bone consists of mineral and organic matter in intimate combination. The former gives hardness and rigidity, the latter tenacity. The mineral matter can be removed by maceration in dilute acids, and the bone thus decalcified is softer, tougher, and more flexible. The organic or animal part may be burnt out, when the bone will retain its original form but be white and brittle. The animal part may be resolved into gelatine on boiling. The Histology 29 mineral 'Or inorganic matter forms about two-thirds of dry bone, and the animal matter one-third. In undried bone, without separation of marrow or blood, there is nearly 50 per cent, of water. The chief organic constituent of bone is col- lagen, which is changed into gelatine on boiling with water ; the chief inorganic constituent is calcium phosphate, Ca3(P04)2. The following table gives the percentage composition of dry bone : — Organic matter 33 '3° I Calcium phosphate . . .51 '04 ,, carbonate . . . 11 30 ,, fluoride . . .2-00 Magnesium phosphate . . i-i5 Sodium salts . .1-20 25. The Marrow. — There are two kinds of marrow found in bone, yellow and red. Yellow marrow fills the medullary cavity and some of the larger cancelli of long bones. It consists chiefly of fat-cells with blood- vessels and a small amount of connective tissue. Red marrow is found in the cancellated tissue of long bones, in the diploe of the bones of the cranium, in the bodies of the vertebrae, the sternum and the ribs. Red marrow consists of delicate connective tissue, numerous small blood-vessels, and a large number of round nucleated cells called mamrw-cells, some large cells known as giant cells or myeloplaxes, and a number of small nucleated cells of a reddish tint. These coloured cells are termed erythroblasts, and resemble nucleated coloured corpuscles found in the embryo. They multiply by karyokinetic division, and are believed to be the cells from which ordinary red blood-corpuscles are developed in the adult, their transformation into biconcave discs being accompanied by a disappearance of the nucleus. They are thought to get into the circulation by passing into the capillaries of the marrow, the walls of which are exceedingly thin, or even imperfect in some places. (See par. 56.) 26. Secreting Glands — Secreting glands are organs in which nucleated cells, spread over a surface in the form of an epithelium, separate or secrete from the blood, or rather from the lymph exuded from the neighbouring capillaries, certain raw materials to form the substances they discharge. The secreting cells are supported by a small amount of connective tissue forming a basement-membrane (par. 13), which may either be continuous or form a mere network, and this basement- membrane (or the cells themselves) is surrounded by lymph that has passed out of a plexus of capillary blood-vessels in close proximity. The essential parts, therefore, are active cells and capillary blood-vessels in close relation to supply the fluid 30 Human Physiology from which certain constituents are drawn to form the secre- tions. A cell charged with its selected or converted contents delivers up its secretion either by exudation or by the burst- ing and destruction of the cell itself. D E Fig. 26. —Diagrammatic Plan of Varieties of Secreting Glands. A. Simple gland. B. Sacculated simple gland. C. Simple convoluted tubular gland. D. Racemose gland. E. Compound tubular gland. The simplest form of a secreting surface is a plain and continuous one, as in various parts of certain mucous mem- branes, but in what are usually called secreting glands an increase of surface is obtained by a recession or inversion of the membrane in the form of a cavity. The tube or cavity, of Histology 3 1 whatever shape, terminates in a blind extremity or extremities, and is lined by secretory epithelial cells having on their outer surface a plexus of capillary blood-vessels. In the simplest form a single involution takes place, consti- tuting a simple gland ; this may be either in the form of an open tube (fig. 26, a) or the walls of the tube may be dilated so as to form a saccule (fig. 26, b). These are named the simple tubular or saccular glands. Or, instead of a short tube, the involution may be lengthened to a considerable extent, and then coiled up to occupy less space. This constitutes the simple convoluted tubular gland, an example of which may be seen in the sweat glands of the skin (fig. 26, c). If, instead of a single involution, secondary involutions take place from the primary one, as in fig. 26, d, the gland is then termed a compound one. These secondary involutions may assume either a saccular or tubular form, and so constitute the two subdivisions —the compound saccular or racemose gland, and the compound tubular. The racemose gland in its simplest form consists of a primary involution which forms a sort of duct, upon the extremity of which are found a number of secondary involutions, called saccules or alveoli, as in Brunner's glands. But, again, in other instances, the duct, instead of being simple, may divide into branches, and these again into other branches, and so on; each ultimate ramification terminating in a dilated cluster of saccules, and thus we may have the secreting surface almost indefinitely extended, as in the salivary glands (fig. 26, d). In the compound tubular glands the division of the primary duct takes place in the same way as in the racemose glands, but the branches retain their tubular form and do not terminate in saccular recesses, but become greatly lengthened out as in the kidney (fig. 26, e). In the simple gland the blind termination of the duct is often called the 'fundus,' but in the compound glands we have duct, intermediate portions, and terminal recesses termed 'alveoli' or 'acini.' Racemose glands have often a lobular structure, the lobules being held together by the branch- ing ducts and interlobular connective tissue. Their alveoli or acini are sometimes almost filled by the secreting cells, only a 32 Huvtan Physiology very small cavity being left in the centre to communicate with the excretory duct that ultimately opens into a cavity lined by mucous membrane or on the surface of the skin. Besides blood-vessels glands have lymphatic vessels that begin in the lymphatic space around the alveolar cells. Nerves are found in the connective tissue, and it has been asserted that in some cases they terminate in the secreting cells. The varying appearance of gland cells when loaded with their special products and when empty, as well as the influence of the nervous system on the secretory process, will be described later (figs. Ill, 114, &c.). The pressure from behind, or vis a tergo, of the accumulating secretion is believed to be the main cause of its discharge, though the contraction of the muscular tissue in the wall of the duct where it exists doubtless gives some aid. 27. Mucous Membrane. — Mucous membranes are the soft and highly vascular membranes that line the internal passages communicating with the exterior. They are moistened with a transparent slimy alkaline fluid termed mucus, which is discharged from secreting cells on the surface of the membrane or in special glands communicating with the surface. Mucus consists of 95 per cent, of water, and contains a small quantity of a peculiar compound (probably globulin with animal gum) termed mucin. Mucin has a ropy consistence, and is precipi- tated with alcohol. The other solids are mineral salts and organic bodies. The various mucous membranes will be described in detail when treating of the organs of which they form a part, but the following general remarks from Gray's ' Anatomy ' may be now studied with advantage : — ' Mucous membranes line all those passages by which the internal parts communicate with the exterior, and are continu- ous with the skin at the various orifices of the surface of the body. They are soft and velvety, and very vascular, and their surface is coated over by their secretion, mucus, which is of a tenacious consistence, and serves to protect them from the foreign substances introduced into the body with which they are brought in contact. ' They are described as lining the two tracts— the gastro- Histology 33 pulmonary and the genito-urinary ; and all, or almost all, mucous membranes may be classed as belonging to and continuous with the one or the other of these tracts. ' The external surfaces of these membranes are attached to the parts which they line by means of connective tissue, which is sometimes very abundant, forming a loose and lax bed, so as to allow considerable movement of the opposed surfaces on each other. It is then termed the submucous tissue. At other times it is exceedingly scanty, and the membrane is closely connected to the tissue beneath ; sometimes, for example, to muscle, as in the tongue ; sometimes to cartilage, as in the larynx ; and sometimes to bone, as in the nasal fossae and sinuses of the skull. ' In structure a mucous membrane is composed of corium or DERMIS and epithelium. The epithelium is of various forms, including the squamous, columnar, and ciliated, and is often arranged in several layers. This epithelial layer is supported by the corium, which is analogous to the derma of the skin, and consists of connective tissue, either simply areolar, or contain- ing a greater or less quantity of lymphoid tissue. This tissue is usually covered on its external surface by a transparent structureless basement-membrane, and internally merges into the submucous areolar tissue. It is only in some situations that the basement-membrane can be demonstrated. The corium is an exceedingly vascular membrane, containing a dense network of capillaries, which lie immediately beneath the epithelium, and are derived from small arteries in the submucous tissue. ' The fibro-vascular layer of the corium contains, besides the areolar tissue and vessels, unstriped muscle-cells, which form in many situations a definite layer, called the muscularis mucosae. These are situated in the deepest part of the membrane, and are plentifully supplied with nerves. Other nerves pass to the epithelium and terminate between the cells. Lymphatic vessels are found in great abundance, commencing either by caecal extremities or in networks, and communicating with plexuses in the submucous tissue. ' Embedded in the mucous membrane are found numerous D 34 Human Physiology glands, and projecting from it are processes (villi and papilte) analogous to the papillae of the skin. These glands and processes, however, exist only at certain parts, and it will be more convenient to defer their description to the sequel, where the parts are described as they occur.' CHAPTER II MUSCULAR AND NERVOUS TISSUE 28. We soon learn that the movements of the body are produced by means of muscles in the limbs and trunk, and that some of the internal organs are also made up in great part of muscular tissue. A special property of this tissue is its power of contraction under an excitation or stimulus ; and since we find that all the muscles are supplied with nerves, we come to the conclusion that contraction normally takes place as the result of a nervous impulse arriving at a muscle. The close connection of these two tissues leads us to treat them together in a separate chapter. Besides, both the muscular and nervous systems intervene so often in the great functions of circulation, respiration, digestion, and secretion that some preliminary acquaintance with these systems is necessary before treating of these functions. The two chief varieties of muscular tissue are — (i) unstriped or smooth muscular tissue, called also involuntary because it is found in organs whose contraction is not under the control of the will; (2) striped ox striated muscular tissue, called also volun- tary because it is found in the skeletal muscles, which are under the control of the will. Striped muscle is also found in certain parts of the internal ear and in the upper half of the pharynx. Intermediate in structure and properties is the striated muscle of the heart. Ordinary striped muscle contracts most quickly; next comes the muscle of the heart ; and plain or unstriped muscle contracts most slowly. 29. Unstriped or Plain Muscular Tissue.— Unstriped Muscular and Nervous Tissue 35 muscle is found in the muscular coat of the alimentary canal below the middle of the oesophagus, in the trachea and bronchi, in the middle coat of the arteries, the coats of many veins and the larger lymphatics, in the bladder and ureters, and in the ducts of glands. It also occurs in the iris and ciliary muscle, and in the true skin, especially between the bases of the papillae. When it contracts in the skin under the influence of cold or fear, or any other stimulus, it causes the papillae to become unusually prominent, thus producing the peculiar roughness termed 'goose skin.' (See par. i6i.) Plain or non-striated muscular tissue is composed of spindle- shaped nucleated cells, somewhat flattened, and having a length seldom more than -5^ of an inch and a breadth about one- eighth of the length. The oval nucleus exhibits the intranuclear network and one or two nucleoli. The cell-substance is longitudin- ally but not transversely striated, and each cell seems to have a dehcate sheath. Between the fibres there is a small quantity of cementing substance. The fibres are collected together into bundles ensheathed in the con- nective tissue. The small blood- vessels run in the connective tissue, and from these capillaries pass between the individual fibres. Non-meduUated nerves are supplied to plain muscular tissue from the sympathetic or ganglionic system, and this tissue responds but slowly to a stimulus, the contraction last- ing several seconds and spreading as a wave from fibre to fibre. 30. Cardiac Muscle. — The heart, though an involuntary muscle, contains fibres that present transverse marks or striae. But the fibres are smaller than those of voluntary muscle, and Fig. 27.- Muscular Fibre-cells from Human Arteries. Magnified 350 diameters. (KuUiker.) (i, rf, nucleus ; B, a cell treated with acetic acid. 36 Human Physiology the striae are not so well marked. The fibres consist of quad- rangular cells joined end to end, and many of these have branches uniting with neighbouring fibres. Each cell has a clear oval nucleus near the centre, with one or more nucleoli (fig. 28). The cells have no investing sheath. Like the skeletal muscles, the heart has an abundant supply of blood-vessels. It has also a rich supply of lymphatic vessels, occupying the interstices of the muscular network. Its nerve-supply will afterwards be considered (par. 83). 31. Voluntary or Striated Muscle. — Each voluntary muscle Fig. 29. — Transverse Section from the Sterno-mastoid in Man. Magnified 50 times. ei, external perimysium ; i, fasciculus ; C", internal perimysium ; rf, fibre. Fig. 28 —Muscular Fibres from the Heart, magnified, showing their cross striae, divisions, and junctions. (Schweigger- Seidel.) The nuclei and cell-junctions are only represented on the right-hand side of the figure. in reality forms an organ, com- posed chiefly of a mass of con- tractile fibrous tissue called muscular, and of other tissues and parts that may be regarded as accessory. Such a muscle as the biceps or gastrocnemius, for instance, consists of a mass of red flesh surrounded by a sheath of areolar connective tissue called the external perimysium, and from the inner surface of this divisions pass off into the interior and inclose bundles of fibres named fasciculi. The fasciculi are of various sizes, each fasciculus having its sheath of perimysium, the tissue extending even between the individual fibres as endomysium. Besides Muscular and Nervous Tissue 37 serving to bind together the fibres and fascicuH, the areolar tissue just described serves to conduct and support the blood- vessels and nerves in their ramifications in the muscle. This areolar tissue between the fibres and bundle is also continuous with that of the tendon in which the muscle terminates, a further connection between muscle and tendon being effected by the fibres of the tendon becoming united with the sarcolemma Fig. 30.— Two Human Muscular Fibres. Magnified 350 times. In the one, the bundle of fibrillse © is torn, and the sarcolemma (a) is seen as an empty tube. Fig. 31.— Muscular Fibre of a Mammal, examined fresh in serum. Highly magni- fied. (E. A. S.) This figure was drawn with the surface layer of muscular substance accurately focussed, the lateral portions having been added by gradually sinking the focus. The nuclei are seen on the flat at the surface of the fibre, and in profile at the edges. or sheath of the muscular fibre. In the body of the muscle the fibres, which are a little over an inch in length, often fail to reach an end ; they are then connected with the fibres of the connective tissue by their sarcolemma. When a muscle con- tracts, therefore, the fibres which end directly in tendon pull on the tendon, while those that do not so end pull on the tendon indirectly by means of the connective tissue in the 38 Human Physiology body of the muscle, for this tissue is continuous with that of the tendon. Tendons are attached to the periosteum of bones. 32. Muscular Fibrea.— We now turn to the individual muscular fibre : for the structure, composition, and properties of the fibres constitute knowledge of muscular 'tissue. By careful teasing a shred of muscle with a needle single fibres may be isolated and examined under the microscope. A muscular fibre is usually cylindrical in shape, on the average jjjth in diameter and a little more than an inch long. Each fibre has a trans- parent elastic sheath called the safcokmma (distinct from the areolar tissue previously mentioned), inclosing a contractile substance. The contractile substance is marked by alternate dim and light stripes running across the fibre, which accounts for the name striped or striated muscle. A dark line, known as ' Dobie's line ' or ' Krause's membrane,' can be seen with very high powers passing through the clear band, while through the centre of the dim band a clearer line, called ' Hensen's line,' is sometimes visible in an extended condition. On carefully focussing the surface of a fibre, rows of apparent granules are seen at the boundaries of the light streaks, w'' " ' '' '' Mines uniting the granules. These lines are Fig. 32. — Transverse Sections of Muscle Fibres. (E. A. S.) A. Transverse Section of a Mammalian Muscular/Fibre showing Cohnheim's areas. Alcohol preparation. Three nuclei are \ isible under the sarcolemma. B. An isolated Disk of Leg-muscle of a Beetle treated with dilute acid. The disk is seen partly on the flat, partly in profile, and exhibits the net-like appearance of the sarcoplasm in the transverse section of the iibre ; the meshes represent the areas of Cohnheim. most conspicuous in the muscles of insects. They indicate the longitudinal elements or muscle-columns (sarcostyles) which compose a fibre, and are probably due to an interstitial substance between the columns termed sarcoplasm. If a transverse section of a fibre be examined with a high power, it is seen to be subdivided into small angular parts, the areas of Cohnheim. These represent sections of the muscle-columns of which the fibre is composed. After death, or on being hardened with certain reagents, as chromic acid or alcohol, a fibre may easily be split up into smaller threads ox fibrilla,^ a term that is here used to signify the same as the term muscle-columns or sarcostyles above. Filires may also, with suitable ' The X.ex'ca. JU'rilla; at fibrils is used by some au'.hors to indicate still nner elements, which are supposed to constitute the muscle-columns them- selves. Muscular and Nervous Tissue 39 reagents, be split up into transverse discs. Each fibre thus appears to be composed of a mass of fibrils (sarcostyles) embedded in interfibrillar substance or sarcoplasm, and composed of alternate segments of dim and clear substance. The columns of muscular substance are the actual con- tractile elements of the tissue. Besides the tubular sheath of sarcolemma and the striated substance, a muscular fibre shows reticulated nuclei surrounded by a variable amount of granular protoplasm. A nucleus with its adjacent protoplasm is called a muscle-corpuscle. 33. Blood-vessels of Muscular Tissue. — The muscles are abundantly supplied with blood-vessels. The arteries, accom- Fiu. 33.— Living Leg-muscle ofWater Beetle. Highly magniBed. (E.A.S.) J, sarcolemma ; a, dim stripe ; hj bright^ stripe ; c^ row of dots in bright stripe, which are enlargements or thickenings on the longi- tudinal septa of sarcoplasm. These septa are represented by tlie longitudinal lines, d. The continuity of these lines through the bright stripe is difficult to see in the fresh fibre, but after treatment with acid it becomes quite distinct. Fig. 34. — Capillary Vessels of Moderately magnified. (E Muscle. A. S.) panied by the associated veins, enter at various points, branch in the areolar tissue between the fasciculi, and at length terminate in capillaries that form an oblong network around the fibres on the outside of the sarcolemma. These capillaries are very small, and from them is exuded the fluid by which the muscular tissue is nourished. There is also in the connective 40 Human Physiology tissue lymph spaces that form the commencement of the lymphatic vessels of the tissue, so that every muscular fibre is surrounded by capillary blood-vessels and lymph spaces, and the nutriment passes from the blood by means of the lymph through the sarcolemma to the substance of the fibre, waste products passing from the fibre into the blood in the opposite direction. During muscular action the blood-supply of a muscle is increased, for the arteries are so arranged that they do not undergo compression during contraction, while the venous blood and lymph are squeezed out as the muscle contracts. 34. Nerves of Muscular Tissue. — Voluntary muscular tissue receives a supply of nerve-fibres, chiefly medullated, from the cerebro-spinal system. The nerves branching in the connective tissue first form plexuses, and then gradually divide until a single nerve-fibre enters each muscular fibre, the primitive sheath of the nerve-fibre fusing with the sarcolemma, while the axis cylinder of the nerve passes through it and ends in a terminal ramification called an end-plate on the substance of the fibre (fig. 35). Each fibre appears to receive one end-plate about its middle, and as the fibres of a muscle are but about an inch in length, the end-plates are distributed over the length of the muscle ; so that a nervous impulse along the different fibres going to a muscle reaches the different parts about the same time, and sets up a simultaneous contraction in the organ. Plain muscle is supplied with non-medullated nerve-fibres on which knob-like endings have been described in some cases. Fig. 35-- -Motor Nerve-endings in Snake's Muscle. (Waller.) 35. Chemical Composition of Muscle.— From a perfectly fresh muscle the contractile semi-fluid substance of the fibres can be squeezed out from the sheaths of sarcolemma, and it is then called muscle-plasma. Muscle-plasma Muscular and Nervous Tissue 41 is a slightly alkaline viscid fluid which, if exposed to the ordinary temperature of the air, coagulates, separating after some time into a transparent muscle- clot and a watery fluid, muscle-serum. Muscle-clot consists of a substance called myosin, which is a proteid substance belonging to the class called globulins, as it is coagulable by heat and soluble in saline solutions. The formation of myosin from muscle-plasma is probably due to a ferment, just as in the case of the formation of fibrin from blood -plasma. In the formation of myosin by the clotting of muscle-plasma an acid called sarco- lactic acid is developed, no acid being formed during the formation of fibrin in blood clotting. The serum contains three different proteid bodies, some extractives, and two pigments, haemoglobin and myohaematin. After a muscle has been removed from the body some time, or even in the body after general death, the muscular tissue becomes dead. When a muscle dies it becomes rigid and more opaque, loses its irritability, and undergoes chemical changes. Rigor mortis (the stiffness of death) is the condition of the muscles that follows the death of man or other animal, and it is due to the coagulation of the muscle-plasma within the sarco- lemma sheaths of the muscular fibres. The onset of this rigidity in man varies from some minutes to a few hours, though it is usually complete in four or five hours after death. It is accompanied by evolution of COj and heat. After a time — one to five days — it passes away and the muscles become soft and flaccid. The earlier it occurs the sooner it passes off. The cause of the disappearance is said to be due to the putrefaction that begins. A living muscle derives nutriment for building up the complex coagu- lable plasma from the lymph that passes out of the blood — the lymph holding in solution the various kinds of food-material taken up in the ali- mentary canal and oxygen absorbed in the lungs — and continually gives off CO.j and nitrogenous waste. When in action it consumes an increased quantity of oxygen, produces more carbonic acid, iis well as some other products, and acquires an acid reaction. On passing into a condition of rigor mortis the complex living molecule breaks up, a solid clot of myosin forms, a sudden increase of COj occurs, and sarcolactic acid appears. 36. Muscular Contraction. — By removing the gastrocne- mius muscle from a recently killed frog, whose tissues retain their vitality longer than those of a warm-blooded animal, the living muscular substance may be submitted to experiment. The muscle is left attached to the femur above and to the tendo Achillis below, with a length of the sciatic nerve that goes to the muscle exposed. Such an isolated muscle with its attached nerve is called a muscle-nerve preparation. On stretching such a muscle it is found to be extensile and elastic. Its elasticity is slight but perfect, that is to say a sinall force will extend it, but it returns exactly to its original length when the force is removed. The elongation of a muscle is not proportional to the weight used, but diminishes in proportion as the weight 42 Human Physiology increases. But the most important property of muscle in the Hving state is its contractility or power of shortening when irritated by a stimulus, its volume or bulk remaining the same. Normally the muscle contracts in response to a stimulus from the central nervous system, but other kinds of excitation or stimuli may be applied. These may be mechanical (pricking or pinching), thermal, chemical, or electrical, and the stimulus may be applied to the nerve going to the muscle or to the muscular tissue direct. It should be noted that a healthy muscle when at rest is contracted or retracted to a small degree, this small amount of contraction being known as muscular tone, and is due to the influence derived from the central nervous Fig. 36. -(Waller.) /, lever cut short passing to drum : m, wires from secondary coil of battery to muscle, nt \ N, wires across which the nerve of the muscle is laid. (For a drum see fig. 73.) system, for on cutting its nerve the muscle becomes longer and more relaxed. That the muscular fibres have in themselves the power of contraction is proved by the use of the drug curare, which paralyses the motor nerve endings in muscle, whilst the muscular tissue still responds to direct stimulation. Ammonia also destroys the nerve and yet stimulates muscular tissue. To study the phenomena of contraction a graphic repre- sentation (called a muscle-curve) of the contraction and relaxa- tion of the muscle is obtained by a myograph. This instrument consists of a drum covered with a coat of lampblack revolving at a definite rate, against which the point of a light lever attached by a thread to a muscle can be brought. When the drum is Muscular and Nervous Tissue 43 moving and the muscle is quiet a horizontal white line is drawn on the paper ; but when the muscle contracts the lever rises falling again during relaxation (see fig. 36 and 37). Fig. 37 represents a muscle-curve obtained by sending a single induction shock from an electrical apparatus through the sciatic nerve to the gastrocnemius muscle of a frog, the time occupied in tracing the curve being indicated by the small curves below, each of which represents -pUth of a second. A study of the curve of a simple muscular contraction or twitch shows three phases :— (i.) A phase termed the latent ieriod, during which no apparent change takes place. This interval is said to be taken up by the propagation of the impulse along the nerve and by the preparatory changes in the muscle. It be- comes longer as the muscle becomes fa- tigued. (2.) A phase of shortening or contrac- tion, during which the _ . c.- , „ , ^ ■ ,„ , . _, , . , Fig. 37.— A Single Muscular Contraction. (Frogs lever rises. The height Gastrocnemius.) nf rnntrnrt-inn Himin Fro]" J '° = .'s *e latent period ; from 2 to 3 the period 01 coniraction aimin- of shortening; from 3 to 4 the period of relaxation. ishes as the muscle tires, though the period remains the same. (3.) A phase of relaxation of the muscle or return to its original length, during which the lever falls. This period also becomes longer as the muscle tires. From the time-marking below it is seen that the latent period in a fresh muscle occupies about TB-sth of a second, the phase of contraction ^-J^j, and the phase of relaxation y|^ second. A single muscular twitch is thus completed in about -[^uth of a second, but it must be noted that the time varies, being much longer in a fatigued muscle. By sending in a second shock before relaxation sets in we shall get a second curve added to the first, and on increasing the rapidity of the shocks each succeeding contraction will start 44 Human Physiology from some part of the preceding one and raise the lever to a greater height. When the shocks increase more rapidly still, the individual shocks, visible at first, fuse together into one continuous curve, the lever having attained a maximum height, where it remains until the muscle is exhausted if the shocks continue. A muscle in a state of continuous contraction is said to be in a condition of spasm or tetanus. Tetanus may occur as the result of disease (lockjaw) or be produced by poisons such as strychnine. A fatigued and exhausted muscle cannot act owing to the accumulated products of action, and if left to JC j)e r sec. '^^:^..,f^^mmM\mNm i/ V V V Fig. 38,— Composition of Tetanus. Stimuli caused by a spring interrupting primary circuit by vibrating in and out of a mercury cup ; the vibration frequency is increased by shortening the spring. rest, may recover by the circulating blood bringing fresh nourish- ment and carrying off the accumulated waste products. Every contraction of a voluntary muscle in the living body is con- sidered to be tetanic in character, a sudden jerk being in reality a tetanic contraction of short duration. A good example of a clear case of tetanus is seen when a person takes hold of the handles of a strong galvanic battery in action. Cramp or tetanus is produced in the muscles of his fingers by the rapidly repeated contractions in the muscles bending his fingers, and he cannot let go. Besides doing mechanical work during contraction, the muscle produces heat and undergoes an electrical change. In Muscular and Nervous Tissue 45 other words it sets free energy, the energy being obtained from that stored up in the muscular tissue and derived from the food. 37. Summary of Changes occnrrmg dnring UnBcular Contrac- tion. — When a muscle contracts the following changes occur : — (a) Structural : — (I.) The whole muscle shortens, thickens, and hardens. (2. ) The muscle-columns or sarcostyles of the fibres contract, the dark discs encroaching on the clear intervals. (3. ) The blood supply is increased, while venous blood and lymph are squeezed out. (^) Physical :— (I.) There is a slight rise of temperature, the venous blood from an active muscle being warmer than that from a muscle at rest. (2.) The electrical conditions of the muscle are changed, a negative variation of the natural muscle-current taking place. (3. ) The extensibility of the muscle is increased — that is to say, a given weight stretthes a contracted muscle more than the same muscle at rest. («■) Chemical : — (i.) The chemical changes occurring in resting muscle become more active during contraction. (2.) Carbon dioxide (CO^) is suddenly evolved in greater quantity, and the amount of oxygen absorbed is increased, but not in proportion. (3.) Sarcolactic acid (C3H5O3) is produced, and the muscle becomes acid to litmus paper. (4.) Sugar is produced, presumably from the muscle glycogen. 38. General Arrangement of the Nervous System. — The nervous system, whose general function is to guide, regulate, harmonise, and co-ordinate the other functions of the body, consists of (i) masses of nervous matter situated within the bony cranium and spinal canal, forming what is known as the cerebrospinal system ; (2) cords of nervous matter termed nerves, extending from the cerebro-spinal system to the muscles, sense organs, and other organs of the body ; (3) smaller masses of nervous matter called ganglia, lying along nerves situated in the neck, thorax, and abdomen, and constituting the so- called sympathetic system. The cerebro-spinal system consists of the brain, divided into cerebrum or large brain and cerebellum or small brain. The brain, with its twelve pairs of cranial nerves, will be treated of in a later chapter. Cpnnected with the brain is the spinal cord, which is united with a portion of the brain in the cranial cavity known as the medulla oblongata. Passing from the spinal cord are 31 pairs of spinal nerves, 8 cervical, 12 dorsal, 5 lumbar, 5 sacral, and I coccygeal. The sympathetic system consists 46 Human Physiology in the main of a double chain of small swellings, or ganglia, on each side of the front of the spinal column, connected with each other and with the spinal nerves. Cords, sometimes called sympathetic nerves, pass from these ganglia to the viscera and blood-vessels, the movements of which are involuntary. These nerves on their way to the viscera sometimes unite with each other to form interlacing networks or plexuses, on many of which collateral ganglia are found. The sympathetic system is not really an indepen- dent system, as was once supposed, for we now know that its ganglia have a distinct relation to certain fibres that leave the spinal cord, and that its nerves are under the control of some part of the central system. Nervous substance is found, under the microscope, to consist of two different structural elements, _/?i5r«j and cells. The fibres are found in the nervous cords and also in certain parts of the brain and spinal cord ; the cells are mainly confined to the cerebro-spinal centre and the ganglia, although some are present at the terminations of the nerves of special sense. A nervous cord or nerve is found to be composed of bundles of nerve-fibres run- ning side by side, bound together and inclosed by a sheath of connective tissue. According as the nerve-fibre has a sheath of medullary substance or is without the sheath, it is spoken of as a medallated fibre or non-medullated fibre. According to their function, nerves and nerve- fibres are classified as (i) afferent ot centripetal, (2) efferent or centrifugal. Nerve-fibres which convey impulses from a centre to some part of the body are efferent nerve-fibres ; nerve-fibres which convey impulses to a centre from some part of the body are afferent fibres. The term motor is sometimes used instead of efferent, and the term sensory instead of afferent ; but these terms are not strictly interchangeable, for im- pulses passing along fibres to the central system may give rise to ettects that do not result in sensation, e.g. reflex action ; and impulses pass- ing along fibres from the central nervous system often produce effects other than movement, e.g. secretion or inhibition. A nerve-trunk, as the Fig. 39.— The Spinal Cord vagus, which contains both kinds of fibres is N'r:i)ft'og«\t';:S ^P""^^" °f -,? -^f^ nerve. J^^ also speak of Sympathetic Chain on one somatic z.'o.Asplanchmc nerve-fibres. Somatic (Gk. sid V v, pons Varolii, below which is the medulla oblongata ; c i to8, the cervical nerves ■ d r to 12, the dorsal nerves ; L i to 5, the lumbar nerves ; s i to 5, the sacral nerves : 6 the coccygeal nerve ; x , the terminal fibre of the cord ; « to j:, the sympathetic chain, show ing the connection with the spmal nerves. "', auuw Muscular and Nervous Tissue 47 soma, body) nerves supply such body structures as the muscles, bones, skin, &c. ; splanchnic (Gk. splanchna, viscera) nerves are distributed to the viscera — heart, lungs, alimentary canal, &c. The viscera of the body are inner- vated by nerve fibres Vifhich leave the spinal cord with a medullary sheath, run forward into the sympathetic or more distal ganglia, and there lose their sheath as they issue from the ganglion as non-meduUated fibres. Before describing the microscopical structure of nerve-fibres and nerve- cells we will return to the spinal cord, to obtain some general ideas of the functions of nervous tissue and the close connections that exist between the central nervous system and the various other ' tissues of the body. Examination of a portion of the spinal cord shows that it is cylindrical in shape and separated into right and left halves by median fissures. Nerves are given off from each half, each nerve arising by two roots, a posterior and anterior root. The posterior root has a slight swelling or ganglion just before it unites with the anterior root, while still in the spinal canal. The section of the cord shows the interior to contain grey matter arranged somewhat like the letter H, and this grey matter is composed mainly of cells (fig. 40). The external white matter is composed of fibres. Byhelpofthe Fig. 40. — Section of Spinal Cord, showing mode of origin of Spinal Nerves, i. Anterior median fissure ; 2, posterior median fissure ; 3, antero-lateral impression ; 4, postero-lateral groove ; 5, anterior root ; 6, posterior root : 6\ ganglion on posterior root ; 7, united mixed nerve. annexed diagram (fig. 41) we can learn something about the distribution of the nerves springing from a typical region of the cord. The diagram repre- sents one lateral half only, the corresponding half being similar. From the horns of the grey matter arise the two roots of the spinal nerve N. Experiments have shown that the nerve-fibres of the posterior root r are afferent fibres and that the fibres of the anterior root A are efferent, the former conveying impulses to the cord and the latter carrying impulses from the cord. The nerve-trunk N must therefore be a mixed nerve. The mixed trunk undergoes many divisions, but the greater portion of these, represented by n', pass to the skeletal muscles M, or to the sensory cells of the skin in a definite part of the body, s. An important branch of the mixed nerve-trunk N is the small branch V, known as the ramus com- municans. The fibres from this pass into one of the ganglia of the sympathetic chain 2, where some of the fibres appear to become connected with the nerve-cells of the ganglion. From the sympathetic ganglion fibres pass out to supply the viscera (or internal organs), these fibres often passing on their way through collateral ganglia. The greater part of these sympathetic or splanchnic fibres probably carry impulses from the central system to the plain muscular fibres of the viscera {m), while others are afferent, and carry impulses to the central system from the sensory cells (s) of the internal organs. Some terminate in other ways, x. 48 Human Physiology One portion of the ramus coninmnicans in the thoracic region is the grey ramus comniunicans, r V. It is shown in the diagram running back from the gangUon 2 to the spinal cord, and giving off a branch v tii, which runs in connection with the spinal nerve to the muscular tissue of the blood- vessels, »2'. Thenervesthat thus regulate the calibre of the blood-vessels are spoken of as vaso-motor nerves. The upper end of the spinal cord, known as the medulla oblongata or spinal bulb, lies in the cranium, and may be regarded as the lowest division of the brain (fig. 42). It is pyramidal in shape, with its broad end upwards, and surmounted by a pan of the brain called the pons Varolii. On the anterior surface of the lower part certain fibres coming up from the spinal cord may be seen to decussate with (cross) eacli other before going to the highest parts of the brain (decussation of pyramids). From its front and sides may be seen issuing the sixth to the twelfth cranial nerves. These nerves have their ulti- mate or deep origin in particular parts of the grey matter of the bulb. In the posterior part of the medulla is found a diamond-shaped space termed thej^ar/A ventricle, which is continuous below with the central canal of the spinal cord, and the roof of which is formed by a thin membrane. The pointed lower end of the fourth ventricle, from its shape re- sembling that of a pen-nib, is called the calamus serif tonus. In the grey matter forming the floor of the fourth ventricle are found groups of cells forming the nerve-nuclei of the cranial nerves just mentioned. One of these numbered X, and termed the vagus or pneumogastric, is very important, being intimately asso- ciated with the functions of circulation, respiration, and digestion. The import- ance of the medulla as a nerve-centre for controlling and regulating various offices is the reason that we have introduced this short account of it at this stage. By a nerve-centre we must understand a gan- glion cell or group of cells capable of receiving, modifying, and discharging nerve impulses, and thus acting for the performance of some function. 39. Nerves.— A nerve is a whitish cord formed of a number of nerve-fibres bound together by connective tissue. In a small nerve we may find a single bundle, or funiculus, of fibres in a tubular covering, but in larger nerves, as the sciatic, several bundles, q\ funiculi, are united together by a common covering Fig. 41.— (After Foster.) -I ca Fig. 42.— View from before of the Medulla Oblongata, Pons Varolii, Crura Cerebri, and other central portions of the Encephalon. Natural size. (Allen Thomson.) On the right side the convolutions of the central lobe, or island of Reil, have been left, together with a small part of the anterior cerebral convolutions ; on the left side these have been removed by an incision carried between the thalamus opticus and the cere- bral hemisphere. I', the olfactory tract, cut short and lying in its groove ; II, the left optic nerve in front of the commissure ; II', the right optic tract ; Th^ the cut surface of the left thalamus opticus ; C, the central lobe, or island of Reil ; 5y, fissure of Sylvius ; x x , anterior perforated space ; e, the external, and i, the internal corpus geniculatum ; k, the hypo- physis cerebri or pituitary body ; tc, tuber cinereum with the infundibulum ; a, one of the corpora albicantia ; P, the cerebral peduncle or crus; III, close to the left oculo-motor nerve ; x , the posterior perforated space. The following letters and numbers refer to parts in connection with the medulla oblongata and pons. PV, pons Varolii ; V, the greater root of the fifth nerve ; + , the lesser or motor root; VI, the sixth nerve; VII, the facial; VIII, the auditory nerve; IX, the glosso-pharyngeal ; X, the pneumogastric nerve ; XI, tfae spinal accessory nerve ; XII, the hypoglossal nerve ; C I, the suboccipital or first cervical nerve ; /a, pyra- mid ; o, olive ; d, anterior median fissure of the spinal cord, above which the decussa- tion of the pyramids is represented ; ca, anterior column of cord ; r, lateral tract of bulb continuous with cf, the lateral column of the spinal cord. E 50 Human Physiology of connective tissue. This common sheath, formed of white and elastic fibres, passes between the funicuH of nerve-fibres and supports the fine blood-vessels distributed to the nerve. It has received the name of epineurium. Each funiculus has also a sheath of connective tissue of a lamellar nature, called the /en««« WOT. The nerve-fibres are also separated from one another by delicate connective tissue called endoneu- rium, which serves to support the capillary blood-vessels Fig. 43. — Section of a Part of the Median Nerve (Human). Drawn as seen under a low magnifying power. (From Landois, after Eichhorst.) efi, epineurium, or general sheath of the nerve, consisting of connective-tissue bundles of variable size separated by cleft-like areoljE, with here and there blood-vessels ; ^^, lamellated connective-tissue sheaths (perineurium) of the funiculi ; ed, interior of funiculus, showing the cut ends of the meduUated nerve-fibres, which are embedded in the connective tissue within the funiculus (endoneurium). nourishing the nerve-fibres. Lymphatic vessels are also found in this tissue. The branches of a nerve and single fibres passing to their distribution are invested with a delicate continuation of the perineural sheath known as the sheath of Henle. (The nerve-fibres in the brain and spinal cord are held together by a special kind of tissue termed neuroglia (See App.).) Nerve-fibres from one funiculus often pass into another, but in these communications the medullated fibres Fig. 44.— White dullated Nerve-fibres, showing the sinuous out- line and double contours. Muscular and Nervous Tissue remain individually distinct and never anastomose together. Most nerves con- tain two kinds of fibre, though the fibres of the cerebro-spinal system are chiefly medullated fibres, while in those of the sympathetic system non-medullated fibres preponderate. Nerve- trunks themselves receive nerve-fibres {nervi nervorum), which ramify chiefly in the epineurium, 40. Medullated Nerve- fibres. — The separate fine threads composing a nerve are termed fibres. The nerves from the brain and spinal cord, as well as the white matter found in the nerve centres, are seen when teased out and examined under the microscope to consist mainly of fibres known as medullated fibres, be- cause the central core of such a fibre is sur- rounded by a white sheath formed of a sub- stance of a fatty nature and called the medullary sheath. They vary in size from jAj inch in the nerves to the volun- tary muscles to —i^ inch in the nerves from the sympathetic ganglia. Medullated fibres, when examined fresh, appear as glassy threads with a double contour (fig. 44), but when heated with certain stainmg reagents each medullated fibre is found to consist of : { I ) a central core known as the axis-cylinder or neuraxis, and continuous from its origin in a nerve-cell to its distribution. Under high powers of the microscope the axis- FlG. 45. — Portions of two Nerve-fibres stained with osmic acid (from a young Rabbit). 435 diameters. K, R, nodes of Ranvier, with axis-cylinder passing through. a, primitive sheath of the nerve ; c, opposite the middle of the segment, indicates the nucleus and protoplasm lying between the primitive sheath and the medullary sheath. In A the nodes wre wider, and the intersegmental substance more apparent than in B. (Drawn by J. E. Neale.) 51 W-. 52 Human Physiology cylinder itself is seen to consist of exceedingly fine fibrils ; (2) a medullary sheaih, or white substance of Schwann, surrounding the axis-cylinder, but not continuous, for it shows gaps known as the 'nodes of Ranvier. ' This sheath being of a fatty nature, stains dark with osmic acid ; (3) an outside sheath, known as i\ie primitive sheath, or neurilemma. This is not found on the meduUated fibres in the nerve-centres. Where present the primitive sheath shows nuclei about midway between two nodes (fig. 44)- As already stated, the axis-cylinder runs along the middle of a nerve-fibre, and has its origin in a nerve-cell. It is the con- FlG. 45^. — Section across five nerve-fibres. (Magnified 1,000 diameters.) The nerve was hardened in picric acid and stained with picro-carmine. The radial striation of the medullary sheath is very apparent. In one fibre Fig. 45a. — An Axis-cylinder, the rays are broken by shrinkage of the axis- highly magnified, showing the cylinder. The fibrils of the axis-cylinder appear fibrils composing it. tubular. (From Schafer's ' Essentials of (From a photograph. Schafer's ' Essentials of Histology.') Histology.') ducting and important part of a nerve-fibre, and is continuous from end to end of the nerves, while the two sheaths disappear near the central and peripheral ends. The axis-cylinder or neuraxon is, in fact, merely a very fine and long process of a central nerve-cell, at first without any sheath, but soon acquiring a medullary sheath, and then, as it leaves the cord, a primitive sheath. At its peripheral end the connection both of motor fibres (par. 34) and of sensory fibres is made by the axis-cylinder alone, which usually forms a terminal arborescence (see figs. 35 and 49). Many of the nerve-fibres of which the anterior roots of the spinal nerves are composed can be traced into the nerve-cells that lie in the anterior cornu of the spinal cord at one end, while those of the posterior root spring from nerve -cells in the spinal ganglion, and in one direction pass into the posterior cornu of the cord, as will shortly be explained. 41. Non-medullated Nerve-fibres.— Besides the medullated fibres just described there occur in the nerves from the sympathetic ganglia, and to some extent intermingled with the medullated fibres in the cerebro-spinal nerves, pale grey fibres that have no medullary sheath, that do not show the double contour characteristic of medullated fibres, and that do not stain with osmic acid. These non-medullated fibres consist of an axis-cylinder Muscular and Nervous Tissue 53 surrounded by a fine primitive sheath only, beset at intervals with nuclei. The sympathetic nerves consist of non-meduUated fibres. Non-medullated fibres show frequent branching in their course, while meduUated fibres only branch, if at all, near their termination (fig. 46). 42. Nerve-eells. — Nerve-cells occur only in the grey matter of the brain, the spinal cord, and in the nodular masses of nervous matter called ganglia (see Glossary). The chief ganglia are those found on the posterior roots of the spinal nerves, upon the roots of some of the cranial nerves, and upon the trunk and large branches of the sympathetic nerves. Nerve-cells are microscopic masses of granular protoplasm with a nucleus. They vary much in size and shape in the various parts of the nervous system, some having many processes (multipolar), some having but two (bipolar), and some but one (unipolar). The largest nerve-cells (3^ inch in diameter) occur in the anterior horn of grey matter in the spinal cord. Such a nerve-cell shows a cell-body or nucleated mass of protoplasm with ii number of branching processes called dendrons or dendrites, and one process called the neuraxon or axis-cylittder process. The dendrites divide and subdivide, becoming finer and finally ending short, but the neuraxon becomes the axis-cylinder process of a nerve- fibre, and is continued to the organ to which the fibre is distributed. At a short distance from the cell it acquires a medullary sheath, and on its exit from the cord a neurilemma, or primitive sheath. Dendrons are thus dis- tinguished by branching and breaking up as soon as they leave the cell-body, whereas the axon or axis-cylinder process does not branch until near its termination, except that a few fine lateral offshoots, called collaterals, are..given off in some cases (figs. 47 and 174). The typical nerve-cells of the cerebral cortex are multipolar cells of pyramidal shape (fig. 199). Another type of nerve-cell is found in the ganglia of the posterior roots of the spinal nerves. These cells have no dendrons, and, as a rule, show but one process, an axis-cylinder process, which soon bifurcates, forming a T-shape junction, one branch passing cen- trally into the spinal cord, and the other branch passing outwards to its peripheral distribution (see fig. 48). In the embryo this ganglion cell of the posterior root was distinctly bipolar, having two axis-cylinder processes, one growing to the spinal cord, where it ends by branching round one of the multipolar cells, the F1G.46.— Portion of theNetwork of Non-meduUated Fibres of Remak, from the Pneumo- gastricoftheDog. (Ranvier.) «, nucleus ; ^, protoplasm surrounding it ; b, striation caused by fibrils. 54 Human Physiology other growing outwards to the periphery, where it ends in a bush of fibres in the skin, or on a muscle, or in some other way. As development pro- ceeds, the two processes coalesce, and the fibre has a convoluted course round the cell before it bifurcates (fig. 48). Each cell of a spinal J ganglion is enclosed in a connective sheath which shows the nu- clei of the connective tissue corpuscles. The function of the nerve- cells of the ganglion on the posterior root of a spinal nerve is to provide for the nutri- tion of the efferent fibres originating as processes of these cells. This is proved by the ' degeneration method' (par. 47). In the sympathetic ganglia, the nerve-cells are again multipolar, with several dendrons, like some of those of the cord, but with a nucleated sheath like the cells of the ganglia of the posterior spinal roots. One process and one only becomes an axon or axis-cylin- der process, giving rise usually to a non-medul- lated nerve-fibre. In recent years much knowledge of the minute structure of the microscopic nerve-cells has been obtained by examina- tion with very high powers and by special methods of staining. In multipolar cells the nucleus is relatively large and clear with a distinct nucleolus, and the protoplasm is seen to be finely fibrillated, the fibrils passing into the processes. With methylene blue the nucleus and nucleolus take up the colour, and the protoplasm of the cell-body also shows peculiar angular granules that have taken up the stain. These granules, termed ' Nissl Fig. 47.— Diagram of Mr.ltipolar Cell from Grey Matter of Spinal Curd. «, axon or axis-cylinder process ; w?, medullary sheath to same ; c, cell-body showing granules ; n, nucleus with nucleolus ; d, dendrites or branching processes. Processes of one cell often interlace with those of another, or the branching fibrils of a nerve-fibre may interlace with the dendrites of the cell. Fig. 47a.— Diagram of a Bipolar Cell in a Young Embryo. ?t, nucleus. One axis-cylinder process grows inwards to the spinal cord, the other to the periphery. In the adult the two processes coalesce as they leave the cell for a short distance, and then divide again. Compare fig. 48 and the ganglion-cell represented in the diagram fig. 47a. Muscular and Nervous Tissue 55 granules,' vary in size and number in different cells. They are also found to vary in the different cells in size and number with the physiological con- dition of the cells, disintegrating in cells fatigued by long activity, and Fig. 48.— Cell from a Spinal Ganglion. (G. Retzius.) sk^ nucleated sheath of the cell ; «, «', the nerve-fibre which the .single process of the cell, after a number of coils, forms. also in those whose axis-cylinder has been cut, thus furnishing ? means of distinguishing healthy cells from other.s. The NissJ granules occur not only in the cell-body but in the dendrons, though they are absent from the axis-cylinder process. The substance of which the granules is composed is a nucleo- proteid, and seems to be closely connected with the nutrition of the cell. As to the special func- tions of the various nerve- cells due to molecular changes in them, we may enumerate, (a) excitation of motor, secretory, and inhibitor nerve-fibres ; (b) the exercise of a tro- phic or nutritive influence on nerve-fibres ; {c) the production in the brain of that state known as consciousness. Examples of these various functions will come before us in our further course of study. One point further may be noted about nerve- cells. Unlike majiy other cells of the body, they soon lose their power of multiplication, though they retain the power of growth. They continue to lengthen their branches, and even put out new ones. They also possess the power of repairing Fig. 48a — Body of a Nerve-cell, from the Spinal Cord. Stained by Nissl's method. t, axis-cylimler process or axon ; ^, angular granules (Nis.sl granules) in the protoplasm : they are -stained darkly by methylene blue ; c, intergranular substance ; rf, nucleus ; f, a Nissl granule at the point of division of one-tof ihoilendroas. 56 Human Physiology branches, so that when the fibre springing from one is cut, it grows again from the central end unless the gap is very large. This reparation, how- ever, does not occur in the great nerve centres. 43. The Doctrine of Neurones — Although it is convenient to speak of nervous substance as consisting of white matter (nerve-fibres) and grey matter (nerve-cells), yet it is clear from what has been said that these are not distinct substances, for a nerve-fibre is only a special process of a nerve-cell, i.e. all nerve-fibres are branches of nerve-cells. The nerve- cell itself, its branching processes, called dendrons (when present), and the long process or neuraxon that becomes a nerve-fibre, form together a nerve-unit called a neurone. A nerve-centre is an aggregation of neurones arranged in different ways in different parts of the nervous system. Each neurone is considered to be anatomically distinct, and the connection of one neurone with another is thought to be effected by the interlacement (not structural union) of the arborisations of the dendrons or axons of one cell with the cell processes or cell-body of another neurone. There is no actual union of the branches of one nerve-cell with those of another, as was once thought, but the processes of neighbouring cells come so close together that nerve-impulses can pass across from the terminal bush of fibrils of one neuraxon to the dendrites or cell-body of another neurone. This mode of junction by interlacing of processes is called a synapse or clasping, and in this way neurones are linked together to form a nervous path. (Fig. 49 represents diagrammatically a synapse of two neurones. ) Among the properties of a neurone is that of conductivity in all directions. But when neurones are linked together to form a path it is found that nerve- impulses only pass from one neurone to another in one direction, the direc- tion in which the nerve-impulses travel under the conditions of natural life. The cell-body of a neurone contains the nucleus and forms the trophic centre of the cell. Any part of the cell cut off from the part containing the nucleus begins to die, and in regard to the axon or nerve-fibre this degeneration constitutes Wallerian Degeneration (par. 47). In some neurones (the spinal motor-cells) severance of the axon leads to degenera- tion not only in the nerve-fibre but in the cell itself. 44. Beflex Action. — The brain and spinal cord are sometimes spoken of as the great nerve-centres or the central nervous system. As already explained, they contain nerve-cells which give origin to nerve-fibres, these elements being held together by a kind of connective tissue. The two great central organs of the nervous system contain groups or aggregations of nerve-cells that are so related to each other that they appear to act together and to be endowed with the power of regulating some distinct function of the body. The term ' centre ' is applied to a group of cells in the brain or spinal cord that are so related to each other as to subserve a certain function. Thus we speak of a respiratory centre being situated in the medulla oblongata, of a centre of def creation in the spinal cord, and of various other ' centres ' to be described later. Speaking physiologically, the processes of the nervous centres are usually divided according to the mode of action into ' reflex acts ' and ' voluntary acts. ' A reflex action is the immediate efferent response to an afferent impulse, independently of the will, the disturbance set up by a stimulus to an afferent nerve being, as it were, reflected along an afferent nerve to a muscle or gland. The spinal cord and the medulla possess this power of responding to afferent impulses in a remarkable degree, and they may, in fact, be regarded as formed in part of a number of reflex nervous centres. The Muscular and Nervous Tissue 57 spinal cord also acts as a conductor of impulses vid the spinal nerves to and from the brain, as well as to and from the different parts of the cord. As we shall afterwards learn, parts of the brain itself may become centres of reflex action. Many reflex actions go on not only without any acts of the will, but also without our being conscious of them. The mechanism of a reflex action involves the following parts or elements : (i) a sentient surface or peripheral sense organ to receive a stimulus ; (2) a sensory or afferent nerve to convey the impulse or disturb- ance set up to a nerve-centre in the spinal cord or elsewhere. The im- pulse is passed on to the nerve-cell of the centre by the interlacing of the Fig. 49.— Diagram illustrating Reflex Action of Spinal Cord. S skin on which are branching fibrils of an afferent nerve ; G, ganglion on posterior root of spiral nerve from v/hich afferent nerve A/grew, one branch of nerve passing out- wards to periphery and one branch inwards to spinal cord, where it ends in a bush of fibrils round the dendrites of a nerve-cell in anterior cornu ; MC, motor-cell of .spinal cord with dendrites, some of which interlace with terminal fibrils in the cord of the afferent nerve ; E/ efferent nerve terminating in fibrils on muscle M. The arrows indicate direction of nerve-impulse. terminal fibrils of the nerve-fibre with the dendrons or cell-body of the nerve-cell of the centre, for the nerve-fibre is only in structural continuity with the nerve-cell in the ganglion or other place, out of which it grows ; (3) a nerve-cell or cells at the centre that receives the afferent impulse and then, owing to changes set up, sends on an impulse by its axis-cylinder process to a muscle or gland capable of response ; (4) an efferent nerve or nerves to convey the impulse from the nerve-centre ; (5) a muscle or gland in connection with an efferent nerve-fibre. It must be noted that the molecular change constituting a_ sensory nervous impulse, such as that described, may pass on to the brain, for a nerve-fibre may, by its terminal fibrils, be connected with more cells than one, and a nerve-cell may have connections through its dendrons with S8 Human Physiology more than one fibre. Thus the skin sensation may pass on to the brain and excite consciousness, and a voluntary message may pass from one or more brain-cells to the motor-cell of the cord, or the brain may inhibit the reflex action of a lower centre. Such impulses will not pass to the brain along the same fibre all the way, nor will the motor impulse pass along a single fibre. There will be a relay of cells with communicating fibres — i.e. an association of neurones. Motor impulses pass in great part by fibres from the pyramidal cells of the brain cortex to motor-cells of the cord, and thence by the fibres of these last to the muscles. In the case of sensory impulses passing to the brain, the passage is still more complicated. The paths of sensory impulses along the cord to the brain, and of motor impulses from the brain along the cord to the muscle, have been in part determined by the paths of ascending degeneration of fibres and descending degeneration, when the cord is reversed. (See par. 47.) It should be noted that the centre in the cord does not merely reflect the afferent impulses into efferent impulses ; the reflex act often seems to be adapted for a purpose, and to be of a useful nature. The brainless frog's movements on being irritated by an acid are directed to removing the irritating substance. Winking on irritation of the conjunctiva, or at the sudden approach of a missile, indicates a purpose. Sneezing, cough- ing, and vomiting are typical reflex actions of a similar nature. There is a variety of actions occurring rhythmically in the body to which the term automatic is sometimes applied, because they appear to arise in the nerve-centres themselves without any external impulse (auto- matic = self-acting, literally). Such an action is breathing. Breathing is brought about by the action of certain muscles which expand the chest and so cause air to rush into the lungs, followed by the action of another set of muscles which contract the chest and drive out air, and this appears due to the rhythmical dischprge from nerve-centres in the medulla of efferent impulses at the rate of sixteen to eighteen times per minute. But such discharges are not really self-caused ; the centres are no doubt stimulated by the condition of the blood. Keeping the term automatic to refer to those motor reactions that occur in series and are due to rhythmic dis- charges from nerve-centres, we may give as instances of automatism, besides breathing, the activities of the centres that keep up a constant or tonic contraction in the arterial walls and in sphincter muscles, as well as those which provide for regular contraction and relaxation of organs. All these centres are doubtless set in action by impressions from the organs they thus regulate, though the afferent impulses may not always be easy to describe. In a typical reflex action afferent impulses reach a nerve centre and are 'transmitted by the irritable protoplasm of the centre, not simply turned aside, or reflected, into efferent impulses. It is true that there is often a close relation between the strength of the stimulus applied to the afferent nerve and the magnitude of the efferent impulse, as shown in muscular movement, i.e. a slight impulse will usually give rise to slight muscular movement and a strong impulse to more forcible movement. But a very slight stimulus may also be so intensified in a nerve centre, where the arrangement of the reflex mechanism is complex, that the result is the powerful contraction of many muscles. Witness the convulsive fit of coughing when a small particle passes into the larynx. The condition of the centre also modifies greatly the resulting reflex action. Thus strychnia heightens the excitability of the cord to a great degree, and in a brainless Muscular and Nervous Tissue 59 frog poisoned with this drug a slight touch on the skin causes violent con- vulsive contractions in most of the muscles of its body, a discharge of energy from the centre spreading in nearly all directions. In simple cases of reflex action between the part stimulated and the muscle or muscles set in motion, the action is such that the movement appears to be the result of an efferent impulse in a motor nerve that is the companion to the sensory nerve ; in complex cases of reflex action co-ordinated movements are set up that appear adapted to accomplish a purpose, as when a brainless frog attempts to wipe off the irritating acid on its flank. It should also be noted that a stimulus applied to the surface of the skin or other terminal sensory organ leads to stronger and more complex movement than when the stimulus is applied to the sensory nerve directly. Reflex movements being immediate, unchosen motor responses to stimulation of afferent nerves may be performed unconsciously or consciously. When the iris contracts under the influence of a strong light, the excited retina leads to the transference of an afferent impulse along the optic nerve to a centre in the brain, and an efferent impulse is then sent along the third cranial nerve to the circular fibres of the iris, and their contraction follows without our knowledge. Sneezing, on the other hand, is a reflex action set up by irritation of the nasal branch of the fifth nerve conveying impulses to the respiratory centre in the medulla that lead to the action of expiratory muscles, and of this we are usually conscious at the time or may become conscious subsequently. Finally, we may note that, reflex actionsmayoften.be inhibited lungs so far stretched that the two pleural layers are always in apposition, and together with the heart and great vessels completely fill the thorax. If the thorax is opened the distended lungs collapse, owing to the atmospheric pressure on their external surface counteracting that through the trachea. The ' internal capacity of the thorax undergoes rhythmical variations, the movements of the elastic lungs following those of the thorax. By the contraction of certain muscles, in a mode shortly to be Fig. 99. — Small portion of Peritoneal Surface of Diaphragm of Rabbit. (Klein.) Magnified. li lymph-channel below the surface, lying between tendon bundles, /, t, and over which the surface-cells are seen to be relatively smaller, and to exhibit five stomata, s, s, leading into the lymphatic. The epithelium of the lymphatic channel is not represented. described, the thorax is enlarged at intervals, the lungs expand to occupy the increased space, and the pressure in the lungs becom- ing less than that outside in the atmosphere, a rush of air from the atmosphere through the trachea takes place to establish equi- librium of pressure. This act constitutes inspiration. When the muscles that produced inspiration relax, the thorax is brought to its former size, mainly by the elastic recoil of the lungs and chest walls (aided at times by other muscles), and this decrease of Respiration 133 chest capacity causing the pressure inside to be greater than that outside leads to an outrush from the air-passages to the external atmosphere. This constitutes expiration. Inspiration plus expiration constitutes respiration, the respiratory act taking place in an adult about sixteen times per minute. The first part of a respiratory act, inspiration, is essentially a muscular act, enlarging the chest capacity ; the second part of a respira- tion, expiration, is essentially an elastic recoil, aided by the weight of the chest walls. These movements set up differences Maximum insniration ~ Complemcnial air " Ordinary inspiration - TIDAL AIR Ordinary expiration — V Supplemental air - Maximum expiration - Residual air ■ 2000 j^ 120 c c CUT?. in. 500 CC ur 30 cabin ]5U0 -LOO:f Vital capacity Capacity of equilibrium Fig. ioo. CWaller.) Amounts of Air contained by the Lungs in various phases of Ordinary and of Forced Respiration. of pressure between the inside and outside air, and in both cases the movement of air follows the general law ' that air passes from a region of higher to a region of lower pressure.' The amount of air, 30 cubic inches, taken in at each inspiration and given out at the succeeding expiration in ordinary easy breathing is but a fraction, about f, of that which the lungs contain. Besides this tidal air, as it is called, the lungs contain about 100 cubic inches that may be expelled by a forcible expiration, termed supplemental air, and another 100 cubic inches that cannot be driven out by any effort, residual 1 34 Human Physiology air. Moreover, by a great effort of inspiration nearly 120 cubic inches of air, additional to the tidal and called complemental air, may be caused to enter the lungs. The total quantity of air that can be expelled from the lungs after an extraordinary inspiration by an extraordinary expiratory effort is thus equal to 250 cubic inches, and this quantity is spoken of as the ' vital capacity.' Vital capacity is sometimes estimated by blowing as long as possible into a spirometer after the deepest inspiration possible. It usually increases with increase of height, 8 cubic inches for every inch above the average height of 5 feet 8 inches. It is less in women than in men with the same circumference of chest and height in the ratio of 7 to 10. The volume of air taken in during inspiration is rather more than that expired, measured at the same temperature and pressure, some of the oxygen forming other combinations than CO2. The changes in the blood of the pulmonary capillaries have been already described in pars. 56 and 58. Muscular exercise increases the number of respirations and thereby the quantity of air passing in and out of the lungs. There is, there- fore, increased absorption of oxygen by the haemoglobin of the red corpuscles and increased elimination of carbonic acid from the plasma. Walking at the rate of five miles an hour is said to increase the quantity of air respired five times, the increased consumption of oxygen and formation of carbonic acid pro- bably taking place in the muscles employed (see paragraphs 36 and 37 on the changes occurring in muscle during its con- traction). Since the lungs, therefore, are but partially emptied of air during expiration by the removal of the contents of the nose, trachea, and larger bronchi, it is necessary to inquire how the fresh air passes to the air-cells, which are the parts of the lung where the interchange of gases takes place between the atmo- sphere and the blood. The process of diffusio7i effects this. It is by the rapid diffusion of the gases in the tidal air and the stationary air that oxygen is supplied to the alveoli, and carbon dioxide given up to the tidal air. Before describing the passage Respiration 1 3 5 of the air into the blood, and the effect of the blood on the air, we will give a more detailed account of the respiratory move- ments, inspiratory and expiratory. Fig. ioi. — Superficial view of the Muscles of the upper part of the Trunk, from before. (Allen Thomson.) , sterno-mastoid of the left side ; i', i', platysma myoides of the right side ; 2, sterno- hyoid ; 3, anterior, 3', posterior belly of the omo-hyoid ; 4, levator anguli scapulee ; 4', 4", scaleni muscles ; s, trapezius ; 6, deltoid ; 7, upper part of triceps in the left arm ; 8, teres minor ; g, teres major ; 10, latissimus dorsi ; 11, pectoralis major ; n', on the right side, its clavicular portion ; 12, part of pectoralis minor ; 13, serratus magnus ; 14, external oblique muscle of the abdomen ; 15, placed on the ensiform pro- cess at the upper end of the linea alba. 92. Inspiration. — In inspiration the internal capacity of the thorax is increased, and air enters the lungs through the larynx and trachea to equalise the pressure within the lungs and outside. The enlargement of 136 Human Physiology the chest is effected by the action of certain muscles, and the increase takes place (I) in the vertical diameter, (2) in the antero-posterior and lateral dia- meters, i.e. an increase in depth, an increase from back to front and from side toside. The vertical diameter of the chest is increased by the contraction and consequent descent of the diaphragm. This is the arched musculo-tendi- nous sheet separating the cavity of the chest from the abdomen. It is attached all round— to the sternum and ribs in front to the ribs at the side Fig. 102.— Intercostal Muscles of the Fifth and Sixth Spaces. (Allen Thomson, after Cloquet.) A, from the side ; B, from behind. IV, fourth dorsal vertebra ; V, V, fifth rib and cartilage ; i, i, levatores costarum mus- cles, short and long ; 2, 2, external intercostal muscle ; 3, 3, internal intercostal layer, shown in the lower space by the removal of the external layer, and seen in A in the upper space, in front of the external layer : the deficiency of the internal layer to- wards the vertebral column is shown in B . and to the ribs and spinal column behind. To its upper surface are attached the investing membranes of the lungs (pleurae) and heart (pericardium). During its contraction the diaphragm becomes flatter to the thorax, the sides descending most. The thorax thus enlarges in depth and the front walls of the abdomen bulge out owing to the pressure on the viscera in that cavity. From its attachment to the sternum and false ribs it tends to pull these downwards and inwards, but its action in this resjject is counteracted partly by the vertical direction of the fibres attached to the ribs and partly by the elevation of the ribs that accompanies descent of diaphragm. The in- Respiration 137 crease of the thorax from back to front and from side to side is brought about by muscles that elevate the ribs. These muscles are : (i ) The scaleni, (2) the intercostal muscles, (3) the levatores costarum or elevators of the ribs. The scaleni muscles pass from processes of the cervical vertebrae to the first two ribs, and by their action raise or at least fix these ribs. The external intercostals whose fibres run downwards and forwards in the spaces between the ribs so act, when the two first pairs of ribs are fixed by the sca- leni, that the ribs are elevated both in front and at the sides, moving on their articulations with the vertebrae. As they slant downwards, the ribs when raised must thrust the ster- num forward and enlarge the ante- ro-posterior diameter of the chest ; and since they form arches which increase in sweep, at least from the first to the seventh, the elevation of one into the place of another causes the chest to become wider from side to side. Further as the ribs are raised there is some stretch- ing of the costal cartilages, and a certain amount of rotation of the ribs which brings their outer sur- faces more directly outwards, these effects plainly aiding the enlarge- ment of the thoracic cavity. The levatores costatum arise from the tips of the transverse processes of the seventh cervical and the upper eleven dorsal vertebra, and pass obliquely downwards and out- wards, being inserted into the outer surface of the rib belonging to the vertebra below that from which they spring. They are re- garded as muscles of ordinary in- spiration, inasmuch as they assist in elevating the ribs. In extraordinary and forced inspiration other muscles are brought into play to enlarge the chest. The quadratus lumhontm, placed between the last rib and the pelvis, aids the diaphragm by fixing one of its attachments and with the help of other abdominal muscles draws down the lower part of the thorax. The serratus posticus superior, a muscle of the back arising from the spines of the vertebrae in the upper dorsal region, aids in raising the 2nd, 3rd, 4th and 5th ribs ; the sterno-mastoid raises the clavicle ; and the serratus magnus, pectoralis major and minor, all serve to lift the ribs when the arms and shoulders are fixed (see fig. loi). Associated with the respiratory movements of the thorax are move- ments of the nostrils and of the glottis. At each inspiratory movement the nostrils expand when breathing through the nose, returning to their previous condition in expiration. During inspiration the glottis is wide Fig. 103. — Diagram of First and Seventh Ribs, in connection with the Spine and the Ster- num, showing how the latter is carried up- wards and forwards in inspiration. (G.D.T.) The expiratory position is indicated by con- tinuous lines, the inspiratory by broken lines. 138 Human Physiology open, but during expiration it is narrowed by the action of the muscles that move the arytenoid cartilages of the larynx. In laboured inspiration these facial and laryngeal actions are exaggerated. 93. Expiration. — In ordinary easy breathing, expiration or the diminution of the chest cavity and expulsion of air is effected mamly by the elastic recoil of the lung tissue and costal cartilages which had been put on the stretch in inspiration. The diaphragm relaxes and ascends, and the exter- nal intercostal and other inspiratory muscles ceasing to act, the ribs fall down and the walls of the chest return to a condition of rest as a result of the elastic re- action. The abdominal muscles may also assist, and possibly part of the internal intercostals. Much discussion has taken place about the action of the internal intercostals, and diverse views are held regard- ing their action. Some authorities hold that the only office of the inter- nal intercostals is to render the intercostal spaces firm and the whole thoracic cage rigid, but the general view is that the parts of the internal inter- costals between the costal cartilages elevate the ribs and assist in inspiration, and that the lateral portions of these muscles between the bony ribs depress the ribs and assist in expiration. Fig. 104. — The Lower Half of the Thorax, with four Lum- bar Vertebrae, showing the Diaphragm from before. (Allen Thomson, after Luschka.) a^ sixth dorsal vertebra ; 5, fourth lumbar vertebra ; c, ensiform process ; d, d', aorta,passing through its open- ing in the diaphragm ; e, oasophagus ; X opening in the tendon of the diaphragm for the inferior vena cava ; i, central, 2, right, and 3, left division of the trefoil tendon of the diaphragm; 4, right, and 5, left costal part, ascend- ing from the ribs to the margins of the tendon ; 6, right, and 7, left crus _; 8 to 8, on the right side, the sixth, seventh, and eighth internal intercostal muscles, de- ficient towards the vertebral column, where in the two upper spaces the levatores costarum and the external intercostal muscles 9, 9, are seen ; 10, 10, on the left side, subcostal muscles. In forced expiration the abdominal muscles play a more prominent part, their contraction forcing the abdominal viscera and the diaphragm upwards and pulling down the sternum and lower ribs. Their action is aided by that of the triangularis sterni which depresses the costal cartilages, by the serratus posticus inferior, and as the expiration be- Respiration 139 comes more forced by every muscle that can depress the ribs or press the abdominal viscera. Even at the end of the most forced expiration the lungs contain the residual air and are in a more or less stretched condition with the two layers of the pleura in contact. This is proved by the fact that if in an animal or in a corpse the thoracic wall be perforated, the lungs collapse driving out air, the collapse separating the two layers of the pleuras and air passing into the pleural cavity (pnemothorax). Collapse of one lung is dangerous, but collapse of both lungs through openings into both pleural cavities is fatal, since the effect of the respiratory movements is merely to drive air in and out of those cavities instead of renewing the air in the lungs. Before birth the lungs contain no air and exercise no elastic force ; but after birth the alveoli and bronchioles are opened out as the animal begins to breathe, a further expansion taking place through the subsequent growth of the thorax. Expiration follows inspiration immediately, and the two acts take up nearly the same time, though in women and children the expiration is slightly longer. At the end of expiration there is normally a very slight pause, In an adult the number of respirations is i6 to i8 a minute. In a new-born child the number is about 44, and in a child of five, 25 u minute. Muscular effort increases the number, so that in standing even it is more than when lying at rest. In health the number of respirations is to the number of heart beats in the proportion of I to 4 or I to 5. 94. Types of Breathing. — There are three more or less distinct types of respiration, marked by the mode in which the increase of chest capacity is mainly produced. In young children and some men the diaphragm is the chief muscle employed in tranquil inspiration, and as this causes a pro- nounced rise and fall of the walls of the abdomen, it is called the abdominal tyfe. In boys and many adult males, the action of the diaphragm is not so great, and the movement of the ribs is most distinct from the seventh downwards, so that this is spoken of as the inferior costal type. In girls and women the upper part of the sternum and the upper ribs take the chief share in respiration, the walls of the abdomen showing but little motion. This is the superior costal type. t)84i 7,264 3.077 5,103 2,161 3.912 1.657 Food 165 excluded. The amount of nitrogen excreted in the urea of the urine was carefully ascertained in each case, and from this the amount of proteid used up was calculated. Knowing the amount of energy required to raise their bodies the height of the mountain, and calculating the amount of energy set free from the proteid disintegrated, it was found that the energy from the proteid was far short of accounting for the work done in climbing. Besides, there was much muscular energy expended within the body in cardiac and respiratory movements, etc. Hence it is evident that energy was set free from other than proteid substances. Other observations on soldiers and pedes- trians confirm this conclusion. Muscular energy is not derived from the metabolism of proteids alone, nor does it increase the excretion of urea proportionately, for the excretion of urea is either not altered at all, or but slightly augmented. On the other hand work does increase the consumption of oxygen and the output of CO2, and the examination of muscle itself (par. 37) shows that its contraction leads to the discharge ofC02 without any nitrogenous products being evident. It would thus appear that normally the sources of muscular energy are to be looked for in the fats and carbohydrates, and that proteids are mainly used to repair tissue waste, though it must be remembered that energy can be derived from proteids when needful, as is evident from the case of the dog fed on an exclusively proteid diet. I ro. Starvation or Inanition. — When an animal is deprived of nourish- ment it continues to live for some time, but instead of using food to supply its heat and other forms of energy, it uses its own tissues. Excretions con- tinue to pass from kidney, skin, and lungs, and the animal diminishes in weight by loss of fat and flesh from its various organs, the total loss amounting to nearly one-half the body weight in some cases. This loss is accompanied by great thirst, weakness, and pallor. The temperature shows a large daily fluctuation, and gradually sinks to about 7°° F. A torpid condition comes on with mental weakness or delirium until death intervenes. A dog may so live for more than 20 days, but in the case of man death occurs with entire absence of food and drink in from 8 to 10 days. A small quantity of water lengthens the time, and adults last longer than the young. The excretion of nitrogen in the form of urea sometimes falls quickly at first, then reaches a minimum where it remains several days; it rises again when the fat has been used up, and again falls quickly as death approaches. The faeces become gradually less and the secretion of bile greatly diminishes. The intake of oxygen and the excretion of carbonic acid also fall. 1 66 Human Physiology In the loss of weight the different tissues and organs take a very dif- ferent part. The fat suffers most, as may be imagined, seeing that it is mainly a reserve or storehouse vfhich the body utilises in the absence of sufficient food from without. Next to the fat, the spleen and liver dimi- Fat 97 per cent. Spleen 66-7 Liver 537 -TiL-stis 40 I\luscles 3o'5 Blood 27 Kidney 25-9 Skin 20'6 Intestine Lungs 17-7 Pancreas 17 - Bones I3"9 'Nerv. syst. 3'= Heart 2-6 10 20 30 40 50 60 70 60 .3a..ilO» Fig- 108. — Graphic representation of the percentage of diflferent tissues lost during starva- tion ; the shaded areae represent loss, the unshaded areae amounts remaining at death. (According to Voit's analyses.) nish in weight (glycogen ceasing to be stored in the liver). The muscles also lose heavily, while the brain and heart are but little affected, being probably nourished by materials drawn from less important organs. The annexed diagram represents graphically the percentage of loss of weight in various tissues or organs at the close of starvation. CHAPTER VIII DIGESTION III. Mastication. — The first stage in the digestive process consists in the mastication or chewing of the food in the mouth. This operation is of great importance, as the thorough breaking up of the food enables the digestive juices to gain Digestion 167 free access, and thus more easily to exert their action upon it. The act is accomplished by means of the movements of the lower jaw, tongue, and cheeks. The articulation of the lower jaw by means of its two condyles moving in the glenoid fossse of the temporal bones is of such a nature that it is movable not only up and down against the fixed upper jaw, but from side to side, so that the food may be cut by the sharp edges of the incisor and canine teeth, or crushed and ground between Fig. 109.— The Pterygoid Muscles ; the Superficial Muscles, the Zygomatic Arch and a portion of the Ramus of the Jaw having been removed. the surfaces of the bicuspids and molars. The mouth is opened by the digastric and other muscles passing from the lower jaw to the hyoid bone situated at the base of the tongue ; it is closed and the biting movement effected by the temporal muscles, which pass from the temporal bone to the upper end of the lower jaw, by the masseter muscles of the cheeks which arise from the malar bones and adjoining parts of the zygo- matic arch and are inserted into the outer surface of the ramus i68 Human Physiology of the lower maxillary, and by the internal pterygoid muscles. The external pterygoid muscles draw forwards the condyles and thrust the lower jaw forward, but when acting alternately lead to the grinding movement. Mastication is partly a volun- tary and partly a reflex act. The afferent branches of the fifth Fig. no —The Salivary Glands. and ninth cranial nerves convey to the brain the tactile and other sensations produced by food in the mouth, and the efferent impulses pass to the muscles by the motor-fibres of the same nerves. The nerve-centre for mastication through which the reflex action occurs, and by which the various movements are co-ordinated, is situated in the medulla. Digestion 169 112. Insalivation. — During mastication tiie food becomes mixed with the first of the digestive juices. This is the saliva secreted by the three pairs of salivary glands, the parotid, the submaxillary, and the sublingual, and the secretion passes into the mouth by the ducts leading from these glands (fig. no). Small buccal glands in the mucous membrane of the mouth also contribute their secretion to the mixed saliva. The general Fig. III. — Section of a Racemose Gland, showing the commencement of a Duct in the Alveoli. MagniHed 425 diameters. (E. A. S.) «, one of the alveoli, several of which are in the section shown grouped around the com- mencement of the duct, d' ; a', an alveolus, not opened by the section ; ^, basement- membrane in section ; c, interstitial connective tissue of the gland ; rf, section of a duct which has passed away from its alveoli, and is now lined with characteristically striated columnar cells ; j, semilunar group of darkly stained cells at the periphery of an alveolus. position and size of the salivary glands, parotid, submaxillary and sublingual, may be again learnt from fig. no, and we confine ourselves now to their minute structure. Tracing one of the main ducts backwards, it is found to divide and sub- divide in the body of the gland into smaller and smaller ducts, the ultimate ducts terminating in spaces, tubular acini or alveoli lined by secreting epithelium cells. A group of alveoli with the small ducts arising from blind ends are bound together I70 Human Physiology by connective tissue carrying blood-vess.els, lymphatics, and nerves to form a lobule, and the whole gland consists of lobules of various sizes bound together and encased by connective tissue. The interior of the ducts are canals lined with a layer of columnar epithelial cells resting on a basement membrane of connective tissue, with a small quantity of unstriped muscular tissue in the larger branches. The lining epithelial cells of the ducts shows a nucleus near the centre, and the half away from the lumen is seen to be faintly striated. If we examine sections .''■•" *.i 7i "- .»^ Fig. 112. — Section of Dog's Submaxillary, stained. (Kolliker.) fl, duct ; i5j alveolus; c, crescent. of the glands under the microscope, the alveoli present different appearances according as they are cut longitudinally, trans- versely, or obliquely, while the real salivary cells lining the alveoli are soon found to be of two kinds, with difference of structure according to the substances they secrete. This leads to a division of the salivary glands into two classes, (a) mucous glands and (b) serous or albuminous glands. Mucous glands have acini or alveoli lined by large conical cells resting on a basement membrane, and leaving only a small central lumen or opening. These cells do not stain well, secreting a highly refracting substance called mucigen, which is Digestion 171 discharged as mucin. Mucin is a complex slimy body consist- ing of a proteid and an animal guta. The thick and viscid secretion of mucous salivary glands is called mucous saliva. Between the large clear cells that form the lining of the alveoli of the mucous glands and the basement membrane there may often be seen small cells called from their shape crescents or demilunes. The large cells of mucous glands differ according to the condition of activity of the gland. In a resting or loaded condition the cells are larger than during activity, and appear crowded with granules of mucigen (fig. 113). In a discharged gland the cells become less and the lumen of the acinus larger, a few granules only being found in the part of the cell near the open- ing. The sublingual are mucous glands in man. 113. Serous or Albuminous Glands are lined by polyhe- dral granular cells, Reging. Afteractivity. ° ' Fig. 113. — Mucous Gland, which readily stain in all parts. No cells corresponding to the crescents or demilunes of mucous glands are present. The secretion is thin and clear, con- sisting chiefly of water with minute quantities of salts and the fer- ment ptyalin. This limpid saliva contains a proteid called serum- albumin, but no mucin. During the resting or loaded state the cells lining the alveoli of the gland almost close up the lumen and appear densely granular, the fine granules being supposed to be the precursors of the ptyalin ferment. After activity the granules become less numerous (fig. 1 14). The fine, thin, watery saliva secreted by the parotid contains no mucin. The parotid gland of man is an entirely a^lbuminous gland. The submaxillary glands are mixed, being mucous in some parts and serous in others. Some of the small buccal glands in the mucous membrane of the mouth are mucous, and some serous or albuminous. The sublingual glands are almost entirely mucous. The blood-vessels of the salivary glands form a rich capil- 172 Human Physiology lary network in the fibrous tissue around the tubular alveoli, and the lymphatics begin as lacunar spaces in the same tissue. The nerve fibres, which are derived both from the cerebro- spinal nerves and from the sympathetic, appear to pass both to the blood-vessels and to the secretory cells. 114. The Nervous Mechanism of Salivary Secretion. — Experiments on the submaxillary gland of the dog and other animals have taught us the chief facts regarding the effects of nerves on salivary secretion. It has already been shown that this gland receives efferent or motor nerve fibres,(i) from the chorda tympani, a branch of the seventh cranial nerve, and (2) from the cervical sympathetic; and we have further shown that, as regards the blood-vessels, the chorda tympani dilates them, i.e. is vaso-inhibitory, but that the cervical sympathetic constricts them, i.e. is vaso-motor (vaso- constrictor (par. 84). Experiments also prove that fibres of both these nerves act directly on the secreting cells of the gland, and that secretion is not the i . Fig. 114.— Cells from Alveoli of a Serous Gland. A, at rest ; B, after a short period of activity ; C, after a prolonged period of activity. (Langley.) In A and B the nuclei are obscured by the granules of zymogen. passive filtration of a fluid through the cell, but is due to the vital activity of the cell, for secretion continues some time after the blood-vessels are ligatured, and drugs are known that stop secretion without affecting the blood supply. Stimulation of the chorda tympani not only produces increased flow of blood through the gland through dilatation of the blood- vessels, but an increased flow of watery saliva. That the increase of secre- tion is not the mere consequence of increased blood flow, though normally associated with it, is proved thus : (a) The pressure in the duct of the gland is often much higher than the pressure in the arteries, and the greater pressure cannot be entirely due to the less ; (b) Stimulation of the chorda tympani in a decapitated rabbit leads to a secretion of saliva, and cannot then be due to variations of blood pressure ; (c) The drug atropin paralyses secretory fibres, but has no effect on vaso-dilatator action. Stimu- lation of the chorda tympani in an animal poisoned with atropin causes dilatation of the blood-vessels of the gland but no secretion, and hence secretion cannot be a necessary consequence of increased blood supply. Hence the chorda tympani is both vaso-dilatator and secretory. Experiments have also shown that stimulation of the sympathetic not Digestion ^73 only leads to constriction of the blood-vessels, but to a slight flow of thick saliva rich in corpuscles, and to a microscopic change in the structure of the cells indicative of the production by protoplasm of the material of discharge. Hence the cervical sympathetic is both vaso-constrictor and secretory. The natural and normal secretion of saliva is a reflex act. The afferent or sensory nerves are in particular the nerves of taste (lingual branch of the fifth) and branches of the ninth pair called glosso-pharyn- geal. Stimulation of these nerves by the food causes afferent impulses to pass to the nerve-centre of salivary secretion in the medulla, and from this centre efferent impulses are sent out along the vaso-dilatator and secretory fibres of the chorda tyra- pani and along the secretory fibres of the sympathetic, so that the gland becomes flushed with blood and saliva is freely discharged. Affer- ent impulses that lead to increased salivary secretions may also pass to the centre through the nerves of sight or of smell on sight Zlnc/iuUn or odour of savoury food. Mental emotions may also pass from the cerebrum via the centre, some of which, as the thought of a pleasing taste, lead to secretion, and some, as fear, inhibit secre- tion and produce dryness of the mouth. 115. Composition of Saliva. — The quantity of saliva secreted daily averages about a quart. Its rate of flow is greatest when the afferent nerves are stimulated by food and mastication is going on ; at other times only enough is secreted to keep the mouth moist. By inserting a small metallic tube or cannula into the duct, saliva may be obtained separately from each kind of salivary gland. Parotid saliva is a clear watery fluid free from mucin and containing serum-albumen, inorganic salts, and the important organic ferment ptyalin. Sublingual saliva is a viscid fluid distinctly alkaline in reaction, contains much mucin (which is precipitated by acetic acid), in- organic salts, and numerous escaped leucocytes called ' salivary corpuscles. ' The saliva from the ' mixed ' submaxillary gland is somewhat viscid, owing to mucin, and contains ptyalin, though in less proportion than in the parotid. Ordinary saliva is a mixture of these secretions with that from the tubular glands of the mouth Its chemical composition is as follows : — cerv-.'fffmfath. Submax. 05. Tymjifiires] Siibmax gland I SyntpatK.fibres Fig. 115. — Diagram to illustrate the nervous channels to the submaxillary gland of the right side. Chorda tympani fibres pass to it through the submaxillary ganglion. Sympathetic fibres reach it by branches from the superior cervical ganglion which accom- pany the arteries of the gland. (Waller.) 1/4 Human Physiology Chemical Composition of Mixed Saliva (Frerichs) Water 994-IO Solids Ptyalin 1-41 Proteids arid epithelium . . .2-13 Inorganic salts in solution . 2-29 5-9 The chief inorganic salts are sodium and potassium chlorides and cal- cium carbonate and phosphate, the two latter of which sometimes form concretions on the teeth called ' tartar. ' Carbon dioxide is also dissolved in appreciable quantity. Recent experiments have shown that the dear parotid saliva only flows freely with dry food, while the submaxillary saliva which acts as a lubricant owing to its mucin flows with all food. Most of the saliva is re-absorbed in the alimentary canal. 116. Uses and Properties of Saliva. — (a) Mechanical. — It serves to keep the mouth moist, and thus facilitates the movements of the tongue ; it dissolves sapid substances, and thus renders them capable of exciting the nerves of taste ; and it moistens and softens the food, thus aiding deglutition. These mechanical functions are important. (b) Chemical. — Its chemical or physiological action is the transforma- tion of insoluble starch into dextrin and a soluble form of sugar called maltose. This is due to the ptyalin, which is a hydrolytic ferment that causes starch to take up water and become a soluble sugar, the ferment itself undergoing no change in the process. Starch grains consist of starch enclosed in an envelope of cellulose. The saliva acts but very slowly on raw starch, but if it be boiled the cellulose envelopes are burst. The chemical action of the ferment appears to consist of several stages, several varieties of dextrin lieing produced, but the final result has been represented by the following equation : — io(C„H«O,„)+8H,O = 8(C,,H,,O„)-f2(C,,H^0,„) starch water maltose achroodextrin The action of ptyalin on starch has led it to be called an amylolytic or starch-digesting ferment, and the process is seen to be one of hydration or taking up of water. The diastase of germinating barley effects a similar transformation of starch into the sugar termed maltose. Saliva has no action on cane sugar, gum, proteids, or fat. Nearly 50 per cent.of the starch of the food is usually acted on in the mouth if properly masticated. The ferment ptyalin, like, other ferments, acts best at a cer- tain medium temperature (35° to 40° C), is delayed by cold and destroyed by heat, ' and has almost unlimited power if the product of its action (mal- tose) is not allowed to accumulate; 117. Deglutition.— The complicated act of swallowing, by means of which the food is passed from the mouth into the oesophagus, may be divided into three stages, (a) The food, sufliciently ground and moistened with saliva, is gathered together by the muscles of the cheeks and tongue into a mass or bolus and carried to the back of the mouth ; (b) it is next passed down the slope of the tongue and epiglottis through the fauces or Digestion 175 Fig. iis«. — Showing position of soft palate i,s.p.'\ and epiglottis icp.) during (A) respiration and (B) bwallowing. back part of the mouth into the pharynx, the soft palate having been raised so as to shut off the posterior nares or cavity of the nose. The act of raising and drawing back the tongue causes the epiglottis to fall down and close the entrance to the larynx, the vocal cords at the same time coming together and the whole larynx being drawn forward and upward by the thyro- hyoid muscle. (c) The constrictor mus- cles of the pharynx grasp the bolus and move it on into the oesophagus, where, by a successive wave- like or peristaltic contraction of the circular fibres, with a simultaneous short- ening of the longitu- dinal fibres, the mass is forced along to the cardiac orifice of the stomach. The first stage of the de- glutition is voluntary, but the complex set of co-ordinated movements that, follow are due to reflex action. The afferent nerves of the pharynx and oesophagus are mechanically stimulated by the food, and the impulses are reflected from the deglutition centre in the medulla along the efferent nerves to the muscles of these structures. In traversing the oesophagus the food is carried forward not by gravity but by the special kind of action (peristaltic action) in which the tube contracts from above downwards vipon its contents in a ring-like wave (see Glossary, Peristaltic). After the food reaches the gullet the trachea is opened by the elevation of the epiglottis, and at the same time the uvula or soft palate falls into its vertical position. 118. The Palate and Tonsils. — The palate forms the roof of the mouth and floor of the nose, and consists of the hard palate in front and the soft palate behind. The hard palate has a bony basis consisting of the maxillary and palate bones, and is covered by a dense structure formed of the periosteum and mucous membrane. The soft palate is the musculo-membranous movable fold attached to the posterior part of the hard palate and having its lower edge free. Hanging from the middle of its lower border is the conical-shaped process called the uvula, also composed of muscles and covered with mucous membrane (fig. 106). From the base of the uvula and under surface of the soft palate folds of muscle and raucous membrane, the anterior arches or pillars of the fauces descend on each side to the tongue. Other folds run downward and backwards on each side to the pharynx and form the posterior pillar of the fauces, being projections of the palato- pharyngeal muscles covered by mucous membrane. In the triangular recess between the anterior and posterior pillars of the fauces are lodged two oval bodies called the tonsils. A tonsil is a glandular mass of lymphoid tissue. The free surface is covered with stratified epithelium which presents from twelve to fifteen orifices that lead into small recesses or crypts from 176 Human Physiology which follicles branch out into the substance of the gland. In these follicles or nodules active multiplication of lymph-cells occurs, and these pass through the epithelium into the crypt, mingling with the saliva as salivary, corpuscles (fig. 117). 119. The Pharynx and (Esophagus. — The pharynx is the dilated upper part of the alimen- tary tube, extendingfrom the base of the skull to the oesophagus (fig. 106). It is composed of an outer fibrous membrane, striated muscles arranged in two layers, an outer layer of constrictors and an inner layer of elevators, and an internal lining of mucous membrane. The upper part of the mucous membrane is coated with ciliated epithelium con- tinuous with that of the nostrils and Eustachian tubes, but below the level of the soft palate the epithelium is stratified like that of the gullet. Numerous mucous glands are found opening on its surface. The oesophagus or gullet is the tube ex- tending from the pharynx to the stomach. This tube has an outer fibrous cover- ing, a muscular coat con- FiG. 116. — Muscles of the Tongue, Pharynx, &c., of the ngu< Leftside. (Allen Thomson.) , external pterygoid plate ; ^, styloid process ; c, section of SlStmg of an outer layer of lower jaw ; . containing some mucous glands, and an internal pale mucous membrane. A narrow layer of plain longitudinal muscular fibres, the muscularis mucoscB, is found between the mucous membrane and areolar tissue (fig. Ii8). 120. The Stomach and its Structure.— The bolus of food passes from the oesophagus through the left or cardiac opening Digestion ^77 into the stomach, where a juice secreted by the gastric glands acts on certain constituents of the food during the churning movements of its contents effected by the muscular walls, the partially digested food being at length expelled through the right or pyloric orifice into . the duodenum. The stomach when moderately distended is in the adult lo to 12 inches in length and 4 to 5 inches broad in the widest part, and it is capable of holding about six pints. Epitheliunj Tunica Fjg. 117. — Vertical Section of a Cn'Pt of a Human Tonsil X20. (Landois and Stirling.) I, crypt ; epithelium infiltrated with leucocytes helow and on the left, but free on the right ; 3, adenoid tissue with sections fi,/'2,/^^ of follicles of it ; 4, fibrous sheath ; 5, section of gland duct ; 6, blood-vessel. The walls of the stomach consist of four coats, which are from without inwards. (i) The serous coat, derived from the peritoneum. (2) The muscular coat, composed of three layers of non- striped muscular fibres, an outer longitudinal layer continuous with that of the oesophagus, a circular layer most abundant at the middle and at the pyloric portion, and an oblique layer found at the cardiac end continuous with the circular fibres of the oesophagus. N 178 Human Physiology (3) The submucous coat, of loose connective tissue with the larger blood-vessels, lymphatics, and nerves that have passed inwards through the muscular coat. (4) The mucous coat, containing the tubular secreting glands and a network of capillary blood-vessels between the glands. The chief uses of the muscular fibres in the walls of the stomach ap- pear to be the adaptation of its walls to the quantity of food in the organ, the closure of the pyloric ori- fice until the food is acted upon by the gastric juice, and the effecting of peri- staltic movements which mix the contents with the gastric juice until the un- absorbed portions are ready to be driven through the pylorus. The mucous mem- brane of the stomach consists of an epithelial layer and a corium of fine connective tissue with a basement-membrane be- tween, the corium of con- nective tissue being sepa- rated from the submucous tissue by a thin layer of unstriped muscular fibres, the muscularis mucosce The mucous membrane is smooth and soft, of a pale pink colour during life, and in the empty state thrown into folds or rugae. When examined with a hand-lens the internal or free surface of the mucous membrane of the stomach presents a peculiar honeycombed appearance owing to small shallow pits or Fig. 118. — Section of the Human CEsophagug, (From a sketch by V. Horsley. ) The section is transverse, and from near the middle of the gullet, a, fibrous covering ; ^, divided fibres of the longitudinal muscular coat ; £■, trans- verse muscular fibres ; dy submucous or areolar layer ; f, muscularis mucosae ; f^ papillje of mucous membrane ; ^, laminated epithelial lin- ing ; hy mucous gland; z, gland-duct ; m\ striated muscular fibres in section. Digestion 179 depressions -^ to ^-J-^ of an inch in diameter, separated by slightly elevated ridges. These pits are the mouths of the ducts of the gastric glands, and the thickness of the mucous membrane of the stomach is due to the fact that it is closely stud- ded with tubular glands set vertically side by side (fig. 119), and bound together by a small quantity of connective tissue which contains the blood-vessels and lymphatics. At the bottom of the pits or ducts are seen minute orifices, which are the openings of the gastric follicles or secreting tubules that branch from the duct. Examined with the microscope the ducts leading into the walls of the stomach are in all cases seen to be lined by a columnar epi- thelium of mucous secreting cells similar to that which forms the free inner surface of the mucous mem- brane, but the epithe- lium of the glands proper or secreting follicles differs from this, the difference being of a different kind in the glands of the cardiac and pyloric ends. We have therefore two chief varieties of gastric glands, (a) cardiac or peptic glands and (V) pyloric glands. The cardiac or peptic glands are tubular glands with a short duct from which branch two or three long tubules, though some are simple or unbranched. The epithelium of the tubules of the cardiac glands differs from the columnar epithelium of the duct, being composed of two different kinds of cells — cubical granular cells, called central or chief cells, forming an almost continuous layer in the tubule; and ovoid opaque granular cells N 2 Fig. 119. -A Section through the Walls of the Stomach. Magnified 15 diameters. , surface of the mucous membrane, showing the open- ings of the gastric glands ; z, mucous membrane, composed almost entirely of glands ; 3, sub-mucous or areolar tissue; 4, transverse muscular fibres ; 5, longitudinal muscular fibres ; 6, peritoneal coat. 1 80 Human Physiology with clear nucleus, called parietal or oxyntic cells and found scattered among the chief cells down to the blind end of the tubule. The chief cells are believed to be the source of the _ Parietal cells £CALE 100 /£ PYLORIC CARDIAC Fig. 120.— Gastric Glands opening by Ducts lined with Columnar Epithelial Cells on the free surface ofthe Stomach. The Cardiac Gland shows two kinds of gland cells, and the Pyloric Gland one kind. tr^ a deep portion of a gland tubule cut transversely. pepsin in the gastric juice, and the oxyntic or parietal cells the source of the hydrochloric acid. (Gk. oxys^ acid, sharp.) The pyloric glands occur only in the region of the pylorus, have longer ducts with well-defined lumen from which branch Digestion i8i short wavy secreting tubules with narrow necks. The duct is lined with columnar mucous epithelium cells as in the cardiac gland, but the cells of the secretory part are all of one kind, corresponding to the chief cells of the cardiac glands. The pyloric glands secrete pepsin but no acid. As in the case of the salivary glands the appearance of the cells in the gastric glands differs according to their physiological activity. During rest the central cells in both cardiac and pyloric glands are loaded with granules which are discharged into the lumen during activity. After activity each cell is seen to have a clear outer zone, the size of which increases with prolongation of activity. The ovoid pa- rietal cells of the cardiac gland show shrinkage only during digestion. The gra- nules of the central cells fur- nish another instance of a zymogen or ferment precur- sor, in this case pepsinogen. During discharge into the lumen of gland pepsinogen becomes transformed into pepsin. The blood-vessels of the stomach are very numerous, and pass to the organ along its curvatures. The arteries, after giving off branches to form a capillary network in the mucous coat, run through the sub- mucous coat to form a longitudinal capillary network between the tubular glands, terminating at the surface in larger horizontal capillaries around the orifices of the ducts. From this superficial horizontal network the veins run downwards. Lymphatics arise in the mucous membrane and pass into larger vessels in the submucous coat. A gangliated plexus of nerve-fibres with many ganglia is found in the submucous and muscular coats. These nerves of the stomach are derived from the pneumogastric and sympathetic system. Fig. I2T.— Section of the Gastric Mucous Membrane taken across the direction of the Glands (caidiac part). i, basement membrane ; c, central cells ; £?, oxyntic cells; r, retiform tissue (with sections of blood-capillaries) between the glands. Human Dog 994-4 973-06 3'I9 17-13 ■20 30s 2 -08 4-36 •12 2-00 182 Human Physiology 121. Composition and Properties of Gastric Juice. — Gastric juice is a clear colourless acid fluid with a specific gravity of 1-003, and containing only \ per cent, of solids in solution. Its chief constituents are two fer- ments, pepsin and rennin ; free hydrochloric acid; mineral salts, and 99 per cent, of water. That there is free hydrochloric acid in gastric juice has been proved by estimating the total quantity of chlorine and assigning to the metals present the due proportion. A quantity of chlorine remains over, which must therefore be combined with hydrogen to form free hydro- chloric acid. Moreover, chemical tests show the presence of free hydro- chloric acid. Analysis of gastric juice gives the following results in parts per 1,000 : — Water Organic .substances, chiefly pepsin . Free hydrochloric ..... Calcium, sodium, and potassium chlorides Calcium, magnesium, and iron phosphates The total quantity of gastric juice secreted daily has been estimated to- range from 10 to 20 pints and to be nearly one-tenth weight of the body. It is mostly reabsorbed in the small intestine with the food. Gastric juice has been obtained through an accidental fistula or per- manent opening from the outside into the stomach. The mucous mem- brane was excited to action by introducing hard substances, and the liquid secreted was then drawn off for examination. It can also be obtained from the stomach of a recently killed animal by. treatment of the mucous membrane with glycerin and hydrochloric acid. Pepsin can also be pre- pared from the gastric mucous membrane, though probably not entirely isolated from other bodies. It is a proteid-like substance, only acting in an acid medium. The functions of gastric juice have been ascertained by observing the changes undergone by the food in the stomach and by experimenting with the gastric juice obtained from a fistula. It has thus been shown that it acts only on proteids, which it dissolves and converts into peptones. Peptones are distinguished from other proteids by the property of diffusing readily through animal membranes. They are not coagulated by boiling. But no peptone is found in the blood of the portal vein, as during its passage through the vascular walls of the stomach it is converted into serum-albumen. If a piece of lean meat or some shreds of fibrin ba placed in gastric juice and the mixture kept at a temperature of about 40° C. for some time, artificial digestion will go on. In about an hour the fibrin will be in great part dissolved, and by appropriate tests three other forms of proteid will be found. These are acid-albumen or syntonin, albumoses, and peptone. If a solution of pepsin alone were added to the fibrin, no change would appear. If a weak solution ( -2 per cent. ) of hydrochloric acid alone were used, the fibrin would become swollen and transparent merely. Putting the solution of pepsin and the weak solution of hydrochloric acid together would lead to the fibrin being dissolved. Boiling the solution of pepsin, however, would render it inactive. It thus appears that both pepsin and free hydrochloric acid are necessary for the conversion of the proteid of food into peptone, and that the change into peptone takes place in stages, the action being of the nature of hydration, or taking up the elements of Digestion 183 water. We may put the matter thus. Proteid food stuffs acted on by the pepsin and hydrochloric acid of gastric juice are converted in the end into a hydrated kind of proteid called peptone ; intermediate substances termed proteoses being formed in the process. The need for the change from proteid to peptone is obvious when we remember that the food proteids (albumin, globulin, casein, &c.) are mostly insoluble in water, and will not diffuse through animal membranes, while peptones are easily soluble and diffuse readily through membranes. If some albumin, such as white of egg or blood serum, be tied up in a sound bladder, and the bladder put into water, little or none of the proteid will pass into the water. If peptone be used instead of albumin, much of it quickly passes into the surrounding water, and water passes from the outside into the bladder. This diffusive passage of substance through membranes is called osviosis, and evidently plays an important part in the absorption of food from the alimentary canal into the blood (see par. 132). It thus appears that the gastric juice, which is the mixed secretion from the tubular cardiac and pyloric glands of the stomach, consists of a watery solution of two ferments, pepsin and rennin, with salts and free hydrochloric acid, and that its chief action consists in converting the proteids of food into diffusible peptones. In the case of milk, a curd or clot of casein is first formed by the action of the rennin in the gastric juice, and this proteid casein is then turned into a peptone by the action of the pepsin and hydrochloric acid of the juice. Fat is liquefied by the heat of the stomach, but the gastric juice has no action on fat except that it dissolves the proteid covering of the fat cells and thus sets the fat free to mingle with the other contents of the stomach. Gastric juice has no action on carbo-hydrates. It rather stops the conversion of starch into sugar as soon as the acid fluid can affect the food taken in, for the ptyalin of saliva is only active in a neutral or acid solution. It may, however, be some time (one or two hours) after the food reaches the stomach before the acid juice penetrates the swallowed masses and stops the action of the saliva. Gastric juice is very active in dissolving collagen, the main proteid of connective tissue. Animal foodstuffs are thus quickly broken up, muscular tissue being thus broken up into its fibres and its fat cells set free. As gastric juice has an antiseptic action, the breaking up of animal foods enables the fluids to penetrate these foods quickly, so that its antiseptic action may destroy micro-organisms and check putrefaction. Why the stomach does not digest itself during life by its own gastric juice is not very clear. An animal killed during digestion and examined some hours later is found to have its stomach partially digested. In most deaths secretion ceases before circulation stops. In some way, however, the living tissue of the stomach resists digestion during life. The flow of gastric juice is not continuous, for during fasting the gastric glands are inactive. The natural stimulus is the advent of food into the stomach. This causes a copious outflow, and is accompanied by an increased flow of blood, which causes the pale mucous membrane to become red. Even the introduction of indigestible substances, as pebbles 01 mechanical stimulation with a feather, induces a small amount of secretion. Dilute alkalis have powerful stimulating effects. The stomach is connected with the central nervous system by nerve-fibres from the two vagi. It also receives fibres from the so-called sympathetic system, , by branches 184 Human Physiology from the solar plexus of the splanchnic nerves. But there is no proof that any of these nerve-fibres regulate secretion, for it goes on when all nervous connections are severed. There is no nerve passing to the stomach whose stimulation excites a secretion of gastric juice as the chorda tympani does in the submaxillary gland. Possibly there are local centres in the walls of the stomach. Emotional states are known to interfere with gastric secretion, and this shows that the central nervous system must be connected in some way, direct or indirect, with the gastric glands. The acid mixture of finely divided food and gastric juice is called chyme. Fig. 122. — The Stomach, Duodenum, Liver, Spleen, and Pancreas. I, stomach ; 2, pylorus ; 3, duodenum ; 4, liver (under surface) ; s, gall-bladder ; 6, pan- creas ; 7, bile-duct; 8, pancreatic duct; 9, spleen; 10, aorta; 11, portal vein; 12, splenic artery ; 13, splenic vein. Note that in the figure the liver is turned up and the other organs drawn apart somewhat Compare with figs. 90 and 131. 122. Position and Structure of the Pancreas.— The pancreas is a long racemose gland of light colour which lies across the abdomen, behind the lower part of the stomach, opposite the first lumbar vertebra. Its right broad end or head lies, in the cavity of the duodenum, while its left narrow end or tail lies tm the top of the left kidney and in contact with the spleen. The structure of the pancreas resembles closely that of the salivary glands, being, like them, divided into lobes and lobules, with ducts and acmi. The gland has a thin connective-tissue capsule which sends septa or divisions between the lobules, and these septa carry blood-vessels and nerves. The secretory nerves of the pancreas have been proved to be derived from the vagus, for suitable stimulation of the vagus has been observed to produce an increased flow of pancreatic juice. Digestion i8s The main pancreatic duct runs from left to right through the whole length of the gland lying near the anterior surface (fig. 122A). In the head it curves downwards and then passes into the duodenum, where it runs obliquely and is joined by the common bile-duct, so that the two have a common orifice in the descend- ing portion of the duo- denum. The main pancreatic duct, formed by the junction of ducts from lobules, begins in the tail. It is joined along its length by smaller con- tributory ducts from Fig. 122A. — Pancreas, showing near the anterior surface its main duct and tributaries, and the connection of the duct with the duct from the gall-bladder. ti, main duct ; ^, common bile-duct ; c, duct entering duo- denum. Other lobules, and by a supplementary duct from the lower part of the ' head of the gland. Each lobule of the pancreas, as in the salivary glands, contains one of the ultimate branches of a duct, terminating in a number of blind alveoli or acini, with narrow lumen. The acini are more tubular in character and more numerous relative to the ducts than in the sali- vary glands, and the con- nective tissue of the gland is somewhat looser. The smajl ducts in a lobule that lead to the secreting tubes or alveoli have a distinct lumen lined with a single layer of flattened cells, resting on a membrana propria ; but the convo- luted tubular acini are lined with cubical granu- lar cells, the lumen be- tween the cell-layers being scarcely visible. The gra- nular contents of the cells are considered to be pro- ducts of the activity of the cell-protoplasm (zymogen) which is about to be trans- formed into ferment in active secretion. The increase in the size of the outer clear zone of the pancreatic cells, as the granular contents are discharged transformed into ferment, has actually been observed during life in the pancreas of the rabbit, which is scattered between the layer of the mesentery, so that individual lobules may there be microscopically examined (fig. 124). 123. Composition and Properties of Pancreatic Juice. — Pancreatic juice is a clear, viscid alkaline fluid, having a specific gravity of i"03O, Fig. 123. — Section of the Pancreas of the Dog. (Klein.) rf, end portion of a duct leading to an alveolus a, lined with cells. 1 86 Human Physiology coagulating with heat owing to the presence of albumen. It contains when obtained from a newly opened duct — Water about 90 per cent., solids about 10 per cent. The solids include proteid substances, inorganic salts, especially sodium carbonate, and four ferments, viz. ; — trypsin, a proteolytic or proteid -digesting ferment ; amylopsin, an amylolytic or starch-digesting ferment ; steapsin, a fat-splitting ferment ; and rennin, a milk-curdling ferment. If obtained from a permanent fistula the secretion in time becomes more watery. Secretion is normally excited by the chyme, and is probably a reflex act, but the nervous channels are not known. The presence of the four ferments makes the pancreatic juice the most important digestive juice of the body. Action on Proteids. —fioteids are converted by pancreatic juice into albumoses and peptones as in the case of gastric juice. But trypsin acts only in an alkaline medium, and pepsin only in an acid medium. Hence the first stage of the change into peptone is alkali-albumin and not acid- .B. Fig. 124. — Part of an Alveolus of the Rabbit's Pancreas. A, at rest; B, after active secretion. (From Foster, after Kuhne and Lea.) a, the inner granular zone, which in A is larger and more closely studded with fine granules than in B,^ in which the granules are fewer and coarser ; ^, the outer trans- parent zone, small in_.^, larger in B, and in the latter marked with faint striaj ; c, the lumen, very obvious in £, but indistinct in A ; d, an indentation at the junction of two cells, only seen in B. albumin. Trypsin also acts more powerfully than pepsin, the proteid not being first swollen and then dissolved as in the case of gastric juice, but gradually eaten away and dissolved at the edges. Further, trypsin carries the process of digestion further, decomposing the hemi-peptone into the simpler products leucine and tyrosine. (See par. 128.) Action on Carbohydrates. — Carbohydrates as starch are converted by the amylolytic ferment of pancreatic juice into the same products, dextrin and maltose, as those produced by the ptyalin of the saliva. But the action is much more rapid and more powerful. Glycogen, or animal starch is affected in the same way. Action on Fais.—Ym^h. pancreatic juice contains a ferment that causes fats to take up water and split up into glycerin and a free fatty acid. The fatty acid then combines with an alkali present to form what is called a soap, the whole chemical change being called saponification. The presence of a soap in the contents of the duodenum assists in breaking up the rest of the fat into fine particles, as it forms a thin layer outside each liquid Digestion 1 87 fat globule, and thus prevents them running together again. This me- chanical division of oils and fats into finely suspended particles is called emulsification. Pancreatic juice, therefore, emulsifies and saponifies fats. Action on Milk. — Pancreatic juice also contains a ferment similar to rennet, which has the power of forming a curd in milk. Complete removal of the pancreas leads not only to the loss of pan- creatic juice in the intestines but to a condition of diabetes (par. 147). It thus appears that, besides its function of producing pancreatic juice, it has some other function related to the general nutrition of the body, so that this is disturbed by removal or disease of the gland. It appears, in fact, to be a general truth that each organ not only does its own special work, but that it is concerned in the general metabolism of the body. (See pars. 150, 152.) One theory is that certain glands produce an internal secretion, which passes away in the lymph, and is then distributed to aid in regulating changes elsewhere. 124. Bile.— The bile is the fluid secreted by the cells of the liver from the blood brought to it by the portal vein. As this gland has several other functions, its minute structure will be considered separately, and we shall now treat only of the composition and properties of the bile. This fluid passes into the duodenum by the hepatic duct during digestion, but is carried by the cystic duct to be stored in the gall-bladder when digestion is not proceeding, as it is then easier for the bile to pass in that direction, for the orifice into the duodenum is narrower than the duct and requires some pressure to open it. This required pressure is only great enough when the stimulus of the acid chyme, passing into the duodenum, sets up contraction in the gall-bladder and the gall-ducts. Bile is of a brown or greenish colour, has a bitter taste, is slightly alkaline and slimy from the presence of mucin. Its specific gravity varies from i-020 to 1-040, and its average chemical composition is as follows : — Chemical Composition of Bile from Gall-bladder.^ 100 parts contain ; — Water ... 86 Bile salts ...... . 9 Fat, lecithin, and cholesterin ..... i Mucus and colouring-matter ... .3 Inorganic salts ...... .1 The bile salts are glycocholate and taurocholate of sodium, the former being by far the more abundant. The colour of the bile is due to the two pigments, bilirubin (yellow) and biliverdin (green), the former of which is the more abundant. When the flow of bile is obstructed the colouring matter is absorbed unchanged, and produces the yellow colour of the tissues seen in the disease jaundice. These bile-pigments are made by the liver from the haemoglobin of the red ' Liver bile contains only about 2 per cent, of solids, gall-bladder bile about 12 per cent. , the difference being due mainly to concentration in the gall-bladder and to the mucin obtained from the bladder-cells. 1 88 Human Physiology corpuscles destroyed in the liver, the bile-pigment differing from blood-pigment in not containing iron. Powerful reducing agents remove some of the oxygen from the bile-pigments, and produce hydrobilirubin. This is identical with urobilin, which is found in urine, and with stercobilin, found in the faeces. Hence the bile appears to be the agent by which the effete colouring matter of the blood is excreted. Cholesterin belongs to the class of organic bodies called alcohols, and is found in bile and the brain. It crystallises in rhombic plates, and though containing no nitrogen is probably produced from a proteid. Lecithin is the compound of bile containing phosphorus. 125. Functions of Bile. — Bile is a fluid secreted mainly for aid in digestion, though it is excreted in part also. Most of the bile secreted by an adult (about one quart daily) is reabsorbed in the intestine after it has done its work. A small part is excreted in the faeces, especially in the colouring matter, and a small part of that reabsorbed portion is removed as urobilin by the kidneys. As a digestive agent, the bile appears to act in concert with the pancreatic juice. (i) Bile aids in emulsifying the fats so as to render them ready for absorption by the lacteals. Bile and oil shaken together form a fine emulsion, and the pancreatic juice still further assists the process by splitting up some of the fatty particles into glycerine and a fatty acid, the fatty acid uniting with sodium to form a soap. A soap favours emulsion and diffusion through an animal membrane. Experiment and observation point to the fact that both bile and pancreatic juice working together lead to the absorption of fat, as where either is excluded the absorption of fatty particles is incom- plete. (2) Bile precipitates the albumoses and peptones of gastric digestion, stopping the action of the pepsin in the acid chyme in the duodenum. It thus prepares chyme for the action of the pancreatic juice by neutralising the acidity of the chyme. (3) Bile excites contraction of the muscular walls of the intestine, thus stimulating the intestines to propel forwards their contents. (4) Bile also acts as an antiseptic, preventing undue putre- factive changes in the intestine. Digestion 1 89 1 26. The Peritonenm and Mesentery. — The peritoneum is the serous membrane of the abdomen, and like other serous membranes forms a closed sac (except in the female), one layer of which (the parietal layer) lines the boundary walls of the cavity, and the other layer of which (the visceral layer) is reflected more or less over the contained organs. The membrane is very thin, and its free surface is smooth and moist, being covered by a layer of flattened endothelial cells ; its attached surface is rougher, being connected to the abdominal walls and viscera by areolar tissue. The peritoneum consists of a main cavity or great sac, connected by a narrow neck with a pouch or small sac. A general idea of the peritoneum and its com- plex reflections may be obtained by supposing all the viscera of the abdomen in position and a closed sac of extreme thin- ness to be placed over them. Wherever there is a cleft between viscera, a process of the peritoneum derived from the part of the sac in contact with the viscera is tucked in between them, ' so as to cover the adjacent surfaces of the viscera and separate them from each other, and at the same time, by becoming adherent to the viscera, form an investment for them.' The reflections of the peritoneum from the abdominal walls thus serve to invest and keep in position the various viscera, and the processes or folds thus formed are of various kinds. Some of these folds, as those passing to the liver and bladder, are termed ligaments. Others connected with the stomach are called omenta, the great omentum consisting of four folds of peritoneum stretching from the greater curvature of the stomach to the transverse colon, from which it hangs down in front of the small intestines as a great protecting flap or apron. The great omentum always contains some adipose tissue which in fat persons accumulates largely. The mesenteries are the folds connecting the intestines to the posterior abdominal walls. The mesentery proper is the broad double fold of peritoneum retaining the small intestines in position and con- necting them with the posterior abdominal wall. Between its layers, which are in apposition where it does not invest the intestine, are blood-vessels and nerves, lacteal vessels and the mesenteric lymphatic glands (figs. 131, 132). 190 Human Physiology 127. The Structure of the Intestines.— The intestinal canal from the pyloric end of the stomach is divided into small intestine and large intestine. The small intestine, whose length is about 20 feet in the adult, is divided into duodenum (10 inches), jejunum (8 feet), and ileum (11 feet). The large intestine begins in the right iliac region of. the trunk, where the small intestine joins it at right angles by the ileum, or last part of the small intestine. The opening of the small intestine is provided with projecting folds of mucous membrane that allow material to pass on but oppose its back- ward passage. This opening is known as the ileo-cacal valve, for the dilated beginning of the large intestine is termed the ccecum. Fig. 125. — A Portion of the Small Inte-^tine laid open to show the Folds of the Mu- cous Membrane (Valvulse Conniventes). Fig. 126. — A Small Portion of the Mucous Membr.nne of the Small Intestine. Mag- nified 12 diameters. u, Peyer's glands, surrounded by tubular glands ; b, villi ; c, openings of "the tubular glands. A small narrow blind tube at the end of the ctecum is known as the appmdix (fig. 133). The appendix is a rudiment of a part of the intestine of some impor- tance in herbivorous animals. In man it sometimes becomes the seat of dangerous inflammation owing to indigestible bodies passing into it. From the point of junction of the small and large intestine, the latter passes up on the right side as the ascending colon, passes across below the liver and stomach as the tranverse colon, and then descends 6n the left side as the descending colon. The puckered colon ends below a flexure in a short straight portion called the rectum, the opening of which, called the anus, is guarded by a sphincter muscle, which is normally con- tracted under the influence of a nervous centre in the spinal cord. Digestion 191 The walls of the intestines are constructed of the four coats already referred to, the serous from the peritoneum, the muscular with its internal circular and external longitudinal layer of fibres, the submucous or con- nective tissue in which the blood-vessels and lymphatics ramify, and the mucous membrane lining the interior. A gangliated plexus of non-medul- lated nerve-fibres lies between the two muscular coats, and another in the submucous coat.' In regard to the function of digestion, we require to note carefully the following structures in the mucous membrane : (a) vahiula conniventes ; (1^) three varieties of glands ; (c) villi. The val- vulae conniventes are crescentic folds of mucous membrane arranged transversely in the small intestine from a ,little below the pylorus to the middle of the ileum. They do not disappear when the tube is distended, but form vertical ridges on the interior surface. They serve to increase the surface for secretion and absorption, to check the rate of transit of the liquid products along the canal, and to assist in mingling the particles of food. The glands of the intestine are of three kinds, (i) The glands or crypts of Lieberkiihn are simple tubular glands lined by columnar epithe- lium distributed all over the surface of the small and large intestines, though increasing in size in the large intestine as they approach the anus. In the small intestine the crypts of Lieberkiihn open between the villi. (2) Brunner's glands are convoluted tubules similar in structure to the pyloric glands of the stomach, found only in the duodenum, and their epithelium undergoes similar changes during secretion. Their blind ends are found in the submucous coat, and the duct passing through the muscu- laris mucossE opens on the surface of the mucous membrane. (3^ Peyer's glands are found in the lymphoid tissue between the tubular glands, and are closed sacs or nodules of the size of millet-seeds, and composed of cells and blood-vessels without any ducts. When occurring singly, they are known as solitary glands ; when in groups, they are known as Peyer's patches. These patches occur chiefly in the ileum. Villi are minute thread-like processes found exclusively on the inner surface of the small intestine. They are concerned in the absorption of food, and will presently be described in detail. The large intestine has the usual four coats, the longitudinal fibres of the muscular coat being gathered into longitudinal bands that produce a puckering of the wall. Its mucous membrane is lined by columnar epithelium containing numerous mucous secreting cells, and there are numerous simple tubular glands along nearly the whole length, but no vilh. 128. Succus Entericus or Intestinal Jnice. — Intestinal juice is a thin alkaline fluid that forms the secretion of the numerous minute glands of Lieberkiihn found in the small intestine. It has recently been shown to be a juice of great importance. Besides a power of converting cane sugar (saccharose) into maltose and glucose owing to a ferment called invertin, it is found to reinforce and increase the action of the pancreatic juice on proteids. It is, in fact, doubtful if pancreatic juice alone will act on pro- teids. Intestinal juice has also a powerful action on the proteoses and peptones produced in the alimentary canal, breaking them up into simpler substances owing to the action of a ferment termed erepsin. As these simpler products are not found in the blood or lymph they appear to be re-formed into proteids during absorption. ' The former is known as Auerbach's nerve plexus ; the latter as Meissner's plexus. 192 Human Physiology It will be seen, from what has been said on digestion, that the various processes in the alimentary canal are not isolated phenomena,but that one secretion aids another, and each step is related to and follows on as the result of the previous steps. The amount and power of the various digested juices secreted is also adapted to the kind of food ingested. 129. Digestive Changes in the Small Intestine. — The chyme as it leaves the stomach consists of the starchy matters that are in process of conversion into dextrin and sugar and the sugar thus converted and dis- solved in the fluids but remaining unabsorbed ; the proteid material con- 127. —Vertical Section of a Portion of a Patch of Peyer's Glands with the Lacteal Vessels injected. (Frey.) 32 diameters. The specimen is from the lower part of the ileum «, villi, with their lacteals left white ; b, some of the tubular glands ; c, the muscular layer of the mucous_ membrane ; d^ cupola or projecting part of the nodule ; ^, central part ; /, the reticulated lacteal vessels occupying the lymphoid tissue between the nodules, joined above by the lacteals from the villi and mucous surface, and passing below into ^, the sinus-like lacteals under the nodules, which again pass into the large efferent lacteals, ^ \ ?, part of the muscular coat. verted into peptones and partially converted ; fatty matter broken up and melted ; gastric juice, saliva, and fluid that has been taken as drink ; and indigestible parts of food. In the duodenum the chyme is subjected to the action of the pancreatic juice, the bile, and the succus entericus, and this action continues as the food is driven along the digestive tulse by the peristaltic contractions of its walls. The starchy or amyloid portions of the food, the conversion of which into sugar had been arrested in the stomach by the acid gastric juice, are acted on vigorously by the amylolytic ferment of the alkaline pancreatic juice as soon as the acidity of the chyme is neutralised (the contents of the duodenum become alkaline a few inches from the pylorus), and the sugar in the form of maltose or glucose is dis- Digestion 193 soived in the intestinal fluids and absorbed by the intestinal blood-vessels. The proteid or albuminous substances which have been only partly dissolved and absorbed in the stomach are brought in the intestines under the influence of the pancreatic and intes- tinal juices. The action of the pepsin of gastric juice, which is only active in an acid medium, appears to be de- stroyed by trypsin, for pepsin is precipitated along with the gastric peptones and albu- moses on meeting the pan- creatic juice and bile in the duodenum. Trypsin then continues the digestion of the proteids by converting them and the allied gelatinous mat- ters into diffiasible peptones, which are absorbed as they pass along the intestines by the blood-vessels and lym- phatics. Possibly some of the peptone is split up into the crystalline bodies leucine and tyrosine. If so, these sub- stances leave the body as urea without having been built up into tissue material, and only contributing to the energy of the body heat during the chemical changes that con- vert them into urea. This apparently wasteful expendi- ture of proteid food has been called ' luxus-consumption,' or wasteful consumption. Another important digestive change in the small intestine is the alteration of fat in such a way as to render it ready for absorption. As already described, this is brought about by the combined action of the pancreatic juice and the bile, and consists partly in producing a fine subdivi- sion of the particles of fat called an emulsion and partly of a chemical decomposition by which a soap is formed. Most of the fat thus changed is absorbed by the lacteals of the villi, as will presently be Fig. 128.— Small Intestine, Vertical Transverse Section, with the Blood-vessels injected. (Heitz- mann.) V, avillus ; G, glands of Lieberkuhn ; M, muscularis mucosae ; A^ areolar coat ; i?, ring-muscle (circu- lar layer of muscular coat) ; Z, longitudinal layer of muscular coat ; p, peritoneal coat. O 194 Human Physiology explained ; but a small part of that converted into soap is probably absorbed by the blood-vessels of the intestine. The cane sugar that reaches the small intestine is mostly converted into invert sugar by the succus, entericus before it is absorbed. ' The liquids taken as drink, as wrell as the digestive juices poured into the intestine and containing the dissolved nutrient materials, are also in great part absorbed. The unabsorbed chyme that passes into the large intestine is still half-liquid, but of a light yellow colour and possessed of a distinct fiEcal odour. 130. Digestive Changes in the Large Intestine. — From the absence of villi in the large intestine we may conclude that little absorption of fatty matter occurs in the large intestine, but other digestive changes may continue. It is clear that absorption of the liquid parts continues, for the contents become firmer and more solid as they approach the rectum. Moreover, nutrient material injected into the large intestine is found to nourish the body when food cannot be taken into the stomach. Other changes of a putrefactive nature are produced by minute living beings called micro-organisms (bacteria). These bacteria are unicellular beings that multiply by division and produce changes in the media in which they live. Numerous bacteria are found in the intestines, and in the large intestine especially set up putrefactive processes. These changes are probably checked by the antiseptic action of the bile. Some of these changes may be beneficial, but others lead to the production of such gases as carbon dioxide, sulphuretted hydrogen, and marsh gas in the intestines. 131. The Faeces. — The feeces expelled from the body consist of the indigestible residue of the food, substances taken in too large quantity for digestion, certain excrementitious matter from the digestive juices not absorbed, substances produced in the intestinal canal, and inorganic salts. Among these bodies are : — (a) Cellulose, woody fibre, uncooTted starch, horny matter, bacteria, &c. {b) Mucin, cholesterin, stereobilin, and other pigments (derived from the bile pigments as they disappear when the bile is cut off), excretin, skatol, CjH,N (a nitrogenous body produced by bacteria and the chief cause of the faecal odour), fatty acids. (it) Lime and magnesia soaps, earthy phosphates, &c. The fseces on a mixed diet are about one-eighth the weight of the food. 132. Absorption of Food.— By the secretory activity of the cells in the various glands of the alimentary canal we have seen that ferments are prepared that so act on the proteids, carbo- hydrates, and fats that a fluid is produced in the alimentary canal that contains these alimentary principles of the food in a diffusible condition. How, then, does this fluid pass through the living epithelial wall of the mucous membrane into the underlying capillary blood-vessels and lacteals ? In the first place, we know of a physical process called ///ra/w«, by which is meant the passage of fluids through the pores of a membrane under pressure. Substances that may be obtained in the form ' See Appendix, ' Carbohydrates.' Digestion 195 of crystals, or crystalloids, as they are termed, filter easily when in solution. Glue-like substances, or colloids, as they are termed, filter with difficulty. The greater the pressure the greater the amount that will filter through in a given time. Filtration may possibly occur to a small extent under the pressure exerted on the digested food by the contraction of the intestinal walls. (The condition known as oedema, or dropsy, in which the lymph-spaces of the connective tissues and other structures become charged with abnormal accumulations of lymph, appears to be due mainly to excessive filtration or tran- sudation of this fluid consequent on increased pressure in the capillaries and veins.) In the second place, we can partly explain the passage of the diffusible and dissolved food materials into the blood and lacteals by the physical process called osmosis. This is a kind of diffusion that takes place between two different solutions separated by a membrane. If a strong solution of sugar or salt be separated by a thin medbrane, as dead bladder or parchment paper, from a weak solution, sugar or salt passes from the stronger solution to the weak solution, and water from the weaker solution to the stronger, in consequence of the osmotic current set up. The exchange of fluid particles in osmosis takes place inde- pendently of pressure, and the rate at which osmosis takes place varies as well as the total amount of exchange varies with the differences of concentration in the solution, the qualities of the dissolved substances, and the nature of the separating membrane. Crystalloids pass through a membrane, or dialyse, as it is termed, much more readily than colloids. Sugar passes more easily than peptone. The conditions for osmosis clearly exist in the alimentary canal. On one side of the mucous membrane of the intestine there are relatively concentrated solutions of diffusible salts, peptones, sugar, and soap, and on the other side are the blood-vessels containing proteids scarcely diffusible. Moreover, the circulation of the blood is con- stantly removing the fluid that has imbibed material from the intestine and bringing fresh fluid capable of absorption. Hence we may say that some of the digested food passes into the capillary vessels in the mucous membrane of the intestine by 0.2 1 96 Human Physiology osmotic diffusion, or at any rate that osmotic diffusion is one factor in absorption of digested foOd. But the absorptive epithelium of the intestine is unlike the dead bladder or parchment in being a living membrane, and there are certain facts that make it probable that the vitality of the membrane exerts an influence in the process of food-absorption. Thus a loop of intestine can be isolated in a living animal, and when various substances in solutions of various strength are intro- duced and their disappearance studied, it is found that there is a difference between the ordinary diffusion of these sub- stances through a membrane and their diffusion through the mucous membrane of the living intestine. It thus appears likely that the secretory activity of the living epithelial cells in the intestinal mucous membrane plays an important part in the absorption of food, and that the phenomena do not agree entirely with the physical processes of filtration and osmotic diffusion. Why, then, does the process of digestion essentially consist in rendering foodstuffs soluble and diffusible ? Dr. Foster answers the question thus : ' Because though the cell is not an apparatus for diffusion, diffusion is an instrument of which the cell makes use. When we say that peptone does not enter the blood by ordinary diffusion, we do not mean that diffusion has nothing to do with the matter. The activity of a living cell is an activity built up upon and making use of various chemical and physical processes ; in it the processes of ordinary diffusion play their part as to the ordinary processes of chemical decomposition ; but the cell uses and modifies them for its own ends. If, as we have every reason to believe, the cell of a villus passes the sugar unchanged from the intestine into the blood-capillary, it makes use of diffusion to effect that passage ; and if it does change the proteid into something else before it passes it on, it receives it into itself in the first instance by the help of diffusion. When we say that substances do not enter the blood by ordinary diffusion, we mean that the diffusion which takes place in a living cell is something so different in the results from ordinary diffusion through a dead membrane that it is undesir- able to speak of it by the name.' Digestion 197 What has been said above applies chiefly to the proteids and carbohydrates of the food, and may be taken to refer to the passage of these digested food-principles both through the epithelium cells of the mucous membrane and through the flat endothelial cells of the capillary vessels situated beneath the basement-membrane. The sugar formed by the ferment of the salivary glands and the amylopsin of pancreatic juice is maltose. Before absorption it is changed by the succus entericus into glucose, and as such is found in blood. Lactose is also changed into glucose, as is also most of the cane sugar. Most of the proteid is absorbed as peptone or peptone and albumose, but, as already noted, neither peptone nor albumose is found in the blood, and we therefore conclude that through the activity of the epithehum-cells or lymph-cells, or both, peptone is changed into blood-proteids during the actual process of absorption. As to the fats, it has been shown that they undergo either a chemical change (saponification) or a physical change (emulsification) ; a small part is saponified, and the soaps so formed are absorbed like the other soluble materials. But the greater part is reduced to a fine state of subdivision or emulsified, and these minute particles are absorbed by a special mechanism in the villi. 133. Structure of a Villus. — The absorbing surface of the mucous membrane of the small intestine is increased by the transverse folds called valyulae conniventes, and by the numerous small processes projecting into the interior along its whole length. They are largest and most numerous in the duodenum and jejunum, and become smaller and fewer in the ileum. Some are conical in shape, some cylindrical, and some triangular, and the average size is about -^ inch. Their total number has been estimated to exceed four millions. Each villus consists of a small projection of mucous membrane coated with columnar epithelium continuous with that of the other parts of the intestine, a basement-membrane supporting the columnar epithelium, an interior framework or stroma of adenoid tissue in which is a network of capillary vessels, and a central lymphatic or lacteal with unstriped muscular fibres around it. The central lacteal begins in a 198 Human Physiology blind end near the summit of the villus, and is continued below into a plexus of lymphatics in the submucosa, which joins together to form larger vessels that proceed to lymphatic glands. The endothelial cells that form the walls of the lacteal appear to have openings between the cell-plates that are in connection with the spaces or meshes of the adenoid tissue of the villus, Fig. 129.— Villi of Small Intestine. (Cadiat.) a, small artery ; b^ subjacent lymphatic plexus ; t, blood-capillaries traversing %alli ; d, lacteal beginning blindly near the free end of the villi ; tf, Lieberkuhn's glands. and these spaces are, as usual in this kind of tissue, occupied by leucocytes. The muscular fibres of the villi are derived from the muscularis, and lie for the most part outside the lacteal. Their contraction shortens the villus and tends to empty the lacteal. The network of blood capillaries is supplied by a small artery that enters from the submucosa, and the blood is carried out of the villus by one or two small veins, the veins of the intestines, the inferior mesenteric and superior mesenteric, uniting with Digestion 199 those from the spleen, pancreas, and stomach to form the portal vein which passes to the liver (fig. 131). The single epithelial layer that forms the free surface of the villus consists for the most part of columnar or conical epithelial cells having the outer broader end marked by fine striations. Mixed with the columnar cells that possess this refractive striated border are mucous cells of a goblet shape which discharge mucin from their mouths into the intestine. r34. Absorption of Fat. — After a meal containing fat the lacteals contain a fluid having a milky appearance owing to the presence of fat in a finely divided condition, and this fluid is termed chyle. During the intervals between digestion the lacteals contain ordinary lymph. Chyle is ordinary lymph plus fatty particles absorbed from the intestines, and under the microscope lymph- corpuscles and minute fat globules may be seen, the fat of chyle forming about 5 per cent. Like blood and lymph, chyle will coagulate spontaneously, yielding a soft white clot (par. 64). The richer the meal in fat the greater the proportion of fat in the lacteals, though for every 100 parts of fat absorbed from the alimentary canal only about sixty parts find their way into the thoracic duct. Where the missing quantity goes has not been ascertained. It cannot be found in the blood of the portal vein, nor does it pass into the blood- vessels of the villi, as none can be seen there when their capillaries are examined under the microscope after treatment with osmic acid (an acid which blackens particles of fat). But if sections be made of the villus of an animal killed during digestion and stained with osmic acid, black fatty granules are seen in the epithelial cells and in the lymphoid spaces surround- ing the central lacteal. The fat-globules in the lacteal itself are in most cases much more minute than those seen in the emulsified contents of the intestine, and these exceedingly fine granules of fat are termed the ' molecular basis ' of the chyle. As to the mode by which the emulsified contents of the intestine reach the lacteals much discussion has taken place. It is probably as follows : (i) Absorption of fat into the colum- nar epithelium-cells of the surface of the villus ; (2) passage of the globules thus absorbed through the spaces of the 200 Human Physiology lymphoid tissue of the villus to the lacteal, or carriage of the fat globules by the lymph-corpuscles (leucocytes) through the lymphoid tissue to the lacteal ; (3) passage into the lacteal, possibly through openings between the endothelial cells forming the wall of the lacteal ; (4) a breaking up of the fat-globules thus brought into the lacteals to form the ' molecular basis ' of the chyle. The peristaltic contractions of the intestine exert a pressure that assists the onward flow of the chyle into the valved lymphatic vessels below, while the muscular fibres of the Fig. 130. — Cross-section of an Intestinal Villus. jr, columnar epithelium ; ^, goblet-cell, its mucus is seen partly extruded ; /, lymph- corpuscles between the epithelium-cells ; h, basement-membrane ; c, blood-capillaries 711, section of plain muscular fibres ; c.l, central lacteal. villi derived from the muscularis mucosas exert a kind of pumping action as they contract, squeezing the contents of the tissue-spaces into the villi, and emptying the lacteals and blood-capillaries of the villus into the underlying vessels. As chyle contains a larger proportion of proteids than lymph, it appears to take some share in the absorption of this kind of food, though it is certain that the proteids after conver- sion into peptones are almost wholly absorbed into the capillary blood-vessels of the viUi and other parts of the intestinal mucous membrane. But it must be noted that the peptones formed from the nitrogenous foods by the action of the gastric pancreatic, and intestinal juice, although absorbed into the Digestion 201 capillary vessels that form rootlets of the portal vein in the stomach and intestines, cannot be found as such in any part of the portal blood. They must therefore be reconverted into albumins as they pass through the alimentary or capillary wall. Absorption goes on to some extent in all parts of the alimentary canal, its various sections having been arranged as regards activity of absorption in the following order : — Small intestine, large intestine, stomach, mouth, pharynx, oesophagus. The most active part is undoubtedly the small intestine, especially the upper part, owing to the presence of the valvulae conniventes and the villi. Watery solutions of salt and sugar, with some peptone, are absorbed into the blood-vessels of the stomach. A little water with very soluble matter may be absorbed in the mouth. After the digestive juices by the action of their special ferments have rendered the food- materials soluble and diffusible, absorption takes place partly by osmotic diffusion, and partly through the vital activity of the epithelium of the digestive tract. The cells lining the tract ingest the nutrient food-materials that have been digested, and then discharge the matter into the tissue-spaces. From the tissue-spaces the more diffusible substances pass for the most part into the capillary blood-vessels of the portal system, while the less diffusible fatty particles pass into the central lacteal, and only reach the blood after traversing the lymphatic system. In other words, ' the greater part of the water, salts, sugar, and proteid absorbed passes into the blood-vessels, and by the portal vein to the liver ; the greater part of the fats absorbed passes into the chyle-vessels, and by the thoracic duct into the venous system.' /Peptones (major part) Sugar Into the rootlets of the portal) Salt „ vein are absorbed | Soaps „ Water „ \Fat a trace. /Fats (major part) Into the lacteal vessels J Soaps (small part) are absorbed ] Peptones „ i Water 202 Human Physiology Fig. 131.— Portal Vein and its Branches. In this figure the liver and stomach are tilted upwards ; part of the duodenum and the whole of the transverse colon are cut away. Digestion 203 135. Movements of the Stomach and Intestines. — It has already been mentioned that the walls of the stomach contain non-striped muscular fibres arranged in three layers. The circular fibres are thickened and abundant at the pylorus, and there form the sphincter muscle termed the pyloric valve. When the stomach is empty its walls are usually at rest though con- tracted, and the mucous membrane is pale ; but when food enters, the membrane becomes red owing to increased blood- supply, and movements of its walls begin. These movements serve to bring the mucous membrane in contact with the food, as well as to propel the food forwards. There is a Fig. 132. — Lymphatics of the Intestine. I, Portion of small intestine ; 2, with layer of mesentery ; 3, mesenteric lym]3hatic glands with lacteals (4) passing in, and (5) passing out ; 6, branch of portal vein formed by smaller branches from intestine. rotatory or churning movement in which parts of the wall of the stomach in contact with the food glide to and fro with a rubbing movement, the food and gastric juice being rolled about and mixed together. A periodic peristaltic or wave-like movement also begins soon after the food has entered the stomach, setting up currents in its contents, and after a time this motion becomes so marked towards the pyloric end that the food, now reduced to the condition of chyme, is propelled through the relaxed pyloric valve into the duodenum. Vomiting is a reflex act by which the contents of the 204 Human Physiology stomach are expelled. It may be produced by afferent impulses of various kinds, as tickling the throat, exciting the vagus endings in the stomach by emetics &c. It is generally preceded by a sensation of nausea and a reflex flow of saliva. A deep inspiration is then taken, the glottis is closed and the diaphragm fixed. The abdominal muscles contract and press the stomach against the diaphragm ; the cardiac sphincter of the stomach relaxes, and its muscular walls, with the reversed peristaltic action of the oesophagus, complete the act. The food mingled with the various secretions, and subjected to the absorbent action of the villi and blood-vessel, is forced along the small intestines by slow, rhyth- mical, wave-like contractions of the longitudinal and circular muscular fibres in the intestinal wall. Here is the best example of peristaltic action. By the contraction of the longitudinal fibres a portion of the intestine is drawn backwards, and then the circular fibres, contracting from above downwards, progres- sively narrow the tube and force the contents forwards, just as pressing along a flexible tube containing a liquid moves it onwards.- There is also a swaying or pendulum action of the intestines due to a rhythmi- cal movement of the muscular fibres in their coats. This movement appears to bring about a mixing of the contents, but not a forward motion like the peristaltic movements. At the junction of the small intestine with the large one is placed the ileo-csecal valve to guard against reflux into the ileum. This valve is formed of two semilunar folds of the mucous membrane of the ileum, projecting into the inner and back part of the large intestine, a narrow transverseaperture lying between. Fig. 133. — The Ileo-caecal Valve. at ileum ; b, ascending colon ; c, caecum ; rf, junction of the caecum and colon ; e andy^ loose folds of the mucous membrane, forming the ileo-csecal valve ; ^, vermiform appendix, attached to CKCum. The Lymphatic System 205 : Distension of the csecum stretches the margin of the folds and brings them together, as does also the pressure exerted by peristaltic movements. These movements in the large intestine are more sluggish than in the small intestine, for it is said that it takes about three hours for food to pass along the small intestine, and about twelve hours to pass through the large iiltestine. CHAPTER IX THE LYMPHATIC SYSTEM- 136. Through the thin walls of the blood-capillaries there passes into the spaces part of the blood plasma, which thus comes to bathe the surrounding tissues. This exuded plasma becomes the lymph, and contains in solution absorbed nutri- tious matters, as well as oxygen set free from the hsemoglobin of the red corpuscles. It is thus both a nutritive and respi- ratory fluid for the cells and other form-elements of the tissues, each tissue taking from it the substances it needs for life and activity. The surplus of lymph, together with certain waste products arising from the metabolism of the tissues, is carried away from the tissues, and back to the blood-stream, by a set of absorbent vessels called lymphatics. These vessels have their origin within the tissues, and are found in nearly all parts — even in some parts, as the cornea, which contain no blood-vessels ; but they are, as we shall see, most intimately associated with connective tissue. Under the term lymphatic system are included : (i) the vessels specially called lymphatics or absorbents, together with the glands belonging to them ; (2) the lacteals, which are the l)'mphatics of the intestines, and differ from other lymphatics only in their power of absorbing chyle during digestion, as well as exuded lymph ; (3) the serous membranes which enclose cavities that are in reality large lymph-spaces. The lymph that fills up the interstices, or Spaces between the cells and other tissue-elements of the body, together with that contained in the lymphatic system exceeds in quantity the fluid in the blood-vascular system, and forms more than a quarter of the weight of the body. It is the intermediary or middlenaan between the blood and the tissues. 206 Human Physiology From it all the tissues obtain their oxygen and nutritive materials ; into it are excreted the waste products of tissue activity. Oxygen taken into the alveolar capillaries of the lungs and nutritive materials absorbed from the alimentary ««»». V^VT'-i ^C■},. 137. Origin and Structure of the Lymphatics.— The lymphatic vessels are those vessels which, beginning as minute channels in the tissues, collect and return into the circulation the fluid passed out of the capillaries, after it has supplied nutritive matter and oxygen to the tissues and received cer- tain products of tissue waste. These absorbent vessels thus form a system for draining the tissues, and, uniting in their course to form wider tubes, they at last combine into two large terminal trunks which open on each side of the thorax near the heart at the junction of the jugular and subcla- vian veins — the trunk on the left side being the largest and most impor- tant, and known as the thoracic duct. The fluid lymph after entering the lymphatic vessels always moves in one and the same direction— towards the point of discharge into the venous system — for the larger vessels are pro- vided with small semilunar valves, the free edges of which are turned towards the heart, so that any reflux towards the tissues is impossible. Valves are also placed at the entrance of the lymphatic trunks into the great veins. Some lymphatics appear to begin in the form of a plexus or network of fine tubes as in membranes, but many of these vessels commence in irregular lacunar spaces in connective tissue, and from these spaces (which are the same as thos? containing connective-tissue corpuscles and wandering leucocytes), often called lymph-spaces or lymphatic canaliculi, as they are partly occupied by this fluid , the lymph passes into more definite channels termed lymph-capillaries. It is not clear whether there is direct open com- munication between the cell-spaces and the lymphatic capillaries or not. The lymph-capillaries form a network in the various organs of the body, and have for their walls a single layer of endothelial cells with sinuous margins. They are generally larger than blood-capillaries, have numerous anastomosing branches, and soon join to form the lymphatic vessels pro- per. As the vessels become larger they resemble in structure the corre- sponding veins, except that their three coats are thinner and the valves more numerous. During their course towards the point of discharge the lym- phatic vessels become connected with the peculiar structures termed lymphatic glands (par. 138). Other modes of origin of the lymphatics are found besides those de- scribed above. Thus the lacteals or lymphatics of the intestine begin in blind extremities in the villi, as already described. Other lymphatics arise by means of free stomata on the walls of the larger serous cavities, which are not the absolutely closed spaces they were once thought to be (see par. go, fig. 99), but lymph-lacunse from which the fluid is drained away through the minute stomata into the network of lymph capillaries lying in the subserous tissue. In the central nervous system minute arteries are found, having around them for a small distance a lymph-capillary in the form 208 Human Physiology of a tubular sheath, so that the plasma that leaves the blood-vessels enters at once into this tubular space and is then carried away into the regular lymphatic canals. This mode of origin is known as ' perivascular lymphatic. ' As the lymph in the lymph-spaces is free it gets into direct contact with the tissue-elements and percolates between the cells, thus reaching parts of tissues devoid of blood-vessels. In this way it supplies nutriment to the epithelium of mucous membrane, to the interior of cartilage, the cornea, and the innermost cells and substance of bone. The total quantity of lymph and chyle that passes into the blood in 24 hours is estimated to be in a well-fed animal equal to the whole quantity of the Fig. 135.— Portion of Serous Membrane of Diaphragm (pleural) from tlie Rabbit, treated with nitrate of silver after removal of superficial epithelial layer. (Recklinghausen. ) c, c, cell-spaces of tissue ; d, d, commencing lymphatic vessels connected at b, b with the cell-spaces. blood in the body, about half of this coming from the lacteals and half from the other lymphatics of the body. [The watery solution of salts and proteids that leave the blood-plasma to become lymph passes from the blood-vessels into the lymph-spaces by a process or processes not fully undferstood. It is partly a process of os- motic diffusion through a porous membrane, partly a process of filtration under pressure through a similar partition, and partly a process depending on the activity of the living endothelial cells of the capillary wall. That it is not mere diffusion is shown by the passage of about one-half the indif- fusible proteids of blood-plasma into lymph ; that it is not mere filtration The Lymphatic System 209 under pressure is shown by increase of pressure not always being accom- panied by increased passage of fluid as in an ordinary filter, and by the pas- sage of waste products in the opposite direction, from the lymph-space into the blood. That the capillary wall plays an important part will be evident F1G.136.— Nitrate of Silver Preparation from Rabbit's Omentum. (Klein.) Magnified. u, lymphatic vessel ; b, artery ; c, capillaries ; d, branched cells of the tissue which are , seen to be connected both with the capillary walls and, as at e, with the lymphatic. The cells are, in this instance, stained by the nitrate of silver. when we consider that it is a living, changing thing continually acting upon and being infltienced by the blood. When peptone is injected into the circulation of a living animal it leads to an increased flow of thicker lymph, a more concentrated plasma passing through the vessel walls, and the peptone disappearing at first by being transferred froim the blood to the lymph. The increased lymph formation and the transference of the 2IO Human Physiology peptone to the lymph point to an cutive process of secretion by the cells of the capillany walls. Lymph capillaries unite to form vessels having the general structure of veins, but the thinness of the walls of these lymphatic vessels makes them difficult to see when empty. When the lymphatic vessels are distended they have usually a beaded appearance owing to their numerous semilunar valves. Lymphatic vessels are arranged in a superficial set near the surface of the body, just beneath the integument, and a deep set in the interior. The lymphatics of any part or organ exceed the veins in number, but are of smaller size. They also anastomose or intercommunicate more freely than veins. Both the lacteals that convey the chyle from the intestines to the thoracic dutt, and the general lymphatics that take up the lymph from all other parts of the body, become connected in their course with lymphatic glands. These lymph glands occur in groups around joints, in the recesses of the neck, in the organs of the thorax and abdomen ; a large number placed in the mesentery being connected with the lacteals. As these glands effect important changes in the fluid flowing through them, their structure must now be described.] 138. Lymphatic Glands. — Lym- phatic glands are small compact bodies, varying in size from a hempseed to a kidney bean, placed in the course of the lymphatic vessels. Through them the lymph and chyle passes in its onward course. They- exist in great numbers in the mesentery and alongside the great vessels of the abdomen, thorax, and neck. Some are found in the axilla and groin, but none below the elbow or knee. At a shght depression on one side, the hilum, blood-vessels enter and leave the gland. The efferent lymphatic vessels also leave at the same spot, but the afferent lymph-vessels enter at various points of the periphery. Section of a lymphatic gland shows an external cortical substance of light colour and an internal darker medul- lary substance coming to the surface at the, hilum. The gland consists of (i) a fibrous envelope or capsule of connective and muscular tissue from which a framework of processes (trabeculae) passes inwards and divides the gland into Fig. t37.^A Lymphatic Gland (i) is shown with (2) affer- ent vessels, and (3) efferent vessels. The numerous valves cause the beaded ap- pearance of the lymphatic vessels. The Lymphatic System 211 spaces (alveoli) with free communication, having the form of converging chambers in the cortex, but diminishing in size and regularity of shape in the medulla ; (2) the proper glandular substance, a mass of lymphoid tissue (i.e. a fine network of fibres with lymph-corpuscles in the meshes) occupying the central part of the spaces, and thus leaving a narrow channel, bridged across by cells and fibres between the lymphoid tissue and the trabeculse ; (3) a free supply of blood-vessels, the arterial a.i Fig. 138. -Diagrammatic Section of Lymphatic Gland. (Sharpey.) a.l, afferent ; e.l, efferent lymphatics ; C, cortical .substance ; M, reticulating cords of medullary substance ; l.s, lymph-sinus ; r, fibrous coat sending trabecule, tr, into the substance of the gland. branches running in the trabecular framework, and breaking up in part in capillary networks in the glandular substance ; (4) the afferent and efferent lymph-vessels. The afferent lymphatic vessels enter the lymph-sinuses of the cortex after passing through the capsule, and the lymph is slowly carried along through the channels in the substance of 212 Human Physiology the gland' towards the hilum, taking up in its passage many lymph-corpuscles, probably derived from the actively multi- plying leucocytes that occupy the meshes of the adenoid tissue. The small spaces or alveoli of the medullary part of the lymphatic gland contain adenoid tissue crowded with leuco- cytes of various sizes, and separated from the fine trabeculae by channels of open reticular tissue forming the lymph-sinuses or lymph-paths, the whole of the lymph-sinuses forming an intercommunicating labyrinth throughout the gland. Active cell-multiplication by karyokinetic division goes on in the Fig. 139. — Section of Lymphatic Gland Tissue. rf, trabeculse ; h, small artery in substance of same ; c, lymph-paths ;, d, lymph-corpus- cles ; f , capillary plexus. small portions of tissue crowded with leucocytes, and some of these cells pass into the lymph-stream of the lymph-sinuses, as it is found that lymph-corpuscles are more numerous in the issuing lymph than before. Leucocytes are also found, and appear to multiply in other localities containing adenoid tissue as in the tonsils, certain parts of the air passages, the solitary glands and Peyer's patches of the intestine, the spleen, &c. One function of lymphatic glands, therefore, is the produc- tion of leucocytes that become the white corpuscles of the blood. Besides the increase in corpuscles in lymph that has The Lymphatic System 213 passed through the glands, it becomes more coagulable, the lymph in the roodets of the system having but a feeble power of coagulation. The same changes occut in chyle after it has passed through the lymph-glands of the mesentery. Before entering the mesenteric glands, chyle is but an opaque milky fluid, hardly coagulable at all ; but after passing through the glands there is a diminution of its molecular basis, numerous white corpuscles and the elements of fibrin being also then present. The lymph leaving a lymphatic gland therefore differs from that entering it by being more coagulable, by being freed from accidental matters, and by being enriched with numerous young leucocytes. The-absorbent power of the lymphatics is taken advantage of when ointments are rubbed into the skin, or when a drug is administered by a hollow needle inserted under the skin, and the fluid forced in by a syringe (hypodermic injection). Thf absorbent vessels gather up the matter and pass it into the blood. That the lymphatic glands act as a sieve and arrest deleterious matters is evidenced by their inflammation after a poisoned wound. Thus a wound in the foot that introduces poisonous matter may inflame the lymphatic glands of the thigh. 139. The Thoracic Duct. — The thoracic duct is the common trunk that receives the lymphatics from the lower limbs, from the abdominal viscera (except the upper part of the liver), from the walls of the abdomen, from the left side of the thorax, from the left lung, the left side of the heart, the left arm, and the left side of the neck and head. It is from 15 to 18 inches long, and extends from opposite the second lumbar vertebra to the root of the neck. Its diameter is nearly \ inch, but at its lower end it is dilated into an expansion called the receptaculum chyli. Its course is somewhat tortuous, and it, contracts and enlarges at irregular intervals. Throughout its course it is supplied with valves, and these are more numerous in its upper part. Its termina- tion at the junction of the left jugular and subclavian veins is also guarded by a valve of two segments which allows the contents to pass into the veins, but prevents any reflux. Its 214 Human Physiology _ Siaht UymaJtatie Fig. 140.— The Ihoracic and Right Lymphatic Duct. walls consist of three coats, an epithelial lining of flattened lanceolate cells, out- side of which are longitudinal elastic fibres ; a middle coat of connective tissue, outside of which are numerous muscular fibres arranged trans- versely and longitu- dinally ; and an ex- ternal coat of alveolar tissue with isolated' bundles of muscular fibres. The muscular character of the wall of the thoracic duct is therefore well marked. 140. Conditions affecting the Amount of Lymph and Chyle. — ■ Various causes combine to determine the amount of lymph in the lympha- tic spaces and vessels of any region of the body. Thus the mere position of a limb, as when the hand is kept hanging down for a time, may lead_ to an increase of lymph in the spaces and channels of the organ, as is evidenced by the swollen limb and tense skin. A tight bandage causes a similar swelling en the peripheral side of the ligature. This swell- ing, though partly due to the dilated blood-ca- The Lymphatic System 215 pillaries consequent on the hindrance to the return flow along the veins, is mainly caused by the unusual fulness of the lymph-spaces and vessels in the skin and subjacent tissues. Active exercise long continued may produce a similar result, for muscular activity not only increases the formation of lymph but promotes its more rapid outflow. The tendons and fasciae of muscles have numerous small stomata that absorb the lymph from the muscles into lymphatic vessels. Increase of blood -pressure in the capillaries of any area due either to arterial dilatation or more effectively to obstruction of the venous outflow usually increases the transudation of lymph, though the condition and activity of the living cells of the capillary wall is of great importance, and may prevent the increased outflow of lymph. A great increase in the amount of chyle occurs during digestion after a full meal, the lacteals becoming white and distended, though collapsed and containing only a small quantity of clear lymph during a period of fasting. Abnormal conditions of the vascular system may lead to an excessive transudation of lymph, with an undue accumulation of lymph in the lymph- spaces. This lymph congestion is spoken of as aiema, or dropsy. 141. Inflammation. — Irritation of a part of the tissue of any organ leads to increased flow of blood to that part ; the vessels dilate and become congested, and the white corpuscles cling to the sides and begin to emi- grate through the vessel wall. If the irritation is great, a morbid condition termed inflammation is set up. This is characterised by swelling, heat, and redness, accompanied by marked vascular changes and much exudation of plasma and corpuscles into the injured tissue. The phenomena of inflamma- tion have been observed by the aid of the microscope in the transparent parts of animals where it has been set up. A frog's tongue or web spread out for microscopical examination shows the normal circulation through the arterioles, capillaries, and small veins. A severe irritant sets up the following inflam- matory changes. A dilatation of the small arteries and a quickening of the blood-stream is first noticed, more vessels also becoming visible. After a time the stream becomes slower, and the dilated vessels are seen to contain two layers of corpuscles — an inner axial layer of red corpuscles and a layer of white corpuscles adhering to the walls. Leucocytes or white corpuscles also crowd the capillaries, and they soon begin to pass in quantity through both veins and capillaries into the surrounding tissues. Small buds or processes first appear on the outer side of the wall, until the whole cor- puscle passes outside by amoeboid movements (see fig. $8). With the emigration or diapedesis of numerous white corpuscles there is great exu- dation of coagulable plasma or plastic lymph, and the swelling that marks an inflammation is produced. The redness is due to the unusual quantity of blood in the affected part ; so also is the heat, which is not really greater than the maximum heat of the blood of the interior. If the inflammation subsides the white corpuscles cease to emigrate, the stream of blood quickens again, and the normal circulation is again set up. The migrated corpuscles and surplus lymph pass away eventually into the lymphatic channels. But if the inflammation continues, complete arrest of blood-flow may occur, some red as well as white corpuscles may pass into the tissue, and the accumulated leucocytes degenerate into pus-cells and form an abscess which must be opened if it fail to burst naturally. Dr. Metschnikoff of Paris explains the phenomena of inflammation as essentially a reaction of the leucocytes contained in animal bodies to the presence of injured tissue or intrusive particles. The irritation caused by 2i6 Human Physiology lesion of tissue or the presence of foreign bodies — bacteria and other micro- organisms — leads to active movement of leucocytes to the spot, and these, migrating from the vessels, devitalize or engulf and digest the disintegrated tissue or foreign particles, and so prepare the -v&y for repair of tissue. The white corpuscles that thus combat irritative agents are called phago- cytes, and diapedesis is one form of struggle against offending particles. The process has often been watched, and leucocytes have been seen to take up the irritating bodies or found containing the dead and dying microbes. If the phagocytes succeed in devouring and removing the irritating substances, they disappear probably by being carried off into the lymph-stream ; if the invading particles are too powerful, the corpuscles themselves are destroyed and collect in the tissue as pus-cells.' 142. Movements of Lymph. — Several causes combine to promote the onward flow of lymph towards its place of discharge into the blood. In the first place, there is a vis a tergo, or pressure from behind, derived from the blood-pressure, i.e. ultimately from the heart. The lymph in the lymph-spaces must be under a higher pressure than the blood in the great veins into which the thoracic duct opens, and where we know that the pressure is very low, and any fluid will flow from a region of high pressure to a region of lower pressure. In the next place, the numerous valves present in the lymphatic vessels are so arranged that every pressure exerted on the tissues in muscular movements must assist in driving forward the lymph. The respiratory movements also assist in drawing the lymph onwards as they assist the flow of the blood towards the heart, every inspiration diminishing the pressure in the vessels within the thorax (par. 102). Further, the muscular fibres in the walls of the lymphatics may probably undergo rhythmical contractions, while the intestinal movements and the contraction of the muscular fibres of the villi appear to assist in driving the chyle from the lacteals into the valvular lymphatics below (par. 134). Lastly, as the lymph-vessels gradually unite to form larger vessels, and finally end for the most part in the thoracic duct, the sectional area of which is less than that of the combining vessels, an in- creased pressure must result from this union of trunlis into one. The respiratory, movements also favour the flow of lymph through the thoracic duct, as they do the current of blood in the veins of the thorax. But the movement is very slow — far slower than the slowest blood movement in the capillaries. ' The term Chemiotaxis is applied to the physical and chemical stimuli that determine the wandering of leucocytes, so that the process of inflam- mation is said to be a chemiotactic one, since it consists of the gathering together of the migratory cells to parts of the living tissue that have been injured or invaded by foreign particles, as if the disintegrated tissue or disease-producing particles were attractive to them. And the process of destroying the injurious microbes is believed to be a chemical one, the source of the destructive agent being in the cells thus acting under chemical stimuli. The Liver and the Ductless Glands 217 CHAPTER X THE LIVER AND THE DUCTLESS GLANDS 143. The liver is the largest gland in the body, and weighs from fifty to sixty ounces. It is situated just under the diaphragm, chiefly on the right side, being under cover of the ribs in front and in contact with the posterior wall of the abdomen behind. Its upper surface is convex, and in relation with the under-surface of the diaphragm ; its under-surface is concave and in relation with the stomach and duodenum. The thin anterior border corresponds with the margin of the ribs in the standing position, but the whole organ rises and falls somewhat with the breathing. The liver is retained in position by five ligaments, four of which are folds of perito- neum that attach it to the diaphragm. On the under irregular surface five fissures are seen, which divide the organ into five lobes. They are the longitudinal fissure, the fissure of the ductus venosus (a vein in the foetus communicating with the inferior vena cava) the transverse fissure, the fissure for the gall-bladder, and the fissure for the inferior vena cava. The two chief lobes are the right and the left, the right being five or six times larger than the left. A serous coat derived from the peritoneum invests the greater part of the liver and is closely adherent to a fibrous coat or capsule beneath, which covers the entire surface. At the transverse fissure this fibrous coat is continuous with Glisson's capsule, an areolar investment that surrounds the portal vein, hepatic artery, and hepatic duct, and accom- panies these vessels in their branches in the interior of the organ. The vessels of the liver are : (i) the hepatic artery, which comes off from the coeliac axis, a branch of the abdominal aorta, to supply the organ with nutrient blood ; (2) the portal vein, which conveys to the liver the venous blood of the stomach, spleen, and intestines ; (3) the hepatic veins which arise in the substance of the organ and convey the blood from the 2n Human Physiology liver into the inferior vena cava. (4) The hepatic ducts which arise by minuteb ranches in the organ, and convey away the bile. (5) Numerous lymphatics. (Note five ligaments, five fissures, five lobes, five kinds of vessels.) The nerves of the Uver are derived from a plexus of the sympathetic and from the pneumogastric. The bile is carried out of the liver by two hepatic ducts which leave the right and left lobes at the transverse fissure. These combine, and from the united duct thus formed the cystic Fig. 141.— The Liver. Under Surface. duct passes on to the tapering end of the gall-bladder. The united duct now known as the cotnmon bile duct passes on and enters the duodenum very obliquely along with the pancreatic duct. The gall-bladder is a musculo-membranous pear-shaped bag lodged under the right lobe of the liver, about four inches long and one inch broad in its widest part. Its walls consist of three chief coats, an external serous coat, a middle fibrous or areolar coat containing plain muscular fibres, and an internal mucous membrane with columnar epithelium, that secretes a The Liver and the Ductless Glands 219 viscid mucus. Bile is formed continuously by the cells of the liver, and during the intervals of digestion it enters for the most part into the gall-bladder, from which it is discharged shortly after food is taken. The ducts have a fibrous external coat and an internal mucous coat. 144. Minute Structure of the Liver.— The surface of the liver shows to the naked eye small spots about the size of a pin's head. These mark the surfaces of the lobules, and the liver is made up of innumerable lobules, vifith interlobular connective tissue, each of which is about 55 inch in dia- meter. Microscopic examination shows that each lobule consists of nume- rous polygonal cells about j^g inch in diameter, and that among these Fig. 142. — Diagrammatic Representation of Two Hepatic Lobules. Tlie left-hand lobule is represented with the intralobular vein cut across : in the right- hand one the section takes the course of the intralobular vein. ^, interlobular branches of the portal vein ; h, intralobular branches of the hepatic veins ; j, sublobular vein ; c, capillaries of the lobules passing inwards. The arrows indicate the direction of the course of the blood. The hver-cells are only represented in one part of each lobule. hepatic cells are seen minute branches of the portal vein and the hepatic artery, with the commencement of a hepatic vein. Minute channels for the bile are also seen to begin between the hepatic cells. The blood-vessels of the liver (portal vein and hepatic artery) enter the liver on its under surfaces, where also the bile duct leaves the organ. The brandies of these three vessels are invested with connective tissue (Glisson's capsule), which is continuous with that surrounding the organ, and the three vessels continue together to the outside of the lobules, the connective tissue there terminating in septa dividing the lobules. The parts or spaces enclosing these three sets of vessels are called portal canals. We will follow the course of each set of vessels in detail. The portal vein accompanied by the hepatic artery enters the liver at the transverse fissure, and is eventually distributed between the liver- 220 Human Physiology lobules, forming mUr-loh\i\a.r veins. From the interlobular veins surround- ing each lobule a close network of capillaries passes into the lobule, the meshes of the network being occupied with rows or columns of cells. These lobular capillaries converge towards the centre of the lobule, to form a central or inira-lohula.i vein. Fig. 143.— Lobule of Rabbit's Liver, vessels and bile ducts injected. (Cadiat.) a, central vein ; i, b, peripberal or interlobular veins ; c, interlobular bile-duct. Each intralobular vein passes through the lobule to its surface, and joins similar veins from other lobules to form sublobular veins. The sub- lobular veins unite into larger and larger trunks, and end at last in the hepatic veins, three large veins which leave the liver at the back and open into the inferior vena cava. The Liver and the Ductless Glands 221 The portal vein, which thus breaks up into capillaries like an artery, has comparatively thick and muscular walls, and neither it nor its branches, nor the branches of the hepatic veins, contain any valves. The hepatic artery divides into branches, which accompany the branches of the portal vein and bile duct until it comes between the lobules. These branches are much smaller than those of the portal vein, and smaller than the accompanying bile duct. In their course they give off capillaries to supply the branches of the portal vein, the larger bile ducts, and the cap- sule, these capillaries termi- nating in the hepatic veins that enter branches of the portal vein. Other capilla- ries are said to break off from the interlobular por- tion of the artery, and enter- ing the lobule unite with the capillaries of the portal vein near the margin of the lobule. There are thus two sets of vessels carrying blood to the lobules, but only one set re- turning blood. The hepatic ducts com- mence in minute passages between the cells of a lobule, and are called bile capilla- ries. These bile capillaries form in the lobule a closer capillary network than that of the blood-capillaries, and as they run between the opposed surfaces of adjacent cells, while the blood- capil- laries run along the edges of the cells, the two sets of vessels are kept separate. The bile capillaries radiate to the circumference of the lobule, and form interlobu- lar ducts between the lo- bules. The interlobular ducts unite, and entering Glisson's capsule accom- pany the branches of the portal vein, until by union they form the two main ducts which leave the liver at the transverse fissure. The large bile ducts possess a fibrous and elastic coat, with some fibres of unstriped muscular tissue arranged circularly. The liver cells are polygonal in shape and about ^^th of an inch in diameter. They are arranged in columns between the meshes of the blood- capillaries, and under high powers the protoplasm and nucleus of each cell is seen to be fibrillated. Their appearance differs according as the body i§ fasting or digesting food. In fasting they are finely granular and cloudy. Fig. 144. — Microscopical Section from the Liver of a Child Three Months Old, hardened in Chromic Acid. The hepatic cells (p), with their single nuclei, are separated from the capillary wall by a sniall mter- vening space. The capillaries («) contain closely compressed coloured, and a few colourless, blood- corpuscles. A few elongated nuclei belonging to the capillary wall are seen. Within the line of junc- tion (septuml, betweeii two hepatic cells, ihe trans- verse section of a biliary duct is seen as a small transparent space (r). There is also one at the angle where several of these cells come into contact {d). 222 Human Physiology but after a meal, especially of starchy food, coarse particles of glycogen are visible (fig. I45). It will now be seen that each lobule of the liver contains all the ele- ments of a gland — protoplasmic cells to form a secretion, blood-capillaries in close relation with the cells to provide material for secretion, ducts to carry away the secreted products. But, as we shall presently see, the cells of the liver perform other functions besides that of secreting the bile. The liver, in fact, discharges at least four functions : — ( i ) Its cells secrete bile which passes away by the hepatic ducts. (2) Its cells form glycogen or animal starch, which passes in an altered form into the blood-stream. (3) It takes part in the formation of urea and other excretory products. (4) It is concerned in the destruction of the red corpuscles of the blood. Moreover, it is believed to assist in arresting and changing deleterious matters in the blood. 145. Secretion of Bile, — The secretion of bile appears to be continuous, but the rate is much influenced by food. The bile does not exist as such in the blood, but is elaborated by the secretory activity of the hepatic cells from constituents in the portal blood. Passing into the bile capillaries and onwards into the bile ducts, it is carried along the common bile duct into the duodenum during digestion, but during periods of fasting it returns along the cystic duct into the gall-bladder, where it accumulates until food is again taken. The rate of secretion rises rapidly after meals, then falls slightly for a time, after which there is a more gradual rise and fall. The discharge of bile in quantity shortly after the ingestion of food is probably a reflex action, the presence of the acid chyrne in the duodenum acting as a stimulus and leading to contraction of the muscular walls of the gall-bladder and bile ducts. The pressure of the bile in the bile ducts is very low, though nominally a little higher than in the portal veins — a proof that it is not a mere filtrate from the blood, though the amount is influenced by the quantity of blood sent to the liver and by blood pressure. If the pressure in the ducts be much increased by any obstruction, the bile is reabsorbed from the distended ducts into the lymphatics of the liver, and this lymph entering the thoracic duct, the bile thus passes into the blood and produces the condition called jaundice, when the skin and white of the eye are yellow, while the fasces are light-coloured. The quantity secreted daily is about sixteen ounces. Its appearance and chemical composition have been described in The Liver and the Ductless Glands 223 par. 124. Some of the elements of bile {e.g. the colouring matters and cholesterin) are probably merely separated from the blood, but the more special constituents {e.g. the bile salts, sodic glycocholate and sodic taurocholate) are formed by the activity of the hepatic cells. Its chief uses In digestion are to assist in emulsifying fat, so as to enable it to pass through the intes- tinal membrane without alteration, to precipitate the peptones and form a sticky deposit on the valvulae conniventes, thereby giving time for absorption, to stimulate the muscular walls of the intestine, and to act as an antiseptic or hindrance to putre- faction (see par. 125). 146. Glycogenic Function of the Liver. — On killing a well-fed animal and at once removing the liver, suitable methods of analysis show the presence of a carbohydrate body called glycogen or animal starch. It can also be seen under the microscope in a section Of the liver of such an animal, stored up as granules in the liver-cells. Glycogen has a formula CjHijOj, or some multiple of this, and is distinguished from ordinary starch by giving a wine-red colour with iodine. If the liver be left several hours in a warm place, little or no glycogen will then be found, but abundance of sugar (dextrose). By some means, therefore, the glycogen soon changes into sugar after death, and the same change of glycogen into sugar is believed to occur during life, though more slowly. But where do the liver-cells obtain their glycogen, for the blood contains none ? Glycogen is formed in the liver from the sugar and some other absorbed food materials in the portal blood. A process of dehydration or taking away the elements of water from the sugar, CsH,jO„, may be one method by which the liver-cells produce it. During digestion, especially after a meal rich in carbohydrates (starchy and saccharine food), the blood entering the liver by the portal vein contains more sugar than the blood leaving the liver by the hepatic veins, but in the intervals of digestion the hepatic venous blood contains about twice as much as the portal venous blood, the average amount in portal blood being 1 per l,ooo, and in the blood of the hepatic veins 2 per l,ooo. It thus appears that the liver regulates the amount of sugar in the blood, storing it as glycogen during digestion and reconverting this glycogen into sugar to be discharged by the hepatic veins for use by the tissues during the time that no food is ingested. This storage is rendered necessary by the fact that if the sugar in the blood rises above '3 per cent, the excess is excreted by the urine. As evidence of the above function of the liver, it may be mentioned that if a solution of sugar be slowly injected into a branch of the portal vein, no sugar appears in the urine, but if the sugar be injected into the jugular vein its presence in the urine may be easily detected. The source of glycogen in the liver is undoubtedly the food, for in the liver of a starved animal glycogen is practically absent. That it is derived from carbohydrates especially is shown by the large quantities that accumu- late in the liver-cells in animals fed on such a diet, the liver thus forming a storehouse of carbohydrate material. But it is also derived from proteids, as has been proved by feeding a dog on proteid material freed from all 224 Human Physiology Fasting. FiG. 14S. After Food. Liver-cells of dog after a thirty-six hours' fast, and fourteen hours after a full meal — in the latter case swollen with glycogen. (Heidenhain.) carbohydrate. It is not derived from fats, because the glycogen soon dis- appears from the liver of an animal fed exclusively on fat. The destination of the glycogen of the liver is to some extent still under discussion. But the most generally accepted view is that it is conveyed from the liver as sugar to undergo combustion in the tissues. Heavy muscular vfork soon leads to the disappearance of hepatic glycogen, and the amount of sugar in the venous blood of an active muscle is slightly less than in its arterial blood. Thus the sugar produced by the liver-cells from the stored glyco- gen is consumed by muscle, and the ' sugar-cycle ' of a well-fed animal is as fol- lows : — The sugar absorbed from the alimentary canal enters the portal blood, is in great part stored as glycogen in the liver-cells, is gradually reconverted into sugar that passes away by the hepatic veins, is consumed by living muscle and discharged as CO2 and HjO. Besides existing in the liver, glycogen is also found to a small extent in muscular tissue, especially in the skeletal muscles, where it possibly forms a local reserve to supply the energy of muscular contraction. It is especially abundant in the rudimentary muscles of the embryo, where it appears to become transformed during development into the contractile sub- stance of the muscular fibres. [Glycogen may also be regarded as a source of heat, for it is found to disappear in a few hours from the liver of a rabbit that is kept in a cold bath.] 147. Diabetes. — When the amount of grape-sugar (dextrose) in the blood rises above -3 per cent. , the sugar is excreted by the kidneys and appears in the urine. The presence of grape sugar in the urine is spoken of zs glyco- suria, or diabetes. Temporary diabetes may be produced in a well-fed animal by a puncture made in the lower part of the floor of the 4th ven- tricle. For a day or two the animal passes sugar in its urine, and then sugar ceases to be excreted. In a starved animal the puncture does not lead to sugar in the urine. The glycosuria in this case, therefore, appears to be caused by the conversion of the glycogen of the hepatic cells into sugar so rapidly that the excess is excreted by the kidney. The nature of the nervous influence that thus causes glycosuria is not definitely known. The puncture is effective even after section of the vagi, so that these nerves cannot be the channels along which the influence is conveyed to the liver. Glycosuria, or temporary diabetes, may also be caused by several drugs, as strychnine, phlorizidin, &c. Phlorizidin will produce glycosuria in starved animals which are free from carbohydrates, and in this case the sugar must be formed directly from proteid. In the disease known as diabetes, or diabetes mellitus, the blood con- tains an abnormal amount of sugar, and large quantities are passed in the urine. This is probably due to the incapacity of the tissues to consume the sugar, or to the inability of the liver-cells to retain glycogen, for in The Liver and the Ductless Glands 22 ^ severe cases sugar continues to be excreted even when carbohydrates are excluded from the food. It has been recently found that the pancreas has some peculiar relation to the sugar function of the body, for if this organ be removed from a dog the animal becomes affected with severe diabetes that continues until death ensues. It is thus concerned in the metabolism of the carbohydrates. On the formation of urea in the liver, see par. 155. 147a. Ductless Glands. — Under this term are included certain duct- less organs lying in different parts of the body which produce an internal secretion that passes directly into the lymph or venous blood in order to be carried and distributed to other parts of the body. These glands do not form a connected system ; they vary much in structure, and their internal secretions are of diverse characters. The ductless glands include the spleen, the thyroid gland, the thymus gland, the supra-renal capsules, and the lymphatics. The structure and function of the lymphatic glands and their internal secretion of lymph cells to furnish a supply of colourless corpuscles, have been already described. By examination of the structure by extirpation or removal, by the study of the gland in disease, and by injection of extracts of the gland, the nature and function of the others have been, in part at least, discovered. It may be remarked that some of the glands which possess ducts along which an external secretion is discharged form an internal secretion, i.e. one passed directly into the blood, as well. This is the case with the liver, for as we have lately seen, it not only secretes bile which is passed along its duct into the duodenum, but it stores glycogen to regulate the sugar supply of the body, and assists in the production of urea. The pancreas also produces an external secretion and an external secretion, enabling the body to utilise the sugar supplied to the blood (par. 123). 148. The Spleen. — The spleen, thyroid, thymus, and suprarenal capsules, are often spoken of as blood-glands, from the peculiar changes they are believed to bring about in the blood circulating through them. As they have no excretory duct, but pour their products into the blood-stream, they are also spoken of as ductless glands. In this respect they are closely related to the lymphatic glands already described. The spleen is the largest of these so-called ductless glands. It is situated at the left or cardiac end of the stomach, between it and the diaphragm, has a deep bluish-red colour, a somewhat oval shape, and measures about five inches in length and three inches in breadth. On the internal concave surface is the hilum or recess where vessels and nerves enter and leave the organ. The spleen is invested with an external serous coat derived from the peritoneum, while under this is a tough fibrous and muscular capsule which is reflected inwards upon the vessels of the hilum. The general structure of the organ is similar in many respects to that of a lymphatic gland. From the inner surface of the capsule numerous partitions or trabecul:^ pass off and join those arising from the covering of the blood-vessels, the whole organ being thus divided into a large number of irregular spaces. In those spaces formed by the interstices of the fibrous framework lies a soft mass of dark red substances, the spleen pulp. This is found to consist chiefly of cells, the branching processes of Q 226 Human Physiology one kind forming a delicate networlt of small communicating spaces in the interior of the larger spaces formed by the trabeculje of the capsule. These small spaces, formed by branching flattened cells, contain blood, granular corpuscles resembling lymph corpuscles, and large amceboid cells containing coloured blood-corpuscles in various stages of transformation. The splenic artery, a branch of the aorta, divides into several branches that enter the organ at the hilum and ramify in the interior. As the small arteries leave the trabecular sheaths, their external coat of connective tissue becomes gradually changed into adenoid or lymphoid tissue, the altered coat having here and there spheroidal swellings termed the Malpigkian corpuscles of the spleen. These small bodies, visible to the Jleen, as seen Fig. 146. — Vertical Section of_a small Superficial Portion of the Human Splei with a low power. (KoUiker.) A, peritoneal and fibrous covering ; h, trabecule ; c, ,:, Malpighian corpuscles iri one of which an artery is seen cut transversely, in the other longitudinally ■ d injected arterial twigs ; £, spleen pulp. ' ' naked eye as whitish specks in a section of the spleen, are from -L to jL inch in diameter, and in minute structure are found to consist of delicate adenoid reticulum traversed by minute capillaries from the small artery, and containing lymph corpuscles in the meshes. The small arteries terminate in capillaries that traverse the pulp in all directions, and it has been shown that in many cases the capillary vessels terminate in the reticu- lated tissue, so that the blood thus comes into actual con- tact with the tissue elements. Venous channels arise in the pulp, and convey the blood that has escaped into the interstices formed by the branched capsules into veijis The Liver and the Ductless Glands 227 wnich run in the fibrous trabeculee and ultimately leave the organ at the hilum. > 149. Functions of the Spleen ( i ) The spleen appears to take some share in the formation of blood -corpuscles. The splenic vein certainly contains more colourless corpuscles than the splenic artery, and according to some coloured corpuscles are also added, the new-formed corpuscles entering the blood as it filters through the spongy network of the pulp. That colour- less corpuscles are produced within the organ seems to be proved by the enormous increase in their number when the spleen is enlarged in the disease termed leucocythsemia. (2) There is evidence to show that the spleen destroys and removes worn-out red corpuscles and particles of foreign matter, chemical analysis revealing the presence of nitrogenous extractives probably thus derived. The large amoeboid splenic cells Fig. 147. — Thin Section of Spleen Pulp, highly magnified, showing the mode of origin of a small Vein in the Interstices of the Pulp. z/, the vein, filled with blood-corpuscles, which are in continuity with others, bl^ filling up the interstices of the retiform tissue of the pulp ; w, wall of the vein. The shaded bodies amongst the red blood-corpuscles are pale corpuscles. seen in the splenic pulp often contain pigment granules and fragments of red corpuscles that finally disappear, the spleen thus playing the part of a purifying filter. (3) Towards the end of digestion the spleen increases in size, owing chiefly to an increase of granular albuminous plasma in the organ, a diminution gradually following as this material becomes less. Uric acid is almost constantly found in the organ. It thus appears that the spleen has some relation to the storing and elaborating of nitrogenous food materials absorbed during digestion. It is a remarkable fact, how- ever, that removal of the organ from an animal produces no serious effect, the only apparent result often being an enlargement of some of the lymphatic glands to make up for the absence of the spleen. To assist the circulation through the network of the splenic pulp, the muscular tissue of the capsule and trabeculse has the power of rhythmic contraction. When the organ is enclosed in a plethysmograph, or volume recorder, its bulk is found to undergo slow variations, each contraction and expansion occupying about a minute. The muscular activity of the spleen is under the control of the nervous system, a rapid contraction being Q 2 22!: Human Physiology brot(ght about directl)' on stimulation of the vagus or splanchnic nerves, as well as by stimulation of the splenic nerves themselves. These latter nerves have their centre in the medulla, as stimulation of the medulla either directly or by asphyxiated blood induces contraction of the organ. The ner- vous system also in some way regulates the flow of blood through the spleen. 150. The Thyroid Gland. — The thyroid consists of two lobes of isolated ductless alveoli, bound together by connective tissue. It is situated be- neath the muscles of the neck on each side of the larynx and trachea, a cross- piece in front of the trachea uniting the two lobes (fig. 67). Each lobe is somewhat conical in shape, about two inches in length and one inch in breadth. The gland is covered by a thin layer of areolar tissue, and from this capsule fibrous partitions pass inwards and divide the organ into spaces occupied by spherical or oval vesicles. Each vesicle is closed and lined by a layer of cubical epithelial cells resting on a basement membrane, and is filled either with a clear glairy fluid or n more solid material termed Fig. 148. — Section of the Thyroid Gland of a Child (Quain's Anatomy). Two complete vesicles are seen. _ In the middle of one of the interspaces a blood-vessel is seen. Between the epithelial cells there are small cells like lymph corpuscles. colloid. A rich supply of blood is furnished by the two thyroid arteries, the capillaries forming a dense plexus round the vesicles. Numerous lymphatics arise in the organ, and an abundant supply of nerves pass off from the cervical ganglia of the sympathetic. Little is known as to the function of the thyroid. It probably forms an internal secretion in the vesicles which is carried ofif by the lym- phatics and poured into the blood. In the disease known as goitre there is great enlargement of this organ, often accompanied by a peculiar form of idiocy termed cretinism. Disease of the organ or its extirpation also fre- quently results in changes in the composition of the blood, an accumulation of matter in certain tissues, and distressing nervous symptoms. The disease myxedema, occurring in adult life, is connected with waste or atrophy of the thyroid, and is marked by excess of connective substance in the subcu- taneous tissues and by general stupidity. An extract of sheep's thyroid, or eating the raw organ, has been found to relieve the disease, so that the gland must be in some way connected with the healthy nutrition of the body. The Liver and the Ductless Glands 229 151. The Thymus. — The thymus gland is a temporary organ placed over the trachea behind the upper part of the sternum. At birth it weighs about half an ounce, increases in size during the first two years, and then shrivels away gradually, being represented in the adult by a small portion of fatty substance. The organ is surrounded by a capsule from which fibrous partitions pass inwards, dividing the body into lobes and lobules and carrying the blood-vessels. Each small lobule is further subdivided into follicles by delicate connective tissue, while each follicle consists of a cortical portion containing lymphoid cells and a medullary portion con- taining granular cells and the concentric cells of Hassall. The thymus is probably concerned in some way in producing changes in the chemical or cellular character of the blood. Coloured corpuscles and haemoglobin granules are said to be found in the issuing lymph. Various extractives can be obtained from the gland, as well as a peculiar proteid body of the nature of globulin, which causes intravascular clotting when injected into the veins. But the early development and rapid disappearance of the thymus suggest that its chief functions are performed during that period of life when growth and tissue formation are most active. 152. The Suprarenal Bodies. — The suprarenal capsules are two small bodies of a flattened triangular shape resting upon the upper surface of the kidney (fig. 149). They are about i| inch long and weigh about 2 drams. A section through a fresh organ shows an external or cortical zone and an internal or medullary zone. Each capsule is invested by a fibrous coat that sends vertical divisions into the cortex. The cortical portion thus consists of a fibrous framework with cells arranged in columns, the blood-vessels running between in the fibrous septa. In the medullary part the framework of connective tissue has more the arrangement of a close network, and the cells of the medulla lying in the interstices are irregular and branching. The arteries supplying these bodies break up into capillaries in the fibrous septa of the organ, forming in the medullary part venous sinuses which converge and ultimately form the large vein which emerges at the centre of the gland. Nerve fibres with ganglion cells in connection, whose mode of termination is unknown, are numerous. Complete removal of the supra- renal capsules leads to an alteration in the composition of the blood, to great muscular weakness, to nervous prostration, and in man when diseased there is a peculiar bronzing of the skin besides. Their removal is also fatal to the animal experimented upon after a short time. They thus appear to decompose effete pigment in the blood and secrete something necessary to muscular tone. CHAPTER XI EXCRETION. — THE KIDNEYS AND THE SKIN 153. Elimination of Waste Products, — It has already been shown that the various food-materials taken into the alimentary canal are prepared by the digestive processes for being taken into the blood, that by absorption into the rootlets of the portal vein, or into the lymphatics of the intestines, they 230 Human Physiology are carried into the blood-stream, and that by exuding through the thin walls of the systemic capillaries the blood-plasma or lymph carries the absorbed material to the cells and other tissue elements, to serve for their nourishment. Oxygen taken up in the alveoli of the lungs is also being continually carried to the tissues. As a result of tissue activity or metabolism, which is mainly a process of oxidation, certain waste products are formed which enter the venous blood, either directly in the capillaries or indirectly by the lymphatic circulation. Carbo- hydrates, fats and proteids are converted into carbon-dioxide, water, urea, and other allied nitrogenous bodies. Certain inorganic salts, as sulphates and phosphates, are also produced by oxidation of the sulphur and phosphorus in some articles of food. These waste products must be removed from the body in some way, and their elimination is effected by special parts termed excretory organs, the effete substances so removed being called excretions. As already shown, the lungs serve as the chief channel for the elimination of carbon -dioxide, as well as for a considerable amount of water in the form of aqueous vapour. The kidneys also remove a large quantity of water in which are dissolved nearly all the urea and allied bodies, with the main portion of the salts, the liquid being known as the urine. A variable amount of water, with a small quantity of salts and carbon-dioxide, is elimin^ated from the blood by the skin in the form of sweat. The faeces also contain, besides undigested portions of the food, a small amount of waste products mainly derived from the bile ; for the bile, though chiefly a secretion to aid digestion and so reabsorbed by the intestine, furnishes a small amount of excrementitious matter that colours the faeces. Even a small portion of the reabsorbed bile is afterwards excreted in the urine as urobilin (par. 1 24), or other colouring matter of the urine. 154. Structure of the Kidneys.— The two kidneys are situated in the abdomen close to its hinder wall and behind the peritoneum and intestinal canal, one being on each side of the spinal column. They are surrounded by a mass of fat and loose areolar tissue, and capped by the suprarenal bodies. Each kidney, which has a characteristic shape, is about four Excretion. — The Kidneys and the Skin 231 inches long, two to two and a half broad, and about one inch thick. The weight varies from four to six ounces. They are invested by a tough filjrous capsule which can be easily removed from the substance of the organ, being only attached by fine processes of connective tissue and minute blood-vessels. The deep longitudinal fissure named the hilum, on the internal concave border, allows the passage of the blood-vessels, nerves, and ureter into and out of the organ. On making a vertical section of a kidney, the naked eye distinguishes a deep red outer cortical portion of the kidney sub- stance and a pale red inner medullary or pyrami- dal portion, the latter being composed of about twelve papillae or pyramids, the apices of which project into a wide funnel-shaped sac called 'Cn& pelvis. The pelvis is the dilated ex- tremity of the ureter or excretory duct, and is divided into several trun- cated branches called calyces, or infundibula, around each apex of the pyramids. The cortical matter lying outside also invests the bases of the pyramids and sends processes between them known as the columns of Bertin. It is soft and friable and shows small granules due to the presence of Malpighian corpuscles. The medullary portion, which is divided into a boundary layer and papillary part, is denser than the cortex, and exhibits stria or radial streaks passing into the cortex, and there Fig. 149. — Vertical Section of Kidney. 232 Human Physiology known as medullary rays, or pyramids of Ferrein. The portion of the cortex lying between the medullary rays is termed the labyrinth, and exhibits the red granules just mentioned. Careful examination with a hand lens shows small openmgs at the apex of each papilla, and on squeezing the part under examination a drop or two of urine exudes. Microscopic examination shows that the kidney is made up of tubules, blood-vessels, and a small amount of connective tissue. The organ is in fact a compound tubular gland, the tubuli uriniferi, or urine carry- ing tubules, arising within the cortex, and these, after pursuing a complicated course and uniting with other tubules to form col- lecting tubules, discharge their fluids into the calyces of the pelvis, through the minute open- ings at the end of each pyramid. Both cortex and medulla are thus largely composed of these urini- ferous tubules. Those parts of the tubules in the medulla have a straight direction, and those in the cortex for the most part a contorted arrangement, with some groups of straight tubules passing from the medulla to form the medullary rays (fig. 151). Each uriniferous tubule con- sists of a layer of epithelial cells resting on a basement membrane, the blood being brought into close connection with these cells at certain parts of the tubule in ways presently to be described. The tubules begin in the cortical part of the kidney in a cup-shaped expansion about -^^ inch in diameter, termed the Malpighian capsule (fig. 153), each capsule enclosing a bunch of convoluted blood-capillaries termed a glomerulus. The tubule leaves the cap- FlG. 150.— Section through part of the Dog's Kidney. (Ludwig.) p, papillary, and^, boundary zones of the medulla ; r, cortical layer ; h, bundles of tubules in the boundary layer, sepa- rated by spaces, ^, containing bunches of vessels (not here represented), and pro- longed into the cortex as the medullary rays, ;« ; c. intervals of cortex, com- posed chiefly of convoluted^ tubules, with irregular rowsof glomeruli, between the medullary rays. At apex of pyra- mid A, excretory ducts open into a calyx of the pelvis. Excretion. — The Kidneys and the Skin 233 sule by a narrow neck (2), becomes twisted on itself to form \h& first con- voluted tubule (3), ' straightens some- what to form the spiral tubule (4), and then passes directly down with diminished width into the medulla to form the descending loop of Henle (5). Be- fore reaching the apex of the pyramid the tubule from the loop of Henle (6) by an upward turn runs towards the cortex, parallel to its previous direction, as the ascending loop of Henle (7, 8, 9), again enters the cortex in a wavy manner as the zigzag tubule (10), and again takes on a contorted course as the second convoluted tubule (11). Now narrowing into 3.Junctional tubule (12), it joins a straight or collect- ing tubule (13), which passes through the medullary substance (14), and unites with other col- lecting tubules to form a duct of Bellini, or excretory duct, that opens at the apex of the papilla, thus allowing the secreted urine to be discharged into the pelvis, or expanded upper end of the ureter. The epithelium cells lining the tubules vary in character in Fig. 151. — Diagram of the Course of Two Uriniferous Tubules. (Klein.) A, cortex ; B, boundary zone ; c, papillary zone of the medulla ; d, a!^ superficial and deep layers of cortex, free from glomeruli. For the explanation of the numerals, see the text. 234 Human Physiology the different parts ; three varieties may be noted. In the capsule the epithelial cells are squamous or flattened, and are reflected over the glomerulus (fig. 152), there being thus two layers, the inner of which is fused with the glomerular loops. In the first convoluted tubule, in the spiral tubule, in the ascending loop of Henle, in the zigzag tubule, and in the ' second convoluted tubule, the epithelial cells are granular m m Fig. 152.— Tubules from a Section of the Dog's Kidney. (Klein.) rtj capsule, enclosing the glomerulus ; «, neck of the capsule ; c, c, convoluted tuhules ; b, irregular tubules ; d^ collecting tube ; e, e, spiral tubes ; yj part of the ascending limb of Henle's loop, here (in the medullary ray) narrow. cubical cells, and show a fibrillated or rodded structure. In the descending loop of Henle and in the straight collecting part of the tubule the cells are clear cubical cells. Thus the cells of the convoluted and irregular parts of the tubules exhibit the characters of active secreting cells, and their protoplasm is believed to be concerned in extracting from the blood the chief organic substances of the urine, while the cells lining the collecting and discharging parts of Excretion. — The Kidneys and the Skin the tubule are similar to those ^ seen in the conducting part of a gland. The kidney is abundantly supplied with blood. The renal artery, a direct branch of the aorta, enters the organ at the hilus, dividing in its passage into several branches that pass round the pelvis into the kidney sub- stance between the pyramids in the columns of Bertin. On reach- ing the boundary between the cor- tex and medulla these branches divide, and spreading laterally form incomplete anastomosing arches at the bases of the pyra- mids. From these arterial arches vessels proceed on the one hand outwards towards the cortex and inwards to the medullary pyra- mids. The vessels from the arterial arches that run outwards to the cortex between the medul- lary rays are the interlobular 235 Fig. 153.— Minute Structure of Kidney. Fig. 154.— Diagram of the distribution of the Blood-vessels in the Kidney. (Ludwig.) ai, ai^ interlobular arteries ; vi, vi, in- terlobular veins ; g; glomerulus ; vs, stellate vein ; ar, z'r, arteriae et venes recta forming bundles, ai) and vb ; "vp, venous plexus in the papillae. 236 Human Physiology arteries. In their outward course the interlobular arteries give off lateral branches that form the afferent vessels of the Malpighian bodies, an afferent vessel entering the capsule of a uriniferous tubule and forming the glomerulus. From each glomerular tuft a somewhat smaller efferent vessel passes out of the capsule, and again breaks up into a network, of capillary vessels over and between the tubules. This double capillary network — a first capillary distribution in the capsules to form the glomeruli and a second capillary distribution to form the vascular network over the tubules — repeats on a small scale the peculiarity of the portal circulation. The branches of the arterial arches that run off to the medullary substance in a straight direction through the pyramids as the arterice rectm, break up into a plexus of capillaries with elongated meshes. From these arise vena rectcB that run into venous arches corresponding to the arterial arches at the boundary of the cortex and medulla, these venous arches also receiving the interlobular veins that are formed by the union of the capillaries from the Malpighian bodies, and by some capillaries formed by small branches of the interlobular arteries that do not enter the glomeruli. The small veins at the surface of the cortex being arranged in a star-shaped manner, are spoken of as the vena stellata. It will be observed that there are at least two ways by which the blood can pass through the kidney without traversing the glomeruli, (i) through the straight vessels of the medullary portion, whose blood supply is distinct from that of the cortex ; (2) through the small branches given off from the interlobular arteries of the cortex, that break up into capillaries without entering the capsules of Malpighi. The lymphatics of the kidney arise in lymph spaces of the scanty framework of connective tissue in the organ, and the fluid collected into vessels emerges from the organ by lym- phatics that make their exit either at the tubes or through the capsule. The nerves derived from the renal plexus and lesser splanchnic nerves form small trunks with ganglia, and for the most part accompany the blood-vessels. Little is known as to their mode of termination. No secretory nerves similar to those found in the submaxillary gland have been discovered. Excretion. — The Kidneys and the Skin 237 155. The Urine. — Fresh urine is a clear straw-coloured fluid of peculiar odour and acid reaction. It consists of water holding in solution urea and other solids, and has an average specific gravity of I '020. The specific gravity, being dependent on the amount of solids relative to water, may vary from i -002 after drinking much wa'er, to i '040 after abstinence from liquid, and after copious perspiration. Any cause which draws off a large quantity of fluid from the body through any other channels than that of the kidney, e.g. the skin, or the lungs, or the bowels, lessens the quantity of water in the urine and increases the specific gravity, for the amount of solids discharged daily by the kidney is fairly constant. Thus in cold weather when perspiration is scanty the urine is more abundant and more dilute than in summer, when the amount of water vapour discharged from the skin is great. A healthy adult passes on the average about 50 fluid ounces or 2.\ pints of urine daily, the amount of solids in sohition being about 2 ounces, or 4 per cent. Urea is the chief solid, forming 2-2 per cent. Sodium chloride comes next, forming I per cent. Other solids dissolved in small quantity are acid sodium phosphate, phosphates of calcium and magnesium, alkaline sulphates, uric acid, creatinin, &c. Nearly two- thirds of the solid substances are organic bodies. A quantity of the gas carbon-dioxide, with a small amount of nitrogen, is also held in solution in urine. The acidity of urine is not due to the presence of free acid, but to acid sodium phosphate. (The sulphuric acid is combined to form sul- phates, the phosphoric acid to form phosphates, and the uric acid to form salts termed urates. ) After standing for some time in contact with air urine becomes alkaline owing to the conversion of urea into the alkaline body, ammonium carbonate. The conversion of urea into ammonium car- bonate, by taking up two molecules of water, is brought about by a micro organism. The colour of urine is due to pigments concerning the nature of which much discussion has arisen. According to some authorities the chief colouring matter is the pigment urobilin, which is said to be a derivative of bile-pigment. The average daily quantity of the chief constituents of urine in twenty- four hours is thus stated by Waller : — per 1000 per diem Water . 960 . 1440 c. cm. Urea _ . . 20 30 grams Uric acid j fo75 Hippuric acid I 2 • i 075 Creatinin J li-5 Phosphates l f3-o Chlorides I . • 15 • • 7-5 l3-o Sulphates Mucus and Extractives f 3 Urea, the chief solid constituent of urine, is an almost colourless body, which can be obtained from a solution in water or alcohol in the small silky four-sided prismatic crystals or in delicate white needles. Its chemical composition is represented by the formula CO (NH,).^. About 33 grams (500 grains) are excreted daily by an adult man, but the 238 Human Physiology amount varies with circumstances, being increased by large quantities of nitrogenous food and diminished by a vegetable diet. Active muscular exercise is also said to produce a slight increase of urea, though the ex- cretion mainly increased by muscular activity is COj from the lungs. There is no doubt that urea represents the chief end-product of the changes undergone by the proteid or nitrogenous food stuffs, just as carbon dioxide represents the chief end-product of the caxbon contained in the food, and water, the chief end-product of the chemical change undergone by hydrogen. The production of carbon dioxide and water can be brought about by the simple process of oxidation. But the stages intermediate br^tween proteids and urea are complex and ill understood, while urea differs from COj and H^O in being only a partially oxidised product. It is thought that creatin ' may be a waste product of muscular tissue that passes into the blood and forms a precursor of urea. Another source is believed to be such substances as leucin and tyrosin, which are two of the products that may arise from pancreatic digestion in the ali- mentary canal. These substances being absorbed in the intestines are carried by the portal vein to -^^V Bk \ ^^C^'^k '^^ \\se.x and there converted into ^^^^^^^ KM ^^^C\ "'^^ ^5' '^^ hepatic cells. The ^^^^^ />K \ ^-Jv^^ ^^^^ ^^ "^^" taken up by the ■^^ a l/I' i\\\\^V^!>. hepatic veins, and passing into the general circulation is ulti- mately eliminated by the kidneys. That some urea is formed by the liver seems evident, because (l) when leucin is introduced into the alimentary canal the amount of urea in the urine is increased, though leucin itself does not ap- pear ; (2) in acute yellow atrophy of the liver, in which the liver- cells lose their function, leucin and tyrosin partially replace the urea of the urine ; (3) on passing blood through a liver from a recently killed animal, the amount of urea in the blood is increased ; (4) when the circulation of an animal is so altered that the blood from the portal vem is made to flow into the inferior vena cava, without passing through the liver, the amount of urea in the urine is con- siderably lessened. Other tissues, as the spleen and the lymphatic glands, are thought to take part m the formation of urea, but beyond what has just been said little is known as to the stages intermediate between the highly complex proteids absorbed by the blood and the comparatively simple nitro- genous end-product urea. One important point seems well established. Observations on starving animals show that the proteid in food is partly built up from the exuded blood-plasma into living protoplasm to supply the tissue waste (tissue or organ proteid), but is chiefly acted upon by the living tissues and converted into products that give rise to urea without having formed an integral part of the living substance (circulating proteid). . The ' Creatin is C.,H„ NjOo. The removal of H^O leaves Creatinin. Fig. 155. —Crystals of Urea. a, four-sided prisms : b, indefinite crystals, such as are usually formed from alcohol solutions. Excretion. — The Kidneys and the Skin 239 increase of urea excretion after the ingestion of proteid food, and the pro- portion that the urea discharged bears to the proteid food consumed, indi- cates that the urea excreted is primarily derived from the circulating pro- teid, that is, from the recently ingested proteid food, and only secondarily from the disintegration of muscle and other organs. As Dr. Waller observes : ' From proteid to urea there are three usual roads : ( i ) the short cut via leucin and tyrosin in the intestine, and urea in the liver ; (2) the high road via circulating proteid ; (3) the long, narrow way via cir- culating and organ proteid. The centre of action is the living tissue element, which, while undergoing little change as to its own proteid, effects considerable change in the proteid solution which soaks through and around it.' That urea is not formed but is eliminated by the kidneys is proved by the result that follows extirpation of these organs. In this case urea accumulates in the blood and tissues. Further, the blood nor- mally contains urea — i in 4000 — and the renal vein carrying blood away from the organ has been found to contain less than the renal artery that leads to it. Uric acid (CjHjNjOs) occurs in normal urine in small quantity as a potassium or sodium salt, and is a less completely oxidised product of proteid metabolism than urea. 156. Secretion of Urine. — Two more or less distinct pro- cesses may be distinguished in the secretion of urine by the kidney, (i) A process mainly of filtration, by which the water and some of the highly soluble salts pass from the blood in the capillaries forming the glomeruli into the capsule at the beginning of the uriniferous tubules. (2) A process of true secretion, by which the urea and other specific constituents of the urine are secreted by the fibrillated epithelium cells of the tubuli from the blood in the second set of capillaries ramifying over the convoluted tubules. A Malpighian body, as already explained, consists of a glomerulus or tuft of capillaries lying inside the capsule that forms the dilated end of a uriniferous tubule, the efferent or outgoing vessel of the capsule having a smaller calibre than that of the afferent or ingoing vessel. Hence the blood in the capillary vessel will be under high pressure, and fluid will pass through the filter formed by the thin capillary walls and the layer of flattened cells covering them into the cavity of the capsule where pressure is low. This view is supported by the fact that the quantity of urine secreted increases with increase of blood-pressure in the renal arteries, or rather with increased blood-flow through the kidney. But urine differs materially from the fluid that could be obtained as a filtrate from blood, especially in the absence of serum-albumin in which the blood 240 Human Physiology is rich. It is evident, therefore, that the process is not one of physical filtration alone, but one in which the epithelial cells of the tubules take an active part. Various observations show that the amount of urine depends largely on the blood-pressure in the glomeruli. Increase of blood-pressure in the glomeruli with increase in the amount of fluid excreted may be produced by — (1) An increase in the general blood-pressure, due to increased action of the heart or to the constriction of the small arteries of the skin or organs other than the kidney. (2) Dilatation of the renal artery unaccompanied by compensating dilatation elsewhere. Such dilatation may be caused by section of the renal nerves, a result which shows that these nerves normally exercise a vaso-constrictor action. It is probable that these nerves also contain dila- ting fibres, so that there appears to be a local nervous mechanism in the kidney controlling to some extent the flow of blood. (As compression of the renal vein arrests the secretion of urine, while at the same time the pressure in the glomeruli may increase, it is more cor- rect to say that the secretion of urine increases or diminishes with increased or diminished blood-flow through the kidney.) Diminution of blood-pressure in the glomeruli with diminution of fluid secretion of urine may be produced by — (i) A lowering of the general blood-pressure of the system due to diminished force of the heaj t's action, or to dilatation of the small arteries of parts other than the kidneys. (2) A constriction of the renal artery by stimulation of the renal nerves. The second part in the process of the secretion of urine — the separation of the specific organic constituents, urea, uric acid, &c. from the blood — is effected by the epithelium cells of the convoluted tubules. It has been pointed out that the cells lining these portions of the uriniferous tubules resemble in certain respects the active secretory cells of other glands, while those lining the Malpighian capsules and portions of Henle's loops are simple flattened cells ; and further, that these relatively large secretory cells are surrounded by a set of capillaries derived from the breaking up of the efferent vessels from the glomerular tufts. Thus the differences of epithelium in the uriniferous tubules, and the differences in the capillary blood supply at different parts of the tubule, forcibly suggest different modes of action in the different parts. The fluid filtered or secreted from the blood in the glomerular tufts into the capsules passes along the tubules, and washes down the urea, &c. excreted Excretivn. — The Kidneys and the Skin 241 by the cells of the convoluted tubules. This view of the nature of urinary secretion is confirmed by various experiments. When the blue pigment sulphindigotate of sodium is injected into the blood of a mammal it is excreted by the kidneys, rendering the urine blue, and the course of its excretion has been traced, since it colours the cells through which it passes. Sections of the kidney of an animal so treated, and killed at appropriate times, reveal the presence of this pigment in the fibrillated cells of the convoluted tubules, but not at all in the capsules. After a time it is seen in the lumen of the tubules also, being washed down by the watery iluid from the capsule. Moreover, stoppage of the action m the capsule, either by destruction of the glomeruli with cauterisation or by section of the cervical spinal cord, which reduces blood-pressure so low as to stop the flow of urine, leads to the discovery of the pigment in the cells alone, the water to carry it along not having been secreted. The same experimenter (Heidenhain) has also traced sodium urate into the convoluted tubules, and crystals of uric acid have been observed within the renal epithelial cells of birds. Not only, therefore, does the structure of the epithelium cells of the convoluted tubules suggest a secretory activity similar to that of the cells lining the alveoli of a salivary gland, but direct experiments support this view. Yet the case of such glands as the salivary and gastric glands, as well as the pancreas, differs from that of the kidney in the fact that their specific constituents— mucin, pepsin, trypsin, &c.— are elaborated in the cells of the alveoli from antecedent substances in the blood, while the urea and some other important constituents of the urine are simply removed from the blood brought to the kidney, having been preformed elsewhere. There is evidence, however, to show that the small quantity of hippuric acid daily excreted in the urine is formed by the kidney-cells, from a combination of the benzoin acid in the blood and the glycin produced by its own cell metabolism. 157. Variations in TTrinary Secretion. — It has been stated that the aver- age quantity of urine in 24 hours is about 50 ounces of water and 2 ounces of K 242 Human Physiology . solids. But the proportion of water to solids, and therefore the specific gravity and colour of urine, vary within wide limits. The kidneys and skin are so correlated that, with the normal daily supply of water, an in- crease in the amount discharged by the one diminishes the amount dis- charged by the other as already stated (par. 155). ' It has also been shown that the amount of fluid removed into the capsule depends largely on the amount of blood passing through the organ, and as the kidney has its own vaso-motor nerves, with both vaso-constrictor and vaso-dilator fibres, under the control of the central nervous system, it is evident that nervous influences may increase the flow of clear watery urine by determining a greater flow of blood with increased pressure. Such an effect is produced by fear and other emotions, as well as by the nervous affection termed hysteria. Section of the renal plexus, or nerves passing to the kidney around the renal artery, causes an increase of watery urine owing to the great rise of pressure within the glomeruli. The nerve centre for the renal nerves lies in the medulla in the floor of the fourth ventricle, just in front of the origin of the vagus nerves. Puncture of this part also increases the flow of watery urine {diabetes insipidus']. Close to this centre lies the centre for the vaso-motor nerves of the liver, and injury to this part leads to diabetes mellitus, copious urine containing sugar (par. 147). By means of an instrument termed the renal oncometer, the actual volume of the living kidney may be observed to undergo variations, an increase of size resulting from dilatation of the renal vessels and a diminu- tion resulting from their contraction. By registering these variations of volume on a revolving cylinder at the same time that the arterial blood- pressure is recorded, it is found that the kidney curve rises and falls in or- dinary circumstances with the blood-pressure curve. This close connec- tion between the volume of the kidney and the supply of blood to it enables the latter to be estimated by means of the oncometer. Increase of volume is followed by increased secretion of urine, and decrease of volume by decreased secretion. It is also found that changes in the com- position of the blood soon affect the renal circulation and volume of the kidney. Injection of water into the blood or rapid absorption through the alimentary canal produces local dilatation of the renal vessels. Certain drugs called diuretics are efficacious in promoting the flow of urine. Thus urea itself is a diuretic, and when injected into the blood is excreted along with watery urine by the cells of the convoluted tubules, even when secretion in the glomeruli has been arrested by section of the spinal cord. Other diuretics act differently. Sodium acetate produces an increase in kidney volume and increased secretion of urine without any increase in general blood-pressure, even when the renal nerves are cut— a result which appears to show that changes in the composition of the blood may produce local dilatation and increased secretory activity, by acting upon some vaso-motor mechanism within the kidney itself or directly upon its blood-vessels. Such changes in the blood probably explain the increased secretion after meals and the diminution during fasting and sleep. Abnormal constituents of urine are sugar, albumin, and bile. In normal urine there is only n mere trace of grape-sugar or dextrose, but under certain conditions the amount becomes greatly increased with great increase of water also. This constitutes the disease glycosuria, or diabetes mellitus. The physiological conditions giving rise to this affection and its artificial production have been treated of in par. 147. In diabetes insipidus, which probably depends on some derangement of tne central vaso-motot Excretion. — The Kidneys and the Skin 243 control of the kidney, there is a copious secretion of watery urine without sugar. The presence of albumin in urine is indicative of the condition termed albuminitria. Various causes bring about this affection, and it can be produced experimentally by ligature of the renal vein for a short time, the stoppage of the,blood-flow so interfering with the activity of some of the cells that they seem unable to prevent the passage of the albuminous constituents of the blood-plasma. Resistance to the outflow of bile into the intestine leads to its absorption into the lymphatics of the liver, and thence by the thoracic duct into the blood. From thence the bile-pig- ments and bile-acids pass into the tissues and urine, producing the condition termed jaundice. 158. Passage of the Urine into the Bladder. — Partly owing to the high pressure under which it is secreted, partly through gravity, and partly owing to the rhythmical peristaltic contraction of the ureters, the urine is driven on through these tubes into the bladder. These tubes are from sixteen to eigh- teen inches in length, and about the diameter of a goose quill. They are formed of an outer fibrous coat of connective tissue with blood-vessels and nerves, a middle muscular coat with longitudinal and circular unstriped fibres, and an inner mucous coat having several layers of stratified transitional epithelium. Reflux from the bladder into the ureter is prevented by the oblique manner in which these tubes enter, a sort of valvular opening being thus formed. 159. The Bladder and Urethra. — The bladder, which is situated in the pelvis, has an average capacity of about twenty ounces. Its structure is very similar to that of the ureters, though the mucous and muscular coats are thicker. From its narrow end, or neck, a canal termed the urethra passes to the outside. At the beginning of the urethra the circular non- striped muscular fibres become thicker, but the urethra is kept closed by a transversely striped muscle termed the sphincter urethrm, which consists of circular fibres extending along the urethra to its middle, the tonic contraction of this muscle being maintained reflexly by a centre or centres in the lumbar part of the spinal cord. The urine gradually accumulates in the bladder until the tension in the organ leads to the contraction of its muscular walls, and the sphincter relaxing, expulsion of the urine (micturition) follows. In young children the whole act is reflex. With advancing age there is acquired more 244 Human Physiology or less control of the act. Thus, when the bladder becomes distended, the afferent impulse passing into the cord may thence reach the brain, but by an effort of the will the sphincter muscle may be assisted and the expulsion arrested for a time. Or the will may inhibit the sphincter, and assist the reflex con- traction of the mus- cular walls of the bladder by producing contraction of the abdominal muscles. 1 60. Structure of the Skin. — The skin forms an external covering for the deeper tissues over the whole body, and consists of a super- ficial layer termed the epidermis or cuticle, and a deep layer termed the corium, dermis, or cutis vera (true skin). As al- ready described (par. 6 ), the epidermis consists of layers of stratified epithelial cells united together by a small amount of cement substance. The dermis, or true skin, is made up of an interlacing net- work of white fibrous connective tissue, with some elastic fibres, numerous blood-vessels, lymphatics, and nerves. Below, the skill gradually becomes blended with the subcutaneous tissue through a layer of areolar tissue of varying thickness containing fat-cells. Certain structures, as the nails, the hairs, Fig. 156. — The Kidneys, Bladder, and their Vessels, Viewed from behind. R, right kidney ; U, ureter ; A, aorta ; Ar, right renal artery ; Ve, vena cava inferior : Vr, right renal vein ; Vu, bladder ; Ua, commencement of urethra. Excretion. — The Kidneys and the Skin 245 the sebaceous glands, and the sudoriparous glands, may be termed appendages of the skin. At the orifices of the body the skin gradually passes into mucous membrane, soft membrane moistened by their secretion mucus, and composed like the _ Fig. 157. — A Sectional View of the Slcin (magnified). skin of corium and epidermis, though the epidermis differs greatly in different parts. The corium, or true skin, whose connective tissue shows a more open texture in its deepest parts, bears on its upper sur- face numerous small papillae which project up into the epi- 246 Human Physiology dermis, the inmost layers of epidermal cells, or rete Malpighii (rete mucosum), being moulded over the papillse and forming processes between them. The papills are highly sensitive vascular eminences of a conical or club shape, about —^ inch in length. On the general surface of the body they are com- paratively few in number, but in sensitive situations, as the palm of the hand and fingers, they are numerous, and being arranged in parallel curved lines, form the elevated ridges seen on the free surface of the epidermis. The papillae contain Fig. 158. — Section of Human Epidermis with Two Vascular Papillae of the Corium. (Heitzmann.) BP^ loop of capillary vessels in papilla ; V, rete mucosum ; PL, stratum granulosum ; M, stratum corneum ; Z* to/, duct of sweat gland passing through the epidermis. capillary loops derived from a small artery in the cutis, and most of them have also one or more terminal nerve fibres, those of the hands and feet terminating as touch corpuscles (par. 49). It will be remembered that fine nerve fibrils pass into the epi- dermis, where they end either between the cells or in the deep epithelial cells themselves (fig. 6). 161. Hairs and Nails. — Hairs are modifications of the epidermis devel- oped in pits— the hair follicles — that often extend down into the subcutane- ous tissue. The substance of the hair consists of a central pith or medulla, a fibrous horny part, and an external cortex or cuticle. The medulla Excretion. — The Kidneys and the Skin 247 formed of angular cells is often wanting. The fibrous part consti- tutes the chief part. It consists of long tapering cells united to form fusiform fibres. The cuticle con- sists of thin flat scales overlapping like tiles. The part of the hair lying within the follicle and termed the root ends in a knob of soft grow- ing cells fitting over a vascular papilla. The pit or follicle is formed of two coats, an outer or dermic coat continuous with the corium, and an inner epidermal coat termed the root sheath. The hair grows from the bottom of the follicle by the multiplication and elongation of the cells covering the papilla. Connected with the hair follicles are small bundles of involuntary muscu- lar fibres forming the muscles of the hairs, or arrectores pili (fig. i6o). Passing from the upper part of the corium on the side to which the hair slopes to the bottom part of the follicle, the contraction of this B / c Fig. 159. — Piece of Human Hair (magnified). A, seen from the surface; B, in optical section, c, cuticle ; f^ fibrous substance ; m, medulla, the air having been expelled by Canada balsam. Fig. 160.— Hair Follicle in Longitudinal Section. (Biesiadecki.) a, mouth of follicle; ^, neck; c, bulb; d, e, dermic coat ;/, outer root sheath ; g, inner root sheath ; /i, hair ; k, its medulla ; /, hair knob ; jn, adipose tis- *" sue ; «, hair muscle ; o, papilla of skin ; /, papilla of hair ; j, rete mucosum, continuous with outer root sheath ; ep horny layer ; t, sebaceous gland 248 Human Physiology muscle not only erects the hair, but also raises one part of the general skin surface and depresses another, thus producing the roughened condition known as ' goose-skin. ' Nails are also modifications of the epidermis implanted by a portion termed the root in S. groove of the skin, and growing from a modified por- tion of the true skin termed the bed, or matrix, of the nail. The nails con- sist of horny cells having a laminated airrangement, the deepest layer lying in contact with the papillae of the matrix. The growth of the nail, like that of the hair and the epidermis, is effected by the successive production of new cells at the root and under surface. 162. Sebaceous Glands. — The sebaceous glands are simple saccular glands found all over the skin, except in the palm of the hand and the sole of the foot. The duct usually opens into a hair follicle (fig. 160, t), though in a few situations it opens free on the surface. Both the duct and its alveolar expansions are lined by secretory cells, some of which become charged with fatty matter. Excretion appears to take place by the rupture or disinte- gration of the loaded cells, the transformed cells and their contents being pushed out of the duct through the hair follicle on to the surface of the skin by the newly generated cells behind. The secretion (or excretion) called sebum appears, when fresh, to be an oily substance that sets on cooling. Under the microscope it is seen to consist of fatty particles, epi- thelial cells, and crystals of organic bodies. As to the physiological cha- racter of the material Professor Foster says : ' The secretion of sebum, in fact, is a modification of the particular kind of secretion taking place all over the skin, and spoken of as shedding of the skin. It is chiefly the chemical transformation which is different in the two cases. In the skin generally the protoplasmic cell substance of the Malpighian cells is trans- formed into keratin, in the sebaceous glands it is transformed into the fatty and other constituents of the sebum. Some, perhaps, may hesitate to apply the word secretion to such a process as this ; but as we shall see later on, the formation of milk, which certainly deserves to be called a secretion, is a process intermediate between the secretion of saliva or gas- tric juice and the formation of sebum.' The chief purpose of the seba- ceous secretion appears to be to lubricate the hairs and to keep the skin moist and supple. Two or more sebaceous glands may be connected with a single hair follicle. 163. The Sweat Glands.— The sweat glands, called also sudoriparous or sudoriferous glands (Latin sudor, sweat), are found in the human skin in nearly all parts of the body, their total number exceeding two millions. They are most numerous on the palm of the hand and sole of the foot, largest in the axilte (armpits) and groin. In the neck and back there are com- paratively few. The orifices of the ducts or pores are about -^ inch in diameter, and may be seen with a hand lens. Each gland consists of a single tube with a blind end forming a close coil about -^^, inch in diameter, situated in the deep part of the true skin or in the subcutaneous fatty tissue. This is the Excretion. — Tlie Kidneys and the Skin 249 secreting portion of the gland, and it is surrounded by a net- work of capillary blood-vessels. From the coil the duct passes towards the surface as a conducting portion in a somewhat wavy manner through the corium, but spirally through the epidermis (fig. 157). The secreting portion of the coiled tube consists of a fine basement membrane, a layer of longitudinally disposed fibres usually regarded as muscular, and a single layer of columnar epithelium cells lining a central cavity or lumen. The effe- rent or conducting tube, which in- cludes about one- fourth of the coiled part, has a base- ment membrane, an epithelium con- taining two or thre layers of cells, an internal delicate lining, and finally the central lumen. It has no muscular layer and is smaller than the secreting tube (see fig. 161). The part of the duct passing through the epi- dermis is merely a channel through the epithelium cells. The wax formed in the external passage or meatus of the ear appears to be a mixture formed from the so-called ' ceruminous ' glands (which are modified sweat glands), and from the sebaceous glands of the passage. 164, Composition and Amount of Sweat. — Sweat has been obtained for examination by enclosing a limb in an air-tight caoutchouc bag. As thus obtained it is mixed with a few scales shed from the epidermis and with a small quantity of Fig. 161.— .Section of a Sweat Gland in tlie Skin of Man. i, a, secreting tube in section ; h, a coil seen from above.; c, c, efferent tube ; d^ intertubular connective tissue with blood-vessels, i, basement membrane ; 2, muscular fibres cut across ; 3, secreting epithelium of tubule. 2 so Human Physiology fatty matter from the sebaceous glands. It consists of water with only i -2 per cent, of soUds in solution. These are chiefly sodium chloride, various fats and fatty acids, and a trace of urea. Its reaction when pure is alkaline, but acid when mixed with sebaceous matter. A small amount of carbonic acid, about ten grains in 24 hours, passes off from the skin, and an equiva- lent amount of oxygen is absorbed. Cutaneous respiration in man and other mammals is thus very slight. Hence the death of such an animal as the rabbit, when its skin is covered with impermeable varnish, cannot be due, as was once thought, to arrest of cutaneous respiration. It seems rather to be due in such a case mainly to the rapid loss of heat from the surface, for the animal can be preserved for a considerable time by wrapping it in cotton wadding. In frogs and other amphibia that have a thin naked skin cutaneous respiration is active and important, as they continue to live some time after the lungs have been removed. As long as the excretion is small in amount it passes off the surface at once, and is called insensible perspiration. When the secretion forms drops on the skin, either owing to increase in amount or prevention of sufficiently rapid evaporation, it is spoken of as sensible perspiration. The relation between these two kinds of perspiration in similar conditions of the body depends chiefly on the rapidity of evaporation, and this is dependent on the degree of saturation of, and the amount of movement in, the surrounding air. The total amount secreted by a man in 24 hours may be said to average about 1,000 grams, or 2 lbs., but this amount is increased in a hot atmosphere, by active muscular exercise, and generally by any cause that increases the circulation in the capillaries of the skin. That the amount of sweat bears a certain inverse relation to the amount of urine excreted has already been pointed out (par. 155). The regulation of the temperature of the body by the evaporation of sweat is explained in the chapter on Animal Heat. 165. The Nervous Mechanism of Perspiration As already stated, dilatation of the cutaneous blood-capillaries leads to increased secretory activity of the sweat glands, and constriction of these vessels leads to diminished activity. Such changes Excretion. — The Kidneys and the Skin 251 may be brought about not only by external heat relaxing the vessels of the skin and external cold contracting them, but by the vaso-motor nerves that regulate the blood supply of the skin. But apart from variations in the vascular supply, it appears certain that there are special secretory nerve fibres passing to the sweat-glands, stimulation of which causes an increase of sweat. These fibres appear to be contained in the same nerve trunks as fibres having vaso-motor and other functions. Thus, if the peripheral end of the divided sciatic nerve in the cat be stimulated, drops of sweat appear on the. hairless sole of the foot, even when there is constriction of the blood-vessels, or even in the amputated limb. Other secretory fibres have been found in other nerves, as the ulnar. These special sweat nerves appear to spring from nerve-centres in the spinal cord, a dominant centre being said to exist in the medulla. A venous state of the blood, as in dyspncea, appears to excite the sweat centres. So also do certain drugs called sudorifics, as pilocarpin and strychnine. Sweating may also be produced reflexly by stimulating certain sensory nerves. Thus the pungency of mustard in the mouth often causes perspi- ration on the face. Certain emotional states, as fear, produce sweating through the agency of the central nervous system. Some observers have described structural changes as occurring in the secreting cells of the coiled part of the tubule, but these are not so well marked as in the case of the salivary glands, as they discharge neither mucus nor proteid material. 166. The Mammary Gland.--The rounded eminences situated on the breast form the lacteal or mammary glands, by which milk is formed during the process of lactation. The gland tissue is composed of large divisions or lobes subdivided into lobules, and connected together by areolar tissue and blood-vessels. From fifteen to twenty small channels termed lactiferous ducts open separately at the nipple. Each of these main ducts has a small dilatation or reservoir at a short distance from its orifice, and as it passes inwards it is found to arise by the union of smaller branches from the various lobes and lobules. During lactation the finest branches are seen to terminate in microscopic saccular pouches or acini. Each acinus of the gland consists of a basement membrane lined by a simple layer of epithelial cells. When the gland is inactive, the cells are flattened and show a single nucleus. But during the activity of the gland the secretory cells undergo peculiar and characteristic changes. The cells swell and become cylindrical, the nucleus often divides, and fatty granules appear in the cell protoplasm at the edge near the lumen. This portion of the cell becoming more promi- 252 Human Physiology nent is at last detached, the fatty granules being shed into the fluid in the lumen to become the microscopic milk-globules, and the discharged proto- plasm dissolving to form the proteid of the milk. The water, salts, and milk-sugar are also secreted by the cells from the blood or lymph bathing the basement membrane, and discharged into the lumen to be carried off by the ducts. It is thus seen that in the case of milk secretion the superficial part of the cell is cast off and forms j^art of the secretory product, while the basal part is left to grow and become an active cell once more. Fig. 162. — Dissection of the lower half of the Female Mamma during the period of Lactation. (From Luschka.) «, «, a, undissected part of the mamma ; i, the mammilla ; 2, areola ; 3, subcutaneous masses of fat ; 4, loculi in the connective tissue which supports the glandular substance ; 5, three lactiferous ducts passing towards the mammilla where they open ; 6, one of the sinuses or ampullae ; 7, some of the glandular lobuleswhich have been unravelled ; 7', others massed together. The regular secretion of milk begins three or four days after birth of the young. For the first two or three days the fluid, termed colostrum, is of a yellowish colour and coagulates on boiling. It contains a number of particles, ' colostrum corpuscles,' which consist of cells filled with minute fat-globules that are larger than the milk-globules. A gradual transition into true milk takes place. As already stated, milk is a perfect food for infants, since it contains all the three kinds of food-stuffs in proper pro- portions. For adults there is too large a proportion of water, as about 1 1 pints per day would be required to supply the food-stuffs of a normal diet (see pars. 106 and 107). 253 CHAPTER XII ANIMAL HEAT 167. General Statements.— The food that we take contains a store of potential energy which is converted into the kinetic form of heat and motion by the chemical changes that go on in the tissues. These chemical changes, included under the term metabolism, are of various kinds, but the great source of heat is the katabolic process termed oxidation. As already remarked, two kinds of processes are continually going on in the body. Protoplasm is being formed out of the food materials, a process called constructive metabolism or anabolism, and unaccompanied by any production of heat ; and protoplasmic materials are constantly being oxidised, a process known as katabolism, and resulting in the evolution of heat and mus- cular movement with production of carbonic acid, water, and urea. The oxidation process, as stated in treating of muscular tissue, is not direct and immediate, but complex, and occurring in several stages. Yet the result is the same, for we know that the oxidation of a given weight of any substance liberates the same amount of energy, whether the process takes place in one stage or in several. A gram of carbon, whether free or combined, on being oxidised sets free a definite and known amount of heat. A calorie or heat-unit is the amount of heat required to raise the temperature of one gram of water 1° C. The number of gram-calories or heat-units liberated by one gram of various materials has been estimated. In round numbers we get : Hydrogen Carbon Fat 3.45° 8,100 9,000 Carbohydrate Proteid Urea 4,000 5,000 2,205 These numbers supply the physical heat-value of i gram of the above substances, i.e. the amount of heat produced when complete combustion takes place in a calorimeter. But the physiological heat value of proteid is not the same as its physical heat value, as it is not completely oxidised in the 254 Human Physiology body. Each gram of proteid yields one-third gram of urea, and hence we must deduct the heat value of this from that of each gram of proteid (from 5,000) to get the physiological heat value of a gram of proteid : 5,000 — 735=:4,265. The estimates that have been made of the heat required to maintain the temperature of the body, and replace the loss by evaporation and radiation in a given time, are found to correspond closely with that derived from the quantity of the food supplied. Production of heat and discharge of heat, or income of energy and expenditure of energy, balance. Add to this the physical truths known about the production of heat during the combustion of various substances, and there can be no doubt that the source of heat in-the body is the chemical processes there taking place, especially the process of oxidation. Were there no loss, the amount of heat produced in the body in an hour would raise its temperature 2°. Helmholtz has estimated that about 7 per cent, of the heat produced in the body is converted into external mechanical work, that nearly 75 per cent, is lost by evaporation and radiation from the skin, and about 18 per cent, passes off by the lungs and excreta (see par. 170). 168. Temperature of the Body.— As regards temperature animals are divided into two great classes : 1. Warm-blooded or homoiothermal animals (Greek homoios, like), those which have a constant temperature. This class includes mammals and birds, birds having a higher tem- perature than mammals. 2. Cold-blooded or poikilothermal animals (Greek poikilos, variable), those whose temperature varies with the medium in which they are living, being only about a degree above the varying medium. This class includes reptiles, amphibians, fish, and invertebrates. The average surface temperature of a healthy human body taken in the axilla is 98-6° F., 37° C. (in the blood of the interior about 2° F. higher), an approximate uniformity being brought about by the circulation of the blood, which carries heat from the parts where it is produced and distributes it to the parts where heat is lost. In the interior of the body, A nimal Heat 255 especially where chemical changes are most active, the tem- perature is somewhat higher than at the surface, where heat is being constantly lost. The warmest organs of the body are the liver, the brain, and the muscles ; the coolest parts are the skin and the extremities, the difference amounting to more than 1° C, or about 2° F. Children have a temperature about ^° C. above that of an adult, and in all persons of regular habit there is a slight diurnal variation from a maximum at 3 p.m. (37'5° C.) to a minimum at 3 a.m. (36-8° C). Active muscular exercise may raise the temperature of the body i ° C, a feeling of great warmth being produced by the increased blood supply to the skin consequent on the dilatation of its vessels. In fact, om feeling hot or cold is due to the state of the cutaneous capillaries. When these vessels are full of warm blood the sensory nerves terminating in the skin are affected with the sensation of heat, but when these vessels are contracted and comparatively empty, we feel the cold of the external air Fig. 163. — A Clinical Thermometer. affecting the nerves of the skin. Thus our feeling is no real guide to the body temperature as a whole, which in health is always about the same, however hot or cold we. feel. 169. Where Heat is Produced. — Though heat is produced wherever chemical changes go on, the chief tissues in which heat-production occurs are the muscles, the glands, and the nervous centres. Every manifestation of muscular energy has been found to be accompanied by evolution of heat and carbon dioxide, and, as the carbon dioxide is given off though in a less degree even by resting muscles, it is evident that active chemical change with production of heat must also go on in muscles at rest. The secreting glands, especially the liver, are also the seat of metabolic processes that result in heat, and the temperature of an organ is greater during activity than during rest. Blood leaving the liver is hotter than that entering this organ, probably the hottest in the body during active digestion and absorption, while stimulation of the chorda tympani so increases the activity of the sub- maxillary gland that the saliva in its duct is found to have a temperature one degree higher than the blood in the carotid artery. The brain is also a source of heat, as the temperature of the blood leaving this organ when mental effort is going on is distinctly higher than the temperature of the blood entering it. Certain physical processes, as friction of the blood against the vascular walls, friction of muscles and tendons, and electrical currents, contribute a small amount to the heat production of the body. 256 Human Physiology As, therefore, the production of heat depends on chemical action rising and falling as that rises and falls, and as it has been ascertained that in a cold atmosphere more carbonic acid is given off than in a hot one, it is evident that the supply of heat is increased in a cold medium and dimi- nished in a hot medium. 170. Where Heat is Lost. — As the temperature of the surrounding air is below that of the body in all temperate climates, heat will be lost from the surface of the body by conduction and radiation, as well as by evapo- ration of the perspiration. By conduction heat passes to the air in con- tact with the body and from one air particle to another ; by radiation heat passes from the body into the surrounding medium by producing in it progressive waves of energy derived from the particles of the body. When the external temperature is below that of the body, the loss of heat by conduction and radiation, together with the heat consumed in warming the respired air, may be almost sufficient to remove the heat not required for maintaining the body at its normal temperature, except during times of vigorous activity. Active muscular exercise leads to increased chemical changes, with increased activity of respiration and increased perspiration. Where the external temperature is higher than 98'5° F., as in the tropics or in a Turkish bath, no heat will be lost from the body by conduction and radiation, but again from the surrounding medium will occur. In these circumstances a large quantity of heat is lost in evaporation of water from the skin and lungs. The conversion of water into water- vapour uses up a large quantity of heat. In hot air the capillaries of the skin dilate, the vessels becoming flushed with blood, the sweat glands increase in activity, and evaporation of the sweat goes on rapidly. When the secretion of sweat is but small, it evaporates as fast as it reaches the surface, and the skin remains dry — insensible ferspiratimi. When the secretion is abundant, sweat may be formed faster than it is evaporated, and it then appears on the skin in drops — sensible perspiration. Even in cold weather, after severe muscular exertion and consequent production of heat, sweat is produced in abundance, so that evaporation leads to the removal of this excess of heat. It should be noted, too, that the degree of moisture in the air is of great importance. Air almost saturated with water-vapour interferes with the evaporation from the skin, and thus hinders the loss of heat in this way. But in dry air a high temperature may be borne for some time, especially if liquid be taken freely, as perspiration is then copious and evaporation takes place rapidly. Clothing is used in temperate and cold climates to keep in the heat of the body by protecting the skin from the chilling influences of the cold air, which, if allowed to come freely into contact with the body, would carry off its heat. The clothing checks conduction and radiation. The low conducting power of atmospheric air renders a number of layers of clothing with sheets of air between them more efficacious than one thick layer. ' Heat is dissipated by radiation and evaporation from the surface of the body ; in hot weather the skin is flushed with blood and moist with per- spiration, and superfluous clothing is put off ; in cold weather the skin is pale and dry, and extra clothing is put on ; in the first case the dissipation of heat from the body is accelerated, in the second case it is retarded. ' — Waller. It has been estimated that the loss of heat by the skin in the two ways already mentioned — i.e. (l) by radiation and conduction in an atmosphere below that of the body temperature, and (2) by the conversion of sweat Animal Heat 257 into water-vapour, amounts to 77 per cent, of the total loss. By respira- tion, in which the inspired air when cooler than 98 '6° F. is warmed and in which much water-vapour evaporates from the lungs, 20 per cent, of the total loss occurs. About 3 per cent, of the total heat loss passes off in the urine and faeces. A small quantity of heat is expended in warming the food and drink ingested. 171. Regulation of the Temperature of the Body.— As the bodily temperature of man is nearly constant, notwith- standing the variations of temperature and moisture in the surrounding medium and variations of bodily activity, there must be some regulating mechanism that brings about this uniformity. It is mainly effected by variations in the amount of heat lost from the surface of the body under varying con- ditions, though some small variation in the amount of heat generated may occur. On the mechanism by which the tem- perature is kept nearly uniform in all latitudes, Dr. Hale White thus writes in Nature : — ' The temperature of a man at the equator is within a degree Centigrade the same as that in the Arctic regions. This is because, in the first place, in the Arctic regions the loss of heat from the body is very slight, and in the tropics it is very great, for (a) in the tropics more perspiration is secreted by the skin, and this, in consequence of the high temperature of the air, evaporates very quickly, and hence the body is kept cool. It is true that in the tropics people may not be observed to perspire freely, but that is simply because as fast as the perspiration is secreted it is evaporated. It is what is called insensible perspiration, {b) More water is secreted by the bronchial mucous membranes in the tropics, and in consequence of the higher temperature of the air it, like the perspiration, evaporates very quickly. The excessive secretion of moisture by the body when the tem- perature of the air is high is shown in a Turkish bath, and leads, in a bath of about two hours' duration, to a loss of weight amounting with some persons to three pounds, and to a great diminution in the quantity of urine secreted, {c) In the tropics the vessels of the skin are more widely dilated than in the Arctic regions ; hence there is more blood in it, and therefore heat is more readily radiated and conducted from the skin to the external atmosphere, (rf) The specific heat of the body is very high, and so it cools very slowly in the Arctic regions. It is very highly pro- bable that in the Arctic regions the quantity of heat produced by the body is much greater than in the tropics. The human body in the tropics must often be the coolest of surrounding objects ; in this case it cannot lose anything by radiation or conduction, but it is kept cool by the rapid eva- poration of perspiration (usually insensible) and fluid secreted by the bronchial mucous membrane. Whether or not a man in the tropics pro- duces any heat under such circumstances has not been demonstrated, but probably, although the production of heat falls very low, it does not entirely cease. ' ' S 258 Human Physiology There are thus variations in loss of heat and variations in production. 172. Variations of Heat-loss and Heat-production. — The skin is the chief organ by which the loss or expenditure of heat is regulated, the regulation being effected (a) by varying the quantity of blood exposed to cooling influences at the surface of the body, {p) by varying the quantity of sweat produced for evaporation. Through the vaso-motor nerves (dilatators and constrictors) its blood-supply may be increased, and result in greater loss of heat by conduction, radiation, and evaporation ; or the supply of blood to it may be diminished and result in diminished loss of heat. It is probable also that the nerves of the sweat glands assist in increasing or checking the produc- tion of the amount of moisture to be evaporated. Moreover, increased temperature of the body as a result of exercise, fever, &c., causes an increase in the number of heart-beats, so that in a given time the whole volume of the blood is more frequently exposed to the cooling influence of the skin. At a temperature of 41° C. (106° F.) the pulse rate reaches no. The other organs by which variations of loss are produced are the lungs. An increase in the number of respirations leads to a larger volume of air becoming heated to the temperature of the body, as well as to an increase of water vapour which has abstracted heat in being evaporated. Animals like the dog, which perspire but little by the skin, pant freely on a hot day, in order that the increased respiratory activity may carry off their surplus heat, the protruded tongue assisting the loss by evaporation from its surface. It has been mentioned previously that cooling of the surrounding medium increases the amount of carbon dioxide excreted, and consequently the amount of heat produced. Heating the surrounding medium has the opposite effects. Cold excites the action of muscles (shivering, &c.) that pro- mote oxidation processes, and also the appetite for food, especially those substances whose physiological heat value is high. The ingestion of all foods leads to increased tissue metabolism and its accompanying heat, A nimal Heat 259 173. Nervoas Control of Heat Frodnction. — The influence of the nervous system on the dissipation and production of animal heat is un- doubted. The regulation of the heat mechanism of the body is probably due to reflex actions of various kinds, and possibly to the action of a special nerve centre. Afferent impulses received at the vaso-motor centre lead to efferent impulses by which the calibre of the blood-vessels of the skin or of the internal organs is altered, with variation of blood supply and variation of heat dissipation. The secretory activity of the sweat glands is also under the control of the central system by means of their nerves. Vaso-motor nerves and the nerves of the sweat glands thus affect the regulation of temperature on the side of loss or dissipation. But be- sides these influences ' physiologists have long suspected that afferent im- pulses arising in the skin or elsewhere may, through the central neiTous system, originate efferent impulses, the effect of which would be to increase or diminish the metabolism of the muscles and other organs, and by that means increase or diminish respectively the amount of heat thus generated.' The evidence adduced in support of a thermogenic (heat-producing) centre that may be reflexly stimulated is as follows : — Warm-blooded animals are so affected by the temperature of the surrounding medium that cold in- duces increased tissue activity with increased production of heat, while warmth diminishes these processes. In cold-blooded animals it is the reverse ; tissue activity rises and falls with that of the surrounding medium. But if a warm-blooded animal be curarised, not only is there paralysis of the motor nerves, but the temperature rises and falls with that of the surrounding medium, the reflex arc being broken at the muscular end. Injury to certain deep-seated parts of the brain is also said to be followed by a rise of temperature and increased heat production, without the mani- festation of any special motor movements. It thus appears that nervous impulses may lead to chemical changes that produce heat without produc- ing muscular action. It is maintained, however, by some authorities that all these heat variations may be explained by the admitted nervous me- chanisms that control vascular and glandular changes, and thus indirectly the production and distribution of heat. If this be so, there is no need to suppose that there is a special set of nerves directly influencing heat production, or a special heat centre in the central nervous system. 174. Fever. — Pyrexia, or fever, is a morbid condition characterised by an increase of the bodily temperature above the normal, with increased tissue waste, owing to disturbance of the mechanism regulating heat formation and expenditure. An increase to 99° F. indicates a feverish condition, 100° F. to 102° F. moderate fever, 104° F. to 106° F. high fever. Various facts go to show that this increased temperature is mainly due to an increased production of heat without compensating loss of heat. Diminished loss may also be a contributing cause, as the skin although hot is often dry at a certain stage of the disturb- ance. But the greatly increased output of COj and urea shows that increased tissue metabolism is going on, and this excessive 26o Hiiman Physiology oxidation must lead to increased production of heat. The wasting character of fever and the incapacity for mechanical work also indicate the transformation of energy into heat. Further, an increased heat production in fever has been ob- served by direct calorimetric measurements. A temperature of 7° F. or 8° F. above the normal, if con- tinued, soon causes death, the heated blood increasing the number of heart-beats and respiratory movements until ex- haustion ensues. The nervous system is also greatly affected, and fails to regulate the bodily functions properly. CHAPTER XIII THE LARYNX AND VOICE 175. The Larynx is the chief part of the organ of voice, and is situated at the upper and fore part of the neck, between the large vessels of the neck, and below the tongue and hyoid bone. It consists of ' a framework of cartilages articulated together, and connected by elastic membranes or ligaments, two of which, projecting into the interior of the cavity, are named the true vocal cords, being more immediately concerned in the production of voice. It possesses special muscles which move the cartilages one upon another, and modify its form and the tension of its ligaments, and it is lined by a mucous membrane continuous above with the mucous membrane of the pharynx, and below with that of the trachea.' 176. Structure of the Larynx. — The cartilages of the larynx are nine in number, three single ones and three pairs : — Thyroid Two Arytenoid Cricoid Two Cornicula Laryngis Epiglottis Two Cuneiform The last two pairs are very small. Only the thyroid and cricoid are visible in front and at the sides of the larynx ; at the back the arytenoid cartilages may be seen surmounting The Larynx and Voice 261 the cricoid cartilages, the epiglottis being placed in front of the upper opening of the larynx. The epiglottis is a leaf-shaped plate of yellow fibro-cartilage, attached by a foot-stalk to the interior of the thyroid cartilage at the front. The epiglottis is of little service in voice production. During respiration it Fig. 164. — Front View of the Laryngeal Cartilages and Ligaments. (Sappey.) 1, hyoid bone ; 2, its large cornua ; 3, its small cornua; 4, thyroid cartilage ; 5, thyro-hyoid membrane ; 6, lateral thyro-hyoid ligament, containing the cariilago triticea, 7 ; 8, cricoid carti- lage ; 9, crico-thyroid me^lbrane ; 10, lateral crico-thyroid ligaments. Fig. 165. — Back View of the Laryngeal Cartilages and Ligaments. (Sappey.) 1, thyroid cartilage ; 2, cricoid cartilage ; 3, arytenoid cartilages ; 4, their mus- cular processes ; 5, a ligament better marked than usual, connecting the lower cornu of the thyroid with the back of the cricoid cartilage ; 6, upper ring of the trachea ; 7, epiglottis ; 8, ligament connecting it to the angle of the thy- roid cartilage. The corniciila are seen surmounting the arytenoid cartilages. Stands upwards, but in swallowing it falls downwards and back- wards, to close the entrance to the larynx. The thyroid is a large ridge-shaped cartilage formed of two lateral halves or wings, open behind, but forming a ridge at an acute angle in front, the prominent point of the ridge 262 Human Physiology forming Adam's apple (pomum Adami). Each lateral piece, or ala, is of quadrilateral shape, and to the outer surface are attached the sterno-thyroid and thyro-hyoid muscles. To the inner surface of the thyroid at the angle is attached the epiglottis, and lower, the true and false vocal cords and the thyro- arytenoid muscles. The upper edge of the thyroid is united by a membrane to the hyoid bone, and its lower border is connected to the cricoid cartilage by the crico-thyroid mem- brane in front, and the crico-thyroid muscle at each side. The posterior edge of the thyroid ends above and below in pro- jections or cornua, the inferior horns or cornua articulating with the posterior lateral portion of the lowest cartilage of the larynx, the cricoid. The cricoid cartilage is named from its resemblance to a signet ring (Gr. krikos). It is situated below the thyroid, its broad part lying be- hind in the gap be- tween the alae of the thyroid, and its nar- row part lying in front, where it is united above to the thyroid cartilage by the crico-thyroid membrane. Its lower border is attached to the first ring of the trachea ; its upper border, at the highest part behind, presents on each side a smooth oval surface for articu- lation with the triangular arytenoid cartilages. The two arytenoid cartilages are like irregular three-sided pyramids in shape, and they rest by their bases on the posterior and highest part of the cricoid cartilage, while their pointed apices turn backwards and inwards, and are surmounted by the two small cartilaginous nodules that form the cornicula laryngis. To the anterior angles of the arytenoid cartilages are attached the posterior ends of the true vocal cords, while the anterior ends are attached in front to the thyroid, in the angle between the two lateral alae or wings (fig. 167). Thyroid C- Arytenoid C' Cricoid C -*-l Axis of rota. tion of crico- \ j thyroid — ' Vocal cord Direction of pull of crico- thyroid Fig. 166. — Diagrammatic Vertical Section of Larynx. The Larynx and Voice 263 The interior of the larynx is wider than that of the trachea. In it we find on each side two ridges formed of folds of tissue covered by mucous membrane and passing from behind to front one ridge below the other on each side. The superior upper ridge on each side is known as 3. false vocal cord, as these ridges take no part in the production of the voice. The lower ridge of tissue on each side is termed a true vocal cord because voice is due to vibrations of the edges of these lower ridges of the larynx. The vocal cords are really bands of yellow elastic tissue which run forwards from the anterior angle of the base of the arytenoid cartilages to be attached in front to the inner surface of the thyroid cartilage where the Fig. 167. — Interioi- of the T-arynx, seen from above (enlarged). two wings of this cartilage unite (fig. 167). Between the two edges of the vocal cords (which are not quite horizontal but incline upwards a little) is a narrow fissure or chink, prolonged behind between the two arytenoid cartilages. This fissure between the right and left vocal cords is termed the glottis or rima glottidis (chink of glottis). In ordinary quiet breath- ing the glottis is V shaped with the opening of the V backwards, but in order to produce voice, muscles make the cords tense, and bring them to- gether. Their edges being made to vibrate by a blast of air from the lungs through the glottis, sound is produced in the air by these vibrating edges. The average length of the vocal cords in men is ~ inch, and in women i inch, while the glottis is about \ inch in width when wide open. Two recesses in the larynx, one between each vocal cord and the false vocal cord above form the ventricle or pouches of the larynx. 264 Human Physiology The muscles of the larynx are arranged in two sets, ciUed extrinsic and intrinsic. The first set serve mainly to elevate or depress the whole organ. At the moment of swallowing, for instance, the larynx is drawn upwards and forwards by the thyro-hyoid muscle (par. 117) just as the epiglottis is made to fall down and close the entrance to it. The intrinsic muscles of the larynx serve mainly to move the cartilages, and thereby to regulate the position and tension of the cords. These muscles are named according to their position and connection, and are for the most part in pairs. Thus the crico-thyroids pass from the external surface of the thyroid forwards and downwards to the outer surface of the cricoid. This contraction, bringing the two cartilages together, will pull up the front of the cricoid cartilage and so depress its back, to which the arytenoid cartilages are articulated, so that the tension of the vocal cords is increased. Certain fibres of the thyro-arytenoid muscles, which lie parallel to the vocal cords, aid in relaxing the cords. The movements of adduction and abduction of the vocal cords, by which the opening of the glottis is regulated, are effected mainly by the muscles connecting the cricoid cartilage with the arytenoid cartilages. The lateral crico-arytenoids adduct the vocal cords, and produce closure of the glottis by rotating the arytenoid cartilages inwards ; the posterior crico-arytenoids abduct the cords, and widen the glottis by rotating the arytenoid cartilages outwards. The single arytenoid muscle that passes between the two arytenoid cartilages behind brings these cartilages together by its contraction, and assists in closing the glottis. We may summarise the action of the various muscles thus— Tensor of vocal cords . Crico-thyroid muscle. ( Lateral crico-arytenoid muscles. Constrictors of the glottis -j Thyro-arytenoid muscles. I Arytenoid muscle. Dilator of glottis . . Posterior crico-arytenoid muscles. Depressor of epiglottis . Aryteno-epiglottic muscle. The nerve supply of the larynx consists of the superior laryngeal and inferior laryngeal branches of the vagus, the former supplying the mucous membrane and the crico-thyroid muscle, the latter the remaining muscles. 177. Voice and Speech. — That the sound of the human voice is the result of the vibration of the true vocal cords which bound the glottis, such vibrations being caused by upward blasts of air and being in turn communicated to the column of air in the passages above, has been proved both by observation of the living subject and by experiment on a dead body. By means of an instrument termed the laryngoscope the interior of the larynx and the movements of the cords may be rendered visible. During ordinary respiration the glottis is about half open and widens slightly with each inspiration ; with a forced inspiration it becomes widely dilated, but when vocalisation takes place and the cords are set in vibration, the glottis becomes a mere chink with parallel sides. In physiology voice signifies the sound produced in the larynx when an expiratory blast of air sets the vocal cords in vibration. In order to pro- duce voice, the chink of the glottis, which is about half open in ordinary respiration, must be rendered narrow by the free edges of the vocal cords being brought close together and rendered parallel. The cords must also be rendered more or less tense. Both the width of the glottis and the tension of the cords are regulated by the action of muscles as just explained— the lateral crico-arytenoids bringing the cords together and The Larynx and Voice 265 making them parallel, while the posterior crico-arytenoids separate them, and the crico-thyroids increasing the tension of the cords while the thyro- arytenoids relax them. The loudness of the voice depends on the force of the expiratory blast, that is, on the amplitude of vibration of the cords. 'Y\\& pitch of the voice depends on — (i) The length of the cords, for the longer a cord is the lower is the note produced, the pitch of a stretched string Fig. 168. — Three Laryngoscopic Views of the Superior Aperture of the Larynx and Surrounding Parts in Different States of the Glottis during Life. (Czermak. ) A, the glottis during the emission of a high note in singing ; B, in easy inhalation ; c, in talking a deep breath ; ^, base of tongue ; e^ the upper free part of the glottis ; f ', the tubercle or cushion of the epiglottis ; ph^ part of the anterior wall of the pharynx behind the larynx ; in the margin of the aryteno-epiglottidean fold w, the swelling of the membrane caused by the cuneiform cartilage ; s, corniculum ; a, tip of arytenoid ; cv^ true vocal cords ; cvs, false vocal cords ; tr^ trachea ; i, bronchi in c. a', b', c', diagrams of the glottis and positions of the arytenoid cartilages in the three states. varying inversely as its length. The vocal cords of women are about one- third shorter than those of men, and hence their voice is of higher pitch. Tenor singers have shorter cords than basses, and sopranos than contraltos in most cases. In males at the age of puberty the larynx enlarges and the vocal cords become longer. Hence the voice becomes deeper and is said to 'break.' (2) The tension of the cords, for the tighter tlie cords are the higher the pitch, and vice versd. Variations in the pitch of the 266 Human Physiology voice can be made by producing changes of tension in the cords as the actions of the muscles are brought under the control of the will. Falsetto, or head notes, are believed to be produced either by the vibration of the edges of the cords only, or by the vibrating portion of the cords being short- ened. In ordinary vocal sounds the vibrations of the cords are commu- nicated to the column of air both in the resonating tubes above the larynx and in the trachea below, and the vibration of this column affecting the chest wall, such sounds are spoken of as chest notes. The quality or timbre (par. 234) of the voice depends on the prominence given to par- ticular overtones or harmonics which accompany the fundamental tone. A voice sound, apparently simple, is in reality composed of a fundamental tone and certain accessory tones or harmonics, and the audible sound pro- duced in the larynx, or primary sounding apparatus, may be so affected in its passage outwards through the adjustable resonance cavities of the pharynx and mouth as to become speech. Different voices uttering the same note differ in quality owing to the different set of overtones that pre- dominate, these overtones being determined not only by the length and physical condition of the cords, but by the structure and form of the throat and mouth. Speech is voice modified by alterations and additions made in the pharynx, mouth, and nose. The sounds formed in the larynx by the vibra- tions of the vocal cords being communicated to the air may undergo modifications from the varying size of the cavities above and the varying position of tongue and lips, and by these changes laryngeal sounds may be transformed into articulate speech consisting of syllables jointed together to form words. Speech may indeed exist without voice (i.e. sounds pro- duced by the vocal cords), as in whispering, when the sounds are produced in the mouth alone. In ordinary speech, however, the sounds or air waves produced in the larynx are moulded and modified in the resonant cavities above. Speech sounds are divided into vowels and consonants. Vowels are the most open and continuable sounds uttered in the process of speech, and they differ from one another not in the pitch of the note pro- duced by the larynx, but in the quality given to the sound by the overtones that are reinforced and modified in the cavity of the mouth. Each vowel sound acquires its special character from the reinforcement or addition of particular overtones as the oral cavity changes shape, and Helmholtz, by means of resonators, has not only been able to analyse the vowel sounds into their component vibrations, but he has succeeded in reproducing them synthetically. When the mouth is adjusted to produce the most open sound, the broad a in/ar, it has a sort of funnel shape with the wide part outward ; for 0, as in more, the vowel chamber is like a bottle with a wide neck ; and for u, as in poor, the chamber is large with a narrow opening at the mouth. With e and i, sounded eh and ee, the mouth has the form of a bottle with a long and narrow neck, formed by raising the tongue towards the hard palate. In pronouncing diphthongs the mouth cavity changes its form as it passes from one vowel sound to the other. Consonants are the closer and less continuable sounds of speech, but there is no sharp line of distinction between vowels and consonants, some of the most open of the consonants having at times the value of vowels. Most of the consonant sounds are produced by irregular vibrations, and are of the nature of noises rather than musical sounds. They are accompanied by a narrowing of some part of the pharynx or mouth, which interrupts and modifies the air vibrations The Larynx and Voice 267 from the larynx, or sets up other vibrations in particular parts. According to the parts used in producing them, they are divided into labials, or lip letters, p, b, f, v, m ; dentals, or teeth letters (made with the tip of the tongue near the teeth), t, d, th ; gutturals, or throat letters (made at the top of the throat with the t^ck of the tongue), k, g. According to their degree of closeness or mode of production, consonants are divided into mutes [checks or explosives') as b and p, which involve a complete shutting off of the passage of the breath ; fricatives (spirants and sibilants), as th, f, V, s, z, in which there is u rustling or friction of the breath through nearly closed parts ; nasals, as n, m, ng, in which the breath is admitted through the nasal passages and made to acquire a peculiar resonance. The tongue, it will be seen, plays only a subordinate part in speech, and its loss only affects the pronunciation of those consonants in which it takes part. Stammering is due to irregular and spasmodic contraction of the diaphragm, which interferes with the expiratory blast of air on which speech depends. Mental disturbances or emotional excitement produce it in some people. Stuttering is defective speech due to inability to manage the larynx and other parts so as to form the proper sounds. CHAPTER XIV THE SPINAL CORD 178. General View of the Cord. — The spinal cord is the cylindrical mass of nervous matter contained in the spinal canal, which is formed by the superposed rings of the vertebrae. It is 17 to 18 inches long, about | inch in diameter, and reaches from the margin of the foramen magnum of the occipital bone to the first lumbar vertebra, where it terminates in a slender thread, iYiefilum terminale, which lies among a mass of nerve roots, termed the cauda equina. Above it is continued into the medulla oblongata, or bulb, which lies within the cavity of the cranium. Passing out from the cord at intervals, on each side, are the thirty-one pairs of spinal nerves, the first pair coming off between the skull and the atlas vertebra, the next pair between the atlas and the axis, and so on. The lower pairs, however, come off close together from the lower end of the cord, and pass downwards in the cauda equina to be dis- tributed to the lower limbs. The spinal nerves leave the bony 268 ^zBr Human Physiology canal by apertures between the vertebrae, termed intervertebral foramina. The diameter of the cord is not uniform throughout, being marked by two enlargements, the upper or cervical, and the lower or lumbar. Two fissures run along the length of the cord and separate it into a right and left half. The anterior median fissure is wider but not so deep as the posterior median fissure, and it contains a fold of the closely investing membrane of the cord, the pia mater, from which blood-vessels pass in to nourish the cord. At the bottom of this fissure is a transverse connecting portion of white substance, termed the an- terior or white commissure. The posterior fissure is not an actual fissure, but a septum of connective tissue and blood-vessels, passing nearly to the centre of the cord, having at its base the posterior grey commissure. Besides the two me- dian fissures, there is a lateral far- row at the line of attachment of the posterior roots of the spinal nerves, named the postero-lateral groove, and the attachment of the anterior roots, though not marked by any furrow and spread over some space, Fig. 169.— Diagrammatic View from before of the Spinal Cord and Medulla Oblongata, including the roots of the Spinal and some 6f the Cranial Nerves, and on one side, the Gangliated Chain of the Sympathetic. (Allen Thomson.) \ The spinal nerves are enumerated in order on the right side of the figure. B^^ brachial plexus ; Cr, anterior crural, O, obturator, and Sc^ great sciatic nerves, coming off from lumbo-sacral plexus ; x , x , filum terminale. w, ^, c, superior, middle, and inferior cervical ganglia of the sympathetic, the last united with the first thoracic, d ; cC ^ the eleventh thoracic ganglion ; /, the twelfth thoracic (or first lumbar) ; below j J, the chain of sacral ganglia. The Spinal Cord 269 may be regarded as indicating another division in the cord. Further in the upper part of the cord there is a slight longitudinal furrow (s, fig. 170), the postero-lateral Jitrrow^ a little distance from the posterior median fissure. Each half of the cord is thus divided into four columns : an anterior column between the anterior median fissure and the anterior roots, a lateral column between the line of origin of the anterior roots and posterior roots, a posterior median column between the line of origin of the posterior roots and the postero-lateralfur- CERVICAL DORSAL Fig. 170.— Section of the Spinal Cord in the Upper Part of the Dorsal Region. (E. A. S.) i a, anterior median fissure ; /, posterior median fissure;/, «, posterior nerve roots entering at the postero-late- ral groove ; a, c, anterior cornu of grey matter ;/, c, posterior cornu ; i, intermedio-lateral tract (lateral cornu) ; /, r, processus reticularis ; c, posterior vesicular column of Clarke ; s, pia-matral septum form- ing the lateral boundary of the postero-mesial column. LUMBAR Fig. 171. - Outline Sketch of Three Sections (X 3). (Waller.) Taken from the cervical,thoracic,and lumbar regions of spinal cord (human). row, and a posterior column between the postero-lateral and posterior median fissure. A transverse section of the cord shows that it consists of white and grey matter. The white matter is on the outside and gives the cord its white opaque appearance ; the grey matter is arranged internally in the form of a crescent in each half, the two crescents being joined in the middle line by a grey com- missure. In the centre of the grey commissure is the central 270 Human Physiology canal of the cord, which is lined in early life by cylindrical columnar epithelium. At the outer side of each crescent the grey matter forms a sort of network, termed ^q processus reticularis. The two horns of the crescents of grey matter are named, from their position, anterior and posterior, and a triangular projection of grey matter in some parts of the cord about the middle of the crescent is called the intermedio-lateral tract, or lateral horn. The grey crescents vary in form in dif- ferent parts of the cord. In the cervical region the anterior cornua are large and broad, and the posterior cornua narrow ; in the dorsal and thoracic regions, both anterior and posterior cornua are narrow ; in the lumbar regions the anterior and posterior cornua are broad (fig. 171). The grey matter relative to white is least in the thoracic and dorsal regions, and greatest in the lumbar regions. 179. Membranes of the Cord. — The spinai cord does not completely fill the spinal canal, for it is invested by three membranes with a space separa- ting the outermost and the middle membrane, and another space between the middle one and the innermost. The outermost membrane is termed the dura mater. It is continuous with that which invests the brain, and is composed of tough connective tissue with a small amount of elastic fibres. In the cranium the dura mater is adherent to the bones, but it is separated from the bony walls of the spinal canal by loose alveolar tissue and a plexus of veins. It sends a tubular sheath along the spinal nerves for a short distance. By slitting up the dura mater and folding it back there is seen a thin delicate membrane around the cord, called the arachnoid membrane. It is continuous with the cerebral arachnoid above, passes for some distance as a sheath over the issuing nerves, and is separated in part from the dura mater by a narrow space termed the sub-dural space. This space contains a small quantity of fluid. On removing the arach- noid there is exposed the pia mater, a vascular membrane closely attached to the surface of the brain and spinal cord, dipping down into the fissures, and sending investments along the nerves. The pia mater is loosely connected with the arachnoid by strands of connective tissue forming a spongy network, but the space between the two is considerable, and is known as the sub-arachnoid space. This space contains a fluid called the cerebro -spinal fluid, and may be regarded as a serous or lymphatic space in communication with the perivascular lymphatics of the small arteries that pass into the nervous tissue of the brain and spinal cord from the pia mater, as well as with the lymph spaces in the nervous matter itself. It is also in communication with the central canal of the cord and the ventricles of the brain by a small aperture through the pia mater in the roof of the fourth ventricle, t\ie foramen of Magendie. Cerebro-spinal fluid differs from or- dinary lymph in the very small percentage of proteids that it contains, in the absence of fibrin ferment, and in the absence of cells ; it probably gets back into the circulation by escaping ioto the lymphatics of the nerves in The Spinal Cord 271 the sub-arachnoid space around their roots, and from them into the general lymphatics of the body. A narrow fibrous band of connective tissue, attached to the pia mater along its whole length on each side between the anterior and posterior roots of the spinal nerves, is joined at intervals by tooth-like projections to the dura mater. This band is called the ligamentum denticulatum. 180. Minute Structure of the Cord. — The white matter of the cord is composed of medullated nerve fibres without the external sheath of Schwann, and running longitudinally. They vary in size in different parts of the cord, those near the surface being usually larger than those near the grey matter. These fibres are supported by a peculiar fibro- cellular tissue termed neuroglia, made of very small cells with many branching fibrils. The neuroglia is abundant at the sur- face of the cord, and extends into Fig. 172. — The Spinal Cord and its Membranes. Fig. 173. — Transverse Section of the Spinal Cord and its Membranes. the grey matter forming the substantia gelatinosa around the central canal and at the tip of the posterior cornu. The grey matter of the cord consists of nerve cells with branching pro- cesses, an interlacement of fine medullated fibres and neuroglia, the cells being arranged in columnar tracts in the crescents. The cells of the anterior cornu, termed the vesicular column of cells of the anterior cornu, are large multipolar cells, each of which has an axis-cylinder process continuous with a fibre of the anterior root of a spinal nerve, the other processes breaking up into a fine 272 Human Physiology meshwork of fibrils. These cells are particularly numerous in the cervical and lumbar enlargements of the cord. The cells of the posterior cornu are much smaller, and any axis-cylinder process they may have does not appear to be connected with the fibres of the posterior root, but passes towards the anterior horn. A group or columnar tract of medium-sized cells, situ- ated at the inner angle of the base of the posterior horn, is known as Clarke's column. Another group is found in the lateral cornu of grey matter. (See also par. 42.) I, cell of anterior cornu ; 2, cell of Clarke's column; 3, 'solitary' cell of posterior cornu ; 4, large nerve fibre. (Drawn to the same scale, viz. x 200 diameters.) ("Waller.) 181. Arrangement of the Nerve Fibres of the White Column in Tracts. — In addition to the arrangement of the -white matter into colitmns marked out by the superficial fissures and furrows, it has been found pos- sible to arrange it into nerve tracts. If a nerve fibre be separated from its cell, it wastes or degenerates, and the degenerated fibres can be distin- guished by their appearance under the microscope, especially after treatment with special staining reagents (par. 47). It has also been found that in the development of the spinal cord the nerve fibres of different tracts acquire their medullary sheath at different intervals, and different groups are thus recognised. Moreover, Gotch and Horsley have been able to distinguish the different paths of nervous impulses by the electrical changes set up. By a combination of these methods various tracts of fibres have been made out, as indicated in fig. 175. If the fibres are found degenerated below a lesion in the cord, the tract is said to be of descending degeneration ; if the degeneration is above the lesion, the tract is said to be of ascending degene- ration. Tracts of descending degeneration are : — ( I ) The crossed pyramidal tract is a descending tract of fibres found in the lateral colum.n at the outer part of the posteripr horn, of grey matter The Spinal Cord 271 throughout the length of the cord. This tract contains rather large fibres mingled with smaller ones, and is known to descend from the opposite side of the cortex of the brain, the fibres crossing at the pyramids of the medulla. Fig. 175. — Section of the Human Spinal Cord from Lower Cervical (A) and Mid-Dorsal (B) • showing the principal groups of the nerve-cells, and on the right side of each section the conducting tracts as they occur in the two regions. (Magnified about 7 diameters.) Uy 0, c, groups of cells of anterior horn ; a, cells of the lateral horn ; e, middle group of cells ;^ cells of Clarke's column; g^, cells of posterior horn; c.c, central canal; a,c., anterior commissure. 274 Human Physiology (2) The direct pyramidal tract is a descending tract of large fibres situated in the anterior column by the side of the anterior median fissure. It gradually diminishes in size on passing downwards, ending about the middle of the dorsal region. It belongs to a portion of the same tract as (i), the fibres not undergoing decussation in the medulla, but passing pro- bably to the opposite side in the cord itself through the anterior white commissure. (3) The antero-lateral descending tract is situated in the antero- lateral column, and contains fibres which are connected with cells in the brain cortex on the same side. (4) The comma descending tract is a small tract in the upper part of the cord, situated in the middle of the postero-Iateral column. Whether its fibres originate from cells higher up the cord or are derived from the descending fibres of the posterior roots is uncertain. Tracts of ascending degeneration are : — (i) The antero-lateral ascending tract of Gowefs, situated at the outer part of the cord, mingles with the corresponding descending tract. On section of the cord its fibres degenerate above the lesion, and it is thought that its fibres, which can be traced upwards to the cerebellum, may arise from cells in the posterior cornu of the cord. (2) The direct ascending cerebellar tract is situated at the outer part of the cord, external to the crossed pyramidal tract in dorsal and cervical regions. Its fibres are thought to be derived from the axis-cylinder pro- cesses of the cells in Clarke's column. (3) The postero-Iateral ascending column, or tract of Burdach, is mainly composed of large fibres that are continuous with the fibres of the entering posterior roots. After passing some distance in the cord numer- ous filaments pass off into the grey matter. (4) The postero-mesial column, or tract of Goll, contains fine fibres also derived from the posterior root fibres and passing up this column into the medulla oblongata, where they terminate among cells of the nucleus gracilis. (5) The tract of Lissauer is a small tract of fine ascending fibres (marked M in fig. 175), close to the posterior roots and derived from them. 182. The Spinal Nerves.— The thirty-one pairs of spinal nerves come off from the cord by two roots at intervals along its length, the portion of cord to which each pair of roots is attached being regarded as a 'segment,' or ganglionic mass of nerve cells, with which fibres are connected. But the segments are fused together into a continuous mass without any mark of separation. Each spinal nerve is attached to the cord by two roots, an anterior or ventral, and a posterior or dorsal, but the two roots after passing through separate openings in the dura mater unite to form a mixed nerve from which branches are given off to the ventral and dorsal parts of the body, as well as a ramus communkans or visceral branch to the sympathetic sys- tem (see par. 38 and App.). Before the two roots of a spinal The Spinal Cord 275 nerve coalesce, a ganglion is found on the posterior root, where the roots lie in the intervertebral foramen. As already stated, experimental excitation shows that the anterior root contains efferent fibres, and the posterior afferent fibres. As will be seen shortly, however, the anterior root contains a few afferent fibres, though the posterior root is entirely afferent or sensory. The anterior root arises by several converging bundles of fibres from the anterior column of white matter, and on fol- lowing the fibres into the grey matter of the cord many of them are seen to be continuations of the axis-cylinder processes of the large cells in the anterior horn. The fibres that can be thus traced are motor fibres for the skeletal muscles. Othei fibres pass by these cells and do not appear to be connected with them. What their nature is cannot yet be stated, though we know that the anterior root contains, besides motor fibres to skeletal muscles, vaso-motor fibres to blood-vessels, and secretory fibres to glands. The posterior root fibres enter the cord in a more compact mass than the anterior root fibres, and after their entrance into the cord separate into two sets, taking different courses. The smallest fibres enter at the tip of the posterior horn, and join the ascending fibres in the tract of Lissauer ; the other set of somewhat larger fibres passes into the postero-lateral white column, proceeding for the most part upwards, and entering either the posterior median tract or the adjacent grey matter. As they enter the cord the fibres of the posterior roots bifur- cate into two principal branches, running upwards and down- wards in the posterior white column or adjacent posterior cornu, and give off collateral branches which run inwards towards the grey matter, and end in a plexus of fibrils around the nerve cells. The fibres of the posterior root originate in the cells of the posterior root ganglia. A section carried through the ganglion of the posterior root in the direction of the nervous cord shows a number of nerve cells with nerve fibres passing between. The cells are unipolar, and a cell process joins a traversing nerve fibre by a T-shaped junction (fig. 48, par. 42), though it is possible that some fibres may pass through without any connection with a cell. The function of 276 Human Physiology the ganglia on posterior roots is to act as centres for the nutri- tion of the nerve fibres of the root. This is proved by severing the connection of the fibres with the ganglion, when the fibres cut off from connection with the ganglion degenerate, while those that remain in connection do not (fig. i??)- Thus section of the posterior root between the ganglion and the cord leads to de- generation of the central part connected with the cord, while the peripheral part connected with the ganglion remains unaffected. Section below the ganglion leads to waste in the peripheral part, but not in the central part. On the other hand, section Fig. 176.— Longitudinal Section through the Middle of a Ganglion on the^ Posterior Root of one of the Sacral Nerves of the Dog, as seen under a low magnifying power. (E. A. S.) a, nerve root entering the ganglion ; h, fibres leaving the ganglion to join the mixed spinal nerve ; c, connective-tissue coat of the ganglion ; d^ principal group of nerve ceils, with fibres passing down from amongst the cells, probably to unite with the longitudinaily coursing nerve fibres by T-shaped junctions. of the anterior root leads to degeneration in the peripheral part, but not in the central, as its nutrition centre is in the cells forming the grey matter of the anterior horn of the cord. Thus the trophic (Greek trophe, nourishment) centre for sensory fibres is in the ganglion of the posterior roots, and the trophic centre for the motor or efferent fibres is in the grey matter of the spinal cord. A few fibres, however, remain unaffected in the peripheral end of a cut anterior root, and the central end contains a few degenerated fibres among the mass of unaffected ones. These are the recurrent sensory fibres. We have already stated that the anterior root is motor or efferent, The Spinal Cord 277 and the posterior root sensory or afferent. But if the anterior root of a spinal nerve be divided, and the peripheral end be stimulated, there is not only movement of the muscles supplied by the nerve, but, in some cases, there is evidence of pain. This is spoken of as recurrent sensibility, and this is due to a few recurrent sensory fibres which leave the cord by the Degeneration of efferent and of afferent fibres below a section of entire nerve. Degeneration of efferent fibres below a section of anterior root. Degeneration of afferent fibres above a section of posterior root above the ganglion. Fig. 177. -Diagrams to illustrate Wallerian Degeneration of Nerve Roots. Degeneration black. Degeneration of afferent fibres below a section of posterior root beyond the ganglion. posterior roots, but turn back into the anterior root. On dividing the posterior root, recurrent sensibility disappears, as we should expect. 183. Functions of the Spinal Cord. — The spinal cord is both a conductor of nerve impulses and a centre for reflex action. From its structural connections with various parts of 278 Human Physiology the body, it is clear that sensory or other afferent nerve impulses from the periphery in the trunk and limbs can only be conducted to the brain and there perceived when the cord is uninjured and intact. The mandates of the will, also originating in the brain, can only pass to their destination along the efferent nerves when the communication through the cord is uninterrupted by disease or injury of the cord. Any disease or injury that affects the whole thickness of the cord leads to paraplegia, or complete paralysis, with loss of sensation and voluntary motion in the parts of the body that receive their nerve supply from the part of the cord below such injury. By experiment and observation the paths of the sensory and motor impulses along the cord have been ascer- tained to a certain extent, although much remains to be learnt. (i) Paths of Sensory Impulses in the Cord. — Afferent impulses con veyed to the cord by the posterior roots pass up some or all of the ascending tracts to the brain, for degenerations occur above the injury in the postero- exteiltial column and the postero-median column on the same side. But the tract taken by different impulses is not definitely determined in many cases. Recent experiments have, however, definitely proved that all sensory impulses do not decussate or cross over in the cord, as was supposed until lately, but that they pass up the same side for the most part, and cross chiefly in the medulla to reach the opposite side of the brain. Mott concludes that sensations of touch, pressure, and impressions of the muscu- lar sense pass up the same side, but that painful sensations pass up both sides. (2) Paths of Motor Impulses in the Cord. — Efferent impulses pass from the brain along the two pyramidal tracts, viz. in the crossed pyramidal tract chiefly, and in the direct pyramidal tract to a less degree, these tracts being undoubtedly the channels of voluntary impulses. For the most part they originate in the cortex of the cerebrum at one side, and cross to the opposite side in the pyramidal decussation in the medulla. A few fibres do not cross in the medulla, but pass down the direct pyramidal tract to decussate in the cord by the anterior white commissure. The vaso- motor impulses to the limbs travel along the lateral columns of the cord on the same side. Hemisection or transverse division of one-half of the cord produces the following results :— Motor paralysis of the muscles of the same side, but not complete in bilateral muscles that act together ; impaired sensation on the same side ; temporary vaso-motor paralysis with vaso- dilatation ; degenerations above and below the lesion on the same side. 184. Reflex Action of the Spinal Cord.— We have already (par. 44) explained the nature of reflex actions, and have pointed out how they may differ in nature and character. The reflex power of the spinal cord is the property which the cells The Spinal Cord 279 of its grey axis have of transforming afferent into efferent impulses. In suoh animals as the frog it is found that the cord alone can carry out numerous reflex acts both simple and complex. A frog from which the brain has been removed recovers from the shock in about an hour, and if protected from all stimulating influences remains still until death occurs. But if a gentle stimulus be applied to the skin of one foot, that foot is drawn up. This is an example of a simple reflex, and illus- trates what is often called the law of unilateral reflection, for in general a slight excitation of a sensitive region causes a reflex movement in the neighbouring muscles. A strong stimulus not only leads to movements of the same side, but in a less degree to movements in the corresponding muscles of the opposite side, and if the stimulus be very strong, or if the cord be in an excitable condition, the sensory impulse may be reflected along most of the motor nerves of the body, so as to produce a reflex spasm. This spreading has been termed the law of radiation or diffusion. Strychnine so affects the grey matter of the spinal cord that the slightest touch on the skin sends all the muscles into this state of spasmodic contraction. If one of the toes of the brainless frog be dipped into dilute sulphuric acid, the leg is only drawn up after some time. This shows that a weak impulse may not in itself be capable of discharging a reflex act, but that a succession of such impulses sent to the cord may combine their influence until a movement is caused. This phenomenon is known as sut?i- mation of stimuli. Complex co-ordinated reflex actions are exhibited by a brainless frog in which stimulation of afferent nerves leads to the discharge of complicated movements involving whole groups of different muscles, the movement being of a protective or purposive character. Thus, stimulating the flank of a frog with acid causes the leg of the same side to be swept over the spot, and if this leg be held, the other leg tries to remove the irritation. So well adapted are these and similar movements to secure the removal of the offending irritant, that they seem at first sight to indicate intelligence as existing in the cord. But the absence of all spontaneous movements which are a 2 So Human Physiology distinguishing mark of intelligence, will forbid this view, and the mechanical nature of reflex acts is well illustrated in the case of a decapitated snake, in which complex movements may be stimulated that indicate an effort to twine round a red- hot iron as readily as round a stick in contact with its body. It thus appears that the disturbance set up when afferent impulses enter the cord spreads in the first place among the cells and network of fibres near the point of entrance on the same side, that it may then cross over at the same altitude to the opposite side, and that in some cases it may even spread along the length of the cord. Excitation of the cord in any part thus tends to spread in various directions, but with a pre- ference for certain paths marked out by the structure and habits of the cord. Reflex actions occur more readily in a brainless frog than in one whose brain has not been removed, and from this it appears that the brain exerts an inhibitory influence on the cord. This inhibition of reflex action that normally follows a slight impulse may also be brought about by placing crystals of sodium chloride on the optic lobes of a frog, or by strongly stimulating any sensory nerve. The influence of the brain in checking the reflex activity of the cord is well illustrated by the greater ease with which reflex actions occur in sleep, and by the absence of any sense of pain for a time that may occur in a soldier wounded in battle, when his mental energies are all centred on the fight. Reflex time, or the time taken to transmute afferent into efferent impulses in the cord, varies somewhat with the nature of the reflex act and with the amount of resistance, and has been estimated at •oi of a second to '06 (par. 46). In mammals, as the dog and man, the spinal cord appears to act habitually under the more direct influence of the brain, and the reflex actions of the cord severed from the brain appear to be of a more simple character than in lower animals. Fur- ther, the cord takes a longer time to recover from the shock of the operation or injury by which its connection with the brain is broken. The vital powers are much depressed, and for some time stimulation of a sensory surface below the lesion fails to evoke the The Spinal Cord 281 simplest reflex act. But after some weeks simple reflex actions are called forth. A dog, whose spinal cord has been severed in the back, performs the reflex movements required for micturi- tion and defsecation, and if its paralysed hind limbs be raised, a gentle push causes it to move fbrwards a few paces before it again sinks into a sitting posture. A man whose cord has been crushed in the dorsal region draws up his legs uncon- sciously when the soles are tickled, but the cord appears to have no power to carry out co-ordinated movements. In certain diseases of the cord more extensive movements are sometimes witnessed, but they are not of the purposeful co-ordinated character such as those described in the frog. A special form of reaction, spoken of as ' tendon-reflex,' is the sudden contraction of a muscle when its tendon is struck. If the knee be half flexed, a sharp tap on the patellar tendon leads to the contraction of the rectus muscle of the thigh with raising of the leg. This 'knee-jerk,' as it is called, is present in health, but absent or exaggerated in certain diseases. Hence its importance to the physician. But it is doubtful whether it is a true reflex act, as the time between the blow and the con- traction is as short as in muscular contraction, though the severance of its nerves from connection with the cord abolishes it. 185. Reflex Centres of the Spinal Cord. — As already intimated, the spinal cord should be regarded as a collection of nervous centres, formed by the union of its segments into a continuous column. These centres subserve various functions, and carry out numerous complicated movements, but it acts for the most part under the controlling and modifying influence of the brain. A tierve centre is to be regarded as a collection of nerve cells that have the power of controlling or modifying some action or func- tion of the body, either automatically as the result of intrinsic changes in the nerve centre, or reflexly as the result of an immediate afferent impulse. The chief special centres of the spinal cord are : — (a) The musculo-tonic centre. — The spinal cord exercises an influence over the muscular system that keeps the muscles of the body in a continual state of slight contraction, a state known as muscular tone. In injury or removal of the brain, the tone of the muscles still remains, but if the spinal cord be destroyed the muscles become flabby and loose. Section of the sciatic nerve in one leg of a frog causes the muscles of that limb to become relaxed — a proof that muscular tone is a continuous reflex action due to continuous afferent impulses. (1^) The deftBcation centre. — The tonic contraction of the sphincter muscle of the rectum is a reflex action of the spinal cord, the centre 282 Human PhysiolQgy being in the lumbar part of the cord. Afferent impulses pass in certain conditions of the lower bowel up to this part, through nerves in the mesen- teric plexus, and are reflected by efferent fibres to the sphincter muscle and to the lower bowel, so that relaxation of this muscle and exi^ulsion of the bowel contents follow. The centre for defsecation in the spinal cord is partially under the control of the brain, so that its action may be either inhibited or augmented. (f ) The micturition centre. — This centre acts in a similar manner to that of the defsecation centre, and its centre is also in the lumbar region of the cord. It is stimulated to action by the presence of urine in the bladder or by impulses from the brain, and the relaxation of the sphincter of the urethra is followed by contraction of the bladder and expulsion of its contents. Fibres also leave the spinal cord by the anterior roots, and join the sympathetic, that regulate the secretion of sweat. Centres subordinate to the principal vaso-motor centre in the medulla are also contained in the cord. Further fibres pass some distance in the cord that pass to the iris and regulate the size of the pupil, and others connected with acceleration of the heart's action. CHAPTER XV THE BRAIN 1 86. The Nervous System.— The nervous system consists of a number of organs, termed nerve centres, nerves, and peri- pheral end-organs. The largest and most important nerve centres are the brain and spinal cord, which together constitute the cerebrospinal system, the brain being lodged in the cranium and the spinal cord in the spinal canal. A smaller system of nerve centres consists of a double chain of ganglia on each side of the vertebral column from the cranium to the pelvis, and is known as the sympathetic system. It is not, however, a system independent of the brain and spinal cord, as once thought, but an outlying part of the same system and in close connection with it, all its fibres being derived from the cerebro- spmal system. The nerves are white cords or threads traversing the different regions of the body, and connecting the nerve centres with one another and with the periphery or outlying parts of the body. The peripheral end-organs are the minute structures at the extremities of the nerves, and are situated in the skin and other sense-organs, in glands, blood-vessels, and The Brain 283 muscles. Under the microscope nervous tissue is seen to consist of nerve cells and nerve fibres in close association, and the structure and function of these two parts have already been in part considered. A previous chapter has also dealt with the spinal cord. We now enter on the study of the complex organ Fig. 178. — The Upper Surface of the Cerebrum, showing its Division into two Hemi- spheres by the Great Median Fissure, and also the Convolutions. called the brain, or encephalon. A careful study of the illustra- tions will greatly aid in understanding the arrangement of the parts of the brain.' 187. General Survey of the Brain— A human brain re- moved from the skull and divested of its membranes, when looked at from above, shows nothing but the convoluted surfaces of the two hemispheres that constitute the cerebrum, ' A good model of the brain will also be of great service. 284 Human Physiology the hemispheres being separated from front to back by a longi- tudinal fissure. By drawing the hemispheres apart they are found to be connected in the middle half, at about half an inch below the surface, by a transverse band of white fibrous matter, termed the corpus callosum. Looked at from the side, the hinder portion of the cerebral hemispheres is seen overlapping the wrinkled cerebellum, or lesser brain. The hindmost part of the brain (for the term ' brain ' includes all that part of the nervous system within the cranium) is the medulla oblongata, which is continuous, through the opening in the occipital bone called the foramen magnum, with the spinal cord. On turning up the base of the brain for inspection, each cerebral hemi- sphere is seen to be subdivided into three lobes, the anterior or frontal lobe separated from the middle lobe by the fissure of Sylvius, the middle lobe divided into parietal and temporo- sphenoidal lobes, and the temporal or occipital lobe. From behind forwards, in and near the middle line, we see, on the under surface or base of the brain, the following parts : — The medulla oblongata, or bulb, pyramidal in shape, overlying the cerebellum, and with its broad end upwards. Certain pairs of the cerebral nerves are seen springing from its surface. ^\\s. pons Varolii, a quadrate mass immediately above the medulla, showing transverse fibres connecting it externally with the two sides of the cerebellum. Internally it is found to be continuous with the medulla (Figs. 42, 179). The crura cerebri, or peduncles of the cerebrum, striated bundles of nervous matter emerging from the pons and entering the under part of each cerebral hemisphere as they diverge. The posterior perforated space, a small triangular plate of brain tissue traversed by many small arteries, and the corpora albicantia, two small white bodies of unknown function, about the size of a pea. Both these structures lie between the di- verging peduncles of the crura. The tuber cinereum, an eminence of grey matter in front of the corpora albicantia, attached to the junction of the optic nerves termed the optic commissure. The infundibulum, a hollow conical process passing from the tuber cinereum to a small reddish body of unknown The Brain 285 function, called the pituitary body. Being enclosed in the dura mater the pituitary body is usually detached when a brain is Frontal Lobe Occipital Lobe Fig, 179. — Base of the Brain. removed from the skull. Lying in grooves on the under sur- face of the frontal lobes of the cerebrum, on each side of the longitudinal fissure, are the two olfactory tracts and bulbs. 286 Human Physiology To get some idea of the inner relations of the various parts, as well as to see the basal ganglia and cavities of the brain, sections may be made in various directions. Fig. i8o represents a vertical longitudinal section along the middle line. The spinal cord is seen to pass upwards into the medulla oblongata, and the central canal of the cord opens out above into a lozenge-shaped cavity, called ^& fourth ventricle. Overhanging the fourth ventricle there is seen in section the cerebellum, its white matter having a peculiar tree-like arrangement, termed arbor vitce. The upward fibres of the medulla and the cross fibres from the cerebellum pass into the pons Varolii, beyond which are seen the crura cerebri. At the upper and back part of the crura cerebri, and separated from them by a small channel passing from the fourth ventricle, are four hemi- spherical masses of nervous matter, called the corpora quadri- gemina, two of which are seen in figs. i8o, i8i, and immediately above them is seen the section of a small conical body, the pineal gland. The small channel called the aquieductus Sylvii leads from the fourth to the third ventricle of the brain. This is a narrow median cavity, each side of which is bounded by the internal surface of an oval mass of matter that projects internally, called the optic thalamus. The third ventricle thus lies between the optic thalami, the grey matter of the thalami being connected across the narrow cavity by the soft commissure (cut end shown white in fig. i8i). In the roof of the third ventricle is the fornix (covered by a double fold of pia mater, called the velum interpositum), a longitudinal arch of white fibrous matter, united behind to the corpus callosum, and in front to a thin partition called the septum lucidum, between the two layers of which is a space termed the fifth ventricle.^ The fornix descends to the base of the brain. An aperture (the foramen of Monro) leads on each side from the front part of the third ventricle by a Y-shaped connection to a lateral ventricle in each cerebral hemisphere. By slicing a brain horizontally down to the corpus callosum, and removing this 1 The fifth ventricle is not regarded as a true ventricle, as it has no connection with the others, all of which are in communication and contain a small quantity of fluid, the cerebro-spinal fluid. The Brain 287 sufficiently, the two lateral ventricles are brought to view. Each lateral ventricle is an irregularly curved cavity, extending Fig. 180. — Vertical Median Section of the Encephalon, showing the parts in the middle line. I. Convolution of corpus callosum. Above it is the calloso-marginal fissure, run- ning out at 2 to join the fissure of Rolando. 3. The parieto-occipital fissure. 4. 4 point to the calcarine fissure, which is just above the numbers. Between 2 and 3 are the convolutions of the quadrate lobe. Between 3 and 4 is the cuneate lobe. 5. The corpus callosum. 6. The septum lucidum. 7. The fornix. 8. Anterior cms of the fornix, descending to the base of the brain, and turning on itself to form the corpus albicans. Its course to the optic thalamus is in- dicated by a dotted line, g. The optic thalamus. Behind the anterior cms of the fornix, a shaded part indi- cates the foramen of Monro ; in front of the number an oval mark shows the position of the grey matter continuous with the middle commissure. 10. The velum interpositum. 11. The pineal gland. 12. The corpora quadrigemina. 13. The cms cerebri. 14. The valve of Vieussens (above the number). 15. The pons Varolii. 16. The third nerve. 17. The pituitary body. 18. The optic nerve. 19 points to the anterior commissure indi- cated by an oval mark behind the number. 288 Human Physiology in the substance of the corresponding cerebral hemisphere for about two-thirds of its entire length, and lined by a prolongation of the ciliated epithelium, which characterises the inner surface of the true brain ventricles. Each lateral ventricle consists of a central cavity or body, and three small cavities or cornua, the body of each lateral ventricle being separated in front from its fellow by the septum lucidum already referred to. The roof of each lateral ventricle is formed mainly by the under surface of the corpus callosum. Into the front portion of the floor of each lateral ventricle a rounded mass of nervous matter, known as the caudate nucleus of the corpus striatum, projects, a deeper part of the corpus striatum, the lenticular nucleus, being embedded in the mass of each cerebral hemisphere. Part of the upper surface of the optic thalamus also enters into the floor of each lateral ventricle, the inner sides of the optic thalami forming the lateral boundaries of the third ventricle. Each thalamus rests upon, and is connected with, one of the crura cerebri, and has on its outer and hind part two small elevations, termed corpora geniculata. Between the lenticular nucleus of the corpus striatum on the outer side and the optic thalamus and caudate nucleus on the inner side, a tract of white fibres (the internal capsule), continuous with the lower or an- terior portion (crusta) of the crus, passes upwards to the cortex or outer layer of the cerebrum, some of its fibres diverging on the way in a fan-like manner to form a mass of white matter, known as the corona radiata. The main bulk of the brain consists of the two large ovoid masses called the cerebral hemispheres, separated by the deep median longitudinal fissure at the bottom of which in the middle portion is the connecting corpus callosum. Each cerebral hemisphere has an outer convex surface in contact with the vault of the cranium, an inner flat surface forming one side of the longitudinal fissure, and an irregular under surface overlapping the basal ganglia in the middle and the cerebellum behind, but resting in front and at the sides on the base of the skull. All these surfaces are moulded into emi- nences which form convolutions, or gyri, separated from each other by fissures, or sulci. The deeper fissures on the surface The Brain 289 mark off each hemisphere into five lobes— frontal, parietal, occipital, temporo-sphenoidal, and central or island of Reil. The brain, like other portions of the nervous system, con- tains grey and white matter, and all its parts are more or less closely connected by numerous nerve fibres. The grey matter consists of nerve cells (one pole or process of which is usually found giving off the axis-cylinder of a nerve fibre), a special jir-Mon, nrpt. luc. ^(irnljc transu.jiss. aTveal strlo- pos^. cotnm., incal boclij Inftvivdlb pit. bodu Corp. alb. Fig 181. — Portion of a Median Section of the Brain, showing the Corpus Callosum, Third Ventricle, Aqueduct and Fourth Ventricle, Pons, Cerebel'um, &c. (G. D. T.) kind of connective tissue termed neuroglia, and minute blood- vessels. It is found on the surface of the cerebrum and cere- bellum, and in small masses witjiin the corpora striata, optic thalami, and other parts of the brain. The white matter con- sists of meduUated fibres without the primitive sheath, of various sizes and arranged in bundles separated by neuroglia. These fibres are sometimes arranged into three different systems XT 290 Human Physiology according to their general course :^i) Diverging or peduncular fibres (projection fibres) which connect the hemispheres with the lower portions of the brain and the cord, and which are in great measure direct prolongations of the axis-cylinders of the nerve cells of the cortex ; (2) transverse or commissural fibres (including the fibres of the corpus callosum, and the anterior and posterior commissures), which connect the two hemispheres together ; (3) association fibres, which connect different struc- tures in the same hemisphere. 188. The Ventricles of the Brain. — As already mentioned, the brain contains certain cavities or ventricles, and the position and relation of these will now be further described. The central canal of the spinal cord opens out in the upper and posterior part of the medulla into a lozenge-shaped cavity <^ ^, the lower end of which is termed the calamus scriptorius, and the narrow upper end of which is continuous with the small channel termed the aqueduct of Sylvius (fig. 181). The roof of the fourth ventricle in the lower part is simply pia n'ater lined by epithelium, but in the upper half the roof is formed by a thin layer of grey matter (the valve of Vieussens), which here unites the converging superior peduncles of the cerebellum. The floor in the lower half is formed by the upper portion of the posterior surface of the medulla, and in the upper half by the posterior surface of the pons. The medulla and the superior peduncles of the cerebellum form the lateral boundaries. The Sylvian aqueduct or passage (iter) from the upper end of the fourth ventricle opens in front into the third ventricle, which lies between the optic thalami. The third ventricle is a narrow oblong cavity with the arching fornix covered by the velum interpositum for its roof, the bodies lying in the space at the base of the brain between the diverging crura for its iloor and the optic thalami with the anterior pillars of the fornix for its sides. A connecting band of soft grey matter, the middle commissure, passes across the cavity between the optic thalami. From the front part of the third ventricle an aperture (foramen of Monro) leads on each side into a large curved cavity in each cerebral hemisphere, termed the lateral ventricle. Each lateral ventricle consists of a body or central cavity, and three horns or cornua— ah anterior cornu or horn, a posterior horn, and an inferior or descending horn. The roof of the main cavity is formed by a part of the corpus callosum. On the outside is the substance of the cerebral hemisphere, and at the inner side the bodies of the two ventricles are separated by the septum lucidum. ' The anterior horn curves from the foramen of Monro somewhat outwards, with a slight inclination downwards into the frontal lobe ; the body comprises that part of the cavity which extends from the foramen of Monro to its bifurcation into posterior and descending horns opposite the splenium of the corpus callosum, and is separated anteriorly from its fellow of, the opposite hemisphere by a thin septum, the septum lucidum ; the posterior horn passes backwards, with a bold curve convex outwards into the occipital lobe ; and the descending horn passes forwards and slightly down- wards, also in a bold curve with its convexity outwards, into the temporal lobe, and extends to about an inch from the apex of that lobe.' The Brain 291 The sephim lucidum consh\s of two vertical layers. of white matter, separating the front part of the lateral ventricles and enclosing the space termed the fifth ventricle. At its posterior part is the arched longitudinal white tract of fibres termed the fornix, the anterior pillars of which can be traced forwards and downwards to the corpora albicantia(fig. 181). 189. The Kembranes of the Brain.— The brain is invested by three membranes or meninges, (l) the dura mater, (2) the pia mater, (3) the arachnoid membrane. The dura mater is the external dense fibrous membrane closely attached to the inner surface of the skull. It is continuous with the dura mater, that cCffftLo anl&fCiJ^ tzu-cLeuS Monro's Foraj t n verUrHT- co:nvm ., posterior fissure ; a..p.^ pyramid ; a, remains of part of anterior cornu, separated by the crossing bundles from the rest of the grey matter ; /, continuation of lateral column of cord; i?, continu- ation of substantia gelatinosa of R*Iando ; p.c.^ continuation of pos- terior cornu of grey matter ; /.g,^ funiculus gracilis. 191. Functions of the Medulla.— The functions of the medulla, or bulb, have already been referred to several times. It acts (a) as a conductor, (b) as a ngrve centre, or rather a collection of mrve centres, (a) All the im- pulses passing between the brain and spinal cord rr.ust pass through the 296 Human Physiology medulla. Efferent impulses travel mainly through the anterior pyramids, where decussation (anterior pyramidal decussation) occurs of those fibres from the cords that have not already crossed in the cord, viz. of the fibres in the crossed pyramidal tract. The fibres of the so-called direct pyra- midal tract are believed to decussate at various levels in the cord by the anterior commissure. The afferent or sensory path is not completely made out, but it is probably for the most part along the posterior pyramids, the fibres of vi'hich terminate in the nucleus gracilis and nucleus cuneatus of the bulb. From these nuclei fibres pass round the front of the medulla to the opposite side in what is termed the superior pyramidal decussation (sensory decussation). Some fibres may decussate in the pons. At any rate decussation of the fibres connecting the spinal cord and brain is com- plete in the crura cerebri, so that all impressions to and from the hemi- spheres of the brain pass across the middle line. Any injury or compres- sion, therefore, of either hemisphere impairs sensation and voluntary motion in the opposite side of the body. A destructive affection of one of the hemispheres usually produces complete motor paralysis and loss of sensibility in the opposite side. (b) The importance of the medulla as a nerve centre is established by the fatal issue that follows its injury or disease. Any sudden displace- ment of the upper cervical vertebrse, as in hanging, so injures the cord and its connections as to produce instant death. So does injury to the medulla itself, especially in the central part. By experiments on the lower animals it has been shown that the whole of the brain except the medulla may be gradually removed while respiration and life continue some time. The same result follows on removal of the spinal cord up to the origin of the phrenic nerve — a nerve arising from the third and fourth cervical nerves and supplying filaments to the diaphragm. Most of the nerve centres of the medulla are reflex centres, but the reflex actions under its control are much more complicated than those of the spinal cord. Unlike the brain proper it discharges no mental functions, initiating none but reflex move- ments. Its most important centres are studied in other parts of the book, and a simple summary is all that can be placed here. (1) The medulla cotAwos, 2. respiratory centre, the centre being bilateral and situated behind the origin of the vagi nerves on each side of the pos- terior aspect of the calamus scriptorius. The efferent nerves are the branches of the vagus distributed to the lungs, and these appear to be stimulated according to the condition of the blood as regards the amount of oxygen and carbon dioxide. The impulse reaching the medullary centre is reflected along the efferent or motor fibres of the phrenics, intercostal and other nerves associated with respiratory movements (see par. 100). Possibly also the venous blood circulating in the medullary centre itself may excite it and lead to its automatic action. (2) Cardiac centres also exist in the medulla, one accelerating the action of the heart through the sympathetic, another inhibiting the action of the heart through the vagus (par. 83). (3) A vaso-motor (vaso-constrictor) centre lies in the medulla, domi- nating the nerves supplied to the unstriped muscle of the arteries, intes- tines, &c. The centre is bilateral and lies in the floor of the medulla, a little above the calamus scriptorius. Under ordinary circumstances ' it keeps the arteries of the body in a state of tonic contraction. Stimulation of the centre leads to contraction of the arteries and a general rise of blood- The Brain 297 pressure ; inhibition of the centre leads to dilatation of the arteries and a great fall of blood-presshre. The vaso-motor centre in the medulla controls subordinate centres in the cord. A special vaso-dilatator (vaso- inhibitory) centre, not acting continuously or tonically, is also believed to exist in the medulla (par. 84). (4) A centre for mastication is believed to be situated in the medulla. The afferent nerves are the sensory branches of the fifth or trigeminal, and VII XII Restiform body Fasc c. cuneatus'- Post. pyramid ' Fig. 186.— Diagram of the Fourth Ventricle, showing Nuclei of Cranial Nerves. (Ziegler.) in, nucleus of third ; IV, nucleus of fourth ; V,, nucleus of motor of fifth ; V._, and V3, nuclei of sensory of fifth ; VI, nucleus of sixth ; VII, nucleus of facial ; VIII,, VIII3 ; nuclei of auditory; IX, nucleus of glosso-pharyngeal ; X, nucleus of vagus ; XI, nucleus of spinal accessory ; XII, nucleus of hypoglossal. the tenth or glosso-pharyngeal, and the efferent nerves are the motor branches of the fifth and twelfth cerebral nerves. (5) A centre for salivary secretion is also found in the medulla, and its reflex activity has been described in par. 114. There are also in the medulla centres for deglutition, vomiting, cough- ing, and dilatation of the pupil. 192. The Cranial or Cerebral Nerves. — The cranial nerves consist of twelve pairs, and appear to arise from the surface of 298 Human Physiology the brain in a double series, passing thence through the base of the cranium to their distribution. Their superficial origin extends from the under surface of the frontal lobe of the cere- brum to the lower end of the medulla (see figs. 42, 179), and the ultimate distribution of all except the tenth (pneumogastric) and eleventh (spinal accessory) is to some part of the head. Their fibres, however, can be traced mto the substance of the brain to some special nucleus of grey matter, which is termed their deep or real origin or root. With one exception (the first or olfactory) the fibres proceeding from the nuclei of origin cross within the cranium, and the nerves are thus functionally connected with the cerebral cortex of the opposite side. The cranial nerves have been named according to the order in which they pass through the dura mater lining the base of the sicuU, as well as according to the parts to which they are distributed, or to their functions. They may be thus enumerated : — 1st. Olfactory. yth. Facial, and. Optic. 8th. Auditory. 3rd. Motor-oculi. 9th. Glosso-pharyngeal. 4th. Trochlear. loth. Pneumogastric or vagus. 5th. Trigeminus. nth. Spinal accessor^'. 6th. Abducens. 12th. Hypoglossal. The first two pairs differ in their origin and mode of development from all the rest, being in reality actual outgrowths, or processes, of the brain itself. According to their function the cranial nerves have been divided into sensory, motor, and mixed, thus : — (a) Nerves of special sense. ist or olfactory, 2nd or optic, 8th or auditory, part of the Sth, part of the 9th. (b) Motor nerves. 3rd or motor-oculi, 4th or trochlear, part of the Sth, 6th or abducens, 7th or facial, 12th or hypoglossal. (4 Mixed nerves. 9th or glosso-pharyngeal, loth or vagus, nth or spinal accessory. "YVz first cranial or olfactory netves are in reality lobes of the brain, and arise by a triple root in the under part of the frontal lobe. The two olfactory tracts lie in a furrow on either side of the median fissure of the cerebrum. On reaching the cribriform plate of the ethmoid bone of the skull they expand into bulbs, from the under surface of which ten or twelve true olfactory nerves pass through the cribriform plate to be distributed to the mucous membrane of the nose, there forming the terminal organ of The Brain 299 smell. Unlike all other nerves, it is supposed that the sensory impressions do not cross to the opposite side of the cerebrum (fig. 179). The second or optic nerves, distributed to the eyeballs, are connected together at the optic commissure, where a partial decussation of fibres takes place. Behind the commissure, under the name of optic treats, the nerves may be traced to their origin in the anterior corpora quadrigemina, the geniculate bodies, and the hinder part of the optic thalami. From their nuclei of origin fibres may be traced to the visual centre in the cortex of the occipital lobe (par. 204). 'M^'"'"*<5!J^ ofcftosite side At the optic commissure fibres from "^ ' the inner or nasal half of each retina F"=- iSj.-Course of the Fibres m the J , , , , 1.1 Optic Commissure. decussate and pass backwards to the opposite half of the brain, the fibres from the outer or temporal half of the retina undergoing no crossing. Thus the right optic trcut contains fibres from the outer half of the right retina and the inner half of the left retina, so that light from objects on the left side of the body is transmitted to the right side of the brain. Similarly, light from objects of the right side passes to the left side of the brain. Hence, section of one optic nerve would produce blindness of -the corre- sponding eye, but section of one optic tract would produce a half-blindness of each retina, hemianopia as it is called. The distribution of the fila- ments of the optic nerve in the eyeball is treated of near the end of par. 204 and in fig. 204. The third or oculo-motor nerves have their origin in clusters of cells in the grey matter of the inner part of the crura cerebri on each side of the aqueduct of Sylvius, where the nerves of the two sides decussate. The nerves of the third pair are purely motor, and paiis by two branches into the orbits of the eyes, to be distributed to (l) the elevators of the eyelids, (2) the superior, (3) inferior, and (4) internal recti muscles. Fibres also pass to the circular muscle of the iris and to the ciliary muscle, to regulate contraction of the pupil and accommodation. 'Y:\itfourth or trochlear nerve arises from a nucleus of large multipolar ganglion cells immediately below the nucleus of the third in the floor of the aqueduct. It is purely motor in function, and supplies the trochlear or superior oblique muscle of the eye. Its section or paralysis leads to squinting inwards and upwards. TheJf^A or trigeminal lurve has two roots like a spinal nerve— a large sensory root in connection with the Gasserian ganglion, and a smaller motor root which has no ganglion. Its nuclei are in the part of the pons forming the floor of the fourth ventricle. The sensory part is distributed to the face, the teeth, the mucous membrane of the nose and mouth, and to the conjunctiva of the eye. Motor filaments pass to the muscles of mastication, the tensor tympani, and tensor palati. The sixth or abducens nerve arises from a nucleus in the floor of the pons, about the middle of the floor of the fourth ventricle. It is exclusively motor, and supplies only the external rectus muscle of the eye. The seventh or facial nerve arises in a nucleus in the pons near that of the sixth, and its fibres emerge from the medulla between the restiform and olivary bodies. It is the motor nerve for all the muscles of facial expression, arid its paralysis or injury on one side leads to a blank look on 300 Human Physiology that side with drooping of the angle of the mouth. In the chorda tympani branch of this nerve are secretory and vaso-dilatator fibres for the sub- maxillary and sublingual glands (par. 114). The eighth or aziditory nerve arises from three nuclei in the floor of the fourth ventricle forming the upper part of the medulla. Passing through the temporal bone by the internal meatus it divides into two branches, one of which passes to the cochlea, and the other to the utricle and semicircular canals of the inner ear. The first is the auditory portion connected with the sense of hearing ; the second branch conveys impulses from the semicircular canals that aid in the reflex maintenance of the equilibrium of the body. Section of the auditory nerve not only causes deafness, but giddiness and other disturbances of equilibrium, that indicate the double function of this so-called eighth nerve. The ninth or glosso-pharyngeal nerve arises from a nucleus in the floor of the lower part of the fourth ventricle. It is mainly a sensory nerve conveying sensations of taste from the hinder portion of the tongue and adjoining mucous membrane of the mouth and pharynx. It has com- munications with the vagus or pneumogastric, with the upper cervical ganglion of the sympathetic, and with the digastric branch of the facial nerve. ' The tenth or pneumogastric nerve ^nervus vagus) has a more ex- tensive distribution than any of the other cranial nerves, passing through the neck and thorax to the upper part of the abdomen. It is composed of both motor and sensory filaments. It supplies the organs of voice and respiration with motor and sensory fibres ; and the pharynx, oesophagus, stomach, and heart, with motor influence. Its superficial origin is by eight or ten filaments from the groove between the restiform and the olivary body below the glosso-pharyngeal ; its deep origin may be traced deeply through the fasciculi of the medulla, to terminate in a grey nucleus near the lower part of the floor of the fourth ventricle, below and continuous with the nucleus of origin of the glosso-pharyngeal. The filaments become united, and form a flat cord, which passes outwards across the flocculus to the jugular foramen, through which it emerges from the cranium. In passing through this opening, the pneumogastric accompanies the spinal accessory, being contained in the same sheath of dura mater with it, a membranous septum separating it from the glosso-pharyngeal, which lies in front.' With the ganglion of the root are connected the accessory part of the spinal accessory nerve, a twig from the glosso-pharyngeal, and an ascend- ing filament from the upper cervical ganglion of the sympathetic. The chief branches of the pneumogastric are the laryngeal, cardiac, pulmonary, and gastric. Most of the functions of the pneumogastric have been already referred to. It carries efferent impulses to the muscles of the pharynx, larynx, oesophagus, stomach, trachea, and lungs ; vaso-motor influences to the same organs, and inhibitory influence to the heart, which is of great importance in the circulation (see par. 83). Among its afferent fibres are those which convey, inwards, impulses from the respiratory and digestive organs, excitor-motor fibres that lead to the reflex phenomena of coughing, vomiting, &c. , excito-secretory fibres, and fibres that convey inhibitory influence (depressor) from the heart to the vaso-motor centre. Section of both vagi in the neck leads to acceleration of heart-beat, difficulty of swallowing owing to paralysis of muscles of pharynx, and The Brain 301 slower but deeper respiration with paralysis of muscles of the larynx. Foreign bodies accumulate in the insensitive larynx and air-passages, usually producing fatal inflammation of the lungs. The eleventh or spinal accessory nerve arises in part from a nucleus Fneitmo-Caatrt'c Spinal AtoetBBr To Stomach Fig. 188. — Course and Distribution of the Pneumogastric, &c. below that of the vagus, and in part from the lateral column of the spinal cord. Motor fibres are distributed from it to the trapezius and sterno- mastoid muscles, and viscero-motor filaments join the vagus. The twelfth or hypoglossal nerve arises from a nucleus near the middle 302 Human Physiology of the floor of the lower end of the fourth ventricle. It is a purely motor nerve, supplying fibres to the muscles of the tongue and the muscles con- nected with the hyoid bone. 193. The Cerebellum. — The cerebellum, orlittle brain, consists of two lateral hemispheres united by a central portion called the vermiform process, seen beneath the medulla in fig. 179. It lies in the posterior part of the cranium, and is separated Fig. 189 -Figure showing the Three Pairs of Cerebellar Peduncles. (From Sankey after Hirschfeld and Leveilld.) On the left side the three cerebeUar peduncles have been cut short ; on the right side the hemisphere has been cut obliquely to show its connection with the superior and inferior peduncles. I, median groove of the fourth ventricle ; 2, the same groove at the place where the auditory striae emerge from it to cross the floor of the ventricle ; 3, inferior peduncle or restiform body ; 4, funiculus gracilis ; 5, superior peduncle — on the right side the dissection shows the superior and inferior peduncles crossing each other as they pass into the white centre of the cerebellum ; 6, fillet at the side of the crura cerebri ; 7, lateral grooves of the crura cerebri ; 8, corpora quadrigemina. from the cerebrum above by a partition of the dura mater called the tentorium. It is connected with the rest of the brain by three pairs of fibrous stalks termed peduncles, or crura. The inferior or lower peduncles are formed by the prolonged' restiform bodies from the medulla. The middle peduncles pass from its two hemispheres fransversely to form- the transverse fibres of the pons Varolii. The superior peduncles connect it with the cerebrum above. Each superior The Brain 303 peduncle, or crus, forms the upper part of the lateral boundary of the fourth ventricle, and is connected with its fellow of the opposite side by the valve of Vieussens, a thin membrane continuous with the white centre of the vermiform process, and forming the roof of the anterior part of the fourth ven- tricle. The cerebellum has a laminated or foliated appearance, and is marked on its surface by numerous transverse curved fis- sures. The cerebel- lum is composed of white matter and grey matter when seen in section. The grey matter lies out- side and forms its cortex, a small nu- cleus of grey matter being also found near the centre of each hemisphere, termed the corpus dentatum. The white matter has a curiously branched arrangement which spreads from a white ' centre. This arrange- ment of white matter is called arbor vitcB (tree of life). The minute structure of the grey matter of the cortex or surface of the Fig. 190. — Section of Cortex of Cerebellum. (Sankey.) *, pia mater ; b, external layer ; c, layer of corpuscles of Purkinje ; d, inner or granule layer ; e, medullary centre. 304 Human Physiology cerebellum shows (a) an outer layer beneath the pia mater membrane, consisting of delicate fibres with small nerve cells and large neuroglia cells ; [b) an inner or granule layer next the white centre, consisting of closely packed granule cells; (c) a middle layer, consisting of a single stratum of large pear-shaped cells (the corpuscles of Purkinje) glj to j^ inch in diameter. From the base of each of these cells an axis-cylinder process passes off to form one of the medullated fibres of the white centre, while from the opposite pole of the cells several processes pass outwards into the outer layer. The function of the cerebellum seems to be that of securing or assisting in securing properly co-ordinated muscular movements, so that in any action, standing, walking, &c., the different muscles employed may each act at the right moment, and with the right force. Other parts of the cerebro-spinal system share in this co-ordinating function, as shown in speaking of the spinal cord. Afferent impulses from the feet, limbs, and other parts stream into the nervous centres during muscular activity, and the office of so co-ordinating the action of the muscles that equilibrium is maintained, locomotion effected, or some definite movement accomplished, appears to be discharged iri many cases by reflex centres situated in the cord, these lower centres being largely regulated by higher centres in the cerebellum. These afferent impulses or sensations that lead to co-ordina- tion of movements are of several kinds, and do not always come into distinct consciousness — muscular sensations that form the basis of the muscular sense, visual sensations through the eyes, and those peculiar impulses that arise in the ampuUary ends of the semicircular canals (par. 241). All these sensory impulses may reach the cerebellum by nerve fibres in one or other of the peduncles, or, in the case of the vestibular branch of the auditory nerve (par. 239), by a strand of fibres connecting it with the cere- bellum. Each half of the cerebellum is connected by fibres in the superior peduncle with the opposite side of the cerebrum, and it is from the cells of the cerebral hemisphere that many impulses arise that give rise to muscular movements (par. 204). Experiment and observation both go to prove therefore that ' the chief function of the cerebellum is to play a very im- portant part in the co-ordination of the actions, nervous and muscular, by which the co-ordinated movements of the body are carried on. ' For we learn — (l) that removal or injury of the cerebellum produces, for some time at least, a lack of orderly executed movements ; (2) that injury to one side causes inclination to fall towards the opposite side owing to failure of muscular control and power on the injured side ; (3) that stimulation of one side of the cerebellum gives rise to muscular contraction on the same side ; (4) that dissection and degeneration show that the connection of the cerebellum with the cerebral hemisphere is a crossed one ; (5) that disease of the cerebellum frequently leads to a staggering gait with loss of muscular power and tone. It is thus clear that the cerebellum acts upon the muscles of the same side of the body in connection with the cerebral hemisphere of the opposite side. The cerebellum does not share in the higher intellectual functions, since it may be completely removed from an •animal without affecting its general intelligence or its special senses. 194. The Pons Varolii.— The pons Varolii lies above the medulla and between the lateral halves of the cerebellum. A section across shows that it consists of transverse and longi- The Brain 305 tudinal fibres intermingled with some grey matter. There are superficial and deep transverse fibres from the middle pe- duncles of the cerebrum, longitudinal fibres arranged in bundles, that pass down to the anterior pyramids of the medulla and then decussate to form the motor tracts of the cord, other longitudinal fibres from the medulla, and grey matter on the Fig. 191. — Transverse Section through the Upper Part of the Pons. (Schwalbe, after Stilling.) Rather more than twice the natural size. ;*, transverse fibres of the pons ; /y, py, bundles of the pyramids ; a, boundary line between the tegmental part of the pons and its ventral partj /', oblique fibres of the lateral fillet, passing towards the inferior corpora quadri^emina ; /, lateral ; /^, mesial filletjyr?-., formatio reticularis; ^./., posterior longitudinal bundle; s.c.p,, superior cerebellar peduncle; v.m.a., superior medullary velum; /, grey matter of the lingula; V 4, fourth ventricle— in the grey matter which bounds it laterally are seen v.d.t the descending root of the fifth nerve, with its nucleus ; s.^., substantia ferru- ginea ; g.c.^ group of cells continuous with the nucleus of the aqueduct. floor of the fourth ventricle forming the nuclei of the seventh or facial nerve, the sixth nerve, and the fifth nerve. The motor fibres from the internal capsule to the facial nuclei are seen to decussate in the pons. Hence injury to one side of the pons in the upper part affects the facial muscles of the opposite side, while injury in the lower part, after decussation occurs, X 3o6 Human Physiology paralyses the muscles of the same side. The motor fibres to the limbs decussate in the medulla. 195. The Crura Cerebri.— The crura cerebri diverging from the upper border of the pons pass into the cerebral hemi- spheres (fig. 42). Each crus on section is seen to consist of two parts separated by dark grey substance termed substantia nigra. The pes, or crusta, is the anterior or lower part, and consists almost entirely of longitudinal fibres, continuous above with some in the internal capsule, and below with some in the pons that go to the anterior pyramids of the medulla. The tegmentum of the crus is the dorsal or upper part, and consists of grey matter and fibres continuous with the formatio reticularis of the pons and medulla, separated by transverse arched fibres and some grey matter. Above these fibres pass into the optic thalamus for the most part. 196. The Corpora ftuadrigemina. — These are four rounded prominences placed in pairs over the aqueduct of Sylvius, and above the pons and crura. The anterior pair is sometimes called the nates and the posterior the testes. They are mainly composed of grey matter, with white fibres externally and some internally. From the outer side of each of these emi- nences a white band termed the arm, or brachium, is continued forwards and outwards. The bands from the posterior pair lose themselves beneath two prominences in relation with the posterior part of the optic thalami, termed the internal ge- niculate bodies. Those from the upper pair pass into the external geniculate bodies and the optic tract. The superior quadrigeminal bodies are indeed intimately connected with the optic tract and the sense of sight. Their destruction leads to blindness, and they appear to contain centres for the contraction of the iris and for accommodation. 197. The Optic Thalami.— The optic thalami are two oval- shaped masses of grey matter, projecting above into the lateral ventricles of the brain, the under surface resting on the tegmentum of the crus. The posterior and inner end of the thalamus is termed the pulvinar, and projects over the arms of the corpora quadrigemina, the geniculate bodies being below and outside. Their inner sides form the lateral bound- The Brain 307 aries of the third ventricle. Their inner surface is united with a prolongation of the tegmental part of the crura cerebri, and on their outer side is the white matter of the internal capsule, formed by fibres from the crusta of the crura that pass into the cerebral hemispheres without entering the optic thalami. The optic thalami consist of grey matter with many nerve cells nm Fig. 192. — Section through the Superior Part of one of the Superior Corpora Quadri- gemina, Crus Cerebri and the Adjacent part of the Optic Thalamus. (After Meynert.) J, aqueduct of Sylvius ; gr^ grey matter of the aqueduct ; c.q-s^ quadrigeminal eminence, consisting of /, stratum lemnisci ; 0, stratum opticum ; c, stratum cinereum; Tk^ thalamus (pulvinar) ; c.g.i^ t-.^.*?, internal and external geniculate bodies ; br.s^br.i, superior and inferior brachia ; f, fillet ; p.l^ posterior longitudinal bundle ; r, rapli6 ; III, third nerve ; «. Ill, its nucleus; l.p-p, posterior perforated space ; s.«, substantia nigra — above this is the tegmentum of the crus with its nucleus, the latter being indi- cated by the circular area ; cr, crusta, or pes. of the crus ; II, optic tract ; M^ medullary centre of the hemisphere ; ».r, nucleus caudatus ; st^ stria terminalis. and white fibres mostly lying on the surface. The optic thalami are connected with the posterior or sensory paths of the spinal cord through the tegmenta of the crura, and from their outer part fibres pass onwards to the cerebral hemispheres. 198. The Corpora Striata. — Each corpus striatum consists of two parts — a pear-shaped part projecting into the lateral ventricle of the same side in front of the optic thalamus, called 3o8 Human Physiology the caudate nucleus, and a part embedded in the white sub- stance of the cerebral hemisphere called the lenticular nucleus. Between the two parts is seen in a deep section the white Fig. 193. — Horizontal Section of Brain. fibres of the internal capsule. Outside the lenticular nucleus is another band of fibres termed the external capsule, beyond which is a thin lamina of grey matter termed the claustrum. The Brain 309 The claustrum lies next to a lobe of the cerebrum in the fissure of Sylvius termed the central lobe, or island of Reil. In section a corpus striatum shows a striped appearance, owing to diverging white fibres being mixed with grey matter. 199. The Internal Capsules. — The internal capsule is a broad band of white fibres, which for the most part connects the cortex of the brain with the crusta and bulb below. It lies between the lenticular nucleus of the corpus striatum on the outer side and the caudate nucleus and optic thalamus on the inner side, the fan-like expan- sion of its fibres into the heniisphere being spoken of as the corona radiata. In horizontal sections the internal capsule shows a bend termed the knee, or genu, a part in front of the genu called ihe front limb and a part behind the knee called the hind limb, and the fibres of the cap- sule curve away in many directions to the various parts of the cerebral sur- face. The internal capsules contain : — (i) The fibres of the pyramidal tract. These can be traced from their origin in the motor areas of the cerebral cortex around the fissure of Rolando through the middle third of the in- ternal capsule into the crusta, or pes, of the crus cerebri, thence into the pons and the anterior pyramids of the medulla. At the lower part of the medulla most of the fibres decussate as they pass into the spinal cord, where they form the crossed pyramidal tracts. Some of the fibres of this tract, how- ever, pass to the nuclei of the cranial motor nerves in the pons and medulla. The uncrossed fibres form the direct pyramidal tracts. Fig. 194. — Horizontal Section through the Optic Thalamus and Corpus Striatum. (Natural size.) V.I., lateral ventricle, anterior comu ; t.c, corpus cal- losum ; j./.j septum lucidum ; a.^., anterior pillars of the fornix ; v. 3, third ventricle ; th,, thalamus opticus ; s.t,, stria pinealis ; ».c., nucleus caudatus, and «./., nucleus lenticularis of the corpus striatum ; /.c, internal capsule ; g.^ its an§:le or genu ; n.c, tail of the nucleus caudatus appearing in the descending comu of the lateral ventricle ; cL claustrum ; /, island of Reil. 3io Human Physiology Cortex (2) Fronto-cortical fibres. These originate in the frontal convulutions anterior to the motor area, and pass down in the anterior third of the cap- sule through the crasta into the pons, where they appear to terminate in grey matter. (3) Temporo-occipital fibres. These fibres take origin in the temporal and occipital regions of the cortex, and, passing through the posterior third of the capsule, terminate in the outer part of the pons. Besides the above tracts of fibres that connect the cortex of the hemispheres with the crus, the internal capsule contains fibres from the nucleus caudatus of the corpus stria- tum that terminate in the pons, as well as fibres from various parts of the cortex that terminate in the grey matter of each optic thalamus. The pyramidal tract of the internal capsule is doubtless concerned in conveying voluntary motor impulses from the cerebral cortex to the muscles. The tract is well marked out by the degeneration of nerve fibres that sets in on injury to the motor area, the descending de- generation showing that the fibres have their trophic centres in the cells forming the grey matter of the cortex. As the motor pyramidal tract decussates, the fibres for the face crossing in the pons and those for the limbs just below the anterior pyramids, it follows that lesions or injuries of the motor areas of the cortex or of the internal capsule will be followed by paralysis of the muscles on the opposite side of the face and body. This motor paralysis on one side is termed hemiplegia, or 'one-sided stroke.' The muscles affected vary with the extent and situation of the lesion, but it is worthy of note that the paralysis is limited to voluntary move- ments, and affects the bilaterally associated re- flex movements but little. As the posterior part of the hind limb of the internal capsule contains numerous sensory fibres connected with the opposite side of the body, total de- struction of the internal capsule gives rise to loss of both motionand sensation on the opposite side of the body. Many cases of ordinary hemi- plegia, or motor paralysis, of one half of the body involve more or less hemiancesthesia, or sensory paralysis of one side. 200. The Cerebrum. — The term cerebrum in the restricted sense describes the two large ovoid masses of grey and white matter called the cerebral hemispheres, that overlap all the rest Capsule Crus Pons Bulb Cord Fig. 195. — Diagram to illus- trate Degeneration in Motor Pyramidal Tract, through internal capsule, crus, pons, anterior pyramids of me- dulla, to lateral column of cord on opposite side. The Brain 311 of the brain.' The two hemispheres are completely separated by the great longitudinal fissure, except at about the middle half of their extent, where they are united at a depth of about I inch by the transverse fibres of the corpus callosum. The surface of each hemisphere presents a peculiar folded fm. JS^t. Fig. 196.— Human Brain ; Lateral Aspect of Left Hemisphere. (After Eclcer.) appearance, being thrown into gyri or convolutions by depres- sions termed sulci or fissures. In this way the superficial area of the hemispheres is greatly increased. The deeper fissures ' In anatomy the term cerebrum usually include? all the brain in front of the cerebellum and pons, i.e. not only the cerebral hemispheres, the corpora striata, and optic thalami, but also the corpora quadrigemina, and the crura cerebri. 312 Human Physiology divide the outer surface of each hemisphere into five lobes, whilst others separate the convolutions of each lobe from one another. It is a great help in describing the brain to refer to these lobes and fissures. The fissure of Rolando begins near the middle of the longitudinal fissure at the vertex, and passes on the outer surface of each hemisphere obliquely downward and forward towards the great fissure of Sylvius, which begins on the under surface of the hemisphere and passes upwards and backwards. The frontal lobe of the brain lies in front of the fissure of Rolando and above the fissure of Sylvius. Smaller fissures divide it into four main convolutions — first or superior, second or middle, third or inferior, and the ascending frontal convolution (fig. 196). The parietal lobe is limited in front by the fissure of Rolando and beneath by a part of the fissure of Sylvius. It is divided into ascending parietal convolution, superior parietal convolution and inferior parietal convolution, which last is subdivided into supra-marginal convolution round "".he end of the Sylvian fissure, and angular convolution round tne end of a temporal fissure. The temporal lobe, lying below the horizontal part of the fissure of Sylvius, shows three parallel convolutions, termed the first or superior temporal, second or middle temporal, and third or inferior temporal. The occipital lobe is small and lies at the posterior end of the cerebrum, separated from the parietal lobe by the perpendicular parieto- occipital fissure. It shows three convolutions, a superior (occ. 1), a middle (occ. 2), and an inferior (occ. 3). The central lobe, or island of Reil, can only be seen by pulUng apart the edges of the fissure of Sylvius, when the concealed convolutions forming the lobe would be exposed. A view of the inner mesial surface of a hemisphere shows a long fissure beginning below and running backwards some distance above the corpus callosum, to terminate behind the upper end of the fissure of Rolando. This is known as the calloso-marginal fissure. Between this fissure and the surface of the hemisphere is the marginal convolution, or gyrus mar- ginalis, and between the calloso-marginal fissure and the corpus callosum is the callosal convolution, or gyrus for meatus. The posterior end of the gyrus fornicatus comes downwards The Brain 313 and then forwards under the name of hippocampal convolution, or gyrus hippocampi. At the end of the temporo-sphenoidal lobe is seen the hooked uncinate convolution, or gyrus uncinatus. Between the posterior end of the calloso-rharginal fissure and the parieto-occipital fissure is the quadrate lobule, while below this, between the parieto-occipital and calcarine fissure, is the wedge-shaped mass termed the cuneate lobule. 201. Structure of the Cerebrum. — The cerebral hemi- spheres consist of white and grey matter, the white pervading Fig. 197. — Human Brain ; Mesial Aspect of Right Hemisphere. (After Ecker.) the middle of the hemisphere and extending into the convolu- tions, the grey forming a layer or cortex from g- to | inch in depth on the outer surface of the convolutions. The white matter consists of medullated nerve fibres arranged in bundles and supported by neuroglia. They vary in size in different parts, but are mostly smaller than those of the cord and medulla. The different systems have already been described. The grey matter on the convoluted surface is arranged in a continuous layer, but divided into strata by lighter lines. A 314 Human Physiology section of a convolution shows to the naked eye the appearance illustrated in fig. 198. There is (i) a thin coating of white matter most conspicuous on the convolutions within the great median fissure ; (2) a layer of grey or reddish-grey matter ; (3) a thin whitish layer ; (4) a yellowish-grey stratum sometimes showing a thin whitish line ; (5) the central white matter of the convolution. Seen under the microscope, a section of the cortex shows five' layers, (i) a superficial layer showing a few small ganglion cells with abundance of neuroglia ; (2) a layer of small pyramidal cells ; (3) a thick layer of large pyramidal cells, each with a process passing towards the surface from the pointed apex, processes passing off laterally, and a process from the centre of the base which becomes con- tinuous with the axis- cylinder of a nerve fibre. These pyramidal nerve cells increase in size to- wards the deeper part, and bundles of nerve fibres are seen passing downward from this layer into the white matter ; (4) a layer of granules and small irregular nerve cells ; (5) a layer of fusiform cells arranged for the most part parallel to the surface (fig. 199). In different regions the various layers bear different relations to each other. Above the fissure of Rolando, in the motor area, the large pyramidal cells are well developed ; in the occipital region the granular layer is well marked and the large cells few ; while the fusiform cells are more abundant in the Sylvian fissure than elsewhere. The axis-cylinder processes of the pyramidal cells pass into the medullary centre to form either association fibres connecting -other parts of the cortex of the same hemisphere, or commissural fibres through the corpus callosum to the opposite hemisphere, or projection fibres either to the corpus striatum and optic thalamus, or by way of the internal capsule to the crura, pons, medulla, and spinal cord. Fig. 198. — Sections of Cerebral Convolutions. (After Baillarger.) The parts are nearly of the natural size, i, shows the five layers ordinarilyseen in the cerebral cor- tex when carefully examined with the naked eye ; 2, the appearance of a section of a convolution from the neighbourhood of the calcarine fissure. The Brain 31S 202. Fnnctions of the Cerebrnm. — The cerebral hemispheres, especially the grey matter of the cortex, are the seat of consciousness, memory, in- telligence, and volition. Various facts may be cited in support of this statement. (l) We find that those races of men that are the most intelli- gent have the heaviest cerebrum and the most fully developed convolu- tions. (2) Among vertebrate animals we find that the degree of intelligence and the higher mental faculties increase in the same general proportion as the size of the cerebrum relative to other parts of the brain increase. (3) Imperfect development of the cerebrum in man is accompanied by im- becility and idiocy. (4) A severe injury to the cerebral hemispheres, as in concussion of the brain, suddenly deprives a man of his mental faculties. (5) Removal of the cerebral hemispheres in one of the lower animals deprives it of all intelligent spontaneous action. Thus, ^frog which has lost its cerebral hemispheres, though capable of performing many complex movements when the right sensory stimulus is applied, has lost all power of spontaneous movement. It can sit up in its natural attitude, breathing quickly, but if undisturbed will remain motionless for an indefinite time. If placed on a board and the board" be lifted, it will crawl up to a position of equilibrium ; if pinched, it will jump away, avoiding any obstacle in its path ; if placed in water it will swim until an object is put before it to rest on, as the C9ntact of the. fluid sets up the appropriate reflex actions. But it manifests no hunger, makes no effort to obtain food, and shows no sign of fear. It is a, mere machine performing certain purely reflex acts, that may be. foretold, on suitable external stimulation. But the com- plete animal acts very differently, and initiates movements apart from sensory stimuli, going through complex acts spontaneously. Its reactions to outward stimuli vary. ' Led by the feeling of hunger, too, he goes in search of insects, fish, or smaller frogs, and varies his procedure with each species of victim. The physiologist cannot by manipulating him elicit croaking, crawling up a board, swimming, or stopping, at will. His conduct has become incalculable. We can no longer foretell it exactly.' ' A pigeon without cerebral hemisphere behaves much in the same way as a frog. Left to itself, it remains still, though it can be made to fly if properly stimulated. It will starve to death on a heap of corn, though it begins to eat on holding its beak in it. "Its emotions and intelligent movements no longer exist. ' James, ' Text Book of Psychology.' Fig. 199. — Transverse Section through the Cortex Cerebri. Five-laminated or Motor Type. X so diam. (Meynert.) 3i6 Human Physiology In higher animals fewer observations have been made, as removal of the hemisphere often produces a fatal shock. Goltz has described the behaviour of the dog after loss of the cerebral hemispheres. After recovering from the surgical operation, the animal walked and moved about in normal fashion, though often • wandering restlessly. It slept at night and a loud sound awoke it. A sudden light caused it to close its eyes, and an injury to one foot led it to limp about on three legs, so that co-ordination of both usual and unusual movements was possible. At first it had to be fed, but after some months it would help itself on being started. But it was absolutely wanting in the higher intellectual faculties. No heed was paid to the barking of other dogs or to the caressing ofits master. It had no memory, and never seemed to learn that it was being taken out of the case to be fed. Thus the animal was capable of com- phcated movement, could take nourishment, and had sensation of taste, hearing, and sight. Otherwise it was in a complete state of imbecihty. While the lower centres, therefore, may receive afferent impulses and by an unconscious reflex act discharge efferent impulses to muscles that carry out many complex acts, the complexity and importance to life of these acts increasing as we pass from the spinal cord to the medulla and lower parts of the brain, yet it is evident from experiment and patho- logical experience that the hemispheres or cortex of the cerebrum is the part of the nervous system in which sensory or afferent impulses become converted in some way into mental impressions that give rise to conscious perception, and that leave behind vestiges of some kind which as recol- lected ideas form the basis of intellectual activity. Further, it is evident that from the cortex alone there can issue those impulses termed voluntary or volitional. It is worthy of note that the mental operations denoted by such terms as perception, memory, imagination, and reasoning, though confined to the cerebral hemispheres, may be carried on with but one hemisphere. Cases have occurred in which one hemisphere has been destroyed by disease, and yet the mental faculties have been apparently unimpaired. Yet in such a case, we know, from what has been said about the motor and sensory paths in the brain, there must be complete paralysis of the opposite side of the body, no sensory impulses being received from that side, and the brain being unable by an effort of the will to send impulses to the muscles of that side. It has already been pointed out that there are certain unconscious reflex actions of the spinal cord and medulla that appear inborn and involved in their very structure. Such reflex actions may be termed natural or original. Now it is found that by the help of the brain new systems of reflex paths may be set up in the nervous structures and we ihay become possessed of many acquired or artificial reflex acts. Actions which in the first instance and for some time require close attention and a continuous effort of the will, become at last so ingrained in the nervous structure that a single sensation or a single impulse from the brain is sufficient to start a whole train of actions. Our voluntary and reflex acts shade into each other gradually, a series being often connected by one or both kinds. Strictly voluntE^ry acts must be guided by idea, perception and volition, but in habitual actions conscious effort is not required beyond giving an impulse that starts a series. A habit is a reflex dis- charge from some nervous centre, the most complex being, as Prof. James says, ' nothing but concatenated discharges in the nerve centres, The Brain 317 due to the presence there of systems of reflex paths, so organised as to wake each other up successively — the impression produced by one muscular contraction serving as a stimulus to provoke the next, until a final impression inhibits and closes the whole chain. ' And Prof. Huxley says : — ' The possibility of all education is based upon the evidence of the power which the nervous system possesses of organising conscious actions into more or less unconscious, or reflex, operations. ' 203. The Fibres of the Brain.— It has already been noted that there are three kinds of fibres in the medullary centre of the brain : (a) transverse or commissural fibres connecting the two hemispheres together, viz. the fibres of the corpus callosum starting in the cortex, and connecting chiefly the frontal and occipital lobes, and the fibres of the anterior commissure con- necting chiefly the two temporal lobes ; {F) association fibres connecting near or distant parts of the same hemisphere, and {c) peduncular, longitudinal, or projection fibres forming the efferent or afferent channels between the cortex of the cerebrum and the lower parts of the brain and spinal cord. These pro- jection fibres are continuous in part with those of the lower or ventral part of the crus cerebri termed the crusta, or pes, and in part with those of the dorsal part termed the tegmentum, ' the latter probably indirectly through the corpus striatum and optic thalamus:' These fibres are in great measure direct pro- longations of the axis-cylinder processes of cells in the cerebral cortex, as is proved by the descending degeneration that occurs when injury of a particular area of the cerebral cortex takes place (fig. 19s). The trophic centres of the fibres of the cells must therefore be in the grey matter of the cortex. Of the pro- jection fibres passing through the pes or crusta of the crura cerebri, the bundle forming the pyramidal tract is best known. From the parietal region of the cortex around the fissure of Rolando, and known as the motor area, fibres pass downwards through the corona radiata into the internal capsule between the nucleus lenticularis on the outside, and the nucleus caudatus and the optic thalamus internally. From the knee and hind limb of the internal capsule the fibres pass into the crusta of the crus cerebri, thence into the pons, where they are split up by the interlacing fibres of the pons ; from the lower border of the pons they pass into the medulla to form its anterior pyramids. 3i8 Human Physiology At the lower part of the medulla most of the fibres cross to the lateral column of the opposite side of the spinal cord to form the 'crossed pyramidal tract.' A few fibres from the medulla do not cross, but are continued on the same side down the cord to form the ' direct pyramidal tract ' ; but, according to many observers, these cross over gradually in the cord through its anterior white commissure. At various levels of the cord the fibres make connection with the multipolar ganglionic cells of the anterior cornu of the cord, and through them with the fibres of the anterior roots, which, as we have learnt, are the nerve fibres conveying efferent impulses, especially voluntary impulses, to the muscles. The pyramidal tract thus making connection with the spinal nerves will evidently gradually diminish in size as it passes down the cord. It should also be noted that, in passing through the lower parts of the brain, the pyramidal fibres of the internal capsule give off fibres to the motor cranial nerves that cross the middle line to the respective nuclei of these nerves, as shown in the diagram (fig. 200), where the course of the fibres to the seventh or facial nerve is indicated. Other fibres pass from the cortex through the internal capsule into the crusta of the crura, but they have not been traced further down than the pons. Still other longitudinal fibres proceed from the cortex of the temporal and occipital regions, and pass round the internal capsule to the tegmentum of the crus. Fibres are also found to connect the tegmentum and the optic thalamus, and from the outer side of the thalamus fibres pass to most parts of the hemisphere. It is not possible to trace the fibres of all the columns of the cord to a termina- tion in the cortex of the cerebrum. While some of the sensory impulses cross over in the cord, others are conducted by fibres which remain on the same side. The fibres of the postero- median column of the cord end in the grey matter of the nucleus gracilis of the bulb ; those of the posterior external column end in the nucleus cuneatus. From both these nuclei fibres pass off to the opposite side above by the superior or sensory pyramidal decussation. Thus, all or nearly all the fibres connecting the spinal cord with the brain decussate in some part of their The Brain course. So also do the fibres proceed- ing firom the nuclei of origin of the cra- nial nerves. Hence, injury to or destruc- tion of one hemi- sphere involves both complete motor para- lysis and loss of sen- sibility on theopposite side of the body (par. 199). 204. Localisation of Function in the Brain. — Not only does the cerebral cor- tex of one side act as the receiver of sen- sory or afferent im- pulses, and the dis- penser of volitional efferent impulses to the opposite side of the body, but it has been found that par- ticular places in the cerebral cortex are the areas from which particular voluntary movements are set up, or in which par- ticular sensory im- pressions are per- ceived and recog- nised. The evidence of the localisation of function in the brain Fig. 20O. — Diagrammatic Vertical Section through the Brain, to show the Course of the Pyramidal Fibres, &c. . L A F, centres for leg, arm, and face on motor cortex ; I. c, internal capsule ; o. t, optic thalamus ; c. c, corpus callosum ; c. N, caudate nucleus of corpus striatum ; L. N, lenticular nucleus ; cr., cms ; p. v, pons Varolii ; py, pyramids of medulla in which motor fibres decus- sate ; o, olivary bodies of medulla ; k, restiform bodies of medulla ; L. p, fibres of lateral or crossed pyramidal tract* in the lateral column of the cord ; A.p, anterior or direct pyramidal tract in the anterior column of the cord ; vii, superficial origin of seventh or facial nerve, with fibres passing to opposite' side of cortex ; I. r, island of Reii at the bottom of s, the Sylvian fissiure ; cbl.] cerebellum. 320 Human Physiology may be thus briefly stated. Electric stimulation of the brain surface of each hemisphere leads not only to muscular move- ments of the opposite side of the body, but special parts of the cortex are associated with special muscular movements of certain limbs. Extirpation of that part of the cortex associated with certain move- ments also leads to loss of power in executing these movements. Further disease of a part, or a tumour pressing on a certain part of a surface, olten produces paralysis of special muscles. The general result of such Fig. 20I.— Human Brain ; Lateral' Aspect of Left Hemisphere. To illustrate Cortical Localisation of Function. (Waller.) experiments and inquiries has led to the term ' motor areas ' being applied to certain parts of the brain cortex. In a similar way parts have been found specially connected with the development of certain sensations, and these are known as ' sensory areas ' of the brain. Motor Areas.-— -'^y excitation of the convolutions around the fissure o{ Rolando co-ordinated movements of certain muscles or groups of muscles are produced. Hence this part of the surface is called the ' motor area.' Moreover, the degeneration method proves that from this area motor impulses normally set out (see par. 199), and observation of the variations The Brain 32i in the normal phenomena that follow disease of these parts supports the conclusions. These movements occur on the opposite sides of the body, but are often bilateral. Further special movements are associated with particular spots in this region. Thus, on the outer surface stimulation of the upper part around the fissure of Rolando is associated with movements of the leg, of the middle part with movements of the arm, of the lower parts with movements of iheface and mouth. On the inner surface of each hemisphere movements of the face, arm, trunk, and leg are represented on the marginal convolution (which is merely the median aspect of the frontal and parietal convolutions) from back to front (fig. 202). These areas represent particular movements of the joints rather than of particular muscles, though it should be noted that localisation is not strict. Fig. 202.— Human Brain ; Mesial Aspect of Right Hemisphere. To illustrate Cortical Localisation of Function. (Waller.) for areas overlap somwhat. Extirpation of any area leads to loss of volun- tary movement in the part regulated by that area, though recovery, more or less complete, of this movement may occur after some interval. In calling these particular parts of the cortex the motor areas for arm movements, leg movements, &c., it is not meant that the voluntary move- ments of these parts originate in these special centres without any kind of stimulus entering these areas. Sensations of some kind, probably common sensations from the skin, and muscular sensations from the muscles, enter these motor areas of the cortex, and then lead the will to produce movements of a particular quality or force. A particular interest attaches to the third frontal convolution on the left side, as a lesion in this part is always asso- ciated with the loss of power of speech, or motor aphasia as it is called. This ' part is the same, or close to the motor centre for the muscles of the tongue and y 322 Human Physiology mouth, and is known as the speech centre. It appears that in most persons —right-handed people— the delicate co-ordinated movements of speech are regulated from a particular part of the left hemisphere, the corresponding part of the right hemisphere remaining dormant or uneducated for this office. When this portion of the brain is the seat of injury, there is total loss of voluntary speech. There is no loss of voice, for the patient can laugh and cry, and even sing, but either he is unable to utter any words at all, or he uses wrong ones, speaking incoherently and unintelligibly. He may recog- nise his mistakes, but cannot avoid them. In this motor aphasia, too, the power of writing is often, though not always, lost, as most people silently articulate the words as they write. Sensory areas. — Another form of aphasia is termed sensory dphasia, the patient being unable to recognise either spoken words (' word-deafness') I'nhaerv. I), sun. tmrh-. ■^ulv'maf \ 1 "/■ h}-ar]i. siiH, cn.tctd.\) , inf. , nerve papilla with tactile corpuscle, t. The latter exhibits transverse fibrous markings, d^ nerve passing up to it ; /,/, sections of spirally winding nerve fibres, {a) Free nerve endings. — In all parts of the epidermis and in the cornea fine nerve fibrils are found, derived from the splitting up of the axis-cylinder of a single nerve passing up from the underlying dermis. These nerve fibrils terminate by free ends between the epithelial cells of the upper part of the Malpighian layer, the free ends being sometimes provided with small swellings (fig. 6). In some parts non-meduUated nerve fibrils are found to terminate in a small enlargement Touch, Temperature, Muscular Sensations, &c. 329 applied to oval nucleated cells of the epidermis, called ' touch- cells.' {b) Tactile corpuscles. — The tactile or touch corpuscles of Wagner are small oval bodies averaging about -^^ inch long and -^ inch broad, found in the papillae of the true skin. They are very numerous in the palm of the hand and the sole of the foot, especially in the fingers and toes, less numerous on the back of the hand, lips, and tongue, and scanty on the under surface of the arm and eyelids. In a large portion of the skin they are absent. Each touch corpuscle is covered outside by layers of connective tissue arranged in transverse layers, within which is the core showing elongated nuclei. One or more me- duUated nerve fibres pass to the lower part of the cor- puscle and wind round it two or three times, and then losing the sheath, the fibre or fibres enter the in- terior of the corpuscle, the axis-cylinders of the nerve ending in small enlarge- ments (fig. 51). (c) End-bulbs. — The end-bulbs of Krause are small oblong or rounded corpuscles from -^-^ to -t^-tt inch long, into the interior of which the axis-cylinder of a nerve fibre passes, and terminates in a coiled mass or in a bulbous extremity. There is a capsule of connective tissue with a core of granulated material, and the nerve sheath of Henle with the neurilemma appears to become continuous with the capsule. End-bulbs are only found in the conjunctiva of the eye, in the mucous membrane of the mouth, and a few other parts (fig. 52). (d) Pacinian corpuscles. — -These bodies are visible to the Fig. 207.-j-End-bulb from the Human Con- junctiva. (Longworth.) b, core, the outlines of (7, entering fibre-branch- ing, and its two divisions passing to terminate in the core at d. a, nucleated capsule its cells are not seen naked eye, being from yV to | inch in length, and ^V to inch in breadth. They do not occur in the skin proper, but 1 330 Human Physiology in the subcutaneous connective tissue of the pahn of the hand and the sole of the foot, including the fingers and toes, along the nerves near the joint, in the nerves of the mesentery, &c. Their structure, resembling much that of an end-bulb, has already been described (par. 49). Being so deep seated it is doubtful whether they can be connected with cutaneous sensations. As to the function of these various nerve endings but little is known (see par. 211). 208. Touch proper. — Tactile sensations may be distinguished into (fl) sensations of simple pressure, and {b) sensations of locality, (a) Mere contact of a body with the skin leads to a slight sensation of touch, and this sensation becomes more acute as the pressure increases up to a certain limit. The least pressure that can be felt, or the smallest difference of pressure that can be appreciated, varies in different parts of the skin, bmall weights are allowed to press on the skin of various parts, different weights being used one after another, and the sensations are noted. Measured in this way the greatest acuteness of the pressure sense for a single pressure is found on the forehead, temples, and back of the hand, which detect a pressure of "002 gram. The skin of the fingers detects a pressure of -005 to '015 gram. The greatest sensitiveness to differences of pressure, when small weights are used, is found on the skin of the forehead, lips, and cheeks, which appreciate a difference of pressure ^ of the first pressure ; then come the back of the last phalanx of the fingers, the palm of the hand, and the forearm, which distinguish differences of i to i. Small intermittent variations of pressure, as feeling the pulse, are better noted with the tips of the fingers than with the skin of the forehead. Individual sensations of pressure following each other with sufficient rapidity, as in touching the blunt teeth of a rapidly rotating wheel, fuse into one continuous sensation. {b) When an object touches our skin we not only experience a pressure sensation of greater or less intensity according to the amount of pressure on the part pressed upon, but we are aware of the part that has been touched. This has been called the sense of space or locality. The power of localisation is much finer in some parts of the body than in others, and the parts of the skin most sensitive to the power of discriminating the locality are not the same as those most sensitive to pressure. This power of localisation probably depends on the number of sensory nerves in the part affected ; for the fewer the fibres in a given area, the more Ukely it is that adjacent points will act on only one and produce but one sensation, and the greater the fibres in a given area, the more likely it is that the different points will be distinguished and the locality determined. The usual mode of testing the power of discriminating points in contact and the sense of locality is to place the two blunted points of a pair of com- passes on a part of the skin and determine the smallest distance at which the two points are felt as one impression ; or, the points being kept at the same distance and drawn over the skin, it is ascertained whether the two points give the sensation of approaching or receding. The results obtained in different parts of the skin are set forth below. Touch, Temperature, Muscular Sensations, &c. 331 Table of variations of the tactile sensations of space or locality in different parts, the measurement indicating the least distances at which two points can be separately distinguished : — Tip of tongue . ^in or I mm Under surface of end phalanx of third forefinger ^ in. or 2 mm Under second „ . iin or 4 mm Red part of lip . . . . iin. or 4 mm Tip of the nose . Jin. or 6 mm Middle of dorsum of tongue • gin- or 8 mm Palm of hand .... • A''^- or 10 mm Back of hand .... . I iin. or 28 mm. Dorsum of foot near toes . . liin. or 37 mm Upper or lower parts of forearm . liin. or 37 mm Back of neck near occiput . 2 in. or 50 mm Upper dorsal and mid-lumbar re gions . 2 in. .or 50 mm Middle part of forearm . . Zjin. or 62 mm Middle of thigh _f j: . ajin. or 62 mm It will be noted that the power of discriminating separate points in space is most acute in those parts of the body that carry out the widest and most rapid movements. Moistening the skin increases this sensitive- ness to separate points, but cold and a bloodless condition of the skin blunts this sensibility. Exercise improves it to some extent, though Mr. F. Galton says that the alleged superiority of blind persons in sensitiveness of touch is not great, as ' the guidance of the blind mainly depends on the multitude of collateral indications to which they give heed, and not in their superiority in any one of them. ' The power to localise our sensa- tions with reference to the surface of the body, and to indicate the position of the touching object, enables the brain to construct a tactile field on the surface of the skin to some part of which the body touching us is referred. 209. Setsations of Temperature. — Temperature sensations as felt by the skin and certain partsof the mucous membrane, as the mouth, are of two kinds, (a) sensations of heat, and (b) sensations of cold. These are quite unlike each other as well as unlike those of pressure. Minute areas are found on the skin in which sensations of heat are felt more acutely than in adjoining parts, and such areas are termed ' hot spots ; ' other areas called ' cold spots ' are particularly sensitive to cold. These spots seldom coincide, nor do they correspond with the points most sensitive to pres- FlG. 208.— Cutaneous ' cold ' spots (verti- cal shading)and 'hot' spots (horizontal shading) ; anterior surface of the thigh. (Goldscheider.) 332 Human Physiology sure ; cold spots are more abundant than hot spots, ^nd the spots are generally arranged in lines often somewhat curved, though scattered points between which other sensations may be produced are frequent. A mode of finding these spots is to use a pointed pencil of copper. On dipping it into hot water and touching parts of the skin, points may be found very sensi- tive to heat, other points not giving this sensation. With the pencil made cold by ice, spots may be found sensitive to cold but not to heat. Such experiments appear to indicate that the nerve fibres from these two kinds of spots are specifically different. With regard to the two kinds of temperature sensations and the general cutaneous surface, it should be noted that the sensations of heat and cold can only be felt through the nerve terminals in the skin. Direct stimula- tion of the nerves, as when the epidermis is removed, produces only a sensation of pain. So, also, irritation of the trunk of the ulnar nerve by dipping the elbow into very hot water or a freezing mixture will not only affect the skin of that part with a sensation of heat or cold, but the stimu- lated trunk of the underlying fibres of ulnar nerve will cause a sensation of pain which, in accordance with a general law, is referred to the peripheral terminations of the fibres in the arm and hand. With regard to variations of temperature it is found that — (a) Bodies of the same temperature as the part of the skin to which they are applied give rise to no thermal sensations. {¥) The parts of the body having the sense of temperature most acute are, in order, the tip of the tongue, the eyelids, the cheeks, the lips, and the hands, (c) Small differences of temperature— about |° C. — are readily appre- ciated by the sensitive parts when the temperature lies near that of the body, (rf) Though the power of the skin to recognise changes of tempera- ture is very great, yet our power of estimating absolute tempera- ture by skin sensation is small. Our own feeling of warmth depends on the state of the cutaneous blood-vessels, full vessels leading us to feel hot and empty vessels to feel cold. Hence a body of the same temperature gives a different sensation according as the skin is full or empty of the warm blood. («) Illusions in this sense are common, a cold weight feeling heavier than J, warm one, a good conductor like metal feeling colder than a piece of wood of the same temperature, &c. 2IO. Pain, Common Sensation. — What is called ' common' or ' general sensibility ' appears to arise from a number of obscure sensory impulses proceeding from the skin and other parts of the body, and these sensations inform us in a vague manner as to our general condition. If these impulses become intense, we have the sensation of pain, so that painful sensations may be regarded as resulting from the excessive stimulation of the nerves of common sensation, though every violent stimula- tion of any sensory nerve appears to provoke pain. As every kind of over-stimulation, mechanical, thermal, chemical, or Touch, Temperature, Muscular Sensations, &c. 333 electrical, may excite pain on the surface of the body, pain may be placed among cutaneous sensations, though it may also occur in almost all other organs. A slight inflammation makes every organ keenly sensitive to pain. Pain may be caused by stimulating a sensory nerve at any part of its course, but the sensation is felt at the nerve termination in accordance with the law of the peripheral reference of sensations. Pains vary in quality and intensity according to the nature and strength of the stimulus and to the excitability of the nerves affected. In violent pain the sensation often appears to spread and to render localisation difficult. Sensations of common sensibility and pain are distinct from other sensations. One great difference between common sensation, which informs us of the general condition of the parts of the body, and of which hunger and thirst are but examples informing us of the condition of the stomach and palate respectively, and the sensations of touch and temperature, appears to be that the latter have special nerve endings. 211. runctions of Cutaneous Nerves. — Having learnt that through the skin we experience three different kinds of sensation, those of touch, of temperature, and of common sensibility, rising to pain in certain cases, we may inquire whether there are special nerves for each of these. That there are functionally different kinds of nerve fibres in the sensory nerve trunks of the skin is shown from the following considerations : — 1. In some cases of nervous disease sensibility to touch in certain parts of the skin has been lost, while the power of distinguishing temperature has remained, and vice versd. 2. In other cases common sensibility and pain have been lost over certain areas while the sense of touch has remained ; and, in general, one or other class of cutaneous sensations may be lost while the others remain. 3. The conducting fibres of these different kinds of sensation run, to a certain extent, in different paths in the cord, and have different central endings. 4. The different modes in which the power of appreciating tactile differences, temperature differences, and sensation of pain is distributed on the surface of the skin, indicate that different nerve fibres perform these, distinct offices. 5. That the ' hot spots ' of the skin do not coincide in position with the ' cold spots ' shows that there are points of the skin having nerve fibres sensitive to heat but insensitive to cold, and vice versd. A leg ' sent to sleep ' by pressure on its sciatic nerve is found at a certain stage to be more sensitive to cold than to heat, while in some nervous diseases a part is found more sensitive to heat than to cold. These facts point to distinct nerve fibres for these different temperature sensations. It appears, therefore, that the skin contains nerve fibres of four different 334 Human Physiology kinds, or fibres performing four different functions— pressure, heat, cold, common sensibility or pain. Whether each of these sets of fibres have distinct terminal organs is another question. For the general sensations conveyed by the fibres of common sensibility or pain no end-organ for the nerve appears to be needed, for we know that pain can be produced by^ pinching, heating, &c. the open surface of a wound or the cutaneous nerves themselves. But such stimulation gives no sensation of pressure or heat, only pain ; and this suggests that there must be some special mode of ending for the nerve fibres carrying sensations of pressure, heat, and cold. What has been said above about the different parts of the skin in which these sensations are most acutely developed, and about the loss of one sensation while the others remained, indicates that there are three different sorts of terminal organs for these sensations. Moreover, various experi- ments and considerations show that the special sensations can only be developed by special terminal organs, that stimulation of the terminal organs by their appropriate stimuli (or of the special nerve centres them- selves) is needed to arouse the specific sensation, stimulation of the nerve fibres not effecting this. The various modes of nerve termination in the skin are described in par. 207. As to the part taken by the various end-organs but little is known. The more complex end-organs in which the extremities of the sensory nerves are buried in soft material surrounded by envelopes of con- nective tissue seem adapted for communicating to nerve fibres slight varia- tions of external pressure and giving to them a rhythmic character. But nothing definite can be said about the part played by end-bulbs and Pacinian bodies. The touch corpuscles being numerous in the papillse of those parts where the tactile sense is most acute, and few and scattered on the legs and trunk where this sense is dull, would appear to be the end- organs specially helpful and concerned in touch sensations, but their proved absence from certain parts where sensations of pressure can be appreciated shows that they cannot be essential nor contain the only nerves taking part in touch sensations. Probably the fine nerve fibrils terminating among the cells of the Malpighian layer of the epidermis are concerned in the sensation of pressure, and it appears certain that some of these fibrils or the cells connected with them are the instruments by which the altered condition of the skin becomes sensible to temperature sensations, as it is found that heat or cold applied to the nerve fibres underlying the skin only gives rise to a sensation of pain. 212. The Muscular Sense. — A special sense called the muscular sense, with afferent fibres proceeding from the muscles to nerve centres, is thought to exist, and to enable us to judge of the activity and degree of contraction of the muscles. When we examine our own consciousness, we find that we are aware of the position of any part of our body, even with the eyes closed, and during movements of the limbs we estimate the character and force of the muscular movements. A little con- sideration teaches us that the afferent sensory impulses from a moving limb, or in lifting a weight, may be of various kinds — Touch, Temperature, Muscular Sensations, &c. 335 impulses from the stretched or relaxed skin, sensations of pres- sure when raising a weight or moving a resisting object, as well as impulses from the contracting muscles and from the joints brought into action. Muscular sensations are, therefore, closely conjoined with cutaneous and other sensations, so much so that some have maintained that there is no muscular sensation inde- pendent of the action upon the skin. But this contention fails when we learn that cutaneous sensations may be lost or greatly impaired while the power of co-ordinated muscular movement remains, and that a certain disease of the spinal cord may lead to 'locomotor ataxia,' or failure of power to use the muscles with the proper degree of force, though the tactile, temperature and pain sensations may be unimpaired. Besides, a muscle is not only supplied with fibres that end in 'motorial end- plates,' but with afferent fibres, some of which are described as ending in fine fibrils among the muscular fibres, and these may serve for the afferent impulses of the muscular sense. At times, also, we have a sense of fatigue that seems produced by the condition of the muscles. Besides impulses derived from the muscle itself, it has been thought that the tendon and ligaments may supply impulses that enter into the ' muscular sense.' Tendons are known to have peculiar sensory nerve endings that resemble the end-plates of muscles (par. 34). The muscular sense contributes largely to our knowledge respecting the external world. Though the muscular feelings are rather difficult to localise, they are delicate, and enable us to discriminate slight differences in the range and force of movements. Muscular movements are an important factor in acquiring a knowledge of the space relations between things generally, though how the muscles help in these space-percep- tions, whether by their own sensations, or by awakening sensa- tions of motion in the skin, retina, and articular surfaces, is still undecided. It is in combination with other senses that this sense plays its most important part, for in almost all sensations muscular sensations form an element. In taste, the movement of the tongue is of importance ; in smell, the air carries the particles into the nose ; in listening, the muscles of the tympanum contract, or the head is moved in the direction 336 Human Physiology of the sound. But movement is of greatest importance to the senses of touch and sight. The delicacy of the sense of touch stands in a definite relation to the mobility of the different parts of the body, those parts being most mobile that are most delicate of touch. Combined with tactile sensations, muscular sensations give rise to composite sensations that enable us to estimate small differences of weight and small amounts of resistance, and also render possible the knowledge and relations of the parts of surfaces and solids that enter into the idea of form. Combined with visual sensations (for every position of the eye is accompanied by muscular sensations from the muscle of accommodation and the muscles that move it) they produce changes of the visual field that aid in building up our complex ideas of the objects of the outer world. 213. The Organ of Taste.— The sense of taste is located chiefly on the upper surface of the tongue, though it is also found in part of the soft palate and its arches, which receive branches of the glosso-pharyngeal nerve — the principal gus- tatory nerve. The tongue is a muscular organ covered with mucous membrane. Its base or root is connected behind with the hyoid bone, with the epiglottis, and with the fauces or part of the mouth leading into the pharynx. Its under surface is connected with muscles, but the tip, sides, and dorsum or back are free. The tongue plays an important part in the actions of chewing, swallowing, and articulate speech, its motor nerve being the hypoglossal. Its sensibility to touch has just been described. The convex dorsum of the tongue is marked by a slight furrow or raphe along the middle line, that ends in a depression termed the foramen ciBcum. The mucous membrane of its under surface is thin and smooth, but that of the upper surface is covered with large papillae, giving it a rough appearance. The epithelium of the mucous membrane is thick with strati- fied layers of cells, and the dermis, which is highly vascular, is raised in many places to form the well-marked lingual papillae, some of which are readily seen on examination. These papillae are of three kinds : — (i) About ten or twelve circuni- Touch, Temperature, Muscular Sensations, &c. 337 vallate papihcB, -^^ to ^ij inch wide, arranged in two rows, and diverging forwards from the middle of the back part of the tongue in the form of a wide V. Each circumvallate papilla consists of a circular projection with a broad free surface, and a smaller attached end, and each is surrounded by a narrow trench or fossa, on the outside of which the mucous membrane is raised to form a wall or vallum. The substance of the papilla consists of corium or dermis, formed of dense connec- FiG. 20Q, — Section of Circumvallate Papilla, Human, The figure includes one side of the Papilla and the adjoining part of the Vallum. (Magnified 150 diameters.) (Heitzmann.) .£", epithelium ; G, taste-bud ; C, corium with injected blood-vessels ; M, gland with duct. tive tissue containing blood-vessels and nerves, and covered by stratiiied epithelium. The papillae contain smaller or secondary papillae. In the epithelium covering the sides of the papillae, small oval or flask-shaped bodies termed taste- duds or taste-bulbs are seen in a section under the microscope, iig. 209. (2) Fungiform papillcB scattered over the surface of the tongue, but most numerous at the sides and front. These are club-shaped, with a narrow base and of bright red colour, owing to their rich blood supply. (3) Conical or filiform 338 Human Physiology papilla scattered over the whole surface of the tongue, and forming the smallest and most numerous papilla. They are small conical eminences, and are sometimes fringed with epithelial threads or filaments (fig. 211). The tongue contains numerous small tubular glands open- ing on the surface, most of which secrete mucus, though some yield a serous secretion. 214 Taste-Buds.— Taste-buds are found in the epithelium on the lateral surfaces of all the circumvallate papillje, in the epithelium of the Fig. 210. — Section of Fungiform Papilla, Human. (Heitzmann.) E, epithelium ; C, corium ; Z, lymphoid tissue ; M^ muscular fibres of tongue. Fig. 211.— Section of Two Filiform Papillffi, Human. (Heitzmann.) (Letters as in previous figure.) surrounding vallum, in many fungiform papillae, here and there in the general mucous membrane of the tongue, and on the under surface of the soft palate and epiglottis. They are oval clusters of epithelial cells lying in the epithelium, and set vertically to the surface, and having their broad base resting on the dermis portion of the mucous membrane and their neck opening at a pore on the surface. Each bulb or bud is composed of two kinds of cells, gustatory cells, and supporting or sustentacular cells. The gustatory cells are small spindle-shaped cells with a central nucleus, having an outer process passing from one end to terminate as a fine hair that projects through the gustatory pore, and an inner process, which is believed to be continuous with a nerve fibril. The fibril, in fact, may be considered to take its origin in the gustatory cell, and a similar arrange- Touch, Temperature, Muscular Sensations, &c. 339 ment will be noticed in the organ of smell. The sustentacular cells are long and flattened, with tapering ends. They are found between the gustatory cells, and also form a sort of covering for the taste-bud. That the taste-buds by means of the gustatory cells are end-organs of taste appears evident when we learn that they are connected with fibres of the glosso-pharyngeal nerve, that the sense of taste is chiefly found where they are most abundant, and that their cells resemble those of other sensory epithelium. But taste sensations are also distinctly felt near the tip of the tongue where no gloEso-pharyngeal fibres have been found, but where fibres from a branch of the fifth cranial nerve are distributed. The lingual branch of the fifth must therefore be considered a gustatory nerve also. Nferve filaments from the chorda tympani also seem connected with the sense of taste, as de- struction of this nerve within the tympanum has been followed by loss of taste on the same side of the tongue. But the con- nections of the nerve of tasle are but imperfectly understood. It would seem that specific tastes have spe- cific nerves, for different parts of the tongue are more sensitive to certain tastes than others ; the back to bitter, the tip to sweet and salt, the sides to acid, the middle to hardly any. Further, weak electric currents applied to the tongue awaken sensations of different kinds in diffe- rent parts, and cocain applied to the tongue in increasing doses is said to abolish sensations of all kinds in the follow- ing order : general sen- sibility and pain, bitter taste, sweet taste, salt taste, acid taste, tactile sensations. 215. Sensations of Taste. — In order that the end-organs of taste may be stimulated, and give rise to a sensation of taste in the brain, sapid sub- stances must be in solution. Such substances are most active at about the body temperature, and when dissolved in fluid or by the saliva, the sensa- tion arises in about \oi a. second on the average. Gustatory sensations may be divided into bitter, sweet, salt and acid. There are also tastes described as metallic. The delicacy of the sense of taste is shown by the power to detect I part of sulphuric acid in l,ooo of water. Quinine, common salt, and sugar are less easily detected, but in the order given. Chewing the leaves of an Indian plant (Gymnema sylvestre) destroys the Fig. 212.— Section through the Middle of a Taste-Bud. (Ranvier.) gustatory pore ; s, gustatoryr cell ; r, sustentacular cell ; tu^ lymph cell, containing fatty_ granules ; e, superficial cells of the stratified epithelium ; «. nerve fibres. 340 Human Physiology sensibility to bitter and sweet, but leaves the power to discern acid and saline bodies. The union of taste and smell gives rise to the composite sensation termed 'Cas.Jlavour of a substance. 2 1 6. The Organ of Smell.— The olfactory organ is found in a portion of the mucous membrane lining the cavities that are situated between the base of the cranium and the roof of the mouth. The internal part consists of two chief cavities called nasal fossae, opening in front into the air by the nostrils or anterior nares, and behind into the pharynx by the Fig. 213. — Section of the Nasal Cavities, seen from behind. J, frontal bpne ; 5, perpendicular plate of the ethmoid bone ; between 4 and 4, ethmoidal cavities ; 5, middle turbinated bone; 6, inferior turbinated bone ; 7, vomer ; 8, malar or cheek bone. two posterior nares. The middle wall of each fossa or nostril is formed by a vertical partition, having a smooth surface, but the outer wall is convoluted. The roof of the cavity is formed by the cribriform plate of the ethmoid bone of the cranium. From this bone a central vertical plate passes, and is continued downwards by the vomer and by gristle to complete the par- tition between the nostrils. The outer or side wall of each chief cavity is formed in part by two scroll-like bones from the ethmoid, and in part by a third similar bone attached to the upper jawbone. These three turbinated bones thus pro- Touch, Temperature, Muscular Sensations, &c. 341 duce three spaces called the superior, middle, and inferior meatus, and these meatuses communicate with small cavities or sinuses in the bone. The cavities of the nose are lined by mucous membrane continuous with that lining the pharynx and Eustachian tube, and prolonged on each side through the lachrymal canal to the eye. The structure of the nasal mucous membrane (or Schneiderian membrane, as it is called from the anatomist who first explained its secretory function) differs in various parts. These parts are {a) vestibular region, or entrances to the air Fig. 214. — Section of Olfactory Mucous Membrane. (Cadiat.) a, epithelium ; ii, glands of Eowman : c, nerve bundles. passages. Its mucous membrane contains numerous seba- ceous glands and hair follicles, from which stiff hairs named vibrissas spring. {d) The respiratory region includes the lower meatus of the nose, and is lined by thick mucous mem- brane having numerous mucous glands and a stratified ciliated epithelium, (c) The upper or olfactory region is the region specially connected with the sense of smell, and is covered with a soft mucous membrane of a yellowish colour. This is found on the anterior two-thirds of the superior meatus, the middle meatus, and upper third of the septum. Fig. 214 shows 342 Human Physiology a vertical section of the mucous membrane of this region. The dermis portion of the olfactory mucous membrane is very thick, contains numerous blood-vessels and nerve fibres, and a large number of peculiar tubular glands, opening on the surface between the epithelial cells. The epithelium of the olfactory region contains cells of two kinds : — (i) Long cylindrical epithelium cells •with broad nucleated portions coming to the surface, and forked processes stretching to the corium or dermis. These are supporting or sustentacular cells. (2) Long spindle-shaped cells with a nucleated central part, from which there passes to the surface a slender filament bearing a free cilium. Fig. 215. — Cells and Terminal Nerve Fibres of the Olfactory Region. (M. Schultze.) Highly magnified. I, from the frog ; 2, from man ; a, epithelial cell, extending deeply into a ramified process ; h, olfactory cells ; c, their peri- pheral rods ; e^ their extremi- ties, seen in i to be prolonged into fine hairs ; d, their central filaments. Tl ... i''.- Nerves of the outer wall of Nasal Fossx. I. Nclwuik of the branches of the olfactory nerve. while another filament passes down to the corium, where it is lost among the nerve fibrils, with one of which it becomes connected. These are the olfactory or rod cells. Round basal cells are also found among the lower parts of the other cells. The nerve fibrils which are non-medullated at the base of the Touch, Temperature, Muscular Sensations, &c. 343 epithelium of the upper third, or olfactory region of the nose, pass through the cribriform plate of the ethmoid bone to end in the olfactory bulb or first pair of cranial nerves that rest on this plate (par. 298). This is the proper nerve of smell, and its fibres may be seen forming a brush-like expansion on the upper and middle turbinated bone, as well as on the septum before they enter the mucous membrane to become connected with the olfactory cells and the real end-organs of smell. Branches of the fifth cranial nerve also proceed to all parts of the mucous membrane of the nose, but only endow it with common sensibility, for the sense of smell is confined to the parts containing fila- ments of the olfactory nerve, and the sense of smell may be lost while those of common sensation and pain remain. 217. Sensations of Smell. — Odoriferous particles carried in the inspired air into the lower nasal cavities pass by diffusion into the upper chambers, and coming into contact with the olfactory epithelium give rise to the sensation of smell. By vfhat physical or chemical process the excitation of the olfactory cells is effected is not understood. The par- ticles must be of extreme minuteness, as will be evident when we remember that air containing an odour may be filtered through cotton wool, and it will still be odorous, and a grain of musk will diffuse its perfume for years without any appreciable loss of weight. It also seems that the particles must be dissolved in the small amount of fluid secreted by the mucous membrane, for if the membrane be too dry or too moist the sensation is not excited. A cold in the head leading to excessive secretion of fluid of altered quality prevents the particles from exerting their action, and smell is abolished. The intensity of an odour varies with the number of odorous particles and the extent of the olfactory epithelium. Sniffing introduces more particles and spreads them with more force over a wider area. The delicacy of the sense is extremely ereat, as — 5 of a grain of musk ■' _ y a ' 30,000,000 ^ can be perceived. Odours are difficult to classify, except into pkasant, indifferent, and unpleasant. Irritating vapours like ammonia not only affect the sense of smell, but stimulate the sensory fibres of the fifth nerve. CHAPTER XVII THE EYE AND THE SENSE OF SIGHT 218. Appendages of the Eye. — The eyeball is situated in a bony socket termed the orbit, which is padded with fatty tissue. Certain structures connected with the eye for its pro- 344 Human Physiology tection are spoken of as appendages. Of these the eyelids and the lachrymal apparatus are the chief. The eyelids con- sist of dense fibrous tissue {tarsus), covered externally by ordinary skin, and internally by a mucous membrane termed the conjunctiva. Beneath the skin are the muscular fibres of the orbicularis muscle for closing the lids, and in the upper eyelid there is in addition the levator palpebrce for elevating this lid. Beneath the conjunctival membrane near the inner surface of each lid, and embedded in the fibrous tissue, are some parallel rows of a variety of sebaceous glands known as Miibomian glands. On turning up the eyelids they may be seen through the conjunctiva like tiny strings of pearls, and from their minute openings at the edge of the lids an oily secretion passes. Along the free edges of the lids curved hairs called eyelashes grow from large hair follicles, to which other sebaceous glands and modified sweat glands are attached. Continuous with the skin at the edge of the eyelids and lining their inner surface is the delicate mucous membrane termed the conjunctiva. From the lids the conjunctiva is reflected over the globe of the eye, becoming adherent to the sclerotic coat, but its epithelial portion only passing over the cornea. The conjunctiva of the lids is thus thicker than the other portions, very vascular, freely supplied with nerve filaments, and has a number of mucous glands where reflection begins. But the chief liquid for keeping the surface of the eye moist is supplied by the secretion of the lachrymal gland. This gland is of an oval shape, about the size of a small almond, and is situated at the upper and outer part of the bony orbit, with its under surface resting on the eyeball. The gland is a compound racemose gland, and consists of several lobules, the acini of which are lined by cylindrical granular epithelium, the structure being similar to that of a serous salivary gland (par. 113). Its watery secretion issues by several small ducts that open on the inner surface of the upper lid, spreads over the eyeball, where its overflow is usually prevented by the oily Meibomian secretion on the edge of the lids, and then collects at the inner angle of the eye. The fluid passes off at two small openings, the puncta lachrymalia, into small canals that unite to form a sac. The Eye and the Sense of Sight 345 The sac opens below into the nasal duct which runs in a groove of the superior maxillary bone, and terminates in the lower meatus of the nose. The lachrymal secretion is under the control of a centre in the nervous system. Various sensory impulses, as pungent smells or irritating vapours, may produce reflex stimulation of this centre, and lead to such copious secretion that the liquid overflows the lower lid in the form of tears. Certain strong emotions often act in a similar way. 219. General Structure of the Eyeball. — The globe-like eyeball consists of seg- ments of two spheres of diflferent sizes. The front portion, forming about one- sixth of the eyeball, is a seg- ment of a small sphere, and the pos- terior portion, form ing about five -sixths of the ball, is a seg- ment of a larger sphere. It is com- posed of three in- vesting coats or tunics, within which lie fluids and solid bodies which, from their action on rays of light, are called the refracting media or humours. The three investing layers or tunics are (i) the outer tunic, consisting of sclerotic and cornea ; (2) the middle tunic, consisting of choroid, iris, and ciliary processes ; (3) the inner tunic or retina, spread out on the hinder portion of the choroid. The refracting media or humours are from before backwards : aqueous humour, crystalline lens, and vitreous humour. The iris and crystalline lens serve to divide the in- terior of the eye into two chief chambers, a small anterior Fig. 217.— The Lachrymal Apparatus. Right Side. 346 Human Physiology chamber containing the aqueous humour, and a large posterior chamber containing the vitreous humour. Fig. 2i8. — ^View of the Lower Half of the Right Adult Human Eye, divided horizontally through the middle. Magnified four times. (Allen Thomson.) i cornea ; i', conjunctiva ; 2, sclerotic ; 2', dural sheath of the optic nerve passing into the sclerotic ; 3, 3', choroid ; 4, 4', ciliary muscle ; 5, ciliary process ; 6, placed in the posterior division of the aqueous chamber, in front of the suspensory ligament of the lens ; 7, 7', the iris ; 8, central artery of the retina ; 8', colliculus of the optic nerve ; 8^', loveacentralis ; 9, era serrata ;. 10, so-called canal of Petit ; 11, aqueous chamber; 12, lens ; 13, vitreous humour ; a, a, a, axis of the eye ; 3, 3, h, b, equator. It will be observed that from the pupil being placed nearer the inner side of the axis of the eyeball, a, a, does not pass exactly through the centre of the pupil ; this line falls also a little to the inner side of the fovea centralis. The following letters indicate the centres of the curvatures of the different surfaces, assuming them to be nearly spherical, viz. : t «, of the anterior surface of the cornea ; c^, posterior surface ; / a, anterior surface of the lens; Ip, posterior surface; sc/, posterior surface of the sclerotic ; r a, anterior surface of the retina. The Eye and the Sense of Sight 347 . 220. The Sclerotic and Cornea. — The sclerotic coat is the strong, dense, fibrous membrane forming the posterior five- sixths of the external coat of the eye. Its external surface is white (white of the eye), and receives the insertion of the recti and obliqui muscles ; its inner surface is of a brown colour, and connected by fine cellular tissue to the outer surface of the second coat. The sclerotic is composed of white fibrous tissue, with some fine elastic fibres and numerous connective-tissue corpuscles. Behind, it is pierced by the optic nerve a little to the inner or nasal side, the fibrous sheath of the nerve becom- ing continuous with the sclerotic. Its hinder part around the Fig. 219. — Nerve Fibrils in Cornea and Conjunctival Layer. optic nerve is also pierced by small arteries and nerves — ciliary arteries and nerves — distributed to the sclerotic, choroid, and iris. A small artery for the retina passes in through the middle of the optic nerve, its branches being distributed in the inner layers of the retina only. The cornea is the transparent circular membrane forming the fore part of the outer tunic, and continuous with the sclerotic, which overlaps its margin as the rim of a watch-case overlaps the watch-glass. It projects forward beyond the curvature of the sclerotic, and in old age becomes flattened. Though only about ^ inch in thickness, it is found to consist of five layers : — 34^ Human Physiology (1) A stratified layer of epithelial cells or conjunctival layer, continuous with the epithelium of the conjunctiva lining the eyelids. (2) An anterior elastic lamina immediately underneath the epithelial cells of the conjunctiva. (3) The cornea proper, consisting of about sixty thin plates or lamellae of fibrous connective tissue continuous with that of the sclerotic, and con- taining spaces in which are branched cells, the corneal corpuscles. (4) A posterior elastic lamina, forming a thin homogeneous membrane. (5) A layer of flattened cells lining the anterior chamber of the eye. The cornea contains no blood-vessels except fine capillaries at the circumference, nutrition being effected by the passage of lymph through the branched spaces in which the corneal corpuscles lie. From the ciliary nerves medullated fibres enter the cornea at the circumference, and losing their sheaths, break up into networks or plexuses of axis-cylinders ; froin one of these networks delicate fibrils pass up into the epithelium and end towards the surface between the cells as free fibrils, in a manner similar to the free nerve endings in the epithelium of the skin. It is this nerve supply to the conjunctival epithelium that makes the eye so sensitive to particles of dust, &c. Running round the margin of the cornea in the sclerotic is a small lymphatic channel termed the canal of Schlemm. 221. The Choroid and Iris. — The choroid is the tunic betweep the sclerotic and the retina, and like them found on the posterior five-sixths of the eyeball. It consists cMefly of blood-vessels — the ciliary arteries and veins — connected together by elastic connective tissue, in which lie large stellate corpuscles containing a dark pigment. These pigment cells of the choroid serve to assist in absorbing the light entering the eye. The internal part of the choroid coat contains a dense layer of capillaries derived from the arteries of the outer part, resting on the transparent membrane of Bruch, and these serve to nourish the underlying pigment epithelium of the retina. The choroid coat is continued in front into the iris, but before union it forms a number of radiating folds, or plaits, called the ciliary processes. They form a sort of plaited frill behind the iris and round the margin of the crystalline lens. The inner retinal coat terminates in front, where the ciliary processes begin, by a rjotched edge termed the ora serrata, but the retina is represented over the ciliary processes by two layers The Eye and the Sense of Sight 349 of cells termed the pars ciliaris retina, or ciliary portion of the retina. The ciliary muscle is situated between the outer sclerotic at its junction with the cornea and the folds of the ciliary processes. It arises by a small tendon from the inner surface of the sclerotic, and its plain fibres pass partly radially backwards (meridional fibres), to be inserted into the choroid opposite the ciliary processes, and partly towards the iris in a circular course round its insertion. The ciUary muscle forms Fig. 220. — Choroid Membrane and Iris exposed by the removal of the Sclerotic and Cornea. (After Zinn.) Twice the natural size. u, part of the sclerotic thrown back ; h, ciliary muscle ; c, iris ; e, one of the ciliary nerves ; f^ one of the vasa vorticosa or choroidal veins. Fig. 221. — Ciliary Processes as seen from behind. Twice the natural size. I, posterior surface of the iris, with the sphinc- ter muscle of the pupil ; 2, anterior part of the choroid coat ; 3, ciliary processes. a ring of plain muscular fibres (seen in section in fig. 237) round the eye, between the sclerotic and ciliary processes, and its contraction draws the choroid forwards, and thus relaxes the suspensory ligament of the crystalline lens, so that the lens becomes more convex, and the eye accommodated to vision at different distances (see par. 228). The iris is the thin, circular, coloured curtain suspended in the aqueous Kumour behind the cornea and in front of the crystalline lens. It is perforated by a circular aperture, th^ 3SO Human Physiology pupil, for the admission of light. At its circumference it is connected with the choroid, and in front of this it is united to the cornea by a network of fibres, termed the pectinate D J E : Fig. 222. — Section from the Eye of a Man (aged 30), showing the relations of the Cornea, Sclerotic, and Iris, together with the Ciliary Muscle, and the Cavernous Spaces near the Angle of the Anterior Chamber. (Waldeyer.) Magnified. A, epithelium ; b, conjunctival dermis ; c, sclerotic J d, supra-choroid space and laminae ; E, opposite the ciliary muscle ; f, choroid, with ciliary process ; G, par* ciliaris retinje ; H, cornea; i, iris; k, radiating and meridional, and l, circular or annular ijundles pf the ciliary muscle ; m, bundles passing to the sclerotic; n, Hgamentum pectinatum iridis at the angle, o, of the anterior chamber ; p, line of attachment of the iris ; T, anterior homogeneous lamina of the cornea ; 2, posterior homogeneous lamina, covered with epithelial cells which are continued over the front of the iris; 3, cavern- ous spaces at the angle of the anterior chamber (spaces of Fontana) ; 4, canal of Schlemm, with epithelial lining, and with a vessel, 5, leading from it ; 6, other vessels ; 7, bundles of fibres of the sclerotic having a circular direction, cut across ; 8, larger ones in the substance of the sclerotic ; g, fine bundles cut across, at limit of cornea ; 10, point of origin of meridional bundles of ciliary muscle ; 11, blood-vessels in sclerotic and conjunctiva, cut across ; 12, section of one of the ciliary arteries. The Eye and the Sense of Sight 3SI ligament. In structure the iris consists of a fibrous stroma of connective tissue containing muscular fibres, blood-vessels, and nerves. The back of the iris is lined with two layers of dark pigment cells (the uvea), pigment cells being also scattered through its substance, and according as the pigment is more or less abundant the iris has a brown, grey, or blue colour. The pupil is black because this pigment and that of the choroid and retina absorb the light entering the eye, so that little is reflected out. In the stroma or framework of the iris is found a layer of circular unstriped muscular fibres, forming the sphincter of the iris, the contraction of which will lessen the pupil ; and some authorities find another layer of plain muscular fibres radi- ating towards the cir- cumference, the dila- tator of the iris, the contraction of which widens the pupil. 222. The Retina. — The retina is the delicate transparent membrane that forms the inner tunic of the eye, and upon it the images of external objects are received. It extends forwards nearly as far as the ciliary process, where it terminates in an indented border, the ora serrata, though beyond this it is represented to the tips of the ciliary processes by a thin layer of different structure containing no nerve fibres, the pars ciliaris retinm. The thickness of the retina diminishes from behind forwards, from y'^th to -^\-^'Ca. of an inch. In the fresh eye it is translucent and of a pale pink colour, but after death it becomes opaque. On examining the concave inner surface of the retina there may be seen, in a line with the axis of the globe, an elliptical yellow mark about ^V inch in diameter, termed the macula lutea, or yellow spot, and in the centre of this a slight white hollow termed the fovea centralis. About Fig. 223. — Segment of the Iris, seen from the Posterior Surface after Removal of the Uveal Pigment. (Iwanoff.) a, sphincter muscle ; ^, dilatator muscle of the pupil. 352 Human Physiology -i\fth of an inch to the inner side of the yellow spot, and there- fore on the nasal side of the axis, is the entrance of the optic nerve, which is marked by a pale round disc, termed the optic pore. In the centre of the optic pore is the point from which the central artery of the retina branches to its inner layers. Though so thin, microscopic examination of vertical sections of the retina shows that there are in it eight layers or strata, together with certain fibrous structures which pass through the membrane and connect the several layers together. These layers, but not the connecting fibres, are shown in fig. 224. The names of the layers from, within the eye outwards, that is, starting with the one on which the light first falls, are named as follows : — 1. Layer of nerve fibres. 5. Outer molecular layer. 2. Layer of nerve cells. 6. Outer nuclear layer. 3. Inner molecular layer. 7. Rod and cones. 4. Inner nuclear layer. 8. Pigment cells. The two series marked limiting membranes are not really membranes, but merely the boundary lines of certain long supporting fibres of the retina, termed the fibres of Miiller. These fibres begin by expanded bases that form the internal limiting membrane, arid pass through the layers of the retinal elements to the bases of the rods and cones. Each fibre as it passes through the inner molecular layer gives off processes, and here too is a nucleated enlargement that shows the original cell nature of the fibre. Other nervous-supporting or neuroglial elements occur as small cells in the various layers. The optic nerve, composed of more than 500,000 fibres, enters the eyeball a little on the inner side through perforations in the sclerotic and choroid. Its sheaths blend with the sclerotic and choroid, and its fibres losing their medulla pass as naked axis-cylinders to the inner surface of the retina, where they radiate from the optic pore to form the ' layer of nerve fibres ' of the retina. They extend as far forward as the era serrata, some of them bending backwards to end in the inner nuclear layer, but they are absent from the yellow spot. Outside the network of fibres, over the greater part of the retina, is a single layer of ganglionic nerve cells, one process of which is continuous with a nerve fibre. The cells are more numerous, and in several layers at the yellow spot around the fovea. The inner molecular layer consists of a ■ granular substance in which processes of the ganglionic corpuscles lose themselves. The inner nuclear layer is mainly composed of bipolar nucleated nerve cells, with processes extending into the inner molecular layer in one direction, and through the outer molecular layer as far as the external limiting membrane in the other. The thin outer molecular layer consists chiefly of the inner granules of the rod and cone fibres. Up to this layer the retina may be said to consist of nervous elements ; beyond it is composed of modified epithelium cells. . Layers 6 and 7, termed the outer nuclear layer and the layer of rods and cones, or Jacob's membrane, are properly one layer,' the elements being continuous through the two. It is the sensory or nerve epithelium layer ot The Eye and the Sense of Sight 353 the retina, and consists of elongated nucleated cells of two kinds. The most numerous cells are rows of rod-like structure set side by side, the rod proper being continued towards the outside in the outer nuclear layer by a fine fibre which swells out near the middle into a nucleated enlargement, Outer or choroidal surface 5. Layer of pigment cells 7. Layer of rods and cones , . . Membrana limitans externa 6. Outer nuclear layer 5. Outer molecular layer 4. Inner nuclear layer 3. Inner molecular layer 2. Layer of nerve cells Layer of nerve fibres ^^^^^^^ Meinbrana limitans interna Inner surface Fig. 224.— Diagrammatic Section of the Human Retina. (Schultze. and then passes on to terminate in the outer molecular layer (fig. 224). The rod proper consists of two parts, an outer part striated across and cleaving into discs, and an inner thicker part striated m part lengthwise The cone elements consist of a tapering part swelling into a spindle-shaped enlargement, that is prolonged as a thick fibre through the outer nuclear 354 Human Physiology layer, to terminate by expanded filaments in the thin outer molecular layer. The outer limb of the cones is much shorter than that of the rods, but like that of the rod is transversely striated, and may be made to split into discs. Though the rods are more numerous than the cones, in the centre of the yellow spot there are cones only. It is believed that the inner ramified ends of the rod and cone fibres come into contact with the fine branches of the fibres from the small bipolar cells forming the inner nuclear layer, and that the branched processes of the ganglion cells come into contact with the opposite processes of the small bipolar cells in the inner molecular layer. But no anatomical continuity between these several elements has been found, ' merely an interlacement ot ramified fibrils.' The axis-cylinder process of the ganglion cell is, how- ever, in direct connection with one of the nerve fibres of the layer of nerve fibres. The outermost layer of the retina next to the choroid, and sometimes described with the choroid, as it often comes away when the choroid is de- tached, consists of a single stratum of hexagonal epithelial cells containing black pigment. They are present in all parts of the retina, except at the entrance of the optic nerve. The outer surface of the cells is smooth and flat, but the inner part is pro- longed, on exposure to light, into fine processes that extend between the rods. The pigment granules lie in the inner part of the cell, and, after ex- posure to light, extend along the pro- longed cell processes, where, by their agency, the rhodopsin or visual purple of the retina becomes developed in the outer part of the rods. There is no pigment in the cones. Visual purple is bleached by exposure to light, and the function of the pigment cells ap- pears to be to restore the purple colouring matter after being bleached by light. Light falling on the retina causes the processes of the pigment cells to pass inwards between the rods; in the dark the processes are retracted. (The brownish -yellow pigment of the macula lutea lies in the inner layer and not in the cones, so that the central fovea is quite clear. ) Certain variations in structure in the different farts of the retina must now be noted. At the optic pore, where the optic nerve enters, fibres Fig. 225. — Diagrammatic Representation of some of the Nervous and Epithelial Elements of the Retina. (After Schwalbe.) The designation of the numbers is the same as in fig, 224. The Eye and the Sense of Sight 355 alone are present, and this part is insensitive to light, and called the 'blind spot ' (fig. 229). In the central depression, ax fovea centralis of the macula lutea {yellow spot), there is nothing below the pigmented epithe- Fig. 226. — Pigmented Epithelium of the Human Retina. (Max Schultze.) Highly magnified. a, cells seen from the outer surface with clear lines of intercellular substance between ; b^ two cells seen in profile with fine offsets extending inwards ; c, a cell still in con- nection with the outer ends of the rods. Hum but long slender cones with their oblique prolongations, the cone fibres, in the outer nuclear layer, all the other retinal elements having gradually thinned away (fig. 227). About 7,000 cones are said to exist in Fig. 227.— Vertical Saction through the Macula Lutea and Fovea Centralis ; diagram- matic. (After Max Schultze.) 1, nerve layer ; 2, ganglionic layer ; 3, inner molecujar, 4, inner nuclear, and 5, outer molecular layers ; 6, outer nuclear layer, the inner part with only cone fibres forming the so-called external fibrous layer ; 7, cones and rods. the fovea. The portion of the yellow spot around the macula lutea is marked by its greater thickness, by the numerous bipolar gangUonic cells arranged in layers, and by the large number of cones that it contains A A 2 3S6 Human Physiology compared with rods. Near the yellow spot' the retina contains one cone to four rods ; midway to its termination at the ora serrata one cone to twenty- four rods ; at the peripheral part rods only. When the retina is examined from its outer surface, the pigment cells being detached, the ends of the rods and cones present the appearance of a mosaic, the pattern varying with the part of the retina under examination. This is well shown, together with the relative number of the rods and cones, in fig. 228. It has already been noted that the layer of nerve fibres becomes gradually thinner towards the anterior part of the retina. At the ora serrata the nerve fibres, ganglion cells, and rods have disappeared, and over the ciliary process the retina proper ceases, there being only the layer of cells spoken of as the pars ciliaris retinf2. A central artery passes through the optic nerve to supply the retina with blood. On reaching the inner surface it divides into two branches, an upper and lower, and these subdividing pass outwards to be dis- tributed as capillaries in the inner four retinal layers. The outer retinal layers, including the outer molecular layer and layer of rods and cones, have no blood-vessels. The capillaries pass into veins that follow the same distribution as the arteries. The blood-vessels of the retina have no connection with those of the choroid, except near the entrance of the optic nerve (par. 226). Fig. 228. — Outer surface of the Columnar Layer of the Retina. (Kolliker.) 350 Diameters. ri, part within the macula lutea, where only cones are present ; b, part near the macula, where a single row of rods intervenes be- tween the cones ; c, from a part of the retina mid- way between the macula and the ora serrata, show- ing the preponderance of the rods. 223. The Evidence that Visual Im- pression begins in the Rods and Cones. — Visual impulses begin in the rods and cones on the outer side of the retina, after the rays of light have passed through most of the retinal layers, and the pro- cesses started in these sensory epithelial cells of the retina pass back to the layer of fibres on the inner surface of the retina and thence by the optic nerve to the brain. That it is the outer layer of rods and cones that is thus sensitive to light is proved by the following : — I. At the point of entry of the optic nerve there is abundance of nerve fibres, but light falling on these fibres produces no sensation. It is the ' blind spot.' Its existence can be shown by placing on a piece of white paper a small black cross and a large black spot about 4 inches apart. On shutting one eye and holding the paper 12 to 16 inches away, with the black spot on the outer side, it will be noted that when the cross is looked at the blot disappears, owing to The Eye and the Sense of Sight 357 its image falling on the insensitive blind spot. By using a movable spot, as a quill dipped in ink, and noting exactly the distance of the eye from the white sheet, and the amount of movement of the black spot from visibility to non-visibility, the position of the blind spot with respect to the yellow spot, and its size also, can be calculated. 2. The yellow spot is the region of most distinct vision, Fig. 229.— Section through the Coats of the Eyeball at the Point of Entrance of the Optic Nerve. (Toldt.) VCy dural sheath ; Vm^ arachnoidal sheath, and Vi^ pia-matral sheath of the optic nerve, with lymphatic spaces between them ; C?, O, funiculi of the nerve ; L,, lamina cribrosa ; A, central artery ; S, sclerotic ; Ch^ choroid ; R, retina. The .small letters refer to the various parts of the retina, b being the layer of rods and cones, and / that of nerve fibres. and the spot upon which objects are imaged when they are looked at ; but its central part consists of cones only, no fibres. 3. If a small lighted candle is moved to and fro close to one eye in a darkened room, while the eyes look steadily forward into the darkness, dark branching lines called Pur- kinje's network are seen on a dull-red ground. These are the 3S8 Human Physiology shadows of the retinal blood-vessels thrown upon the sensitive layer of the retina. Various other modes of producing these shadows are known. Now we know that the retinal vessels are distributed in the inner layers (nerve fibres and ganglionic cells) of the retina near the vitreous humour, and the shadows cast behind them must be perceived by sornething posterior to these vessels. This is a clear proof that the external layers of the retina nearest the choroid, that is, the rods and cones, are the elements in which visual impressions begin. It thus appears that the real end-organs of vision, the rods and cones, must be in some way connected functionally, if not structurally, with the nerve filaments that pass to the optic nerve, and it is evident that these rods and cones, being backwards from the light towards the sclerotic, must receive the light waves after they have passed through the internal layers of the retina, except at the fovea, where, all the other layers having thinned oif, the basal fibres of the cones them- selves are directly exposed to the light waves. It is not possible to say whether the cones or the rods are most essential and important, for although there are only cones in the fovea centralis, these are of a more rod-like character than in other parts. Moreover, cones are absent in some animals ; rods are absent in others. Nor do we know how the undulations of light become converted into nervous impulses that give rise to visual sensations. It has been thought that this may be done by the light bringing about chemical changes in the pigment. Light does produce changes in pigment, and we know that the outer limbs of the rods are tinged with a pigment termed visual purple, or rhodopsin, derived from the pigment cells of the outer layer of the retina. If an animal be killed in the dark and its head fixed in front of a window, the pattern of the window will be bleached upon the retina, the rest of the retina remaining purple. With a 4 per cent, solution of alum this pattern can be fixed, and thus serve as a photograph or ' optogram ' taken by the retina. Yet visual purple can hardly be essential to vision, as it is absent from the cones of the fovea, and entirely wanting in some animals that see well. 224. The Refractive Media of the Eye. — The vitreous immour occupies about four-fifths of the eyeball, its convex hinder surface resting upon the retina, while into its concave anterior surface the crystalline lens fits. The vitreous humour is a jelly-like body, composed of water with a little over i per cent, of proteid matter and salts, and the fluid appears to The Eye and the Sense of Sight 359 belong to the same system as the aqueous humour, communi- cation taking place through the suspensory ligaments. It contains no blood-vessels in the adult, and must therefore derive its nutrition from the surrounding vascular structures. A narrow canal passes through it, however, from back to front, terminating at the lens capsule. This canal of Stilling takes the place of an artery existing in the foetus. The surface of the vitreous humour is covered everywhere by a thin, glassy membrane, the hyaloid membrane, which separates it from the retina behind and the crystalline lens in front. At the ora serrata the hyaloid membrane divides into two layers, a delicate layer passing over the front surface of the vitreous body beneath the lens, a more fibrous layer extending over the ciliary processes, to be attached to the capsule of the front surface of the lens as a suspensory ligament, or zonule of Zinn (fig. 237). Other suspensory fibres are attached to the edge and posterior part of the lens capsule. The interstices between these fibres form the so-csWed. canal of Fetit, around the lens. The crystalline lens is a trans- parent solid body of a biconvex shape, but with the posterior surface more con- vex than the anterior. It is situated in front of the vitreous body and behind the iris and pupil, and is surrounded by a transparent elastic membrane, strongest in front, termed the lens capsule. To the lens capsule is fused in front the suspensory ligament. The proper substance of the lens within the capsule consists of a number of concentric laminae, which, after hardening, may be detached like the coats of an onion, each lamina consisting of elongated ribbon-like fibres with serrated edges. The inner laminae are closely applied, so as to form a denser core or nucleus. The fibres, which by development are but elongated cells, run from front to back, but are so arranged that no fibres reach from one pole of the lens to the other (fig. 230). The Fig. 230. — Diagram to illus- trate the Course of the Fibres in the Foetal Crystalline Lens. (Allen Thomson.) rt, anterior ; /, posterior pole. 360 Human Physiology lens contains no blood-vessels, its nutrition being effected by the blood-vessels of the choroid. The aqueous humour is the watery substance found between the cornea and the lens. The space so occupied is divided into two parts by the iris, a large anterior chamber, bounded in front by the cornea, and behind by the iris and pupil ; a small posterior chamber, formed by the triangular interval at the edge of the lens between the ciliary processes, the iris, and the suspensory ligament of the lens. This posterior chamber is in communication with the anterior chamber between the iris and the lens. Aqueous humour is water with but 1-3 per cent, of solids. It is believed to be a very dilute lymph fur- nished by the vessels of the ciliary processes, and continually passed from the anterior chamber at the angle of junction of iris and cornea into certain spaces — spaces of Fontana — that communicate with the canal of Schlemm, and thence with the venous system of that region. Both aqueous humour and vitreous humour thus come and go, exercising some nutritive action on the adjacent structures, and being of mechanical use in supporting and keeping tense the coats of the eyeball. 225. Herves of the Eye. — The optic nerves, or special nerves of sight, originate from the optic tracts in the mode already described (par. 204), each one passing into the bony orbit of the eye through the optic foramen, and finally terminating in the cup-like expansion of the retina. As each nerve passes through the foramen it receives a sheath from the dura mater, which in the orbit splits into two layers, one forming the periosteum of the orbit, and the other forming the sheath for the nerve until it pierces the sclerotic. Other nerves enter the eyeball, piercing the sclerotic around the optic nerve. These are the ciliary nerves, which are mainly branches of the first or ophthalmic division of the fifth cranial nerve. They are accompanied by branches from the sympathetic system. On passing through the sclerotic the ciliary nerves run forward between it and the choroid, and are distributed to the iris and ciliary muscle. Twigs from the first branch of the fifth nerve are also supplied to the cornea, fine fibrils passing between the epithelial cells to the conjunc- tival layer, which confer on it acute sensibility to foreign par- The Eye and the Sense of Sight 361 tides. Injury or disease of this branch of the fifth renders the surface of the eyeball insensible to particles of dust and other stimuli. Vaso-motor fibres to the blood-vessels of the choroid, iris, and retina are included in the ciliary nerves. Fig. 231.— Vessels and Nerves of the Choroid and Iris seen from above, the Sclerotic and Cornea being in part removed. (Testut.) A, optic nerve ; B, sclerotic ; b', the same in section ; c, section of cornea ; d, ciliary muscle ; e, iris ; F, anterior chamber of the eye; i, i, short posterior ciliary arteries supplying choroid ; 2, 2, long posterior ciliary arteries supplying ciliary processes and iris ; 3, 3, anterior ciliary arteries passing to front of eye with the four recti muscles ; 5, large superior external vorticose vein Irom the upper and external quarter of the choroid and ciliary processes ; 6, large superior internal vorticose vein as it leaves the choroid ; 7, some of the smaller vorticose veins (the central artery of the retina is not seen in this figure) ; 4, ciliary nerves passing to blood-vessels, ciliary muscle, and iris. The chief motor nerves of the eye are the third pair of cranial nerves, or motores oculi. Each motor ocuU ?m^-^\^^ all the muscles of the eyeball except the superior oblique and external rectus ; it also sends filaments to the elevator of the eyelid and to the iris and ciliary muscle. Besides controlling the movements of 362 Human Physiology the eyes, therefore, this nerve serves to regulate the amount of light entering the pupil, and brings about accommodation for near objects. A bright light acting through the retina and optic nerve appears to serve as a stimulus to the centre of origin of the third nerve, so that the nerve is thus reflexly excited and the pupil made to contract. 226. Blood-vessels of the Eye. — The eye is richly supplied with blood-vessels. The eyelids and glands are supplied from the palpebral and lachrymal arteries, the conjunctival branches passing also to the edge of the cornea. The tunics or coats of the eyeball are supplied by two distinct sets of ves- sels: — (a) The vessels of the sclerotic, choroid, and iris ; {b) the vessels of the retina. The first set include the short posterior ciliary arteries, which enter the first part of the sclerotic around the optic nerve, and are distributed to the choroid and ciliary processes ; the long posterior ciliary arteries which enter the choroid behind, and running forward are dis- tributed to the ciHary muscle and to the iris, at the margin of which they form a close vascular circle ; the anterior ciliary arteries, which pierce the front part of the choroid and are dis- tributed to the ciliary processes and iris. From the capillaries numerous veins arise which form a vorticose arrangement on the outer surface of the choroid, and unite for the most part into four large trunks that pass out of the choroid about midway between the cornea and the optic nerve. The retina is supplied by a central artery which enters at the middle of the optic nerve and gives off branches to supply its Fig. 232.— Inner Concave Surface of the Retina of the Right Eye. », sclerotic; b^ choroid; c, retina ;(,i, yellow spot imacula- luted) ; 2, optic disc ; 3, 4, 5, 6, branches of the retinal artery,-retinal veins coloured blue ; T, temporal side ; n, nasal side. The Eye and the Sense of Sight 363 inner layers. The capillaries unite to form a central vein. Except near the entrance of the optic nerve the two sets of blood supply to the eyeball are quite distinct. 227. The Formation of an Image on the Retina. — An eye closely re- sembles a photographic camera filled with water. There is a screen, the retina, at the back of a darkened chamber, from which little light can be reflected to blur the image which is formed by a lens with a spherical surface placed in front. A stop or diaphragm, the iris, cuts off outside rays which would also tend to mar the image. The formation of an image by the eye and by the camera is some, what roughly illustrated in figure 233. For a full account of the formation of images by lenses and other bodies, the student must consult a work on Optics. We will, however, give a short explanation of the formation of the retinal image. As long as rays of light travel through the same substance or medium, they proceed in straight lines. When rays of light pass obliquely from a me- dium of low density, as air, into a medium of higher density, as water or glass, such rays are bent or refracted to- wards the perpendicular separating the two media. Rays falling perpendicu- larly on the boundary surface of the media are not bent or refracted. Where a spherical convex surface is met by ray of light travelling from a medium of low density into a medium of greater den- sity, all the rays falling normal or per- pendicular to the surface pass without refraction through the centre of curva- ture or nodal point. Parallel rays in such a case are all brought to a meet- ing- point called the principal focus, and the distance of the principal focus from the boundary surface of the two media varies with the amount of curvature. Other rays diverging from a distant point, and not falling perpendicularly, are brought to a meeting-point or focus by refraction at some distance beyond the principal focus. U 0) s 364 Human Physiology We may now understand how a simple collecting system forms the image of an object. A simple collecting system consists of two refractive media, separated by a spherical surface. Let P Q represent an olyect placed in front of the spherical refracting surface seen in section at S S', C being the centre of curvature, and F the principal focus, or focus for paral- lel rays. The medium on the right of S S' is supposed to be the more refractive. The line O C F is called the optic axis. Consider the rays pro- ceeding from the point P. The line P C P' represents a ray meeting the bounding surface perpendicularly, and passes through the centre of curva- ture without refraction. The ray P S is parallel to O C F, and will be bent Fig. Z34. so as to pass through F. On prolonging it to meet the unrefracted ray it cuts the latter at P', where all other rays, as P S', are also brought to a focus by refraction. At P', there is then formed an image of the point P by the diverging pencil of rays from P. In a similar way the diverging pencil of rays from Q is brought to a focus, and forms an image of Q at Q'. Points intermediate between P and Q of the object send out rays which are brought to a focus at corresponding points between P' and Q'. Thus the image P' Q' of the object P Q is formed, and might be received on a suitable screen. In the eye, however, it will be said that there are several different sur- FiG. 235. faces separating different media, where refraction takes place. This is quite true, for taking the refractive power of the aqueous humour to be the same as that of the cornea, we still have three refracting surfaces — anterior sur- face of cornea, anterior surface of lens, and posterior surface of lens ; and three media — cornea with aqueous humour, the substance of the lens, and the vitreous humour. At each of these surfaces refraction will take place, most at the anterior surface of the cornea, and none at the anterior surface of the biconvex crystalline lens. But by suitable calculation this compli- cated optical apparatus can be reduced to one mean curved surface of known curvature, and one mean medium of known refractive power. There is thus constructed a simplified or reduced eye, which will act and form an image The Eye and the Sense of Sight 365 as in the simple collecting system above explained, where S S' may be con- sidered to be the one surface where refraction occurs, and P' Q' the image formed on the retina (see also fig. 235). It will now be understood how the eye, like a simple collecting system, forms an image of an external object in the field of view, and that this image, owing to the crossing of the rays within the eye, is an inverted image. The size and position of the image of any object depend on the distance of the object, and can readily be found by calculation. But it will easily be seen, without calculation, that as the object is moved the position of the image changes (this may be easily seen if the reader will draw fig, 234. and put the arrow P Q at different distances from the curved surface S S'). In other words, the focal distance, or distance from the refracting surface at which the luminous rays are collected to form an image, varies with the distance of the object from this surface, being less as this is longer and vice Dersd. Hence there must be some means of adjusting the eye for vision at different distances. A photographer can alter the distance of his sensitive screen from the refracting lens, and thus focus for distant or near objects. This cannot occur in the rigid eyeball, and the minute change of this cha- racter that would be required in the eyeball does not seem practicable. Altering the curvature of the refracting surface, however, produces a change in the distance at which luminous rays are brought to a focus so as to form a distinct image, increase of curvature increasing the refracting power, and bringing rays to a focus sooner, and decrease ot curvature having the opposite effects. (See Appendix, ' Ophthalmoscope.') 228. Accommodation. — The power by which the eye is enabled to form distinct images on the retina of both distant and near objects is called ' the power of accommodation.' The need for accommodation is evident when we consider the mode in which images of objects at various distances are formed, and when we remember that we cannot see distinctly both a near and a distant object at the same time. In the normal eye at rest it is accommodated for distant objects, the rays from which are sensibly parallel. This is proved by the •fact that such objects are seen without any effort quite clearly, so that when the mechanism for accommodating the eye is paralysed, and near objects cannot be clearly seen, the images of distant objects still remain distinct. But for distances less than seventy yards, an effort of accommodation is needed. How is this accommodation effected ? It is not by altering the position of the retina, as the globe of the eye has been proved not to alter its shape. It is not b>- altering the curva- ture of the cornea, as was once thought, for the eye can be accommodated under water, which has practically the same refractive power as the cornea. It has been definitely proved l66 Human Physiology that the anterior surface of the crystaUine lens undergoes a change of curvature when looking at near objects, the increased curvature being necessarily accompanied by increased refractive power. This increased curvature of the anterior surface of the crystalline lens in the act of accommodation is proved by observing ' Sanson's images.' If a lighted candle be held in a dark room, a little to the side of a person's eye, an observer looking at the other side of the eye will see with care three reflected images of the flame, (i) A small, erect, and clear image formed by the anterior surface of the cornea, (ii) A larger, but fainter image, also erect, formed by the anterior surface of the lens, (iii) A small, inverted, and faint image formed by the back surface of the lens. If, while the observer is watching these images (it is better to have three pairs of images produced by allowing the light to pass through two holes placed one above the other jj jjj J jj jjj in a piece of cardboard Fig. 236. interposed between the vifith reia«d,accom- With accommodation, candle and the eye), the I, from surface of cornea ; ir, from anterior surface pCrSOn expCniTientCCl OH of lens ; ■:,, from posterior surface of lens. j^^^^ f^^ ^^^ ^^^^ ^^^^ -^^ one line of vision, it will be noted that with accommodation to a near object 11 grows smaller and approaches i. This observa- tion can only be explained by the anterior surface of the lens becoming more convex and bulging forwards. Images i and iii' neither move nor alter, and therefore the cornea and posterior surface of the lens neither move nor alter their curvature. The change in the anterior surface of the crystalline lens is brought about by the action of the ciliary muscle. When the eye is in the condition of rest or relaxed accommodation, the lens is kept somewhat flattened in front by the tension of the suspensory ligament or zonule of Zinn, which passes forward all round from the ciliary processes of the choroid to its attach- ment to the margin of the lens. The ciliary muscle springs from a fixed point at the junction of the cornea and sclerotic, The Eye and the Sense of Sight 1^7 and its smooth fibres pass back to be inserted upon the ciliary and front part of the choroid, so that its contraction pulls for- ward the movable ciliary processes and choroid (but loosely attached to the sclerotic), and thus slackens the suspensory ligament. This lessens the tension of the ligament on the anterior surface of the lens, which then bulges forward and becomes more convex in virtue of its own elasticity. Various experiments show that this explanation is correct. Thus, if the point of a needle be passed through the sclerotic into the choroid of an eye, it is noticed that when the ciliary muscle is stimulated the eye-end of the needle moves back- ward, and therefore the point in the choroid moves forward. Fig. 237. — Diagram to illustrate Accommodation. Sc, sclerotic; C.P., ciliary processes; CM., ciliary muscle; Cs, canal of Schlemm Sp. l, suspensory ligament ; C L, crystalline lens. Stimulation of the ciliary nerves, branches from which pass to the ciliary muscle, has been observed to lead to forward move- ment of the choroid and bulging of the lens. Accommodation for near objects is associated with conver- gence of the eyes upon the object viewed, and with move- ments of the iris that diminish the size of the pupil. When an object is placed nearer to the eye than 4 inches (10 cm. ), it is too near for accommodation, for the rays of light are too divergent to be brought to a focus on the retina. Hence in a normal eye accommodation occurs between an infinite distance, the punctum remotum, or remote point, and 4 inches, the punctiwt proximum, or near point. As age advances the near point gets further away, owing to the power of accommodating becoming less in consequence of the lens growing less elastic and the ciliary muscle becoming weaker. Near objects are seen less distinctly, u book being held further and further from the eye. This defect in old 368 Human Physiology people is called presbyopia. This kind of long-sighted eye must be dis- tinguished from that due to the eyeball being unusually short. The luminous rays from all objects beyond 70 yards' distance are virtually parallel. In a normal eye parallel rays are focussed on the retina without any effort. In viewing nearer objects we become conscious of an effort, especially after the distance becomes less than 20 feet, due to the con- traction of the ciliary muscle in accommodating. A normal eye is said to be e>nmetropic^ or in measure. In a myopiceye distant objects are indistinctly seen, and objects nearer than 5 inches are plainly visible. Such persons are said to be ' short- sighted.' The eyeball is too long— its accommodation being normal — so that distant objects ave brought to a focus in front of the retina. In a Pr- Fig. 238.— Emmetropic Eye. Parallel rays focussed on retina. Pr- Pr Fig. 239. — Myopic Eye. Parallel rays focussed in front of retina, rt, remote point of distinct V . , from which divergent rays are focussed on the retina. Fig. 240. — Hypermetropic Eye. Parallel rays focussed behind the retina. Convergent rays focussed on retina. hypermetropic eye the eyeball is too short, near objects cannot be distinctly seen, and the rays from distant objects are brought to a focus behind the retina unless accommodation is used. Such persons are said to be ' long- sighted.' . Spectacles with scattering or diverging lenses (biconcave) are used to remedy myopia, or short-sight ; converging or convex lenses are used to remedy hypermeiropia, or long-sight. (See App. , Scheiner's Experiment. ) 229. Movements of the Iris. — The pupil or central aperture of the iris undergoes variations of size, thus serving to regulate the amount of light entering the eye as well as acting as a diaphragm to cut off marginal rays. No light enters the eye except through the pupil, the dark pigment or uvea absorbing and cutting off others. The pupil is constricted or lessened, (i) when light falls on the retina, The Eye and the Sense of Sight 369 and the brighter the light the greater the constriction, (2) when near ob- jects are viewed to cut off the widely divergent rays that could not be focussed, (3) when the eyeballs are turned inwards, as this is associated with near vision, (4) under the action of certain drugs, (5) during sleep. The pupil is dilated or widened, (l) when the light becomes less, as in passing into darkness, (2) when the eye becomes adjusted for distant objects, (3) when there is an excess of aqueous humour, dyspnoea, or violent muscular effort, {4) when a sensory nerve is strongly stimulated, {5) under the influence of atropin and some other drugs. Constriction of the pupil is brought about by the contraction of the cir- cular muscular fibres which form the sphincter muscle around the margin of the iris. The contraction of these involuntary fibres is very rapid, unlike that of involuntary muscles generally, and leads of necessity to some extension of the iris inwards with a narrowing of the pupil. Dilatation of the pupil is partly the result of the rebound which occurs when the sphincter fibres relax, and partly the result of contraction of the radiating or dilatator fibres. Some authorities do not recognise dilatator fibres, and attribute dilatation to the removal of tonic constrictor impulses by the inhibitory action of fibres from the sympathetic system. Section of the cervical sympathetic does diminish the aperture of the pupil. The constriction of the pupil when light falls on the retina is a reflex act. The afferent nerve is the optic nerve, the centre is in the brain in or near the anterior corpora quadrigemina, the efferent nerve carrying the constrictor impulses to the circular fibres of the iris is the third or oculo-motor nerve. 230. The Muscles of the Eye. — Besides the levator palpebrcB superioris, which is the muscle by which the upper hd is raised, there are six muscles of the eyeball, viz. the four straight muscles arid the two oblique muscles. The four recti or straight muscles arise behind by a continuous tendinous origin at the bottom of the orbit or bony cavity, and pass forwards, one above, one below, and one on each side of the eyeball, and are inserted by short, membranous tendons into the fore part of the sclerotic coat. The superior oblique or trochlearis muscle springs from the bottom of the orbit, and proceeding towards the front terminates in a tendon that passes through a cartilaginous ring or pulley (trochlea) attached to the frontal bones ; the tendon is then reflected backwards and downwards, to be inserted into the upper part of the sclerotic coat, midway between the cornea and the entrance of the optic nerve. The inferior oblique muscle arises at the lower and front portion of the orbit, inclines backwards and upwards, and ends in a ten- dinous expansion inserted under the external rectus at the outer and posterior part of the eyeball. B B 370 Human Physiology The eyeball is turned outwards b)' the external rectus, inwards by the z«/«r«a/ rectus, upwards\yj \k\s. combined action of the superior rectus and inferior oblique, down- wards by the combined action of inferior rectus and superior oblique. Thus, in the lateral hori- ^^ .^ ''^^^ (\ zontal movements the n ^>_^_ ' 5=- W I /■ interna] and external '^ ^~~ ~~^ V \ y V »r recti muscles are the sole agents, but the superior and inferior recti, while elevating and depressing the cornea, have both a tendency, from the line of their action being to the inner side of' the centre of motion of the eyeball, to produce in- ward direction as well as movement upwards or downwards. 'The simple action of the superior oblique muscle, when the eye is in the primary posi- tion, is to produce a movement of the cornea downwards and out- wards ; that of the inferior oblique, to direct the cor- nea upwards and out- wards, and in both with a certain amount of rota- tion. But these move- ments, caused by the oblique muscles, are precisely those which are required to neutralise the inward direction and rotatory movements pro- FlG. 241, — A, View of the Muscles of the Right Orbit, from the outside, the outer wall having been removed. (Allen Thomson.^ \. B, explanatory Sketch of the same Muscles. orbital arch;^^, lower margin of the orbit; c, anterior clinoid process ; d^ posterior part of the floor of the orbit above the spheno-maxillary fossa ; f, side of the body of the sphenoid bone below the optic foramen and sphenoidal fissure ; f^ maxillary sinus ; i, levator palpebrae supe- noris ; 2, pulley and tendon of the superior oblique muscle ; 3, tendon of the superior rectus muscle at its insertion upon the eyeball ; 4, ex- ternal rectus ; 4', in B, tendon of insertion of the same muscle, a large part of which has been removed ; 5, inferior ^ obliciue muscle crossing the eyeball below the inferior rectus ; 6, inferior rectus ; 7, in B, the internal rectus, and near it, the end of the optic nerve cut short close to the place of its entrance into the eyeball. The Eye and the Sense of Sight 37 1 duced by the superior and inferior recti, and accordingly by the combined action of the superior rectus and the inferior oblique muscles a straight upward movement is effected, while a similar effect in the downward direction results from the combined action of the inferior rectus and superior oblique muscles.' ' So much for the straight movements of the eyeball. In all oblique movements of direction it is found that two recti act in combination with one oblique muscle. Thus the eye is turned upwards and inwards by the combined action of the superior and internal recti with the inferior oblique ; down- wards and inwards by the combined action of the inferior and internal recti with the superior oblique ; upwards and outwards by the combined action of the superior and external recti in combination with the inferior oblique ; downwards and out- ivards by the combined action of the inferior and external recti with the superior oblique. The movements of the eyes are always bilateral. In the upward and downward movements both eyes are turned in the same direction ; in the lateral movements the two eyes may either be directed to the same side, one being abducted and the other adducted, or both may be adducted to bring about the convergence of the visual axes required in near vision. The superior oblique muscle receives its nerve-supply from the fourth or trochlear nerve ; the external rectus is sup- plied by the sixth or abducens nerve ; all the other muscles are under the influence of the third or common oculo-motor nerve. 231. Various Definitions and Explanations. — The optic axis is the line which passes through the common centre of curvature (nodal point) and the centre of the cornea. Prolonged backwards it falls on the retina a little to the inner side of the yellow spot. The visual axis or line of vision is the straight line which joins the centre of the yellow spot with the points on which the eye is fixed. These two lines form a small angle with each other. The visual angle is the angle under which an object is seen. It is in- cluded between straight lines drawn from the extremities of the object to the nodal point. Its size plainly depends on the size of the object and on its distance from the eye. The visual angle is equal to the retinal angle (angle A«B to angles nhva fig. 243 Euclid 1. 15), and the base of the retinal angle gives the size of the image formed on the retina. It is evident that objects of different size at different distances may form the same visual angle, • Quoin's Anatomy. Human Physiology 372 and thus form retinal images "f. ^^^frJifpy/enrtTlf sh^wT ' if Hso size.' How the real size is estimated wi P^f ^"'^j "^jn the visual angle evident that the more f t^^"' '^f^lnow the si e oMhe object, its dis- and the retinal image become If we ™°7^"^,? ^^^06 of the nodal point tance from the nodal point of the eye, and the disanc xhusifAB, fem the retina, we can -^-^^l^^::^:g^i::Tt^A.X^^^^^Z^^ol,^. fig. 244 represent a rnan six iff J^f^^^'^^if l^^en from the properties of the images a ^ on the retma of an observer is givi. ^.^^^.^^^ triangles anb and hn-& 'Ccms. -.—a b : a?z::AB : AK, that is, ^^^a»xAB_ NowffK A« in the reduced eye is about IS mm. or -6 inch. Hence ^ye get, reducing all to inches, _ -6 X 72 ^ _l 1760 X 36 1500 inch nearly. In a similar way it may be found that one of these printed letters (about Jt-th inch in height) will at a distance of one foot y,(; 2.3 form a retinal image of ^igth inch, or -08 mm. Smaller objects than these may be seen at these distances. Acuteness of vidon may be tested by ascertaining the smallest visual angle under which wo points or two fine lines are distinguished as separate. I^J"f ' <=?^f the two objects become one when the visual angle is ess than I or bo The retinal image corresponding to this is about J^ '"^1'' °l '"^^ mm., the li^tance betweel two a<^oining cones This implies that the smaUest ob- iect that could be seen at a distance of 9 inches is jlj inch m diameter, for such an object at such a distance would just form the iriinimum sensible retinal im^e. Sharpness of vision diminishes rapidly with distance from a b =f Fig. 244. the yellow spot. The macula luteals oval, its long axis being 2 mm., and the fovea centralis has a diameter of -3 mm. , , j ■ The7f«/ . _ rS i = o V.S £ O S c o i'" «j o ^ y w rt o _ ?„ J3 iS.i: aj (? .. « 6 e o c u C^ ^"t'- list's •3 Mo 1 .111 «°J| '^o « aj e oj o ™ ■< outer hair cells. Ultimately, however, the fibrils become connected with the hair cells, though it is doubtful whether they actually pass into the cells or merely invest them. It 400 Human Physiology may be regarded as settled that the hair cells, inner and outer, are the true terminal organs of the auditory nerve in which the sensory impulses that pass to the brain are originated, and that the other parts of Corti's organ are merely accessory in function, assisting or guiding in some way the nervous impulses, but not actually giving rise to them. 241. Perception of Vibrations. — Sound-waves collected by the pinna of the external ear pass along the meatus, and set in vibration the tympanic membrane, its peculiar form and structure, being somewhat loosely and unequally stretched and loaded by the tympanic bones, enabling it to take up aerial vibrations of various rates. The vibrations of the membrane are transmitted onward by the chain of ossicles swinging as a whole and acting as a lever, their movement being molar and not molecular. With diminished amplitude, but increased force, the base of the stapes communi- cates the vibrations to the whole mass of the perilymph in the vestibule, each vibration passing from the vestibule along the scala vestibuli, and down the scala tympani to end on the fenestra rotunda, this membrane moving outwards as the fenestra ovalis moves inwards, and vice versd. As the vibrations ascend the scala vestibuli they are also transmitted across the membrane of Reissner to the endolymph of the central cochlear canal and the basilar membrane, affecting in some way the auditory epithelium or hair cells of the organ of Corti, and with them the termina- tions of the cochlear nerve. The vibrations of the perilymph of the vestibule are also transmitted through the membranous labyrinth to the endolymph of the utricle, saccule, and membranous semicircular canals, to reach the epithelium of the maculee of the utricle and saccule, and that of the cristje of the ampullae, thus affecting the terminal filaments of the vestibular nerve. It may be noted that vibrations may be transmitted by the bones of the skull to the tympanic membrane, and onwards by the ossicles to the internal ear. The sound from a vibrating tuning-fork held between the teeth may pass into the labyrinth through the skull bones, and so affect the auditory nerve, when the tympanic membrane is injured. An audiphone is a plate used in this way. Confining our attention or the present to the cochlea which contains the organ of Corti, the most highly specialised portion of the auditory apparatus, and therefore the chief agent in hearing, we may inquire further into the mode in which the vibrations reach the hair cells of this organ, and the functions of its other portions. The note of a musical instrument or the human voice possesses, as we have seen (par. 234), a certain quality or timbre, due to the fact that along with its fundamental tone, determining its pitch, there are upper partial tones or harmonics, of varying intensity and number in different instru- ments. The complex sound thus produced by vibrations of more than one period travels as a complex or composite air-wave. A composite sound or complex air-wave may be analysed by raising the dampers of a piano, and allowing the musical note to resound powerfully before it, when a set of strings is brought into sympathetic vibration, namely, those strings which correspond in pitch to the fundamental tone, and to the several upper tones of the note sounded. ' Now, suppose we were able to connect every string The Ear and the Sense of Hearing 401 of a piano with a nervous fibre in such a manner that this fibre would be excited, and experience a sensation every time the string vibrated. Then every musical tone which impinged on the instrument would excite, as we know really to be the case, in the ear a series of sensations exactly corresponding to the pendular vibrations into which the original motion of the air had to he resolved. By this means-, then, the existence of -each par- tial tone would be exactly so perceived as it really is perceived by the ear.' As the ear doeS' possess this power of analysing complex sounds, and can appreciate the pitch and quality of notes, we may inquire what parts of the terminal organ perform the office of the supposed piano-strings, that is, what parts of the terminal organ connected with the ends of the microscopic nerve fibrils can be set in sympathetic vibration by the various complex waves of sound. Formerly it was thought that, the rods of Corti, which vary regularly in the length and span of their arch from the base to the apex of the cochlea, served for the analysis of sound, each pair vibrating in response to a particular simple tone, and stimulating a particular nerve fibril, or group of fibrils. But their number and variation in size and form do not seem sufficient for the purpose, and in birds, which clearly can appreciate musical notes, there are no rods of Corti in their rudimentary cochlea, though hair cells lie in contact with the basilar membrane. Hence it is now generally believed that the stretched radial fibres of the basilar mem- brane, on which the organ of Corti rests, are the vibrating threads which analyse complex sounds, the rods of Corti assisting the transmission of the vibrations of the basilar membrane to the hair cells, and possibly acting as dampers also, as the chain of ossicles serves both purposes. Abtut 24,000 fibres exist in the basilar membrane, their length varying from "075 mm. at the base to '126 mm. at the apex of the cochlea, and their tension pro- bably undergoing changes, owing to the action of what appear to be muscular cells in the outer ligament of the membrane. Here, then, seems to be an apparatus adapted for the appreciation of pitch and the analysis of complex sounds. A simple pendular vibration reaching the ear excites by sympathetic vibration the fibres of the basilar membrane tuned to its pitch, the shorter fibres near the base of the cochlea vibrating to high notes and the long fibres near the apex of the cochlea vibrating to low notes ; and the different fibres tuned to differences of pitch affect in some Way different auditory cells, which in their turn affect different nerve fibres in order to give rise in the brain to sensations of pitch. Thus in the case of a complex vibration the basilar membrane resolves it into its elements, one fibre taking up the fundamental tone, and other fibres the various harmo- nics or partial tones, the synthesis and appreciation of the sound, as having a certain qualityj being effected in the nervous cells in the auditory centre of the brain. Such is one view of the auditory mechanism, though other explanations and theories have their advocates. Some consider that the sound-wave, on passing into the endolymph of the cochlear canal, impresses the membrana tectoria, and thus sets in vibration the hairs of the hair cells ; but those who regard the basilar membrane as the organ of analysis for pitch and quality believe that the tectorial membrane acts rather as a damper to check vibrations, the auditory hairs serving to convey the damping action to the hair cells when excited in some other way. If, then, the organ of Corti can appreciate pitch and quality, it must be able to appreciate loudness, as the quality of a note depends to some extent D D 402 Human Physiology on the relative intensity of the subordinate upper tones. Moreover, as noises involve elements of pitch, and are but partially distinguishable from musical tones, it would appear that the sensory epithelium of the cochlea can also appreciate them. Having thus found in the cochlea an apparatus sufficient to account for the perception of the intensity, pitch, and quality of sounds, it would appear that the sensory epithelium in the semicircular canals connected with the vestibular division of the nerve must have other functions than the perception of sound. The semicircular canals, in fact, are believed to be the organs of the ' sense of orientation ' which enables us to deter- mine our position in space. In conjunction with sensations from the skin, muscles, and eyes, it furnishes help in guiding the complex co-ordinated movements by which the body is balanced and equilibrium maintained. Their function, therefore, is pro- bably not auditory but 'equilibrating.' Impulses set up in the vestibular nerve endings of the ampuUse of these canals (the cristae acusticse) by the varying pressure of the fluid endo- lymph iH them, give rise to sensations that enable us to become aware of the position of the body. As the planes of the three canals lie in three axes of spaces, a movement of the body or a change of the position of the head may lead to changes of pressure or movements of endolymph that affect the ampullae differently, and so give rise to different impulses in the vestibular branch of the auditory nerve fibres distributed to the ampullae. Injury to the membranous canals in birds and rabbits does not affect the hearing, but produces, especially in birds, some loss of co-ordination of movement, accompanied by movements of the head in the plane of the injured canal and movements of the eyeballs. Meniere's disease, of which the chief symptoms are giddiness and staggering, has been found associated with affections of the semicircular canals. It thus appears that the impressions proceeding from the nerve endings in these canals form an important part of the afferent impulses controlling the mechanism of equilibrium (par. 193), and that this part of the internal ear is not connected with the sense of hearing. The sensory epithelium of the maculae in the utricle and saccule has also been asserted to be connected with The Ear and the Sense of Hearing 403 the sense of movement, and to have little connection with the sense of hearing. It appears, however, certain that the cochlea supplied by the cochlear division of the eighth or auditory nerve is the part of the organ concerned in receiving auditory impressions, and that the semicircular canals supplied by the vestibular branch of the auditory nerve are not concerned in hearing but in receiving impulses set up by various movements of the body. These impulses transmitted from the canals to the cerebellum aid in giving information regarding the direction of movement and the position of the body in space. By means of the impressions made on the ampullary nerve-endings in the semicircular canals, it has been found possible to judge of the direction of motion when the body is being whirled round with the £yes bandaged and the feet away from the ground. Each pair of canals is ' sensitive to any rotation about a line at right angles to its plane or planes, the one canal being influenced by rotation in one direction, the other by rotation in the opposite direction.' When all the canals are injured or destroyed an animal loses the power of balancing itself, and executes erratic movements of all kinds, so that it is plain that the impulses arising in them when transmitted to the cerebellum (par. 193) assist in enabling the body to maintain its equilibrium and in the performance of orderly executed movements. The function of the epithelium lining the utricle and saccule within the bony vestibule is not yet fully determined. On the macula of each membranous chamber are hair-cells, to which nerve fibres are distributed from one or other branch of the eighth cerebral nerve ; but there is reason to believe that the utricle and saccule have a function similar to that of the semicircular canals. 242. Auditory Judgments, — To the sensations of sound the mind adds certain judgments respecting them, forming its conclusions by knowledge previously acquired, and by the aid of other senses. As a visual sensation is referred to some external object, and not to the retina, so a sound is referred to something outside the internal ear. Whether the sound is produced inside our own body, or by some object outside ourselves, is mainly decided by noting whether the sonorous vibrations reach us through the external meatus or through the 404 Human Physiology bones of the head. If the meatus be filled with water the idea of externality to the body disappears, and the sound that then reaches the tympanic membrane seems to originate in the head. We determine the direction of a sound chiefly by the difference of intensity with which it is heard by each ear. If the source of a sound be directly behind or before, our estimate of its position is faulty until the head is turned to one side or the other. The estimate of the distance of a sound depends on previous experience of the habitual quality of the sound at varying distances, and any mode of transference, as by a speaking tube, that checks the law of intensity, diminishing according to the square of the distance, leads us to suppose the sound much nearer than it is. Ventriloquists may deceive us by imitating the character of distant sounds, The slight difference, however, with which sounds affect the two ears does not lead to the idea of two sounds, for they are in some way fused into one. Binaural audition is the ordinary mode of hearing, and appears to aid in the perception of space, though in a far inferior degree to binocular vision. 243. General Remarks on the Senses. — Each of the special organs of sense may be said to consist of three parts : — (i) A receiving organ situated on the periphery and constituted of modified epithelial cells destined to receive a particular kind of impression or stimulus ; (2) a conducting portion consist- ing of a nervous cord or pathway ; (3) a perceptive organ or central nerve cell where the sensation or particular state of consciousness determined by the stimulus arises. Although sense stimuli may be mechanical, electrical, thermal, or chemical, each sense organ has its own normal stimulus acting on the special peripheral end-organ of the sense, this end-organ being usually insensible to other stimuli. For sight, the stimulus is the undulations of the ether, which act upon the epithelial cells termed rods and cones, probably through chemical changes induced in pigment ; for hearing, the stimulus is of a mechanical nature, and consists of the vibrations of the material air ; for touch, the stimulus is the mechanical pressure on the surface of the skin ; for taste the stimulus is the molecular vibrations of a sapid substance in The Ear and the Sense of Hearing 40s solution on the tongue and other paits ; for smell, the stimulus is the odorous particles acting on the olfactory cells in the olfactory mucous membrane ; for the sense of temperature, the stimulus is certain slower ethereal vibrations than those producing the sensations of sight. The stimulus must have a certain intensity, for there may be sounds so low that we cannot hear them, contact so feeble that it does not affect us, &c. The least intensity of stimulus that excites a sensation has been thus estimated for the different senses : — Sense Sight . Hearing . ^ Touch . Taste Smell Temperature Least Appreciable Stimulus jljth of the amount of light from the full moon reflected from white paper The sound of a cork ball weighing I mm. gram falling from a height of i m. metre on a glass plate 90 millim. distant A pressure of -002 gram I part of quinine sulphate in 1,000,000 parts of water acKwooo ^^ milligramme of an alcoholic solu- tion of musk J° C. when the skin temperature is 98'5° C. The maximum limit of sensation is difficult if not impos- sible to state. After reaching a certain pitch of intensity any increase in the stimulus ceases to be perceptible. As to the smallest-appreciable increase of sensation, or the least possible difference in intensity, which gives rise to a sense of difference in succeeding stimuli, much valuable informa- tion has been obtained by Weber and others. In the first place it is found that the increase of stimulus necessary to produce an increase of the sensation bears a constant ratio to the total stimulus, i.e. that the fraction expressing the least perceptible difference between two succeeding stimuli varies for each sense but is constant for that sense. Thus : — Sense Sight Hearing . Touch . Temperature Muscular sense Constant of Difference jijth of the total amount of light Doubtful (one-third ?) One-third of the original pressure Varies with the skin temperature J^th of the weight tried 4o6 Human Physiology Thus, to perceive an increase in a weight of 17 ounces it would be necessary to add one ounce more, and to perceive an increase in a weight of 170 ounces, ten ounces would have to be added, and for 170 lbs. ten lbs. would be the least observable difference, the constant of difference being ^^^h. the original weight. In the case of light a sense of different luminosity would be felt by making a difference of roTjth in the amount of light. In the second place we may note that, though the mtensity of the sensation increases as the stimulus or exciting cause increases, yet it increases in a less proportion. The law expressing the relation betv/een the sensation and the stimulus may be put thus : ' As the strength of the stimulus increases in geometrical progression, the strength of the sensation increases in arithmetical progression.' Thus, if the stimulus increase in the geometrical progression i, 2, 4, 8, 16, the sensation only increases in the arithmetical progression ii 2, 3, 4, 5. This law (Fechner's Law) holds best in regard to light, though even here it is only true within certain limits, i.e. when the strength of the stimulus lies within the middle ranges of the scale of intensities. Time is required for the transmission of a nerve impulse along a nerve to and from the central organ, and we know that the nerve current travels at a rate not exceeding 100 feet per second (par. 46). Time is also occupied in the nerve centres in giving a response, either an immediate motor response as in such a simple unconscious reflex action as winking, in which the reflex time is but o'ci to 0-015 second, or a response show- ing that the subject has become conscious of, i.e. perceives, the sensation. The perception or reaction time for the senses, then, is the interval between the application of a stimulus and the responsive signal that the sensation has been felt. By arranging an electro-magnet so that it marks on a blackened revolving cylinder the moment when a sensory impulse is applied, and the moment when the subject experimented on gives a signal, this interval can be found. It varies with different degrees of attention and health, the varying result being due to the vary- ing time taken by the brain centres, but average values of the The Ear and the Sense of Hearing 407 reaction time for three of the senses are given in the annexed figure from Dr. Waller. Fig. 267. — ij time indicated in T^th sec. ; 2, perception time for sight 'iS sec. ; 3, for hearing "16 sec. ; 4, for touch '14 sec. Normally the retina is excited by waves of light which give rise to visual impulses and then to visual sensations. But the retina may also be excited by mechanical or electric stimuli with resulting visual sensations, and it is also said that the excitation of the optic fibres themselves always produces visual sensations, though this last statement is doubted. Hence has arisen a view termed the specific activity or energy of nerves, which holds that any impulse in a sensory nerve, however excited, will give rise to a sensation specific to that nerve ; that impulses, e.g. of all kinds along an optic nerve give rise to visual sensations, along the auditory nerve to auditory sensa- tions, and so on. But the direct stimulation of the optic fibres is said by some observers not to produce visual sensations, and in the case of the skin it is certain that the direct stimulation of the nerves gives rise to no touch sensation, but rather to painful sensation, so that it would appear that the development of a special sensation must be started in the special terminal organ. It is admitted, however, that stimulation of the various centres by any means gives rise to the specific sensation of the 408 Human Physiology centre. The doctrine of the specific energy of nerves thus becomes modified into the doctrine of the ' specific energy of perve-centres.' Notwithstanding our knowledge of the structure of the sense organs, of the conditions of sensation, of the relation of stimu- lus to strength of sensation, and of the time taken up in ner- vous propagation and nervous elaboration, we know absolutely nothing of the mode by which physical energy becomes trans- formed into mental energy ; nothing of the mechanism occur- ring in the protoplasm of the cells 'within the book and volume of my brain,' nothing of the mode by which thought is related to the forces of external nature. Our senses, even taken collectively, respond only to certain forces of nature, and to these only within certain limits, so that our conception of the universe must be but imperfect and incomplete, and all that the most learned can say is what the Soothsayer in ' Antony and Cleopatra ' says : — In nature's infinite book of secrecy A little can I read. APPENDIX APPENDIX MEASUREMENTS— FRENCH AND ENGLISH I lb. avoirdupois .... = 16 oz. av. = 7,000 grains I oz. avoirdupois . . . . = 437 's grains I lb, troy . = 12 oz. troy = s. 760 grains 1 oz. troy .... = 480 grains I gallon ... . . . = 8 pints = 277 '25 cubic inches I pint , . . . . . . . = 20 fluid oz. I gallon of water at 62° F. (i6°7 C.) weighs 70,000 grains I cubic foot of water at 62° F. weighs . . 62 '326 lb. av. I metre (m. ) . . . . . . = 10 decimetres (dm. ) = 100 centimetres (cm.) = 1,000 millimetres (mm.) = 39 '37 inches I foot = 3o'48 centimetres I inch = 25"4o millimetres I litre (1.) = 1,000 cubic centimetres (c.c.) = 1*76 pints I pint = 568 cubic centimetres I fluid oz = 28-4 ,, I kilogramme (kg.) = 1,000 grammes (g.) = 2-205 lb. av. I gramme . . . . , . . = 10 decigrammes (dg.) = 100 centigrammes (eg. ) — 1,000 milligrammes (mg.) = 15,432 grains I lb. avoirdupois = 453 '6 grammes I oz. avoirdupois = 28'35 ,, I grain = 648 milligrammes The unit of microscopic measurements is the micro-millimetre =Y;ij_jth part of a millimetre, o'ooi mm. = 25^00 '"• nearly. It is denoted by the Greek letter i". I kilogrammetre =7-24 foot-pounds I foot-pound = o'i38i kilogrammetre ( I kilocalorie = 424 kilogrammetres) , Centigrade and Fahrenheit Scales. F. r. F. C. F. C. F. C. :f. c. F. 32" 8° 46-4° 20" 68° 34" 93-2° 38" 100-4° 42" 107-6° .3r« 10 S° 24 75-2 35 95 39 102-2 43 109-4 3rf> 14 57 -2 28 82-4 3" 96-8 40 104 44 III-2 39 '2 i« 64-4 30 86 37 986 41 105-8 4S 113 C F— 72 Equation of Interchange. — ^ 100 180 412 Hutnan Physiology Note on the Microscope and Microscopic Figures. — Microscopes are optical instruments which enable us to see and examine objects which are too minute to be seen by the naked eye. A normal unaided eye can divide an object jjj in. in length at a distance of lo inches. At 5 inches a smaller object may be divided, but beyond 4 or 5 inches the object is too near for accommodation. With a first-rate compound microscope objects as fine as 150000 '"• ™ length may be seen. Microscopes are either simple or compound. With a simple microscope, as a lens, the object is viewed directly ; with a compound microscope two or more lenses are so arranged that an enlarged image of the object formed by one lens (the objective) is magnified by a second (the eye-piece) and seen as if it were the object. For histological purposes a microscope with two ob- jectives—a low power working at about 5 inch from the object and a high power with a focal distance of § inch — should be obtained. Two oculars or eye-pieces of different power are also an advantage. With such an instrument a magnifying power of from 50 to 400 diameters may be obtained. To bring out the minute structure of cells and certain tissues still higher powers are needed. Illustrations of objects seen under such powers will be found in the book, as well as illustrations drawn imder low powers and illustrations of objects not magnified at all, and the student should note when examining a figure the conditions under which it is drawn. In this country the size of microscopic objects is often given in fractions of an inch, but the unit of microscopic measurement employed on the Continent is also' used. This unit is the micro-millimetre, or the one-thousandth part of a millimetre, 0^0000397 inch=5jlgj inch nearly. The letter \i (mu) denotes the micro-millimetre. Thus the diameter of a red corpuscle is said to be j^ in. , or between 7/i and 8jn. It is scarcely necessary to remind the reader that the superficial mag- nification equals the square of the linear magnification. Thus 500 diameters linear = 250,000 superficial enlargement. (The -student is recommended to consult Prof. Schafer's ' Essentials of Histology. ') THE CHEMISTRY OF THE BODY The Chemistry of the Body.— Of the seventy-two elements known to chemists, fourteen enter into the composition of the human body. These are oxygen, carbon, hydrogen, nitrogen, sulphur, phosphorus, chlorine, sodium, potassium, calcium, magnesium, iron, fluorine, and silicon, the first four forming about 85 per. cent, of the whole. Other elements, as manganese and lead, have sometimes been found in small quantities. With the e^tception of oxygen , and- -nitrogen, which are found dissolved in the blood, and hydrogen found as a result of putrefactive processes in the ali- mentary canal, the elements are always united to form chemical compounds. These compounds, o\ proximate principles as, they are called in physiology, are either (a) Mineral or inorganic compounds, or {b) Organic compounds, i.e. compounds of carbon. The inorganic compounds are water, acids, such as hydrochloric acid in the gastric juice, and salts, such as calcium carbonate and calcium phos- phate in bone, sodium chloride in blood and urine, &c. Water is found in greater or less proportion in all the tissues, and forms about two-thirds of the weight of the whole body. Appendix 413 The organic compounds are numerous, and include four important classes, Froteids, 'Albuminoids, Carbohydratss, and Fats, the last two of which are non-nitrogenous, i.e. contain no nitrogen. Froteids, called also ' albuminous ' bodies, form an important class of complex organic compounds containing carbon, hydrogen, oxygen, nitro- gen, and sulphur. They occur in a solid viscous condition or in solution in nearly all the solids and liquids of the body. The proteids, with the exception of hsemoglobin, which is really a compound of the proteid globin with hajmatin, are all amorphous or non-crystallisable. They are insoluble in alcohol and ether. Some are soluble in water, others insoluble. Some are soluble in weak solutions of neutral salts, such as sodium chloride and magnesium sulphate, others soluble only in concentrated saline solutions. It is on these varying solubilities that the classification of proteids depends. The following additional account of the proteids is taken with slight altera- tions from Watts's Dictionary of Chemistry, edited by Morley and Muir. Proteids are never absent from the protoplasm of active living cells, whether animal or vegetable, and they are indissolubly connected with every manifestation of organic activity. A definition of proteids is not possible in the logical sense. Gamgee gives in the following sentences a terse description of these substances, which must take the place of a defini- tion : ' Proteids are highly complex, and for the most part non-crystallis- able, compounds of carbon, hydrogen, nitrogen, oxygen, and sulphur, occurring in a solid viscous condition or in solution in nearly all the solids and liquids of the organism. The different members of the group present differences in physical, and to a certain extent in chemical, properties ; they all possess, however, certain common chemical reactions, and are united by a close genetic relationship. ' In vegetables the proteids are constructed out of the simpler chemical compound's which serve as their food. In animals such a synthesis never occurs, but the proteids are derived diiectly or indirectly from vegetables. By the action of certain digestive juices all proteids are capable of being convertea .into closely allied substances called peptones, which after absorp- tion undergo reconversion into proteids. The various proteids differ somewhat in elementary composition within the limits of the following numbers : C H N S From S'-S 6-9 15-2 0'3 20-9 To S4-S 7-3 17-0 2-0 23 -s Classification of Proteids I. Native Albumins. These are proteids which are soluble in water, and not precipitable from their solutions by saturation with sodium chloride or magnesium sulphate. They are coagulated by heat. The important members of the group are egg albumin, serum albumin, and lactalbumin. II. Globulins. These are proteids which are insoluble in water ; they are soluble in dilute solutions of neutral salts, and are precipitated in an uncoagulated condition by saturation with sodium chloride and magnesium sulphate. They are coagulated by heat. The most important members of the group are : serum globulin, or paraglobulin, as it is also called, fibrino- gen, myosin, crystallin, and globin. III. Albuminates, or Derived Albumins. This name is applied to the 414 Human Physiology metallic compounds of proteids, and also to acid albumin or syntonin, and alkali-albumin. Restricting the term to the two latter substances, they may be defined as proteids insoluble in water or in solutions of neutral salts, but readily soluble in dilute acids or alkalis. Their solutions are not coagulated by heat. The caseinogen of milk may be put in this class. IV. Proteoses. These are proteids which are not coagulable by heat, and most of them are precipitable by saturation with certain neutral salts. They are precipitated by nitric acid, the precipitate dissolving on the appli- cation of heat and reappearing when the solution is cooled. They resemble peptones in being slightly diffusible, and in giving the biuret reaction. They are formed from other proteids as the result of the action of proteolytic ferments on them, being an intermediate stage in the formation of peptones. They are also found in certain animal and vegetable tissues. The best known members of the group are the albumoses, the chief albumoses being proto-albumose soluble in distilled water, hetero-albumose insoluble in water, and deutero-albumose, the most nearly allied to peptones. V. Peptones. These are proteids which are very soluble in water ; they are not precipitated by heat, by saturation with any neutral salt, nor by nitric acid. They are completely precipitated by tannin, by excess of absolute alcohol, and by potassio-mercuric iodide ; incompletely by phos- photungstic acid, phosphomolybdic acid, and picric acid. They give the biuret reaction. Peptones are subdivided into hemipeptones, those which yield leucine and tyrosine as the further result of pancreatic digestion, and antipeptones, those which do not. VI. Insoluble Proteids. This class includes a number of proteids varying in their reactions which cannot be included in any of the foregoing classes, but which all resemble one another in their extreme insolubility in various reagents. This class includes fibrin, coagulated proteid, lardacein, antialbumid, and gluten. Among the general properties of proteids are : 1. Indiffusibility. Solutions of proteids are non-diffusible. They belong to T. Graham's class of colloid substances. Peptones, and to a less extent albumoses, are, however, diffusible. This property of indiffusi- bility enables us to separate proteids from saline admixtures, and also to separate various proteids from one another : e.g. if a mixture of albumin and globulin in a saline solution bedialysed, the salts pass out, the albumin remains within the dialyser in solution, while the globulin, which is in- soluble in water, is precipitated. 2. Action on Polarised Light. Proteids all rotate the plane of polarised light to the left. 3. Heat Coagulation. Most of the proteids are coagulated by heating their solutions, especially the globulins and albumins, as in the familiar instance of the solidifying of the white of an egg on boiling. Serum globulin coagulates at a temperature of 75° C, egg albumin at 72° C, myosin and fibrinogen at 56° C. Among the general tests for proteids are the following : (i) The Xanthoproteic Test. On adding strong nitric acid and heating a yellow colour is produced : this becomes orange on the addition of ammonia. (2) Millon's Test. An acid solution of nitrate of mercury gives a white precipitate, which turns brick-red on boiling. (3) Biuret Test. Addition of a trace of copper-sulphate and excess of Appendix 415 potassium hydrate causes a violet colour. In the case of albumoses and peptones the colour produced is a pink one. The decompositions that proteids undergo in the body lead to their breaking up into simpler bodies, some of which, like urea CO(NH2)2, form a class of crystalline nitrogenous substances. In the alimentary canal the proteids are converted into proteoses (albumoses) and peptones ; this change is probably due to hydration. Under the influence of the pancreatic ferment, a certain class of peptones called hemipeptones are further acted upon, resulting in the formation of leucine, tyrosine, aspartic acid, ammonia, and protein-chromogen (a substance made purple by bromine). Putrefac- tive processes due to bacteria in the small intestine also occur ; these result in the formation of indol, skatol, phenol, and oxy-acids. One of the sources of hippuric acid in the urine of flesh-feeders is the phenyl-propionic acid that results from the putrefaction of proteids in the alimentary tract. After the proteids have been absorbed from the alimentary canal, they become assimilated by the tissues, and there undergo combustion or meta- bolism, the chief ultimate products being water, carbonic acid, and urea. It is probable that glycocine, leucine, and creatine and ammonium car- bonate are intermediate products in this change. It has also been demon- strated, by experiments on animals, that proteid food gives rise to glycogen in the liver, and to fat in the subcutaneous and other tissues. That pro- teids can be converted into fats is also shown by the occurrence of adipocere in the muscular tissues after death. Albuminoids. — The term albuminoids is now restricted to certain nitro- genous substances closely allied to proteids and that resemble proteids in many points, but differ from them in others. The chief members of the group are : Collagen, the substance of which the white fibres of connective tissue are composed. It has been obtained from tendons, ligaments, and areolar tissue. A similar substance derived from bone is spoken of as ossein. On boiling with water collagen becomes converted into gelatin, a substance that sets into a jelly when the solution in hot water cools. Gelatin gives most of the proteid colour tests, but is not precipitated by acetic acid nor by many of the metallic salts that precipitate proteids. Though it is easily digested, being converted into a peptone-like body which is easily absorbed, it will not entirely replace proteids, acting only as a proteid- sparing food. The question of the part played by gelatin, which is an easily digestible substance in nutrition, is very important practically, jellies especially being given to invalids. Volt's chief result showed that gelatin will not entirely replace proteids, but that animals rapidly waste which are fed on it alone; but, in conjunction with a certain small amount of proteid, it is capable of maintaining nitrogenous equilibrum as well as if the only nitrogenous food taken was proteid in nature. These results have been since very generally confirmed. Voit distinguishes between circulating and organic albumin ; gelatin can never yield the latter, but it may replace the former in so far as it prevents the conversion of organic into circulating albumin. Gelatin also diminishes the waste of fat in the body. Mucin is the albuminoid that forms the secretion of certain epithelial cells, and the main part of the intercellular substance of connective tissue. It is viscid and tenacious, precipitated by acetic acid and composed of a proteid and animal gum. Mucin forms the chief constituent of mucus, and gives the sliminess to 41 6 Human Physiology the secretion of raucous membranes. In mucus it is suspended in an alkaline exudation from the blood and mixed with the debris of epithelium cells, and a few white blood corpuscles. The mucin itself is here formed by the protoplasm of certain cells of the epithelium becoming altered, so that it becomes swollen and brightly refracting ; the globule of mucin so formed is discharged, leaving a so-called goblet cell. In mucous glands, such as the submaxillary salivary gland, a very similar replacement of protoplasm by mucin (or mucigen, as it is called when inside the secreting cells) takes place. Mucin is also largely contained in the surface secretion of several invertebrate animals, e.g. the snail. Chondrin is the albuminoid formed from cartilage on boiling it with water, and is probably a mixture of mucin and gelatin. Elastin is the substance of which yellow or elastic fibres of connective tissue are composed. Nuclein is the chief constituent of cell-nuclei, and resembles mucin in its physical characters, but differs chemically in the percentage of phos- phorus that it contains. Nucleo-albumin is a compound of proteid With nuclein, and is found in the protoplasm of cells. Keratin is the highly insoluble substance that replaces the protoplasm in the surface cells of the epidermis, in nails, hairs, horns, &c. Carbohydrates are compounds of carbon, hydrogen, and oxygen, there being two atoms of hydrogen for every atom of oxygen in the molecule.. They are derived chiefly from vegetable tissues, and form important foods, but some of them, as dextrose (grape sa^xi), glycogen, and lactose (milk sugar), are found in animal organisms. Many of them possess the property of causing the rotation of a ray of polarised light, and the direction and amount of this rotation assist in their detection. According to their empirical formula they are arranged into three groups, Amyloses, Glucoses, and Sucroses. They may be tabulated thus : Classification oj Carbohydrates. I. Amyloses (C.H,„0,)„ II. Sucroses or Saccharoses C„H=.0„ III. Glucoses C.H,,0, Starch Dextrin Glycogen Cellulose Cane sugar Lactose Maltose Dextrose Lsevulose Galactose I. Amyloses or Starches, with the general formula (C5H,„05)„, where ' n ' varies and is probably large. The chief amyloses are : 1. Starch, a body found in microscopic granules in many plants, insolu- ble in cold water, forms an opalescent solution with hot water that gives a blue colour with iodine, is changed by the ferments of the saliva and pancreas into dextrin and maltose through a process of hydrolysis or taking up the elements of water. With boiling dilute acids starch may be converted into dextrose. 2. Dexttin, the intermediate product between starch and sugar, two varieties being distinguished, erythro-dextrin which gives a reddish-brown colour with iodine, and achroo-dextrin which does not. Appetidix 417 3. Glycogen, the animal starch found in the liver and muscles. The liver appears to receive its carbohydrate material as dextrose, to store it as glycogen, and to reconvert it by the activity of the hepatic cells into dex- trose as required. 4. Cellulose, a colourless material forming the cell walls and woody fibres of plants, not easily digested by man, and hence starch requires cooking to burst the cellulose envelopes of the grai.as. II. Sncroses or Saccharoses have the general formula C,2H220,,, and include : 1. Cane su^ar, a sugar found in the juices and fruits of many plants and forming an important article of food, crystalline and easily soluble in water. Injected into the blood-vessels it is eliminated unaltered, and there- fore appears to be non-assimilable by the tissues. Hence, when used as a : food, canesugar undergoes a change before absorption, being ' inverted ' ot changed into equal parts of dextrose and Igevulose by the action of a ferment in the intestinal juice. Ci^HjjO,, + H^O = CeH,20„ (dextrose) + C^HijOs (Isevulose). 2. Lactose or milk sugar, a suga,r occurring in milk but not assimilated until changed in some way into the glucoses, dextrose and galactose. , 3. Maltose, the chief sugar formed from starch by the action of the . ptyalin in saliva and amylopsin in pancreatic juices, formed also by the action of malt extract (diastase) on starch. Though very soluble in water, it appears to be converted into dextrose before absorption into the portal blood, either by the action of a ferment in the intestinal juice or by the. activity of the epithelial cells of the intestines. ' The converting action of extracts of the intestinal mucous membrane is strikingly, less than that of the tissue itself ; from this it may perhaps be inferred that the change into dextrose takes place rather during than previous to absorption. This fact corresponds closely to the well-known views as to the changes which peptones similarly undergo during their passage through the walls of the intestine into the neighbouring blood- vessels. ' III. Glucoses have the general formula QH,; O5, and include : 1. Dextrose glucose or grape sugar is a sugar found in certain fruits, in honey, and in minute quantities in the blood and other tissues. It is crys- talline but not so sweet as cane sugar. It is important to note that all the carbohydrates are absorbed from the alimentary canal and reach the blood as dextrose. It is the sugar most readily assimilated. Dextrose possesses the important property of undergoing fermentations, alcoholic under the influence of yeast, when alcohol and carbon dioxide are formed ; lactic under the influence of another micro-organism. Dextrose is found in large quantities in the urine in diabetes. It reduces salts ot copper in alkaline solutions. 2. Lavulose is a sugar occurring with dextrose in many fruits, and . formed with dextrose from cane sugar by inversion. It is distinguished from dextrose by rotating the plane of polarised light to the left. 3. Galactose is one of the sugars formed by hydration of lactose or milk sugar. It ferments with yeast, but not so readily as dextrose. Inosite or muscle sugar is a crystallisable substance found in muscle and other parts of the body in small quantities. Though possessing the same percentage composition as sugar, it does not really belong to the class of sugars, yielding none of the reactions of the class. Fats. — Fats consist of carbon, hydrogen, and oxygen, the same three E E 4i8 Human Physiology elements as in carbohydrates, but the oxygen is in less proportion to the hydrogen than in carbohydrates. Fat is found in considerable mass in some tissues and in small particles in most, its average quantity in a healthy well-fed body being a little more than one thirtieth of the body weight. The three fats that occur in man are palmitin, stearin, and olein, the two former, which are solid at the body temperature, being kept liquid by mixture with the olein. Fats are insoluble in water, but soluble in ether and alchohol. Fats are considered as compounds of glycerin with a fatty acid (palmitic, stearic, or oleic), water being elimi- nated in the act of combination. When heated with a caustic alkali, as sodium hydrate, the fat splits up into glycerin and a compound of the fatty acid with the alkali termed a soap, and both glycerin and the alkaline soaps are soluble. This formation of a soap is termed saponification. Another change —physical, not chemical — that fats undergo is termed «»;«/- sification. Shaken up with some liquids fats are reduced to a fine state of subdivision termed an emulsion. Milk is a natural emulsion. Both saponification and emulsification of fats occur in the alimentary canal. The ferment steapsin of the pancreatic juice saponifies a portion of the fat of food, and the soap so formed aids thepancreatic juice and bile in forming an emulsion of the remainder. While a large part of the fat laid on by an animal during the process of fattening is derived from the fat of the food, yet it must be noted that fat can be formed from proteids, as is shown by the storage of fat during ' nitrogenous equilibrium,' and also from carbo- hydrates, as more fat may be stored than could be derived from the proteids. Ferments. — Certain minute organisms possess the power of inducing definite chemical changes in the fluids or other media in which they live. Thus the cells of the yeast plant can transform sugar into alcohol and carbon dioxide, and to the change thus produced the term fermentation is applied, the agent producing the change being called a ferment. Other micro- organisms can lead to the formation of vinegar or acetic acid firom alcohol, to the souring of milk by the formation of lactic acid, &c. These living organisms are spoken of as organised ferments. In another class of chemical changes the result is brought about not by living organisms, but by chemical substances derived from living cells, these substances possessing the property of inducing chemical transforma- tions in a large mass of certain other substances without themselves undergoing noticeable alteration. Such agents are called unorganised ferments, soluble ferments, or enzymes. The ptyalin of the saliva which changes starch into sugar is an example. The diastose of malt can also convert an indefinite amount of starch into sugar if the product of its activity is not allowed to accumulate too much. Confining ourselves to the unMganised ferments or enzymes of the human body, they may be classified as follows : — (a) Amylolytic, or those which change amyloses as starch and glycogen into sugar. As examples we have the ptyalin of saliva and the still more active amylopsin of pancreatic juice. {b) Proteolytic, or those which transform proteids into proteoses (albu- moses, &c. ) and peptones. As examples we have the pepsin of gastric juice and the trypsin of pancreatic juice. ( squirrel ; 4, hamster. 45 8 Human Physiology Phrenic (Gk. phren, the diaphragm), pertaining to the diaphragm : as the phrenic nerves from the cervical plexus to the diaphragm. Plexus (L. plecto, I plait, plexus, a plaiting), an intricate network, especially of nerves or veins. Pneumograph (Gk. pieuma, breath, air, and grapho, I write), an apparatus for recording the movements of the chest in breathing. Pons (L. pons, bridge), a term applied to the nervous structure connecting the medulla and the crura cerebri. Process (pro, forward, and cedo, I go), a word used to signify a projecting part or prominence. Proteid. A proteid substance is a compound of carbon, oxygen, nitrogen, and hydrogen, with sulphur and phosphorus in very small quantities. Proteids cannot be built up in an animal's body from simpler com- pounds, and must therefore , be suppHed in the food. The proteid food-stuffs are called nitrogenous because they are the only kind containing nitrogen. All proteids that enter the body are in the end broken up to form carbon dioxide, water, and urea. The general properties and tests for proteids are given on page 414. The property of most proteids found in nature to set or coagulate on heating is seen when white of egg is cooked. The xanthoproteic reaction (heating with strong nitric acid and then adding ammonia) may be applied to show the presence of proteid matter in bread and potato. The pro- nounced orange colour that results in the case of bread, and the faint orange colour in the case of the potato, indicate the much smaller proportion of proteid in the potato. For the action of gastric juice and pancreatic juice on proteids, see pars I2i and 123. Protoplasm (Gk. protos, first, plasma, form), the viscid contractile substance that forms the chief portion of cells ; living matter. Owing to its presence in all organised beings, protoplasm has been called ' the physical basis of life.' Chemically it consists of water, proteids, and small quantities of carbohydrates and mineral salts (see par. 2). Fnlmonary (L. pulnio, pulmonis, lung), pertaining to the lungs. Bacemose (L. racemits, a bunch of grapes), resembling a bunch of grapes on its stalk, having ducts which divide and subdivide and end in small sacs, or acini : exemplified in the salivary glands and pancreas. Kamify (L. ramus, a branch), to spread out like branches. Beilex (L. re, back, and fiecto, fiexus, I bend), bending backward. Re- flex action implies the existence of a nervous arc composed of ( i ) an afferent nerve, conveying a stimulus from its starting-point in the periphery, (2) a nerve centre composed of ganglion cells and capable of converting this impression into an outgoing impulse, and (3) an efferent nerve leading from the centre to the part put in action. Eenal (L. renes, the kidneys), pertaining to the kidneys. Eeticulum (L. rete, a net), a network, an interlacement of fine fibres. Bhythm (Gk. rhythmos, measured movement), any regularly recurring motion, as the rhythmic contraction of the heart. Saponify (L. sapo, saponis, soap, and facio, I make), to convert into soap by combination with an alkali ; to decompose oils or fats into salts of the fatty acid and glycerine. Sarcolemma (Gk. sarx, sarcos, and lemma, husk or sheath), the delicate sheath surrounding the fibres of striped muscular tissue. Sciatic (L. ischiatkus, from ischium, hip bone). The great sciatic, the largest nerve in the body, arises from the sacral plexus (fig. 169), and Glossary 459 running through the back of the thigh ends in branches for various muscles of the lower limb. The disease sciatica is marked by severe neuralgic pains along the course of the sciatic nerve. Sensorium (L. sensus, a sense), the part of the nerve centres supposed to receive sensory impression ; the part vifhere sensation resides. Septum (L. sefio, to fence in), a partition, especially a partition between two cavities. Somatic (Gk. soma, somaios, the body), pertaining to the body. Sphygmograph (Gk. sphugmos, the pulse, zxA grapho, I write), an instru- ment for obtaining a graphic representation of the blood pressure in the arteries. Squamous {L. squama, a scale), scaly, shaped like a scale. Starqh. A chemical compound of carbon, hydrogen, and oxygen, belong- ing to the class of compounds called carbohydrates. It is an important food-stuff, and is present in most vegetable products used as food. By itself it is seen to be a white powder consisting of microscopic grains with concentric rings. The granules are enclosed in a coating of cellulose or woody fibre. Starch is insoluble in cold water, but when the water is boiled the granules burst their coating and the starch forms a sort of opalescent solution. The chief test for starch is iodine solution. With iodine solution starch gives an intense blue colour. The presence of starch in bread or boiled potato may be readily seen by pouring a drop or two of iodine solution on these bodies. The various kinds of starch are included under the term Amyloses (see Appendix). For the action of saliva and pancreatic juice on starch, see pars Ii6 and 123. Stimulus (I^. stimulus, a goad or spur, pi. stimuli), an agent which is able to cause reaction in muscular or other tissue on which it acts. It may be mechanical (pinching, pricking, &c. ), thermal, electrical, or chemical. Striated (L. stria, a furrow or streak), striped, provided with strife. Subjective (L. sub, under, xadjado, I throw), pertaining to the subject or conscious self ; internal ; a term applied to sensations which originate within the organism without the agency of an external object. Sugar. A chemical compound of carbon, hydrogen, and oxygen, used as a food-stuflf. The various kinds of sugar are carbohydrates like the starches, and belong to the two classes Sucroses and Glucoses (see Appendix). Dextrose or grape sugar, one of the Glucoses, is found in small quantities in the blood and certain tissues. It is the form to which all carbohydrates are converted when taken into the circulation. Sustentacular (L. sustentare, to hold up, support), supporting. Systole (Gk. syn, together, and stello, I place), the contraction of the heart. Tactile (L. tactus, touch), pertaining to touch or touch sensations. Tension (L. iendo, I stretch), a state of stretching or lightness ; internal pressure or tendency to expand. Tetanus (Gk. tetanos, tension, spasm), a. term used to denote the spasm or prolonged contraction of the muscles due to rapidly repeated stimulations. Tone (L. tonus, a sound, from tendo, to stretch), a sound of a certain pitch ; a moderate state of tension or contraction, as arterial tone ; a state of healthy vigour. 460 Human Physiology TropMc (Gk. trepho, I nourish, trophe, nourishment), pertaining to nutrition. A trophic centre is a group of gangUonic nerve cells pre- siding over the nutrition of certain nerve fibres. Turbinated {L. turbo, turbinis, a top), convoluted ; rolled like a scroll : as the turbinated bones of the nose. Ulnar (L. ulna, the elbow), situated near or in relation to the ulna. The ulnar nerve is a branch of the brachial plexus (fig. 169) descending on the inner side of the arm to the elbovif-joint, and passing thence to the muscles of the palm, the little finger, and one side of the ring finger. Its stimulation by a knock {' hitting the funny-bone ') causes the well-known peculiar numb sensation. Vagus (L. vago, I wander, vagus, wandering), a term applied to the pneumogastric nerve on account of its length and varying distribution. Vaso- (L. vds, vasts, a vessel), a prefix meaning belonging to a vessel. Vaso-motor nerves are nerves regulating the movements of the walls of blood-vessels. Villus (L. villus, a tuft of hair, pi. villi), the hair-like processes of the mucous membrane of the small intestine. Viscera (L. viscus, pi. viscera, the internal organs of the body), the internal organs of the great cavities of the body. Vitreous (L. vitrum, glass), of a glassy nature or appearance. Zymogen (Gk. zUme, ferment, leaven, and gennao, I produce), the peculiar substance of the digestive secreting glands that gives rise to the digestive ferments (see Appendix, p. 419), the ferment precursor. The zymogen in the gastric cells is called pepsinogen ; that in the pancreas is called trypsiiwgen . Neurone,— A neurone is a nerve unit, and consists of an independent complex nerve-cell with branching processes called dendrons and an axon or axis-cylinder process that gives rise to a nerve-fibre. Each neurone is an anatomical independent unit in physiological association with other neurones by means of an interlacement and touching of its dendrons (not, as formerly supposed, by structural union) or by the bush of fibrils at the termination of its nerve-fibre. Neurones are thus linked together so as to allow the transmission of nervous im- pulses in certain definite directions (see par. 43). INDEX The numbers refer to Paragraphs^ and not to pages ABD Abducens nerve, 192 Absorption of fat, 132 Absorption of food, 132; Fos- ter on, 132 Accommodation, 228, App. After-image, 231 Air, complemental, 91 ; com- position of, 96 ; residual, 91 ; supplemental, 91 ; tidal, Ai^^cells, 86, 87 Air-passages, 86 Albumin, 104, App. Alveoli, 26, 86, 87, 112, 133 Amoeba, 3 Ampulla, 238 Amyloids, 104, App. Amyloi>sin, 123, App. Anabolism, 3, 167 Animal-heat, 167-174 ; con- trol of, 173 ; loss of, 170 ; production of, 169 ; regula- tion of, 172 Anus, 127' Apex beat, 69 Aphasia, 204 Apnoea, 100 Appendix, 135 Aqueduct of Sylvius, 190 Aqueous humour, 224 Arachnoid membrane, 179, i8g Arterial circulation, 78 Arteries, 65 ; structure of, 74 ; tone of, 84 ; vessels and nerves of, 74 Arytenoid cartilages, 176 Asphyxia, 100 Auditory hairs, 240 ■Auditory nerve, 192, 234, 239, 240 Auricle, 66 ; left, 67 ; right, 67 . Automatic action, 44 Axis cylinder, 40, 43 Bacteria, 130, App. Basement membrane, r3 Basilar membrane, 238, 240 Bicuspid valve, 67 Bile, I2<^ ; functions of, 125 ; secretion of, 145 CAR Bilirubin, 124 Biliverdin, 124 Bladder, 159 Blind spot, 222 Blood, coagulation of, 60 ; composition of, 59 ; cor- puscles of, 55 ; gases of, 58 ; changes effected in, 97 ; microscopic appearance of, 54 ; physical properties of, 53 ; plasma of, 53, 63 ; quantity and distribution of, 62 Blood current, velocity of, 82 Blood platelets, 57 Blood pressure, 77 ; in kidney. 156 Blood-vessels, 73 Blushing, 84 Bone, 21 ; chemical composi- tion of, 24 ; minute struc- ture of, 23 ; physical pro- perties and general structure of, 21 Brain, 187 ei seq. ; fibres of 204 ; localised functions of, 204 ; membranes of, 189 ; motor areas of, 204 ; sen- sory areas of, 204 ; ventricles of, 188 Bread, 106, 107 Breathing, types of, 94 Bronchi, 86 Bronchial tubes, 86 Buffy coat of the blood, 60 Bulb, see Medulla oblongata C^cuM, 127 Calamus scriptorius, 38, 82, 190, zgi Calorie, 108 Canaliculi, 23 Capacity of chest, gi Capillaries, 33, 65, 75 ; of lung, 88 ; of muscle, 33 Carbohydrates, 104, App. Carbonic acid, 35, 85, 96, 97, 98 Cardiac cycle or revolution, 68 ; summary of events in, 70 CIL Cardiac glands, 120 Cardiac nerves, 83 Cardiogram, 72 Cardiograph, 72 Cartilage, 20 Cartilage, hyaline, 20 Cartilage, white fibro-, ao Cartilage, yellow fibro-, 20 Casein, 104 Cell, I, 2 Cell, bone, 23 Cell, central, 120 Cell, ciliated, 10, 86 Cell, Deiters', 240 Cell, goblet, g, 86 Cell, gustatory, 214 Cell, inner hair, 240 Cell, lymphoid, 18 Cell, olfactory, 216 Cell, outer hair, 240 Cell, oxyntic, 120 Cell theory, i Centres, card io- inhibitory, 83, 190 Centres, in spinal chord, 44, 185 Centres of medulla, 190 Centres, respiratorj-, 100, 193 Centres, trophic, 182 Centres, vaso-raotor, 84, 185, 190 Cerebellum, 187, 193 ; func- tions of, 193 Cerebral nerves, 192 Cerebro-spinal system, 38 Cerebrum, 187, 200 et seg. ; convolutions of, 204 ; cor- tex, 201 ; functions of, 202 ; lobes of, 200 ; removal of, 202 'j structure of, 201 Chemiotaxis, 141 Chemistry of body, App. Cholesterin, 124 Chondrin, 20 Chorda tympani, 84, i r4 Chordae tendinese, 67 Choroid, 221 Chyle, 64, 134 ; conditions af- fecting amount of, 140 Chyme, 121 Cilia, 10 462 Human Physiology CIL Ciliary muscle, 221 Ciliary nerves, 225 Ciliary processes, 221 Circulation of blood, 65 ; in arteries, 78 ; in capillaries, 80; in veins, 81 ; influence of respiration on, 102 Clot, 53, 60 Coagulation of blood, 60 Coagulation of blood, circum- stances influencing, 61 Cochlea, 238, 240 Cold spots, 209 Colon, 137 Colour, 253 Colour-blindness, 233 Colour -vision, 233 Complementary colours, 233 Conjunctiva, 218, 220 Consonants, 177 Corium, 27, 160 Cornea, 220 Corona radiata, n^g Coronary circulation, 67 Coronary valve, 67 Corpora quadrigemina, 196 Corpora striata, 198 Corpus callosum, 187 Corpuscles, 53 Corpuscles, colourless, 5^ Corpuscles, connective- tissue, Corpuscles, Pacinian, 207 Corpuscles, red, 56 Corpuscles, tactile, 207 Cortex cerebri, 201 Corti, organ of, 239, 240 ; rods of, 239, 240 Coughing, loi Cricoid cartilage, 176 Crura cerebri, 187, 194 Crying, 10 1 Crystalline lens, 224 Cutaneous nerves, functions of, 211 Cutaneous respiration, 164 Cystic duct, 143, 124 Decussation of pyramids, 190, IQI Deglutition, 117 Dextrose, 128, App. Diabetes, 147, 157 Dialysis, 132 Diapedesis, 55, 141 Diaphragm, movements of, 03 ; nerves of, 100 Diastole, 68 Diet, suitable, 107 ; effects of various, 109 Distance, perception of, 231 Dropsy, 132, 140 Duodenum, 127 Dura mater, 179, i8g Dyspnoea, 100 Ear, 234 ; externa!, 235 ; in- ternal, 238 ; labyrinth of. GLA 238 ; middle, 236 ; ossicles of, 237 Egg, composition of, 106 Electrotonus, 50 End-bulbs, 49, 207 Endocardium, 67 Endolymph, 238 Endothelium, 5 End-plates, 34 Energy, 107 Energy, kinetic, 108 Energy, potential, 109 Epidermis, 5 Epiglottis, 176 Epithelium, '5, 27 ; ciliated, 10 ; columnar, g ; cubical, 5 ; functions of, 12 ; sensory, II ; squamous, 8 ; stratified, 6 ; transitional. 7 Equilibrium, 193, 212, 241 Eupncea, 100 Eustachian tube, 236 Eustachian valve, 68 Excretion, 153 et seq. Expiration, 93 Eye, blood-vessels of, 226 ; nerves of, 225 ; orbit of, 218 ; refractive media of, 224 Eyeball, appendages of, 218 ; general structure of, 219 ; muscles of, 230 Facial nerve, 192 Faeces, 131 Fat, 19, App. ; absorption of, Fechner's Law, 243 Fenestra ovalis, 236, 237 Fenestra rotunda, 236, 237 Ferments, 120, 123, App. Kever, 174 Fibrin, 59, 60 Fibrin ferment, 60 Fibrinogen, 59 Filtration, 132, 156 Flavour, 215 Food, T03 ; absorption of, 132 ; as a source of energy, 108 ; composition of common, 106 Food-stuffs, classification of, 104 ; nitrogenous, 104 ; non- nitrogenous, 104 ; mineral, 104 Fornix, 187 Fourth ventricle, 38, 187, 188, 190 Fovea centralis, 222 Frog's heart, App. Ganglia, 38 ; of sympathetic nerves, 205 ; on posterior roots of spinal nerves, 38, 51, 182 Gastric juice, 121 Gelatine, 104 Geniculate bodies, 196 Glands, 26 ; albuminous, 112, JAW 113 ; blood, or ductless, 147a: Brunner's, 127 ; cardiac, 120 ; ceruminous, 163, 235 ; Lieberkiihn, 127, 133 ; lymphatic, 138 ; mam- mary, 166 ; mesenteric, 126, 134; mucous, 112; parotid, 113; peptic, 120; pyloric, 120 ; sebaceous, 162 ; 'se- creting,- 26 ; serous, 112, 113 ; sweat, 163 ; varieties of, 26 Glomerulus, 154 Glosso-pharyngeal, 192 Glottis, S'^, 176, 177 Glucose, or grape sugar, App. Gluten, 104 Glycogen, 146 Glycosuria, 147, 157 Goitre, 156 ' Goose-skin,' 161 H>ematin, 56 Haemoglobin, 56 Hair, 161 Harmonics, 234 Haversian canals, 23 Heart, ^t \ beat, frequency of, 71 ; regulation of, 83 ; chambers of, 67 ; depressor nerve of, 84 ; impulse of, 69 ; nervous control of, 83 ; sounds of, 69 ; valves of, 68 ; working of, 68 Heat unit, 108 Hemiplegia, 199 Hepatic cells, 144, 146 Hepatic lobules, 144, 146 Hepatic vessels, 143, 144 Hering's theory, 233 Hiccough, IQI Histology, I Horopter, 231 Hot spots, 209 Hunger, 103 Hyaloid membrane, 224 Hyoid bone, 86, 11 1 Hypoglossal, 192 IliuMj T27 Inanition, no Incus, 237 Inflammation, 141 Infundibulum, 86 Insalivation, 112 Inspiration, gi, 92 Internal capsule, 187. igg Intestinal juice, 128 Intestines, digestive changes in small, 129 ; large, 130 ; structure of, 127 ; move- ments of, 135 Iris, 221 ; movements ofj 229 Jaundice, 124, 145, 157 Jawbones, 11 1 Index 463 JEJ Jejunum, 127 Judgments, auditory, 242 visual, 232 Karyokinesis, 2 Katabolism, 3, 167 Kidney, 154 ; blood supply of, 154 ; convoluted tubules of, 154 ; pelvis of, 154 Knee jerk, 184 Kymograpb, 77 Labyrinth, 238 Lachrymal gland, 218 Lacteals, 136 Lacunae, 23 Lamellae, 23 Lamina spiralis, 238 Larynx, 86, 91, 175 ; structure of, i;?6 Laughing, loi Lecithin, 124 Leucocytes, 55, 138, 141 Ligament, 16 Liquor sanguinis, 54 Liver, 143 ; cells of, 144, 146 ; functions of, 144 ; glyco- genic function of, 146 ; mi- nute structure of, 144 Localisation of functions of brain, 204 Long sight, 228 Loss of material, 105 Lungs, 87 ; blood-vessels and lymphatics of, 88 ; changes effected in, 97 ; nerves of, 89 Lymph, 63, 136, 137 ; con- ditions affecting quantity of, 14a; movements of, 142 ; quantity of, 64 Lymphatic glands, 138 Lymphatics, 136 ; origin and structure of, 137 Macula lutea, 222 Malleus, 237 Maloighian capsule, 154 Malpighian corpuscles of kid- ney, 154 ; of spleen, 148 Mammary gland, 166 Manometer, 77 Marrow, 25 ; corpuscles of, 25, 56. . Mastication, iii Measurements, App. Mediastinum, 87, go Medulla oblongata, 38, i^ ; functions of, 191 ; pyramids of, 190 Meibomian glands, 2t8 Mesentery, 126, 135 Metabolism, 3, 167 Microscope, note on, App. Milk, io6, 166 OXY Mucous membrane, 27, 86, irg, 120, 127, 160 Musc^ volitantes^ 231 Muscle nerve preparation, 36 Muscular fibres, 32 Muscular fibrillae, 32 Muscular movements. 36; co- ordination of, 193, 212, 241 Muscular sense, 212 Muscular tissue, 28 et seq. ; cardiac, 30 ; chemical com- position of, 35 ; contraction of, 36 ; tone of, 36; unstriped or plain, 29 ; voluntary .or striated, 31 Myograph, 36 Myopia, 227 Myosin, 104 Myxoedema, 150 Nails, i6r Nasal duct, 218 Nerve cells, 42 Nerve centres, 44 Nerve current, velocity of, 46 Nerve fibres, meduUated, 40 ; non-medullated, 41 Nerves, 39 ; afferent, 38, 48 ; auditory, 239 ; cardiac, 03 ; classification of, 48 ; cranial, 192; degeneration of, 47,182; efferent, 38, 48 ; electrical phenomena of, 50 ; functions of, 48 ; intercostal. 48 : ori- gin or roots of,si ; peripheral distribution of, 49 ; specific energy of, 243 ; spinal, 38, 182 ; vaso - dilatator, 84 ; vaso-motor, 38, 84. Nervous system, 38, 185 ; general arrangement of, 38 ; influence on respiration, 100 Neuroglia, 39, App. Neurone, 43 Nceud vital, 83 Nucleoplasm, 2 Nucleus, 2 OcuLO-MOTOR nerve 192,225, 230 CEdema, 132 CEsophagus, 119 OifdCtory cells, 216 Olfactory nerve, 192 Omentum, 126 Ophthalmoscope, App. Optic axis, 230 Optic chiasma, 204 Optic nerve, 192 Optic thalami, 187, 197 Optic tract, 204 Optogram, 223 Organ of Corti, 239, 240 Organ of Golgi, 49i 212 Osmosis, 132 Otoliths, 240 Ovum, 3, App. Oxygen, 35, 85, 96. 97. 98 REN Pacinian corpuscles, 49, 207 Pain, 210 Palate and tonsils, 118 Pallor, 84 Pancreas, 122 ; cells of, 122, 123 Pancreatic juice, 132 Papillae, circumvallate, 213 Papillae, conical or filiform, 213 Papillae, fungiform, 213 Paraplegia, 183 Pars ciliaris retinse, 221, 222 Pepsin, 121, App. Pepsinogen, 120, App. Peptic glands, 120 Perception-time, 243 Pericardium, 66, gt Perichondrium, 20 Perilymph, 238 Perimysium, 31 Periosteum, 22 Peritoneum, 126 Perspiration, 163, 164, 16s, 170 Peyer's patche?, 127 Phagocytes, 55, 141 Pharynx, 86, 119 Phrenic nerves, 100, igi Pia mater, 179, 189 Pituitary body, 187 Plasma, 53 Pleurae, go, 91 Plexuses, 38 Pneumogastric nerves, 38, 83, 192 Pons Varolii, 38, 187, 194 Portal canal, 144 Portal vein, 65, 134, 143 Prickle-cells, 6 Proteid, 104, App. Protoplasm, 2, 3 ; composition of, 3 ; structure of, 3 Pterygoid muscles, iii Ptyalin, 115, App. Pulse, 79 Pulse tracing, 79 Punctum proximum, 228 Punctum remotum, 228 Pupil, 221 ; movements of, 229 Purkinje's figures, 223, 230 Pyramidal tracts, r8i, 190, 203 8UADRATUS lumborum, 92 uotient, respiratory, g6 Ramus communicans, 38, 205 Receptaculum chyli, 139 Recurrent sensibility, 182 Rectum, 137 Reflex action, 44, 114, 184 202 ' Reflex action, contrasted with voluntary, 45 Reflex time, 46, 184, 242 Reissner's membrane, 238 Reanin, 123, App. 464 RES Respiration, nature and ob- ject of, 85 ; influence on cir- culation, 102 ; influence of nervous system on, 100 ; in- ternal or tissue, 98 Respiratory movements, qr Respiratory movements, special, loi Respiratory quotient, 96 Respiratory sounds, 95 Restiform bodies, 190 Kete mucosum, 6 Retina, 222 ; corresponding points of, 231, 232 ; forma- tion of image on, 227 ; rods and cones of, 222, 223 Ribs, movements of, 91 Rigor mortis, 35 Rods and cones, 222 ; func- tions of, 223 Roots of spinal nerves, 182 Saccule. 238, 240 Saliva, 113 ; composition of, 115 ; uses and properties of, 116 Salivary secretion, nervous mecbanism of, 114 Sanson's images, 228 Sarcolemma, 31 Scala media, 238 Scala tympani, 238 Scala vestibuli, 238 Scaleni muscles, 92 Scbeiner's experiment, App. Sclerotic, 220 Sebum, 162 Semicircular canals, 238, 240 Senses.general remarks on, 243 Septum lucidum, 187, 188 Serum, 53, 60 Short sight, 228 Sighing, loi Size, perception of, 232 Skin, 6, j6o ; sebaceous glands of, 162 ; sweat glands of, 163 ; nerve-endings of, 207 Smell, organ of, 216; sensa- tions of, 217 Sneezing, loi Snoring, loi Sobbing, loi Solidity, perception of, 232 Sound, 234 Sounds of breathing, 95 ; of heart, 69 Speech, 177 Sphygmogram, 79 Sphygmograph, 79 Spinal accessory nerve, 192 Spinal cord, 38, 178 ; Clarke's column of, 180 ; functions of, 183 ; membranes of, 179 ; refles action of, 184 ; reflex Human Physiology TRO centres of i8,t^ ; structure of, 180 ; tracts of, 181, et seq. Spinal nerves, 38, 182, p. 426 Splanchnic nerves, 39 Spleen, 148 ; functions of. 149 Stammering, 177 Stapes, 237 Starch, 104, 116, App. Starvation, no Steapsin, 123, App. Stercobilin, 124 Stereoscope, 232 Stethoscope, 95, App. Stimuli, 3, 243 Stomach, sti-ucture of, 120; movements of, 135 Stomata, 90, 137 Stuttering, 177 Succus entericus, 128 Sugar, 104, App. Suprarenal bodies, 152, 154 Suspensory ligament of eye, 224 Sweat, 163, 164 ; nervous con- trol of, 165 Sympathetic system, 38, 186, 205 Synapse, 43 Systole, 68 Tactile corpuscles, 49, 207 Taste, organ of, 213 ; sensa- tions of, 215 Taste-buds, 214 Temperature, 168 ; regulation of, 171, 172 ; sensations of,, 2og Tendon, 16 Tetanus, 36 Thirst, 103 Thoracic duct. 139 Thymus gland, 151 Thyroid cartilage, 176 Thyroid gland, 150 Timbre, 177, 234 Tissues, I ; adenoid, 18, 138 ; adipose, ig ; areolar, 15 ; cancellous, 21 ; classifica- tion of, 4; compact, 21; connective, 14 ; muscular, 28 ; nervous, 28 ; letiform, 18 ; supporting, of nerve centres, 43 ; white fibrous, 16 ; yellow elastic, 17 Tone of arteries, 84 ; of mus- cles, 36 Tonsils, iiS Touch, 206 Touch, proper, 208 Trachea, 86 Tracts of nerve fibres, iSi Tricuspid valve, 69 Trigeminal, 192 Trochlear, 192 Trophic nerves, 48 ZYM Trypsin, 123, App. Tubuli uriniferi, 154 Tympanum, 235 Urea, 155 Urethra, 159 Urine, 155 ; secretion of, 156 : variations in amount, 157 Urobilin, 124 Utricle, 237, 238 Uvea, 221 Uvula, 118 Vagus, see Pneumogastric Valve, bicuspid, d-j, 68 Valve, coronary, 67, 68 Valve, Eustachian, 68 Valve, ileo-cascal, 135 Valve, pyloric, 135 Valve, semilunar, 67, 68 Valve, tricuspid, 67, 68 Valvulse conniventes, 127 Vaso-dilatator nerves, 84 Vaso-motor nerves, 38, 84 Vaso-vasorum, 74 Veins, 65 ; distribution and structure of, 76; valves of, 67 Velocity of nerve current, 46 Ventilation, 99 Ventricles, 66 ; left, 67 ; of brain, 188; right, 67 Vestibule, 238 Vibrations, perception of, 241 Villus, 127, 123 ; structure of, 133 Vision, bmocular, 232; direct 231 ; field of, 231 ; indirect, 231 Visual angle, 231 Visual axis, 231 Visual purple, 222, 223 Vital capacity, 91 Vitreous humour, 224 Vocal cords, 176, 177 Voice, 177 Voluntary actions, 44, 45 Vomiting, 135 Vowels, 177 Waste products, 105 ; elimi- nation of, 153 Water, 104 ; quantity excre- ted, 96, 105 ; uses of, 104 Xanthoproteic reaction, App. Yawning, ioi Yellow-spot, 222 Young-Helmholtz theory, 232 Zonule of Zinn, 224 Zymogen, 120, App. S^ottiswoode &^ Co. 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